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The gastropod Cymatium muricinum, a predator on juvenile tridacnid clams
Aquaculture, 48 (1985) 211-221 Elsevier Science Publishers B.V., Amsterdam
211 - Printed in The Netherlands
THE GASTROPOD CYMATIUM MURICINUM, A PREDATOR ON JUVENILE TRIDACNID CLAMS
F.E. PERRON,
G.A. HESLINGA
and J.O. FAGOLIMUL’
Micronesian Mariculture Demonstration Center, Koror, Republic of PaIau 96940 (Caroline Islands) 1Marine Resources Division, Yap State (Federated States of Microneeia) This work was funded by the Pacific Fisheries Development of Marine Resources, Republic of Palau. (Accepted
Foundation
and the Division
3 June 1985)
ABSTRACT Perron,
F.E., Heslinga, G.A. and Fagolimul,
Cymatium 43: 211-221.
J.O., 1985. The gastropod
cinum, a predator on juvenile tridacnid clams. Aquaculture,
muri-
Infestations of C. muricinum were episodic or seasonal in Palau and Yap (Micronesia), with larvae settling from the plankton primarily in late winter and spring. The results of preliminary field trials indicate that clam losses due to snail predation can be kept at acceptable levels by removing snails manually from clam nursery trays.
INTRODUCTION
Technology related to the mariculture of giant clams (family Tridacnidae) has progressed rapidly over the past three years and has been documented in a series of publications (Heslinga and Perron, 1983; Munro and Heslinga, 1983; Heslinga et al., 1984). At the Micronesian MaricultureDemonstration Center (MMDC) in Palau, laboratory larval culture methods have been developed which result in yields as high as 55 000 lo-mm seed clams per 4-month rearing cycle per concrete tank (dimensions 8 X 1.5 X 1 m). Seed clams are then either grown out in land-based raceways or are placed in protective “habitats” on the shallow lagoon bottom near MMDC (Heslinga et al., in press). As all of the major biotechnical constraints connected with giant clam mariculture (spawning, larval rearing, handling and grow-out of juveniles) have been largely overcome, emphasis is now being placed on refining methodology, maximizing spat yields, and improving survival during the grow-out phase of production. Predation is a problem for most mariculture operations. Although giant clams are relatively immune to predation (except by man), juveniles younger
0044~8486/85/$Q3.30
0 1985 Elsevier Science Publishers B.V.
212
than approximately 2.5 years (shell length 15 cm) are susceptible to a number of fish and invertebrate predators (Heslinga et al., 1984, and in press). One method for eliminating predation during the first 2.5 years is to keep the young clams in a land-based raceway system. However, the major drawbacks of this option are that it ties up valuable tank space which otherwise could be used for spawning and spat production, requires the constant running of seawater pumps, and subjects the clams to the possibility of mass mortality in the event of a seawater system breakdown. Alternatively, seed clams can be removed from the larval rearing tanks at a relatively young age and grown out in an ocean-based nursery system which provides protection from predators (Heslinga et al., in press). The nursery system currently in use at MMDC is modular in design and consists of fiberglass trays with polyethelene mesh covers. These covers (mesh size 2.5 cm or smaller) effectively exclude carnivorous fish, octopus, crabs and the large predatory snail Chicoreus rumosus. The present report deals with the only predator of any significance which is able to enter the nursery habitats and kill juvenile giant clams.
Fig. 1. A growth series of the gastropod Cymatium
muricinum.
