Cnidaria

Chapter 5

Cnidaria Lawrence B. Slobodkin†

Patricia E. Bossert

Department of Ecology and Evolution, State University of New York, Stony Brook, New York 11794

Long Island Group Advancing Science Education (LIGASE), State University of New York, Stony Brook, New York

I. Introduction II. General Biology of Cnidaria A.  Body Plan B.  Nematocysts C.  Feeding D.  Reproduction and Metamorphoses E.  Ecological Interactions III. Ecology of Freshwater Cnidaria A.  Hydra B.  Craspedacusta C.  Calposoma D.  Cordylophora E.  Polypodium hydriforme IV. Collection and Maintenance of Freshwater Cnidaria A.  Collection Techniques B.  Maintenance Procedures V. Classification of Freshwater Cnidaria A.  Species Groups of Hydras VI. Taxonomic Key to Genera of Freshwater Cnidaria VII. Selected References

I.  INTRODUCTION Medusae, anemones, corals, and other polyps compose the ancient and remarkably successful phylum, Cnidaria. They occur as fossils in the lithographic stone of the MidCambrian Burgess shale and have not changed very dramatically since then. Fossilized cnidarian embryos are reported from the lower Cambrian[39]. The phylum name Cnidaria is derived from the Greek term for “nail,” based on their possession of nematocysts, which look like rods attached to a round capsule. The other name for the ­phylum †

Deceased

Ecology and Classification of North American Freshwater Invertebrates Copyright © 2010, 2010 Elsevier Inc. All rights reserved.

is Coelenterata, a term alluding to their saclike internal space, the coelenteron. A general overview of the phylum and survey of older literature is provided by Hyman[36] and the papers in various conference proceedings[49]. The freshwater representatives of Cnidaria are small animals belonging to the class Hydrozoa, with relatively few species and somewhat monotonous anatomy. They consist of the following taxa: The common and familiar hydra, a group of secondarily simple, solitary polyps. l The sporadically common Craspedacusta and Limno­ codium, jellyfish with minute, polyploid larvae[11,26]. l Calposoma, a tiny, colonial polyp, which is so small and inconspicuous that it is probably more common than it appears to be[26,59]. l Polypodium, which spends part of its life cycle as a parasite in the eggs of sturgeon and part as an ambulatory, predaceous polyp[47,60–61]; this species has been described primarily from eastern European rivers but should also be found in North American sturgeon. l Various estuarine coelenterates, which may occasionally occur in relatively fresh water; colonial, sessile animals of the genus Cordylophora will serve as an example of these[35,63]. l

Hydra and Cordylophora belong to the order Hydroida of the class Hydrozoa. Craspedacusta, Limnocnida, and Calposoma are classified as members of the order Limnomedusae. The parasitic Polypodium has been assigned to the order Trachylina of the same class (cf. Hyman[36]). The fact that the other three orders of the class Hydrozoa (Actinulida, Siphonophora, and Hydrocorallina) and the other three classes of coelenterates (Scyphozoa, Cubozoa, and Anthozoa) are not represented outside of the sea is curious. There is not even any serious speculation as to why freshwater invasion by coelenterates has been so 125

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severely limited. There is no special osmoregulatory organ in the phylum, but this is not an explanation since its absence has not stopped the successful invaders in this phylum. Because of their small size, soft bodies, and often sessile habits, freshwater Cnidaria are either not collected or not well preserved in most routine collecting procedures. They are, however, widely distributed and can be found in most ponds and streams when a specific search is made. When they are abundant, they can be major predators of small invertebrates and even of tiny fish. In turn, hydra is fed upon by flatworms, and crayfish eat Craspedacusta. Probably other animals prey on freshwater Cnidaria, but this has not been carefully studied.

II.  GENERAL BIOLOGY OF CNIDARIA A.  Body Plan All cnidarians share a simple body plan of a central cavity surrounded by two cellular layers (Fig. 5.1). The endoderm lines the interior cavity, the coelenteron. Between the endoderm and the ectoderm is an intermediate, noncellular mesoglea. Nematocysts aid in feeding and in repelling predators and are present in all freshwater and marine cnidarians. The feeding aperture or mouth leads into the coelenteron, which functions as a gut. By convention, the end of the animal with the mouth is termed oral, the opposite end aboral. The oral aperture serves at different times as mouth and anus. When the mouth is closed, the pressure of fluid in the coelenteron can stiffen the body, even in the complete absence of any hard tissue. The coelenteron, therefore, functions also as a hydrostatic skeleton. The mesoglea varies enormously in thickness among different members of the group. At its most meager, as in

hydra, it is not more than 200 m thick, containing only wandering cells, nonliving fibrous components, and fibers from neuromuscular cells. It is more fully developed in medusae, such as Craspedacusta, and may contain a great deal of collagenous or gelatinous material. The ectodermal body wall has neural and contractile properties and is also the location for ripe nematocysts. The presence of definite cell layers with differentiated functions distinguishes these animals as true metazoa. The absence of mesoderm implies that they do not have organs like higher metazoa; therefore, such terms as “tentacles” and “gut” are used in a functional sense. The basic body plan can be manifested as either a polyp or a medusa (Fig. 5.2). Polyps are typically elongated along the oral–aboral axis. Medusae are approximately bell-shaped and usually have their greatest body dimension perpendicular to the oral–aboral axis. The coelenteron of a polyp is usually deeper than its body is wide, while a medusa is usually wider than its coelenteron is deep. Also, medusae generally have relatively thicker mesoglea. In some species, different generations, and in some cases the same individual organism at different stages of its development, can adopt the form of either a polyp or a medusa. Around the feeding aperture of polyps or the edge of the bell of medusae is typically a ring of tentacles. Tentacles are extensions of the two cellular layers into more or less elongated projections. The coelenteron may or may not extend into the tentacles. Tentacular ectoderm is especially rich in nematocysts, which may be arranged in rosette or ring-shaped batteries. Tentacles are used in food capture, defense, and in some cases, locomotion.

B.  Nematocysts The phylum is characterized by the presence of cnidoblasts, ectodermal cells that produce the cell products called cnidia or nematocysts. Because there are many kinds of elaborately spined nematocysts, these structures are valuable characters for the classification of coelenterates, ­ particularly in such morphologically monotonous groups as hydra.

Nematocyst Mouth

Endoderm Digestive cavity

Mesoglea

Ectoderm Mesoglea

Digestive cavity Endoderm Ectoderm Mouth Tentacles

A Figure 5.1  Cross section through digestive cavity (coelenteron) of generalized cnidarian.

B

Figure 5.2.  Basic body plan of Cnidaria showing (A) polyp and (B) medusa.