The gastropod mollusk Cymatium muricinum (Fig. 1) occurs throughout the Indo-Pacific and is also found in the Western Atlantic (Kay, 1979). The habitat of C. muricinum is reef flats and shallow sandy areas where it preys on a variety of bivalve molluscs (Houbrick and Fretter, 1969). Although C. muricinum is not normally encountered at high population densities in Palau, relatively large numbers of these predators have recently been found feeding on juvenile giant clams inside nursery habitats at MMDC. A similar infestation of C. muricinum was observed when 1000 juvenile giant clams
213
were introduced to the Micronesian island of Yap in January 1984. Because C. muricinum is a potentially serious pest affecting tropical bivalve mariculture systems, aspects of the biology of this predator as well as the relationship between the snail and its giant clam prey are also described. METHODS Specimens of C. muricinum were obtained from the modular habitats comprising the MMDC giant clam nursery system. Snails and clams were brought into the laboratory where predation was observed in detail. The nursery system was searched daily for snails from 20 April 1984 through 11 July 1984. All snails and infested clams were removed from the nursery. Records were kept of numbers and sizes of snails found inside or associated with dead or infested clams. Clam size and species were also recorded. Other species of snails sometimes found on clams along with C. muricinum were also collected and studied. A more quantitative estimate of the destructive potential of C. muricinum was obtained by deploying 1000 young clams on the fringing reef of Yap island. These clams (Tridacna derasa, a shell length 84.13 mm) were distributed in a total of 40 covered trays and were observed for a 5-month period from 27 January 1984 to 3 July 1984. The trays were checked twice every week at which time all visible snails and all dead clams were removed. A cumulative plot of clam mortality was constructed from the data. In the laboratory, size-specific growth rates of C. muricinum were obtained by confining individual snails of various sizes in numbered mesh bags which also contained specimens of T. g&as. Growth data were collected in the form of initial snail size, final snail size and length of time between measurements (10 days). A growth curve was calculated from the mark-recapture data using the Von Bertalanffy computer program of Fabens (1965). The mesh bag isolation technique was also used to determine how much time is required for a single snail to kill an individual clam. Snails of various sizes were placed in numbered bags along with clams of constant size (98.6105.5 mm shell length). The bags were examined daily and the dates of clam deaths were recorded. Possible methods of reducing clam mortality due to snail predation were examined in the laboratory. Since most snail attacks involve penetration of the clam through the byssal gape at the point of attachment to the substratum, the relationship between substratum type and the vulnerability of clams to predation was examined. In each trial, eight specimens of T. derasa (100-110 mm shell length) were placed in an aquarium with eight specimens of C. muricinum (28-31 mm shell length). Four of the clams were byssally attached to basalt gravel while the remaining four were attached to cementcovered paper plates. Dead clams were counted after 10 days of exposure to the snails. Five sets of trials were run. Finally, experiments were carried out to determine if some species of
214
clams are naturally more resistant to snail predation than are others. In these trials, the tridacnid clam Hippopus hippopus was tested against both T. gigas and T. derasa. In each trial, five H. hippopus and five T. gigas or T. derasa of comparable shell length (80-95 mm) were placed in an aquarium with 10 specimens of C. muricinum (28-31 mm shell length). All clams were placed on a basalt gravel substratum. Dead clams were counted after 10 days. Five trials were run for each species pair tested. RESULTS
Gastropods in the family Cymatiidae typically possess larvae which spend long periods of time swimming in the plankton (Scheltema, 1971). When these larvae encounter an appropriate substratum they metamorphose into juvenile snails which probably begin seeking prey almost immediately. Although the larval biology of C. muricinum has not been studied in detail, the protoconch, or larval shell, of this species is quite large (3 mm) and closely resembles those of Cymatium species known to have long-term planktonic larvae. That the larvae of C. muricinum settle directly on maricultured giant clams is strongly suggested by the fact that newly metamorphosed snails are frequently found living inside their prey. These small snails are usually found lodged between the shell and the large fleshy mantle of the clam. As many as
Fig. 2. (A) The empty shell of a maricultured Z! gigae (shell length 138 mm) showing internal shell blisters (arrows) caused by C. muricinum. (B) A 135-mm maricultured specimen of T. gigas being eaten by a 29-mm specimen of C. muricinum (arrow). Note retracted mantle.
215
eight snails have been counted inside a single clam. Newly metamorphosed snails appear capable of crawling directly into the clam’s ventral shell gape rather than having to chew their way through the byssus. Initially, infested clams may show few outward signs of distress. As the snail grows, and as the damage caused by its feeding activity increases, the clam may react by closing its shell valves. Some clams react to the irritation of a small snail by producing a pearl-like blister on the inner surface of one or both shell valves (Fig. 2A). Eventually, a seriously infested clam will begin to gape and retract its mantle away from the shell margins (Fig. 2B). The 135-mm specimen of T. g&m in Fig, 2B died 24 h after the photograph was taken. When a clam has been killed and its meat consumed, the responsible snail or snails then attack nearby uninfested clams. A large snail will generally attack a clam by penetrating the byssal opening (Fig. 3A) with a long extensible proboscis. Such snails are difficult to detect and may remain hidden under the clam until gaping and mantle retraction are observed. Dead or moribund clams often attract a variety of scavenging organisms including the predatory muricid gastropod Drupella rugosa (see Kay, 1979). That D. rugosa acts only as a scavenger and not as a predator on giant clam juveniles was determined by keeping several of these gastropods in an aquarium with specimens of T. gigas and T. derasa. After a period of 1 month, no clams had been injured by D. rugosu. The clams in the trial ranged in shell length from 72 to 110 mm.
Fig. 3. (A) The dorsal hinge area of a maricultured juvenile T. derasa (shell length 105 mm) showing the byssal gape. (B) The dorsal hinge area of a maricultured juvenile H. hippopus (shell length 95 mm) showing the zipper-like structure of the hinge.
216
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Fig. 4. The cumulative mortality curve for a sample of 1000 specimens of T. derasa placed in covered trays on the reef at Yap Island on 27 January 1984. The graph covers the period 8 April through 3 July 1984.
Fig. 4 is a plot of the cumulative mortality experienced by a sample of 1000 juvenile T. derusa between 8 April and 3 July 1984. These clams (initial shell length averaging 84.1 mm) were placed on the fringing reef at Yap island on 27 January 1984. Specimens of predatory snails were first noticed on 16 March when two clams were opened and found to contain several small C. muricinum. It is probable that the C. muricinum infestation began well in advance of this date. In mid April 1984, 34 specimens of C. muricinum were collected from the clam trays and measured. These snails ranged in shell length from 3.3 mm (newly metamorphosed juveniles) to 31.4 mm. The mean + standard deviation for the sample was 10.7 + 5.8 mm. Sixty percent of the sample consisted of young juveniles less than 10 mm in shell length. By June 1984, small juvenile C. muricinum were no longer being found and a sample of 11 snails collected between 5 and 18 June ranged in size from 15 to 35 mm (X = 24.8 + 6.2 mm). By 3 July 1984, 163 clams had been killed by snails or had died from other unknown causes. This cumulative mortality figure corresponds to 16.3% of the clams planted on 27 January 1984. At the end of the trial, the average shell length of the surviving 837 clams was 119 mm. Specimens of C. muricinum maintained in laboratory seawater tanks grew rapidly when fed ad libitum on young giant clams. The Von Bertalanffy growth equation for C. muricinum was X = 40.66(1 - 0.919e-0*347f) where X is shell length in mm and t is age in years. Fig. 5 is the growth curve obtained by solving this equation for various values of t. The growth rates predicted from the Von Bertalanffy model are in accordance with the data on size frequency collected at the Yap study site. Between mid April and mid June, average snail shell lengths increased from 10.7 to 24.8 mm. This 2-month growth increment of approximately 15.4 mm is almost precisely the amount of growth the Von Bertalanffy model predicts for 10.7-mm snails over a a-month period (Fig. 5).
217
12
34
I
I
I
5
67
I
I
8
I
I
1
9lOll12
1
AGE(months)
Fig. 5. The Von Bertalanffy the laboratory.
growth curve for postlarval
C. muricinum
fed ad libitum in
Fig. 6. The relationship between size of C. muricinum and the number of days required for individual snails to kill juveniles of T. g&as.
The relationship between snail size and the length of time required to kill a juvenile T. gigus (98-105 mm shell length) is specified by the equation Y = 13.98 - 0.326X (r = 0.758, P
218 TABLE 1 The effect of substratum type (basalt gravel vs. cement plates) on the survival of juvenile T. derasa subjected to predation by C. muricinum. In each trial, eight clams (shell length 100-110 mm) were placed in an aquarium with eight snails (shell length 25-35 mm). Dead clams were counted after 10 days Trial
Substratum type Basalt gravel
Cement plates
4 4 3 4 4
4 3 4 4 3
-
-
19
18
1 2 3 4 5
TABLE 2 Numbers of juvenile clams (H. hippopus, T. gigas and T. deraeu) killed during lo-day trials when exposed to predation by C. muricinum. In each trial, five H. hippopus were tested against either five T. gigae or five T. derasa. Dead clams were counted after 10 days Trial
H. hippopus
1 2 3 4 5
0 0 0 0 0
T. gigus
-
5 3 4 5 5
0
22
Trial
H. hippopus
T.