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Nematocysts consist of a capsule containing a threadlike tube (Fig. 5.3a). Near the base of the tube is a projection reminiscent of a trigger. Remnants of the living cnidoblast are absent from mature nematocysts. Many nematocysts come to lie on the tentacles, often after having been moved through the mesoglea from some other region. After firing, a nematocyst consists of a long thread with a capsule at its base (Fig. 5.3b). Some nematocysts (the stenoteles or penetrants) are open at the thread tip, giving the appearance of a hypodermic syringe with a long needle. Stenoteles may have a complex of thornlike structures around the base of the thread. Stenoteles eject a neurotoxin which partially paralyzes the prey. Desmonemes (or volvonts), another form of nematocyst, seem to lack poison but rather eject sticky threads that wind about the spines and hairs on the body of the prey, interfering with movement. The capsules of some volvonts remain fixed to the tentacles after firing so that their exploded threads fasten prey to the tentacles as if by many tiny ropes or grappling hooks. At least 17 morphologically distinct forms of nematocysts are present in the phylum (Fig. 5.4). The penetrants and volvonts of hydra come in several forms, classified in terms of capsule size, spination of the threads, and shape and distribution of the basal spines. A single animal may have five or more types of nematocysts. Nematocysts of basically the same type may differ among species in how the thread is coiled inside the capsule prior to eversion. Some may appear like a coiled spring, with gyres at right angles to the longest dimension of the capsule, while ­others are coiled parallel to the long dimension of the ­capsule. Nematocyst structure was initially considered a

central taxonomic character, but recent evidence indicates that details of shape and coiling and also the relative abundance of nematocysts of different types are somewhat variable even within clones of hydra[15]. The microanatomy and function of nematocysts is a rich research area. The enormous interest about nematocysts and their production arises in part from the following observations. On the level of electron microscopy, they are extremely elaborate structures, making them of interest as examples of complex cell differentiation. Also, the mechanism by which the thin thread of the nematocyst everts from its coiled state within the capsule (like an enormously elongated, inverted finger of a glove suddenly turning itself inside out) is a difficult problem in fluid pressures. The poison that is secreted by some nematocysts through the open tip of the thread is of medical interest. Furthermore, the enormous diversity of shapes, sizes, and spination of nematocysts within single organisms and among the different coelenterates poses a problem in cell differentiation and genetics. Readers interested in more details on nematocysts should consult the large review volume by Hessinger and Lenhoff[34], which is too lengthy to summarize here.

C.  Feeding After a prey has been stung and encumbered by the nematocysts, the tentacles move the victim to the mouth, which opens to admit it into the coelenteron. Often the prey is still alive and active, but this ceases as soon as the gastric cells lining the coelenteron secrete digestive juices.

Figure 5.3  Discharged nematocyst from Hydra.

A

B

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D.  Reproduction and Metamorphoses 4

D

E

G

F

2

2

A C

J

H

B 3

3

1

5

P N O

1

2

3

K 6

L

3

7

R

S

3

T

6

M

Q Figure 5.4  Some of the types of nematocysts present in Cnidaria (redrawn from Hyman, 1940); (A) rhopaloneme (only in Siphonophora); (B) spirocyst; (C) same as (B), unraveling (not discharged); (D) desmoneme ((H) and (J) same as (D) but not discharged); (E) atrichous hydrorhiza ((L) same as (E), but discharged); (F) holotrichous isorhiza ((K) same as (F) but discharged); (G) stenotele inside its cnidoblast ((M) same as (G) but discharged); (N) microbasic amastigophore; (O) homotrichous microbasic eurytele; (P) heterotrichous microbasic eurytele; (Q) macrobasic mastigophore; (R) teleotrichous macrobasic eurytele; (S) heterotrichous anisorhiza; and (T) microbasic mastigophore. 1, Capsule; 2, tube; 3, butt; 4, cnidoblast; 5, its nucleus; 6, lid; and 7, stylet.

Since there is no anus, food cannot be passed along the gut while digestion continues (as in higher metazoa). Instead, feeding stops until the digestion process is completed and the indigestible remnants have been regurgitated. In hydra, ingested food decomposes within an hour into a slurry of particles. The role of food vacuoles is reminiscent of the feeding process in protozoa and sponges[5]. The free borders of the digestive cells ingest particles by pinocytosis. Individual food particles are enclosed in vacuoles that are moved through endodermal cells. Eventually, their indigestible residues are ejected by the endodermal cells and returned to the coelenteron to leave ultimately through the mouth.

In the coelenterates, many kinds of reproduction exist[36]. Like all metazoa, they can reproduce sexually. The fertilized eggs may produce larvae differing anatomically and ecologically from the adult sexually reproducing stage. In marine coelenterates, there may be an elaborate succession of larval stages, some of which may form colonies or reproduce vegetatively by budding or fragmentation. In any particular species, one or more of these stages may be missing. There are medusae that produce eggs that go through various larval stages to produce new medusae (Fig. 5.5a). Some polyps produce gonads (Fig. 5.5a), and others bud off medusae that either swim away to become sexual (Fig. 5.5c) or remain attached to their parents and become sexual without ever feeding independently. Polyps or medusae may produce new individuals that may or may not resemble their “parent.” A new individual may separate from its parent, but in many coelenterates, the asexually produced individuals stay attached and form a colony. The development of colonies is absent in hydra, but occurs among all other freshwater Cnidaria. In the anatomically simplest coelenterate colonies, such as the colonial microhydra, “larvae” of the freshwater Craspedacusta, all attached individuals are essentially similar in both form and function[56]. Each has its own mouth and coelenteron and can bud new polyps. Microhydra larvae can produce medusae by budding. Colonies of many coelenterates are much more elaborate. In a common modification of this process, certain members of the colony become gonozooids that neither feed nor have tentacles but instead consist of a stalk rising from the common stolon that buds off medusae. These medusae, in turn, either form gonads while still attached to their colony or leave the colony and then produce gonads. The best-known freshwater coelenterates, the hydra, have lost the medusa stage entirely. In hydra, polyps may produce new polyps by asexual budding or may temporarily switch to sexual reproduction using gonads. Fertilized eggs derived directly from polyps may then hatch to produce new polyps. There are several marine examples but only one freshwater example of medusae budding new medusae. This is in a species presumed endemic to Lake Tanganyika in the great African rift[75].

E.  Ecological Interactions Although some species may gain part of their nourishment from intracellular algal symbionts, all cnidarians are carnivores. Their prey consists primarily of small coelomate animals, organisms that evolved long after the appearance of coelenterates. They generally do not feed on protozoa, nematodes, or sponges, nor will these animals trigger the nematocysts. This raises a curious question, for which there is no evident answer: “What did the first coelenterates eat?”

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Planula

Medusa

Actinula A

Egg Gonad Medusa Egg Female Male

Polyp B

Planula C

Figure 5.5  (A) Sexual medusa produces larva; larva forms new medusa; (B) sexual polyp generates new polyp; and (C) polyp produces medusa which becomes sexual and forms new polyp (redrawn from Barnes, 1966).

Hydra are extremely effective predators. Prey includes crustaceans, worms, and fish and insect larvae. They have been seriously considered as a biological control organism for mosquitoes. Hydra and Craspedacusta can both sting fishes very badly. Fish that are too big to swallow often cannot survive being stung. For this reason, hydra are sometimes serious pests of fish hatcheries. Nematocysts are sufficiently unpleasant that relatively few predators attack coelenterates. Turtles, fish, crabs, worms, echinoderms, and flatworms are among the predators on marine coelenterates. Predation on the freshwater coelenterates is not well studied. Crayfish eat Craspedacusta[20]. Flatworms are reported to eat hydra[36,38], but we have seen small flatworms withdraw from contact with hydra. Some marine coelomates are not affected by nematocysts and are commensal with coelenterates. On coral

reefs, clown fish live among tentacles of large anemones and other fish occur only in close association with the Portuguese man-of-war. In freshwater, Anchistropus, a chydorid cladoceran, has been observed clinging to, and apparently feeding on, the body wall of hydra[30] (Slobodkin, personal observation). The amoeba Hydramoeba hydrox­ ena feeds on hydra[74], and the hypotrichous ciliate Kerona lives on the surface of hydra[36]. Polypodium hydriforme is parasitic on a fish for part of its life[61].

III.  ECOLOGY OF FRESHWATER CNIDARIA Their anatomical simplicity and the ease with which hydra can be cultured in the laboratory make freshwater Cnidaria very important as experimental material in cellular and

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developmental biology, as well as in neurobiology, biochemistry[17], and genetics[64,67]. Hydra also lend themselves to studies in cell growth, morphogenesis, microanatomy, and symbiosis[9,17,21–23]. In the present chapter, we will focus on cnidarian ecology and natural history, which have been less well studied.

A.  Hydra 1.  General Biology of Hydra Hydra are small polyps from 1 to 20 mm in body length. The body is crowned by up to 10 or 12 tentacles. Usually the tentacles are approximately the same length as the body but may be somewhat shorter, particularly in the green hydra, and can exceed 20 cm in length in hungry brown hydra in quiet water. There are no medusae. Reproduction is by budding or by gametes produced from multipotent stem cells in the interstitial cell compartment[73]. The fertilized eggs may enter a resting stage. When development proceeds, the egg immediately develops into a polyp (Fig. 5.5). Hydra are found attached to almost any reasonably hard surface. Slight bacterial films may make surfaces more attractive, but heavy growth of microalgae may be avoided. Water lily stems, charophytes, dead leaves, sticks, and stones are favored substrates. They may appear as single animals or as dense fur-like aggregations. There are reports of fishing nets becoming completely covered with brown hydra, resulting in rashes on the hands and arms of the fishermen[6]. If the wait for food is longer than approximately 12 hr, the hydra begin to change their locations on the substrate[44,62].They have two different methods of movement. Small-scale movements on a substrate may occur by attaching the stretched tentacles to the substrate or in shallow water, to the surface film, releasing the pedal attachment and contracting the tentacles and reattaching. Sufficiently crowded or hungry hydra float off their substrate[48]. They may appear in the plankton or be found floating upside down with the pedal disk in the water surface film and their tentacles trailing. Batha[6] extensively documented the existence of planktonic hydra in Lake Michigan through use of divers and of suitable attachment surfaces suspended in midwater. In addition, the reports of fishnets covered by hydra and observations by Griffing[30] of sudden relocations of hydra within a single pond suggest that hydra are probably much more important components of lake plankton than has been generally realized. Floating animals will sink and settle either from wave and current action or from having just fed. This behavior keeps hydra in areas of abundant food. It is also evolutionarily important as a dispersal mechanism. From the perspective of a naturalist, it has the effect of making it relatively easy to collect hydra at the outflow of lakes and ponds.

They are not tolerant of heavy metals, but they can thrive in even highly eutrophic water. They can live at temperatures from near freezing to 25°C. Several hydra species may coexist in a pond; often, a small green hydra and at least one large brown species will co-occur. Also, strains of hydra may replace each other seasonally. Bossert[8] showed that green hydra collected several months apart from the same small pond had very different size and growth characteristics when maintained under very similar conditions in the laboratory. Despite many years of collection and observation, we have never found a species of green hydra that was consistently larger than any strain of asymbiotic brown hydra, nor have we found any contradictory account in the scientific literature. If it occurs, it certainly seems rare. Some large brown hydra can be caused to become green in the laboratory[59], but it is not clear that this is significant in nature. Hydra occur in freshwater from the Amazon to Alaska and from Siberia to Africa at depths from shallow water to 60 m or more. This cosmopolitan distribution may be a result of the portability of the thecate eggs, or perhaps, as has been suggested by Campbell[15], it might be due to the four species groups (see Section V) having differentiated before the primeval continental masses separated in the Mesozoic era. A study of the hydra of Madagascar[16], which has been an island for more than 100 million years, shows that they differ very little from mainland hydra, indicating that either the rate of evolution of the genus Hydra is very slow or that they are surprisingly well dispersed. One species group (oligactis) is missing from Madagascar. Since there is no obvious reason why hydra of this group should be less mobile than the others, their absence strengthens the interpretation of slow evolution. This implies that hydra are evolutionarily very old.

2.  Hydra Reproduction and Mortality Asexual budding is the primary reproductive mechanism of hydra during periods of population increase. Under optimal conditions of food supply, temperature, and water quality, each adult hydra polyp can produce two buds per day and each bud can mature and begin reproducing in a week or less. Generally, the smaller strains of hydra bud more rapidly than the larger ones, at equal feeding rates. Buds are from 12 to 20% of the size of the mother, varying with the strain. While hydra usually reproduce by producing free-­living buds, under deleterious environmental conditions they may develop testes or single egg ovaries and engage in sexual reproduction. External fertilization by free-swimming sperm occurs while the egg is attached to the body wall of the mother. The embryo may enter a resting period of days or even months before proceeding with direct development into a new polyp. During sexual reproduction, gonads develop along the stalk in place of buds (Fig. 5.6). Mature male gonads are

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Chapter  |  5  Cnidaria

A

1 mm

C

B

Figure 5.6  Hydra. (A) Male gonad; (B) female gonad; and (C) egg (redrawn from Campbell, 1987).

a mound of tissue with a distinct apical nipple from which sperm extrude. Mature female gonads consist of a single large egg cell resting on a cushion of smaller cells. The fertilized eggs are surrounded by a theca, which may be smooth or ornamented or may consist of polyhedral plates. The features of the theca are of taxonomic importance. Sexuality in hydra seems to occur only under deleterious environmental conditions, although the precise cues are unknown. Chemical changes, temperature fluctuations, and perhaps sudden nutritional variation can all induce sexuality at certain times[38,43]. Individual hydra have been reported as unisexual and some bisexual individuals have been noted[36,38]. If conditions improve, sexual hydra can return to asexual reproduction. Buds and gonads may occur concurrently in the same individual. Most studies of hydra sexuality are made on animals that have been maintained in the laboratory. Batha[6] suggested that sexual reproduction is very rare in nature, based on the complete absence of gonads among thousands of brown hydra collected in Lake Michigan. Although we have found eggs in green hydra collected from a small pond and other field reports of gonads do exist[38], it seems clear that in hydra the numerically most important reproductive process is budding. In most plants and animals, population size and genetic recombination are associated through the process of ­sexual reproduction, but in hydra sexual reproduction is not ­significant to population dynamics. It seems important for maintaining genetic heterogeneity and permitting escape from temporarily unsuitable conditions (cf. the chapter on Escape in Time and Space in Slobodkin[70]). In a single pond or stream, most of the animals are likely to belong to a rather small number of vegetative reproductive lines, being genetically identical except for

occasional mutations. Even those that have emerged from eggs are likely to be the result of relatively close inbreeding, since sexuality is usually found in crowded local populations whose members have descended from a very small number of clonal lines. There is no direct evidence on sperm survival in nature, but it seems unlikely that sperm can travel great distances. Hydra that are very small, either because of youth or starvation, will not produce buds. If nutrition is limited, animals may be kept indefinitely in a condition of neither growing nor budding. Both body size and budding rate are proportional to feeding rate up to a point of food saturation. The capacity of hydra to reduce their body size is of ecological interest. Animals with hard skeletons are committed to a particular body size in the sense that they cannot shrink below a given point during periods of food shortage. The inability to reduce size may contribute to death by starvation. By contrast, hydra shrink in size when starved but can be restored to full reproductive size by increasing their food supply. At a given temperature, hydra will come to a steady state body size and budding rate if food supply is sufficient and constant. The time for complete size adjustment to either temperature change or feeding-level change on the part of an individual brown hydra is approximately 3 weeks. Reduction of temperature decreases growth rate but increases body size. These size differences seem to be due to changes in cell number rather than cell size (Fig. 5.7)[32]. Because larger animals have greater food requirements, the smaller green hydra can produce buds when fed one or two Artemia nauplii per day, while the larger brown hydra require from 5 to 10 per day before they will bud.