1 2 3 4 5
0 0 0 0 0 0
derasa
4 5 5 3 4
21
The type of substratum to which maricultured clams were attached did not have a strong effect on susceptibility to predation by C. muricinum (Table 1). Clams attached to solid cement surfaces were as likely to be killed by the end of the 1Oday trial as were clams attached to l-cm basalt chips. In the trials designed to detect differences in susceptibility to snail predation among species of giant clams, H. hippopus proved far more resistant to attack than either T. derusa or T. gigus (Table 2). This difference probably
219
stems from the fact that, unlike other species of Tridacnidae which have a byssal gape which cannot be closed completely (e.g., T. derasa, Fig. 3A), the zipper-like hinge of H. hippopus (Fig. 3B) can be shut tight as an effective defense against predators. DISCUSSION
Life history of Cymatium muricinum Cymatium muricinum is a highly opportunistic predator which will prove difficult if not impossible to completely eradicate as a pest on maricultured tropical bivalves. However, for reasons discussed below, we feel that C. muricinum does not constitute a serious constraint to the expanded development of giant clam mariculture facilities. Opportunistic species are typically adapted to utilize patchily distributed or ephemeral resources. Many bivalve species, including tridacnid clams, appear to be patchily distributed on the reef (Heslinga and Perron, unpubl., 1984; R. Braley, personal communication, 1984). The planktonic larvae of C. muricinum are well suited to locating such prey, and the evidence indicates that these larvae may be able to sense and respond to the presence of living bivalves. The extremely rapid postlarval growth rate of C. muricinum (Fig. 5) permits these snails to approach maximum size (and probably sexual maturity) within 1 year of settlement from the plankton. Cymatium muricinum therefore rapidly complete its life cycle and produces more larvae which drift in the plankton until an additional food source is encountered. Prospects for the control of Cymatium muricinum
Altering the substratum on which juvenile giant clams are grown is unlikely to- be a practical method of reducing losses due to snail attacks (Table 1). Basalt chips are used as a substratum at MMDC to facilitate subsequent handling of the clams (Heslinga et al., 1984). If clams are grown on a solid substratum, the byssal attachment must be broken every time handling is required. Also, if juvenile clams are placed on a solid flat surface, they will tend to move about and form clumps which result in the stunting of some individuals. Therefore, in the absence of evidence that substratum type can have a major effect on snail predation on young clams, we continue to advocate the use of basalt chips as a convenient attachment surface for giant clam nursery systems. Juveniles of the tridacnid species H. hippopus are significantly less vulnerable to snail predation than are T. gigas or T. derasa (Table 2). Hippopus hippopua is a hardy species which lives on relatively exposed reef flats. The postlarvae have byssal threads, but lose all attachment to the substratum at an early age. In nature, juveniles of T. gigas and T. derasa are usually found attached to dead coral rubble some distance above the substratum. In con-
220
trast, H. hippopus juveniles are found resting directly on the sandy surface of the reef flat. This difference in habitat is likely to be related to the relative predator resistance of H. hippopus. Although the hardiness of H. hippopus would seem to make this species a likely candidate for commercial clam mariculture, other factors favor continued emphasis on 2’. derasa and T. gigus. Giant clam mariculture efforts have been concentrated on T. derasa and T. gigus because these two species attain large adult sizes and have extremely high growth rates under culture conditions (Heslinga et al., 1984). Hippopus hippopus, although showing initially rapid growth, has not continued to grow well in the MMDC oceanbased nursery system. This poor performance could be related to the small adult size of H. hippopus or to the energetic cost of the relatively massive shell of this species. The simplest method of limiting damage caused by C. muricinum is a program of manually removing all visible snails and obviously infested clams from the nursery system. Clams infested by large snails are quite easy to spot when the mantle begins to retract from the shell lip (Fig. 2B). Since specimens of C. muricinum become increasingly destructive as they grow, it is particularly important to remove the large snails as rapidly as possible. At the Yap study site, snails were removed from the clam trays at roughly twice-weekly intervals. Given this moderate level of attention, 16.3% of the clams had died by the end of the 5-month trial. It must be remembered that this figure includes all sources of mortality since it was not usually possible to determine cause of death in the absence of a snail being caught in the act. The effectiveness of the snail removal program at Yap is reflected in the decreasing clam mortality rate during the final month of the trial (Fig. 4), and by the fact that only 10 snails were found on the clams from the middle of June until the end of the trial. Infestations of C. muricinum appear to be episodic or seasonal. At the Yap study site, small juveniles appeared on the clams in mid April 1984. Since the average size of the snails at that time was approximately 10 mm, the cohort probably settled from the plankton 2-3 weeks earlier (Fig. 6). By June 1984, the average size of the snails had increased considerably and no new juveniles were being found. These results were paralleled by observations made at the MMDC nursery system in Palau. Early postlarval snails were common during the late winter and spring of 1984, but were no longer in evidence by the summer of that year. Since Palau and Yap are 300 miles apart, the roughly simultaneous appearance of C. muricinum postlarvae at both locations could indicate a late winter or early spring settlement peak for this species. If this suggestion of seasonality is borne out by further observations, it may prove advantageous to schedule outplanting of giant clam juveniles for seasons during which the probability of C. muricinum settlement is low. Because infestations of C. muricinum are episodic and can be controlled through simple manual removal of snails from clam trays, this predator is
221
more likely to be a minor annoyance than a major constraint to the development of commercial giant clam farms in the tropics.
REFERENCES Fabens, A.J., 1965. Properties and fitting of the Von Bertalanffy growth curve. Growth, 28: 265-289. Heslinga, G.A. and Perron, F.E., 1983. The status of giant clam mariculture technology in the Indo-Pacific. South Pacific Commission Fisheries Newsletter, No. 24: l-5. Heslinga, G.A., Perron, F.E. and Orak, O., 1984. Mass culture of giant clams (f. Tridacnidae) in Palau. Aquaculture, 39: 197-215. Heslinga, G.A., Ngiramengior, M. and Perron, F.E. An ocean-based nursery system for tridacnid clams. Aquacultural Engineering. In press. Houbrick, J.R. and Fretter, V., 1969. Some aspects of the functional anatomy and biology of Cymatium and Burma. Proc. Malacological Sot. Lond., 38: 415-429. Kay, E.A., 1979. Hawaiian Marine Shells. Bishop Museum Press, Honolulu, HI, 653 pp. Munro, J.L. and Heslinga, G.A., 1983. Prospects for the commercial cultivation of giant clams (Bivalvia: Tridacnidae). Proceedings of the Gulf and Caribbean Fisheries Institute, 35: 122-134. Scheltema, R.S., 1971. Larval dispersal as a means of genetic exchange between geographically separated populations of shallow-water benthic marine gastropods. Biol. Bull., 140 (2): 284-322.
211 - Printed in The Netherlands
THE GASTROPOD CYMATIUM MURICINUM, A PREDATOR ON JUVENILE TRIDACNID CLAMS
F.E. PERRON,
G.A. HESLINGA
and J.O. FAGOLIMUL’
Micronesian Mariculture Demonstration Center, Koror, Republic of PaIau 96940 (Caroline Islands) 1Marine Resources Division, Yap State (Federated States of Microneeia) This work was funded by the Pacific Fisheries Development of Marine Resources, Republic of Palau. (Accepted
Foundation
and the Division
3 June 1985)
ABSTRACT Perron,
F.E., Heslinga, G.A. and Fagolimul,
Cymatium 43: 211-221.
J.O., 1985. The gastropod
cinum, a predator on juvenile tridacnid clams. Aquaculture,
muri-
Infestations of C. muricinum were episodic or seasonal in Palau and Yap (Micronesia), with larvae settling from the plankton primarily in late winter and spring. The results of preliminary field trials indicate that clam losses due to snail predation can be kept at acceptable levels by removing snails manually from clam nursery trays.
INTRODUCTION
Technology related to the mariculture of giant clams (family Tridacnidae) has progressed rapidly over the past three years and has been documented in a series of publications (Heslinga and Perron, 1983; Munro and Heslinga, 1983; Heslinga et al., 1984). At the Micronesian MaricultureDemonstration Center (MMDC) in Palau, laboratory larval culture methods have been developed which result in yields as high as 55 000 lo-mm seed clams per 4-month rearing cycle per concrete tank (dimensions 8 X 1.5 X 1 m). Seed clams are then either grown out in land-based raceways or are placed in protective “habitats” on the shallow lagoon bottom near MMDC (Heslinga et al., in press). As all of the major biotechnical constraints connected with giant clam mariculture (spawning, larval rearing, handling and grow-out of juveniles) have been largely overcome, emphasis is now being placed on refining methodology, maximizing spat yields, and improving survival during the grow-out phase of production. Predation is a problem for most mariculture operations. Although giant clams are relatively immune to predation (except by man), juveniles younger
0044~8486/85/$Q3.30
0 1985 Elsevier Science Publishers B.V.
212
than approximately 2.5 years (shell length 15 cm) are susceptible to a number of fish and invertebrate predators (Heslinga et al., 1984, and in press). One method for eliminating predation during the first 2.5 years is to keep the young clams in a land-based raceway system. However, the major drawbacks of this option are that it ties up valuable tank space which otherwise could be used for spawning and spat production, requires the constant running of seawater pumps, and subjects the clams to the possibility of mass mortality in the event of a seawater system breakdown. Alternatively, seed clams can be removed from the larval rearing tanks at a relatively young age and grown out in an ocean-based nursery system which provides protection from predators (Heslinga et al., in press). The nursery system currently in use at MMDC is modular in design and consists of fiberglass trays with polyethelene mesh covers. These covers (mesh size 2.5 cm or smaller) effectively exclude carnivorous fish, octopus, crabs and the large predatory snail Chicoreus rumosus. The present report deals with the only predator of any significance which is able to enter the nursery habitats and kill juvenile giant clams.