3.  Hydra Feeding The tentacles extend above and lateral to the body. They are generally motionless except in the presence of potential food or during locomotion. In healthy animals, the tentacles are cylindrical or tapered but never clubbed. The length of the tentacles varies somewhat with species, but considerably more with environmental circumstances. In quiet water, animals with bodies no longer than 2 cm have been observed to constrict their tentacles into thin threads extending at least 20 cm (L. B. Slobodkin, personal ­observation). This is unusual, however, and tentacles equal to three body lengths or less are more typical. Tentacles are quickly retracted if the animal is disturbed or if organisms brush against them. Whole prey or extracts of their body fluids will initiate active waving movements of the tentacles[42]. If a prey organism brushes against the tentacles, nematocysts will discharge, poisoning the prey and attaching it to the tentacle. Other tentacles and their nematocysts then join the attack. In a matter of 1–4 min, the prey will have been pressed against the mouth

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Ecology and Classification of North American Freshwater Invertebrates

13 °C

20 °C

27 °C

Figure 5.7  Relative sizes of hydra raised at different temperatures. Scale shown is the same size at each temperature (redrawn from Hecker and Slobodkin, 1976).

surface by the tentacles and will have entered the coelenteron. Meanwhile, the tentacles for subsequent swallowing may have caught other prey. The number of prey swallowed in one feeding encounter varies with the size of the hydra, the size of the prey, and the previous feeding condition of the hydra. One large Daphnia magna may fill the coelenteron completely, stretching its walls to give the appearance of a Daphnia stuffed into a thin expandable sack of hydra tissue. During the digestion process, the body may become rounded as the swallowed prey is reduced to a slurry. After approximately 1 hr, material is suddenly discharged from the coelenteron through the mouth, the hydra momentarily appearing like a punctured balloon. The columnar shape is then restored, tentacles regain their virulence, and the animal waits for its next meal. There is evidence that several days of starvation will increase the appetite of a brown hydra. Brown hydra can survive more than 40 days without food, and green hydra can live without food 4 months or longer. In general, hydra eat small, open water plankters but are less effective at capturing animals that normally inhabit underwater surfaces. The common cladoceran genera Simocephalus, Scapholebris, and Chydoris and at least some ostracods are immune to the activities of hydra[68]. Hydra can eat very small fish and insects but sufficiently large animals with hard skeletons and strong swimming force can escape after being stung. The long bristles on small midge larvae have been found to impede predation by hydra[33]. Apparently hydra primarily feed on the kinds of prey that they are least likely to encounter. This suggests that hydra are sufficiently important as natural predators that only those crustaceans that have evolved immunity to hydra can coexist closely with them.

4.  Immortality and Regeneration in Hydra Trembley, in the eighteenth century, demonstrated the capacity of hydra to regenerate perfectly, even after severe mutilations. The regenerative powers of hydra make them favorite classroom objects. They can be decapitated, bisected, have their tentacles amputated, or even be turned inside out and in a matter of a few days regenerate missing parts or regain their proper organization. Within a clone, rings of hydra stalk can be threaded on hairs like quoits and may fuse to form a single tube[14]. Even without operations, different accidental conditions in the field or laboratory will produce hydra with various mutilations, more or fewer tentacles, and missing heads, and so on. All of these hydras will reorganize themselves neatly if water chemistry is not deleterious and if they have been reasonably well nourished before the mutilations occurred. Since hydra can regenerate so well, are they potentially immortal? Immortality in any organism is impossible to demonstrate during a research program of finite duration, and therefore the question cannot be unequivocally settled. However, what we generally mean by immortality in organisms is the absence of any signs whatsoever of senescence or permanent scars of the past. Martinez[50] maintained laboratory cohorts of hydra for more than 3 years and demonstrated the absence of any symptoms of senescence. Single polyps have been maintained in our laboratory without budding for at least a year, their lives terminating only from human error. There is no clear evidence for senescence of any kind in hydra. If hydra are starved, they will become smaller, but do not die until they are too small to feed. Newborn hydra are approximately at this minimum size[71]. It seems likely that hydra polyps are potentially immortal. All deaths in hydra can be assigned to such things as alterations in water quality or food supply, temperature shocks, excessively severe starvation, or predation.

5.  Hydra Symbiosis While species of hydra may be difficult to distinguish, there is a very clear distinction between the brilliant green color of some species of hydra and the yellow, brown, and gray colors of those that do not have symbiotic algae. The green hydra are accepted as a taxonomically distinct group and have been assigned their own generic status as Chlorohydra, which we use here interchangeably with “green hydra.” No certainty exists as to the monophyletic character of this genus. The green color arises from Chlorella-like cells, unicellular algae each occupying a vacuole in the endodermal cells of their hosts. Each endodermal cell contains 10–35 algae-laden vacuoles (Fig. 5.8). The precise number varies with species but is nearly constant within a hydra strain and a set of environmental circumstances[8]. Algae are also

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Figure 5.8  An algae-laden endodermal cell from Hydra (viridis) (1000).

found in the central cells of the tentacles but in somewhat smaller numbers. The endoderm of the buds contains algae like those of the mother. Ample evidence exists showing that green hydra gain nourishment from their symbionts. Radioactive tracer experiments have demonstrated that maltose, a secondary photosynthetic compound, leaks from the algal symbionts to their hosts[17,41,52]. Also, there is microscopic evidence that algal cells can be attacked by host lysozymes[21–23]. In competition experiments between brown hydra and green hydra populations, light provides a significant advantage to the green hydra[69]. The algae can be removed from some strains of green hydra by using a variety of techniques, including photosynthetic poisons, prolonged darkness or extremely strong light, and dilute glycerin solutions[55]. Such hydra are referred to as aposymbiotic and are susceptible to reinvasion by algae. It is possible to develop green patches in brown hydra by injecting their coelenteron with algae or by feeding green hydra to brown ones (L. B. Slobodkin, personal observation), but this coloration fades with time. If the association with algae is advantageous, “why are big hydra not green?” Symbiotic relationships involve a delicate interaction between the two partners, particularly when one partner lives within the cells of the other. A primary requirement for stable endosymbiosis is a mechanism

providing balanced rates of mitosis of the host and symbiont cells. If there were no control of the algal rate of increase, they would be expected to kill their host cells by filling them with algae, and if the hydra cells excessively limited the algae, they would be eliminated and the hydra would be brown. While the number of algae per host cell stays constant in all green hydra, maintaining this constancy seems more difficult in the larger green hydra strains. Bossert and Dunn[9] have shown that algal cells are increasing faster than host cells in green hydra strains of all sizes, but the disparity between algal and animal mitotic rates is greatest in the largest strains. Bossert and Dunn[9] and Dunn[23] suggested that algal cells are being actively expelled or digested by all green hydra, but especially by larger strains. Size in green and brown hydra is apparently regulated by the relative amounts of several hormones, some of which activate the formation of buds while others inhibit budding[64–65]. Bossert[8] found that one of the hormones implicated in producing smaller hydra size also inhibits algal mitosis within green hydra cells. Apparently, the same hormonal mechanism that produces small size aids in maintaining the balance between algal cell and hydra cell increases, while a hormonal balance that permits larger size makes control of algae more difficult and, in the largest strains, impossible. More investigation of this problem is needed. Green hydra are never found in nature as aposymbionts and seem to have evolved a dependence on algae. However, these algae have probably not evolved a dependence on hydra. In fact, how the algae benefit from the association is not at all clear. Certainly the Chlorella in a green hydra are immune to being eaten by filter feeders, have an assured source of mineral nutrients, CO2, and nitrogen, and are moved into light as the hydra move. However, the actual rate of increase of algal cells inside hydra is probably lower than those of algal cells outside, and it is not obvious that the number of algal cells contained in the entire green hydra population is a significant fraction of the natural algal population in the lake or pond.