Fig. 1. A growth series of the gastropod Cymatium
muricinum.
The gastropod mollusk Cymatium muricinum (Fig. 1) occurs throughout the Indo-Pacific and is also found in the Western Atlantic (Kay, 1979). The habitat of C. muricinum is reef flats and shallow sandy areas where it preys on a variety of bivalve molluscs (Houbrick and Fretter, 1969). Although C. muricinum is not normally encountered at high population densities in Palau, relatively large numbers of these predators have recently been found feeding on juvenile giant clams inside nursery habitats at MMDC. A similar infestation of C. muricinum was observed when 1000 juvenile giant clams
213
were introduced to the Micronesian island of Yap in January 1984. Because C. muricinum is a potentially serious pest affecting tropical bivalve mariculture systems, aspects of the biology of this predator as well as the relationship between the snail and its giant clam prey are also described. METHODS Specimens of C. muricinum were obtained from the modular habitats comprising the MMDC giant clam nursery system. Snails and clams were brought into the laboratory where predation was observed in detail. The nursery system was searched daily for snails from 20 April 1984 through 11 July 1984. All snails and infested clams were removed from the nursery. Records were kept of numbers and sizes of snails found inside or associated with dead or infested clams. Clam size and species were also recorded. Other species of snails sometimes found on clams along with C. muricinum were also collected and studied. A more quantitative estimate of the destructive potential of C. muricinum was obtained by deploying 1000 young clams on the fringing reef of Yap island. These clams (Tridacna derasa, a shell length 84.13 mm) were distributed in a total of 40 covered trays and were observed for a 5-month period from 27 January 1984 to 3 July 1984. The trays were checked twice every week at which time all visible snails and all dead clams were removed. A cumulative plot of clam mortality was constructed from the data. In the laboratory, size-specific growth rates of C. muricinum were obtained by confining individual snails of various sizes in numbered mesh bags which also contained specimens of T. g&as. Growth data were collected in the form of initial snail size, final snail size and length of time between measurements (10 days). A growth curve was calculated from the mark-recapture data using the Von Bertalanffy computer program of Fabens (1965). The mesh bag isolation technique was also used to determine how much time is required for a single snail to kill an individual clam. Snails of various sizes were placed in numbered bags along with clams of constant size (98.6105.5 mm shell length). The bags were examined daily and the dates of clam deaths were recorded. Possible methods of reducing clam mortality due to snail predation were examined in the laboratory. Since most snail attacks involve penetration of the clam through the byssal gape at the point of attachment to the substratum, the relationship between substratum type and the vulnerability of clams to predation was examined. In each trial, eight specimens of T. derasa (100-110 mm shell length) were placed in an aquarium with eight specimens of C. muricinum (28-31 mm shell length). Four of the clams were byssally attached to basalt gravel while the remaining four were attached to cementcovered paper plates. Dead clams were counted after 10 days of exposure to the snails. Five sets of trials were run. Finally, experiments were carried out to determine if some species of
214
clams are naturally more resistant to snail predation than are others. In these trials, the tridacnid clam Hippopus hippopus was tested against both T. gigas and T. derasa. In each trial, five H. hippopus and five T. gigas or T. derasa of comparable shell length (80-95 mm) were placed in an aquarium with 10 specimens of C. muricinum (28-31 mm shell length). All clams were placed on a basalt gravel substratum. Dead clams were counted after 10 days. Five trials were run for each species pair tested. RESULTS
Gastropods in the family Cymatiidae typically possess larvae which spend long periods of time swimming in the plankton (Scheltema, 1971). When these larvae encounter an appropriate substratum they metamorphose into juvenile snails which probably begin seeking prey almost immediately. Although the larval biology of C. muricinum has not been studied in detail, the protoconch, or larval shell, of this species is quite large (3 mm) and closely resembles those of Cymatium species known to have long-term planktonic larvae. That the larvae of C. muricinum settle directly on maricultured giant clams is strongly suggested by the fact that newly metamorphosed snails are frequently found living inside their prey. These small snails are usually found lodged between the shell and the large fleshy mantle of the clam. As many as
Fig. 2. (A) The empty shell of a maricultured Z! gigae (shell length 138 mm) showing internal shell blisters (arrows) caused by C. muricinum. (B) A 135-mm maricultured specimen of T. gigas being eaten by a 29-mm specimen of C. muricinum (arrow). Note retracted mantle.