6.  Hydra Toxicology With the development of modern techniques, there has been an impressive increase in the number and significance of research programs focused on hydra. For example, like any small, easily cultured animal, hydra have been ­attractive for use in ecotoxicology. In these tests, the survival or death rate of organisms is typically related to concentrations of particular toxins. Toxicity tests using specially prepared hydra can provide evidence as to whether or not a particular toxin is likely to cause cancer. The procedure involves mass culture

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of Hydra attenuata, disassociation of the hydra into individual cells that are aggregated into “artificial embryos.” The small, mixed cell hydra patties organize themselves into complete and perfect animals in less than a week. It has been found that toxins which do not produce cancers are approximately equally toxic to intact hydra and to the reconstituting patties while known carcinogens are much more toxic to the regenerating cell mass than to the intact hydra[37]. The toxicological studies as well as mathematical analyses of hydra population dynamics and evolution[71] are all possible because of the extreme anatomical and behavioral simplicity of hydra[37].

7.  Molecular Biology of Hydra Biology as an experimental science began when Swiss native Abraham Trembley asked a “simple question.” Convinced that the green polyps he had collected were animals, he cut them, recognizing regeneration as a plantlike character and in so putting them to the test discovered regeneration. The understanding that all animals share essentially the same complement of genes has brought this little creature to the center of research involving pattern formation and stem cell differentiation. Scientists working on the Hydra Genome Project have isolated and characterized over 100 hydra genes with many more in progress. The biochemical and genetic properties of Cnidaria are also of significance in studies of the evolution of metazoa. According to a recent review by Galliot and Schmid[28], “From the molecular data accumulated during the past ten years, it appears that among the four Cnidarian classes, the Anthozoans (sea anemone, coral) arose first[12–13,54,66]” (Fig. 5.9a). Despite this finding, the extreme simplicity of hydra anatomy (Fig. 5.9b,c) suggests that it may provide a ­useful molecular model for higher metazoa. Galliot et al.[29] reviewed hydra as a niche to study cell and developmental plasticity. They presented a summary of the cellular and molecular changes that occur during the early stages of regeneration as cells are being reprogrammed (see Fig. 5.10).

The control of basic body form in hydra involves an interaction of homeotic genes, some of which are largely identical with body form genes in mice and fruit flies[2,7,25,27]. The implications of this are that the basic developmental processes of the first metazoa (or perhaps advanced premetazoan ancestors of the first metazoa) evolved a developmental control mechanism which has remained essentially intact throughout all subsequent metazoan evolution!

B.  Craspedacusta The first scientific accounts of freshwater medusae were based on specimens found in the giant water lily tank of Regents Park in London in 1880 (Fig. 5.11a). Two authors described these separately. Lankester named them Craspedacusta sowberii after the discoverer, Mr. Sowber, and Allman named the same organisms Limnocodium vic­ toria after the lily. In the same year and in the same tanks, a tiny colonial hydroid was found. This animal had no tentacles; each polyp terminated in a bulbous capitulum studded with nematocysts. In the center of the capitulum was the mouth. These little polyp colonies (Fig. 5.11b) were initially and correctly assumed to be the larval stage of the medusae. However, very similar polyps were discovered in a water tank in Philadelphia and were described and named as a separate species. It was not until 1928 that it became once again obvious that the polyp named Microhydra ryderi was the larva of Craspedacusta[11]. Since their initial description, Craspedacusta medusae (Fig. 5.12) have been found in many locations around the world, apparently transported with ornamental aquatic plants and with the water hyacinth, a recently spread pest species. They have also been found in locations free of imported plants, for example Lake Gatun in the Panama Canal Zone (Slobodkin, personal observation). Since first documented in the United States in 1908, these unpredictable and sporadic organisms have been reported in 35 states. The jellyfish have been found in lakes, farm ponds, quiet coves of rivers, and old water-filled quarries. When

Endo

Ecto

Porifera (sponges) Ctenophora (combjellies) Cnidaria Ancestral protists

Hydrozoans Cubozoans Scyphozoans Anthozoans Protostomes Deuterostomes

A

B

C

Figure 5.9  (A) Phylogenetic scheme showing the position of cnidarians in the metazoan tree; (B) bilayered organization of the hydra polyp, with only the endodermal (endo) and ectodermal (ecto) myoepithelial cells depicted; (C) external anatomy of the hydra polyp (Galliot et al., 2006)[29].

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EcMEC i-cells

EnMEC

Immediate 0 – 1 hr

CREB

P-CREB

β-catenin, Tct BMP5-8b, CREB Kazall, hydkk 1/2/4 RSK, CREB

Early 0 – 6 hrs

STK, RSK, P-CREB, CBP, prdl-a HMP1, laminin-β1 Heady, wnt, BMP5-8b, nPKC, PKB, RSK, STK, hyp1, prdl-a, hyBra, Tct, CREB, CBP, Cnnos2 Hcol-1 Hcol-IV

RSK, CREB

Early-late ≥ 16 hrs

Late ≥ 40 hrs

Laminin, HMP1

Adult-like pattern

budhead, Tsp1, cngsc

adult-like pattern

prdl-a, cnox-2

cngsc, cnot, RF-amide

prdl-a

RF-amide

Ks1, HMP1 Hym-301, cPKC

ECE, hyAlx, cnox-3

HMP1

Hcol-1

Figure 5.10.  Successive cellular and molecular phases displayed by head-regenerating tip from midgastric amputation up to apical regeneration. The ectodermal myoepithelial cells (EcMEC) are drawn white with light-blue nuclei. The endodermal myoepithelial cells (EnMEC) are digestive cells at the time of bisection (elongated, dark grey with red nuclei), then transiently transformed into blastema-like cells (roundish, dark grey with blue nuclei), and progressively regain their original epithelial organization. Similar modifications of the EnMEC were observed by Murate et al. during regeneration after reaggregation. Tentacle buds start to appear after 40 hr, while the hypostome (dome surrounding the mouth opening) is forming. Arrows represent the amputation plane. For each phase, genes (light colors) or proteins (darker colors) that are specifically up-regulated are indicated with a color code according to the cell lineage where they are expressed: EnMEC (green), EcMEC (blue), i-cells (yellow). Genes tested in functional assays are underlined (Galliot et al., 2006)[29].

they occur, they are locally abundant but they are sufficiently rare to call to public attention[4].

1.  Craspedacusta Life Cycle The most conspicuous stage of the life cycle is the small medusa (0.5–1.5 cm). Except in the Yangtze river system of China, the occurrence of noticeable populations of the medusae of Craspedacusta is sporadic and often surprising to local naturalists. As the water warms in a pond or in the slow current of a stream backwater, a swarm of medusae appears, often where it has never been seen before or at least has not been apparent for many years. These medusae feed on zooplankton. As they grow, the number of tentacles increases from 8 to 12 up to as many as 100. After several weeks of growth, gonads develop in pouches of the radial canals. Fertilized eggs produce a small crawling planula, which then differentiates into a microhydra[56]. The planula consists of two cell layers forming a ­doublewalled, sausage-shaped sac. The differentiation of the planula into a microhydra is rather simple. It stands on end and both a mouth and a capitulum develop on the unattached end, thereby forming a polyp. The new polyps may continue to bud off new microhydra or medusae. Buds may remain attached after they have

developed a capitulum, producing colonies of up to 12 polyps attached to a common stolon. Starvation and severely abnormal temperatures cause the polyps to shrink to a cellular ball, surrounded by a chitin-like membrane. This can persist through the severe conditions and then redifferentiate as a polyp. The buds of medusae appear initially as rounded swellings of the polyp wall (Fig. 5.11c). Over a period of several weeks, they enlarge and develop a central manubrium. An endoderm-lined circular canal with four radial canals leads to the base of the manubrium. Eight tentacles emerge from the circular canal. The medusa bud is initially covered by a layer of tissue that eventually perforates centrally, remaining as the vellum of the adult. The mouth opens into the manubrium. The medusa is by now at least as large as the polyp, to which it is still attached at the aboral end. Eventually, it begins locomotory pulsation and separates from the polyp, completing the life cycle[56]. Medusae usually reproduce sexually: fertilized eggs develop into planulae, which transform into microhydra.

2.  Craspedacusta Ecology The microhydra polyps feed in essentially the same fashion as hydra. Their small size limits their prey, but this is

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Ecology and Classification of North American Freshwater Invertebrates

C

1

Gastral cavity

A

8

Gonad

7

Radial canal

2 Tentacle

Velum

Manubrium

3 9

4 B

5

6

10

11

Figure 5.11  (A) Craspedacusta sowberii, also named Limnocodium victoria (see text); (B) larval stage of (A), also named Microhydra ryderi (see text); and (C) stages in development of Craspedacusta (redrawn from Payne, 1924).

partially compensated for by their colonial growth pattern, which permits several polyps to make a simultaneous attack. Free-living medusae feed on various crustacean zooplankters. They can even kill, but apparently not swallow, the large (0.5 cm) predaceous cladoceran Leptodora[20]. As a medusa feeds, it grows larger, adding additional tentacles at both the vellum margin and its inner edge. Several hundred tentacles are found on mature animals. Medusae appear sporadically in shallow ponds, natural lakes, and artificial reservoirs throughout the north and south temperate zones. Often, entire medusa populations are unisexual. The polyps are so inconspicuous that they probably have a much broader distribution than the literature reports. Many locations containing polyps are reported not to have medusae. This bewildering picture has been clarified by Acker[1] and Kramp[40]. Acker suggested that medusa production by the polyps requires temperatures greater

than approximately 20°C during a period of increasing temperature and adequate, but not enormous, food levels. Other environmental alterations may be significant but have not been tested. These conditions are reminiscent of those that produce sexuality in hydra. Acker and Kramp maintain that the original natural habitat of Craspedacusta sowberii is the Yangtze-Kiang region of China. In the upper river valley, two Craspedacusta species C. sowberii and C. sinensis coexist, whereas only C. sowberii reaches the downstream areas. From this habitat, C. sowberii traveled with water hyacinths and other plants to its present worldwide distribution. Shallow pools exist along the lower Yangtze valley which is subject to large temperature changes and to sudden flooding from the main river during high water. Plankton populations in these ponds fluctuate strongly. In these ponds, medusae are a recurring phenomenon throughout the year. The occurrence of medusae in the spring is so regular

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Figure 5.12  Medusa of the common freshwater jellyfish, Craspedacust sowberii (photograph by K. Dunn, with permission).

that they are given a common name that translates to “peach blossom fish.” The generic name Limnocodium is usually applied to freshwater medusae of the Old World and Craspedacusta to those of the New, but it is not clear if this is not more than a geographic distinction nor has there been enough investigation to determine how many species of freshwater medusae actually exist. Due to their sporadic occurrence, the changes in size and tentacle number with developmental stage, the simplicity of larval anatomy, and the difficulties of preserving specimens, morphologic studies of freshwater medusae are difficult. As in hydra, many species have been described, but perhaps some of these are based on nutritional or developmental history or ­ preservation artifacts. The strongest evidence for multiple species is that sympatric populations of two species of medusae are known from the Yangtze and that a medusa from Lake Tanganyika has an extraordinarily different life cycle. In this latter case, a mature medusa buds new medusae from its manubrium in addition to reproducing sexually[10].

C.  Calposoma This is a colonial polyp not much bigger than a paramecium and similar in general appearance to the microhydra except that it is considerably smaller and has tentacles rather than a capitulum. As far as is known, there is only one species, Calposoma dactyloptera[26]. Some of its properties have been described by Rahat and Campbell[58]. These organisms are so small that their tentacles consist

Figure 5.13  Cordylophora (redrawn from Roos, 1979).

of a single cell, a tentaculocyte, which contains a row of miniscule nematocysts. The full natural history of these animals and the details of their life cycle are not known. Their general anatomy is reminiscent of a miniaturized hydra with stiff tentacles, but this does not necessarily indicate taxonomic or evolutionary proximity.

D.  Cordylophora The genus Cordylophora is an athecate member of the primarily brackish water and marine hydrozoan family Clavidae. It grows as a branching colony up to 5 cm high. The feeding polyps have a conical hypostome on which filiform tentacles are irregularly arranged. Colonies also include gonophores, which produce gonads. There is a chitinous periderm (Fig. 5.13). During periods of stress, the animals regress to metanonts, masses of resting tissue in the hydrorhizae[53]. The metanonts appear when the plant stalks on which they grow begin decomposing in the fall[63]. Cordylophora was first found in the Caspian and Black Seas. It has been suggested that it evolved during the time

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Ecology and Classification of North American Freshwater Invertebrates

that these two bodies of water were connected. It now occurs in rivers of Europe and America, apparently having spread during the last century attached to ship bottoms or ballast. The recent expansion of range may be attributed to the relatively greater speed of seagoing vessels during the last 100 years which would shorten the time of saltwater immersion between brackish water ports[63]. Industrial and navigational developments have extended the region of brackish water in some estuaries, and the increasing pollution of rivers may have duplicated estuarine conditions. This may be expected to encourage the geographic spread and the upriver movement of not only Cordylophora but also other estuarine Cnidaria[35]. The possibility of foreign Cnidaria appearing in habitats where they have never been seen is also enhanced by rapid vessels and the use of water ballast discharged in ports of arrival. Cordylophora is one example of an expanding range. We expect that there are many others, which we have not attempted to survey.

E.  Polypodium hydriforme The remaining freshwater coelenterate that we will consider is the strange and poorly understood Polypodium hydriforme, originally described by Lipin[46–47] and later examined in detail by Raikova[60,61]. This tiny Hydrozoan was first discovered in Eastern Europe as a parasite inside the eggs of the European sterlet (Acipenser ruthenus), a small member of the sturgeon family. It is particularly interesting as it is perhaps the only endoparasitic coelenterate. Obviously there are modifications associated with its parasitic habit that make classification difficult, but it is reported to have only one type of nematocyst. These match the type found in the narcomedusae, an extremely toxic group of hydrozoans.

The development of the larval Polypodium (Fig. 5.14) is closely coordinated with that of the sturgeon egg. A binucleate, single-cell stage is known from immature sterlet oocytes. The two nuclei are unequal in size and chromosome number. The smaller, reportedly haploid nucleus is surrounded by the large polyploid nucleus, which develops into a trophic envelope around the embryo formed by the division of the small nucleus and its surrounding cytoplasm. By the time the host oocyte has started to accumulate yolk, the Polypodium is a two-layered planula approximately 1 mm long. It has a flagellated external layer, which eventually will become the endoderm and an internal layer of ultimate ectoderm, all surrounded by a capsule that serves as a digestive organ for consuming yolk[61]. After a month, the planula has developed into a stolon with internally directed buds and tentacles. At this stage, the Polypodium is a colony consisting of a straight stolon from which projects as many as a dozen knobs, arranged linearly with their apices toward the center of the egg (Fig. 5.14a). Each knob develops two indentations. From each indentation, 12 tentacles, two of which are short and stubby, project into the stolon itself (Fig. 5.14b). As the fish eggs ripen and are released from the fish, the tentacles evert through a slit in the stolon. Simultaneously, the knobs invert, developing a coelenteron lined by what had been the surface exposed to the egg yolk. The stolon breaks up and the knobs now appear as somewhat bifurcated polyps with 12 tentacles on each head and a coelenteron full of fish egg yolk (Fig. 5.14c). The free-living polyps subdivide by longitudinal fission. They crawl on the bottom using tentacles as walking legs, aided by nematocysts (isotrichous isorhiza) which hold the substratum. They feed on turbellaria and oligochaetes. Gonads form on the polyps; both single-sex and hermaphroditic individuals are known. The genital anatomy is

Figure 5.14  (A) Polypodium stolon with apex of knobs directed toward the center of the egg; (B) emerging polyp; and (C) mature polyp (redrawn from Lipin, 1911).