215
eight snails have been counted inside a single clam. Newly metamorphosed snails appear capable of crawling directly into the clam’s ventral shell gape rather than having to chew their way through the byssus. Initially, infested clams may show few outward signs of distress. As the snail grows, and as the damage caused by its feeding activity increases, the clam may react by closing its shell valves. Some clams react to the irritation of a small snail by producing a pearl-like blister on the inner surface of one or both shell valves (Fig. 2A). Eventually, a seriously infested clam will begin to gape and retract its mantle away from the shell margins (Fig. 2B). The 135-mm specimen of T. g&m in Fig, 2B died 24 h after the photograph was taken. When a clam has been killed and its meat consumed, the responsible snail or snails then attack nearby uninfested clams. A large snail will generally attack a clam by penetrating the byssal opening (Fig. 3A) with a long extensible proboscis. Such snails are difficult to detect and may remain hidden under the clam until gaping and mantle retraction are observed. Dead or moribund clams often attract a variety of scavenging organisms including the predatory muricid gastropod Drupella rugosa (see Kay, 1979). That D. rugosa acts only as a scavenger and not as a predator on giant clam juveniles was determined by keeping several of these gastropods in an aquarium with specimens of T. gigas and T. derasa. After a period of 1 month, no clams had been injured by D. rugosu. The clams in the trial ranged in shell length from 72 to 110 mm.
Fig. 3. (A) The dorsal hinge area of a maricultured juvenile T. derasa (shell length 105 mm) showing the byssal gape. (B) The dorsal hinge area of a maricultured juvenile H. hippopus (shell length 95 mm) showing the zipper-like structure of the hinge.
216
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DATE
Fig. 4. The cumulative mortality curve for a sample of 1000 specimens of T. derasa placed in covered trays on the reef at Yap Island on 27 January 1984. The graph covers the period 8 April through 3 July 1984.
Fig. 4 is a plot of the cumulative mortality experienced by a sample of 1000 juvenile T. derusa between 8 April and 3 July 1984. These clams (initial shell length averaging 84.1 mm) were placed on the fringing reef at Yap island on 27 January 1984. Specimens of predatory snails were first noticed on 16 March when two clams were opened and found to contain several small C. muricinum. It is probable that the C. muricinum infestation began well in advance of this date. In mid April 1984, 34 specimens of C. muricinum were collected from the clam trays and measured. These snails ranged in shell length from 3.3 mm (newly metamorphosed juveniles) to 31.4 mm. The mean + standard deviation for the sample was 10.7 + 5.8 mm. Sixty percent of the sample consisted of young juveniles less than 10 mm in shell length. By June 1984, small juvenile C. muricinum were no longer being found and a sample of 11 snails collected between 5 and 18 June ranged in size from 15 to 35 mm (X = 24.8 + 6.2 mm). By 3 July 1984, 163 clams had been killed by snails or had died from other unknown causes. This cumulative mortality figure corresponds to 16.3% of the clams planted on 27 January 1984. At the end of the trial, the average shell length of the surviving 837 clams was 119 mm. Specimens of C. muricinum maintained in laboratory seawater tanks grew rapidly when fed ad libitum on young giant clams. The Von Bertalanffy growth equation for C. muricinum was X = 40.66(1 - 0.919e-0*347f) where X is shell length in mm and t is age in years. Fig. 5 is the growth curve obtained by solving this equation for various values of t. The growth rates predicted from the Von Bertalanffy model are in accordance with the data on size frequency collected at the Yap study site. Between mid April and mid June, average snail shell lengths increased from 10.7 to 24.8 mm. This 2-month growth increment of approximately 15.4 mm is almost precisely the amount of growth the Von Bertalanffy model predicts for 10.7-mm snails over a a-month period (Fig. 5).
217
12
34
I
I
I
5
67
I
I
8
I
I
1
9lOll12
1
AGE(months)
Fig. 5. The Von Bertalanffy the laboratory.
growth curve for postlarval
C. muricinum
fed ad libitum in
Fig. 6. The relationship between size of C. muricinum and the number of days required for individual snails to kill juveniles of T. g&as.