Chapter  |  5  Cnidaria

considerably more complex than that of hydra. These gonads and their accessory structures arise from endoderm. The presumed ovule is diploid and is released into the gastric cavity. The presumed mate gonads become filled with binucleate cells. One nucleus remains haploid, while the other becomes polyploid. These may fall out of the coelenteron, but there are observations of polyps crawling onto young sterlets and placing these gonads on the fish. The transition from fish surface to immature oocyte has not been observed. Polypodium parasitizes at least five species of Acipenser in all of the major rivers of the former Soviet Union. Eighty percent of sterlet (Acipenser ruthenus) and 20% of the sturgeon (A. guldenstadti) are infested, with sporadic infections in other species of the genus. A careful search of North American sturgeon will probably reveal the presence of this cnidarian.

IV.  COLLECTION AND MAINTENANCE OF FRESHWATER CNIDARIA A.  Collection Techniques Hydra are so ubiquitous that their presence should be expected in any reasonably unpolluted body of water. Unfortunately, since they are usually sedentary, they are not easily found in plankton tows. Also, lacking hard body parts, they are often badly damaged by preservatives. A careful examination of suitable substrata is required. If rocks, leaves, and submerged vegetation (e.g., Myriophyllum, Elodea) are collected and placed overnight in a glass or enamel pan and the pan is carefully examined under a low-power dissection microscope, hydra are usually found. Abundance will depend on the seasonal distribution of zooplankton and may vary among lakes. Since hydra float when hungry and attach again after they have been fed, the best place to look for them is often the downstream end of a lake or the pools in the stream immediately below the lake. If there is a dam, this should be examined with particular care. In a typical body of water, there may be at least three species of hydra: a green one, a large brown one, and an intermediate-sized brown one. Collecting of the medusae of Craspedacusta must be done with buckets rather than nets. The greater the sample volume (to prevent anoxia and excess temperature change), the better are the chances of collecting Craspedacusta. A compromise must be found between excessive agitation of the water, which will damage the animals, and insufficient stirring, which will permit them to settle or to become anoxic. The microhydra larvae can be found on fragmentary organic debris, much like hydra, but we have not collected them.

B.  Maintenance Procedures Craspedacusta and Cordylophora can be kept in natural water and fed on field-collected zooplankton. Even

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the free-living stage of Polypodium can be kept in the laboratory and fed on oligochaetes and turbellaria[60]. Dodson and Cooper[20] successfully fed a range of foods to Craspedacusta, including large rotifers and copepods. Generally, it is extremely difficult to maintain jellyfish in aquaria for any length of time. Their tissues are so fragile that they are battered to pieces by most aquarium aerating or stirring systems. The more or less sedentary polyps are therefore easier to study. However, the microhydra larvae of Craspedacusta have not been extensively studied, perhaps due to their small size. Hydra are laboratory animals par excellence. General directions for most of the things one might want to do with hydra are discussed in detail in the papers collected by Lenhoff [42]. They can be maintained in artificial pond water of simple composition and fed either live natural foods such as cladocerans (e.g., Daphnia), copepods, or larvae of midges and mosquitoes. They will not eat protozoa. Although the brine shrimp, Artemia, do not normally occur in freshwater, many pond animals eat them avidly in the laboratory. Many investigators use Artemia nauplii as food for hydra, Craspedacusta, planarians, and other small freshwater invertebrates. The Artemia eggs are collected at the edges of salt ponds and lakes. They are sold in vacuumpacked cans for use by aquarists and may be stored for long periods at cool temperatures. The procedure for feeding hydra with Artemia consists of following the package directions for hatching the brine shrimp (a process requiring about 1 day), draining and rinsing the hatched nauplii free of the saltwater, and adding them to the hydra. The nauplii must be alive and vigorous or the hydra will not eat them. Dead Artemia can be fed to flatworms! Artemia are best drained in a net that can be made by inserting a taut sheet of bolting silk in an embroidery hoop. The Artemia and their salt solution are poured into the net, which should be wet on both sides to speed drainage. The shimmering mass of Artemia and eggs are then rinsed into a new container of hydra culture water. If this is permitted to stand for 15 min, the active nauplii will swim toward a light, leaving the unhatched eggs behind. These active swimmers can be taken with a medicine dropper and fed to the hydra. This keeps salt and hydra separate and also permits elimination of most of the unhatched Artemia eggs. These small prey may be eaten in great numbers. Large brown hydra can eat as many as 100 Artemia at a single meal, and even the smallest green hydra can consume one or two. No more than 10 min after the first prey is swallowed, the swallowing process stops even if some prey remain on the tentacles. We have seen that organisms may brush against the tentacles with impunity for some time after a hydra has been satiated. This suggests either that nematocysts are no longer discharging or that the nematocyst supply has been temporarily exhausted.

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The primary danger in this feeding process is that water contaminated with either dead Artemia or with the regurgitation products of previous feedings is highly detrimental. The live food must therefore be added to the hydra, and after approximately 1 hr, the hydra must be placed in clean water, either by moving them or by discarding the tainted water. Since hydra usually stick to the substrate, the old medium can be simply poured off and new medium added. The glass surface will eventually become coated with bacteria. This can be prevented somewhat by scraping the glass with a rubber spatula before discarding the old medium, but this is not usually effective for more than about 1 week, at which time it is best to provide new, clean containers. The hydra are remarkably sensitive to heavy metals and to detergents. Even a very short length of copper tubing in a water supply will kill hydra. Therefore, in cleaning the dishes, physical dirt is often less dangerous than the detergents. Dishes must be very thoroughly rinsed in metal-free water. To obtain nontoxic water, one may either use water from the collecting site, pretest the water with hydra to make sure that the water is innocuous, or use glass-distilled or deionized water. When rinsing dishes containing hydra, appropriate salts must be added to the distilled water. Various recipes for culture media are available. We use two stock solutions adapted by K. Dunn (personal communication) from solutions developed by Loomis, Lenhoff, and others. Solution A contains 81 g NaHCO3 in 1 L of distilled water. Solution B contains 7.46 g KCl, 20.33 g MgCl2 6H2O, and 147.02 g CaCl2 2H2O in 1 L of distilled water. These stock solutions are added at the rate of 1 mL per liter to distilled water to make artificial pond water. Hydra die at temperatures above approximately 30°C, but most seem healthy at temperatures as low as 5°C. The growth and reproductive rates are proportional to temperature, as are the food demands. Stocks are therefore best maintained at low temperatures except when rapid growth is desired.

V.  CLASSIFICATION OF FRESHWATER CNIDARIA The five groups of freshwater Cnidaria are clearly distinct from each other. Except for the hydras, they are relatively rare and sporadic in their distribution. In North America, there is no evidence, at present, for more than one species each of Craspedacusta, Polypodium, and Calposoma. There may be several species of Cordylophora, but we are considering Cordylophora as one example of a brackish water cnidarian rather than a completely freshwater coelenterate. Our taxonomic key, therefore, consists of five very coarse divisions, one of which, that for the hydras, is then subdivided in somewhat greater detail.

A.  Species Groups of Hydras There are two closely related genera of hydras: the brown genus known as Hydra and the green hydras assigned to the genus Chlorohydra. By convention the term hydra refers to members of both genera, unless otherwise specified. Hydra tend to look superficially similar, except for the distinction between the asymbiotic and green species. On more careful examination, differences become apparent, not only among apparently unrelated species but even within a clone. Asymbiotic animals from the same clone change color depending on the color of their food. All hydra will also change body size as a function of the amount of food and of temperature[32]. Despite the withinclone plasticity, unrelated hydra maintained under identical conditions, side by side in the laboratory, retain consistent differences. Batha[6] noticed that field-collected hydra differ in appearance from their own clonal descendants maintained for long periods in the laboratory. Even animals collected from the same pond at different seasons and kept in the laboratory sometimes show consistent differences in laboratory cultures, despite apparent similarities of the field-collected specimens. The effect of all this is to make taxonomy extremely difficult. Some investigators have tended to describe new species on the basis of rather unstable, variant appearances, while others have despaired of making any precise identification. Campbell[15] has provided a compromise position, in which he has classified the hydra into clearly distinct species groups, each composed of an uncertain number of more closely related species whose precise identity may have to await new techniques of examination. Color, presence or absence of a body stalk, nematocyst shape, and the order of appearance of tentacles on the new buds are the characters used. The structure of the theca around the fertilized eggs, the appearance of gonads, proportions of different kinds of nematocysts, microscopic details of symbiotic algae, and certain physiological characteristics are all likely to be important in subdividing the various species groups. Unfortunately, these subdivisions are difficult for all but the most serious students and professionals. Since the species group is adequate for most purposes, it seems advisable to quote extensively from Campbell’s descriptions. The four species groups are given as follows. 1. The easily recognized group “Hydra viridissima” (also known as Chlorohydra). In addition to the brilliant green color (Fig. 5.15), Campbell listed the following characteristics: “small to moderate body size and tentacles, stalk not distinct from the rest of the column; hermaphroditic; embryotheca spherical, made of polygonal plates; nematocysts very small; tentacles arising simultaneously on buds.” 2. The stalked Hydra or “Oligactis” group: “large size; pronounced translucent stalk in large individuals; long

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3. The “vulgaris” group or common hydra: “11 moderate size; tentacles of moderate length; dioecious or monoecious, sometimes switching between the two conditions; spherical embryotheca ornamented with spines; broad stenotele (capsules); slender holotrichous isorhiza; often slipper-shaped, with the anterior end broadly pointed; tentacles arising on the buds simultaneously or nearly so.” 4. The “gracile Hydra” or “braueri” group: “small to medium size; tentacles moderately short; hermaphroditic; embryotheca and egg flattened, with the embryotheca adherent to the substratum; embryotheca smooth or papillate; tentacles arising simultaneously on the buds; holotrichous isorhiza (capsule) broader than half its length; and stenotele (capsule) plump; body often pale colored during laboratory cultivation.” No single character, not even the nematocysts, can by itself distinguish one group from another. Every group probably contains a large number of species, each of which has been described under a large number of names. Perhaps biochemical procedures may eventually sort them out, but for the moment and for most purposes, strains collected from the field should be classified to a species group and then supplied with enough ancillary description so that other workers can at least know if they have the same or a similar organism. Campbell[15] provided a key to the four groups of Hydra, which we have incorporated in a key to the other freshwater Cnidaria. Figure 5.15  Green hydra with an attached, budding hydra (courtesy of Brigitte Galliot, University of Geneva).

tentacles; dioecious; spherical embryo with simple theca; slender stenotele (capsules) and large, blunt, cylindrical holotrichous isorhiza (capsules); distinct golden color in culture due to yellow crystals in the ectoderm; two lateral tentacles arising before the others in the bud.”

VI.  Taxonomic Key to Genera of Freshwater Cnidaria The polyp and medusa stages of a single species of coelenterate can differ greatly, both morphologically and ecologically. The key has been arranged to give the same results whether one starts with polyps or medusae. Also, the key refers to “species groups” of Hydra as names in parentheses.

1a.

Parasitic in fish eggs ..................................................................................................................................................................... Polypodium

1b.

Nonparasitic ....................................................................................................................................................................................................2

2a (1b).

Medusae .................................................................................................................................................................................... Craspedacusta

2b.

Polyps .............................................................................................................................................................................................................3

3a (2b).

Polyp without basal attachment, with oral surface downward ..................................................................................................... Polypodium

3b.

Polyps with basal disk ....................................................................................................................................................................................4

4a (3b).

Solitary polyps with single circle of multicellular tentacles; no medusa, gonads are body stalk . .................................................................7

4b.

Colonial polyps ...............................................................................................................................................................................................5

5a (4b).

Filiform tentacles, irregularly arranged on conical hypostome; branching colony with gonophores ....................................... Cordylophora

5b.

Colonial; atentacular or tentacles unicellular . ................................................................................................................................................6

6a (5b).

Atentacular, oral capitulum; polyps from common stolon; may bud medusa from polyp; “microhydra” ............................... Craspedacusta

6b.

Minute with unicellular tentacles . ..................................................................................................................................................Calposoma

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Ecology and Classification of North American Freshwater Invertebrates

7a (4a).

Bright green ..............................................................................................................................................................................Hydra (viridis)

7b.

Not green ........................................................................................................................................................................................................8

8a (7b).

Holotrichous isorhiza nematocysts at least half as broad as long; embryotheca without spines .............................................Hydra (braueri)

8b.

Holotrichous isorhiza nematocysts slender or if plump, then embryotheca spined . ......................................................................................9

9a (8b).

Buds acquire two lateral tentacles before others appear; otherwise stenotele nematocysts at least 1.5 times as long as broad ............Hydra (oligactis)

9b.

Buds acquire tentacles in some other order; otherwise stenotele nematocysts less than 1.5 times as long as broad ............ Hydra (vulgaris)

VII. Selected References 08. Bossert P. The Effect of Hydra Strain Size on Growth of Endosym­ biotic Alga. PhD thesis, Department of Biology. Stony Brook, NY: State University of New York; 1987. 09. Bossert P, Dunn K. Regulation of intracellular algae by various strains of the symbiotic Hydra viridissima. J. Cell Sci. 1986;85:187–195. 19. Devries DR. The fresh-water jellyfish Craspedacusta sowberii—a summary of its life-history, ecology, and distribution. J Freshwater Ecol. 1992;7(1):7–16. 20. Dodson SI, Cooper SD. Trophic relationships of the freshwater jellyfish Craspedacusta sowberii Lankester 1880. Limnol Oceanogr. 1983;28:345–351.

28. Galliot B, Schmid V. Cnidarians as a model system for understanding evolution and regeneration. J Dev Biol. 2002;46:39–48. 34. Hessinger D, Lenhoff HM, eds. The Biology of Nematocysts. London: Academic Press; 1988. 36. Hyman LH. The invertebrates: Protozoa through Ctenophora. New York: McGraw-Hill; 1940. 42. Lenhoff HM. Hydra: Research Methods. Plenum Press: New York; 1983 463 pp.. 49. Mackie GO. Coelenterate Ecology and Behavior. New York: Plenum; 1976. 73. Steele R. Developmental signaling in Hydra: what does it take to build a “simple” animal? Dev Biol. 2002;248:199–219.