The relationship between snail size and the length of time required to kill a juvenile T. gigus (98-105 mm shell length) is specified by the equation Y = 13.98 - 0.326X (r = 0.758, P
218 TABLE 1 The effect of substratum type (basalt gravel vs. cement plates) on the survival of juvenile T. derasa subjected to predation by C. muricinum. In each trial, eight clams (shell length 100-110 mm) were placed in an aquarium with eight snails (shell length 25-35 mm). Dead clams were counted after 10 days Trial
Substratum type Basalt gravel
Cement plates
4 4 3 4 4
4 3 4 4 3
-
-
19
18
1 2 3 4 5
TABLE 2 Numbers of juvenile clams (H. hippopus, T. gigas and T. deraeu) killed during lo-day trials when exposed to predation by C. muricinum. In each trial, five H. hippopus were tested against either five T. gigae or five T. derasa. Dead clams were counted after 10 days Trial
H. hippopus
1 2 3 4 5
0 0 0 0 0
T. gigus
-
5 3 4 5 5
0
22
Trial
H. hippopus
T.
1 2 3 4 5
0 0 0 0 0 0
derasa
4 5 5 3 4
21
The type of substratum to which maricultured clams were attached did not have a strong effect on susceptibility to predation by C. muricinum (Table 1). Clams attached to solid cement surfaces were as likely to be killed by the end of the 1Oday trial as were clams attached to l-cm basalt chips. In the trials designed to detect differences in susceptibility to snail predation among species of giant clams, H. hippopus proved far more resistant to attack than either T. derusa or T. gigus (Table 2). This difference probably
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stems from the fact that, unlike other species of Tridacnidae which have a byssal gape which cannot be closed completely (e.g., T. derasa, Fig. 3A), the zipper-like hinge of H. hippopus (Fig. 3B) can be shut tight as an effective defense against predators. DISCUSSION
Life history of Cymatium muricinum Cymatium muricinum is a highly opportunistic predator which will prove difficult if not impossible to completely eradicate as a pest on maricultured tropical bivalves. However, for reasons discussed below, we feel that C. muricinum does not constitute a serious constraint to the expanded development of giant clam mariculture facilities. Opportunistic species are typically adapted to utilize patchily distributed or ephemeral resources. Many bivalve species, including tridacnid clams, appear to be patchily distributed on the reef (Heslinga and Perron, unpubl., 1984; R. Braley, personal communication, 1984). The planktonic larvae of C. muricinum are well suited to locating such prey, and the evidence indicates that these larvae may be able to sense and respond to the presence of living bivalves. The extremely rapid postlarval growth rate of C. muricinum (Fig. 5) permits these snails to approach maximum size (and probably sexual maturity) within 1 year of settlement from the plankton. Cymatium muricinum therefore rapidly complete its life cycle and produces more larvae which drift in the plankton until an additional food source is encountered. Prospects for the control of Cymatium muricinum
Altering the substratum on which juvenile giant clams are grown is unlikely to- be a practical method of reducing losses due to snail attacks (Table 1). Basalt chips are used as a substratum at MMDC to facilitate subsequent handling of the clams (Heslinga et al., 1984). If clams are grown on a solid substratum, the byssal attachment must be broken every time handling is required. Also, if juvenile clams are placed on a solid flat surface, they will tend to move about and form clumps which result in the stunting of some individuals. Therefore, in the absence of evidence that substratum type can have a major effect on snail predation on young clams, we continue to advocate the use of basalt chips as a convenient attachment surface for giant clam nursery systems. Juveniles of the tridacnid species H. hippopus are significantly less vulnerable to snail predation than are T. gigas or T. derasa (Table 2). Hippopus hippopua is a hardy species which lives on relatively exposed reef flats. The postlarvae have byssal threads, but lose all attachment to the substratum at an early age. In nature, juveniles of T. gigas and T. derasa are usually found attached to dead coral rubble some distance above the substratum. In con-
220
trast, H. hippopus juveniles are found resting directly on the sandy surface of the reef flat. This difference in habitat is likely to be related to the relative predator resistance of H. hippopus. Although the hardiness of H. hippopus would seem to make this species a likely candidate for commercial clam mariculture, other factors favor continued emphasis on 2’. derasa and T. gigus. Giant clam mariculture efforts have been concentrated on T. derasa and T. gigus because these two species attain large adult sizes and have extremely high growth rates under culture conditions (Heslinga et al., 1984). Hippopus hippopus, although showing initially rapid growth, has not continued to grow well in the MMDC oceanbased nursery system. This poor performance could be related to the small adult size of H. hippopus or to the energetic cost of the relatively massive shell of this species. The simplest method of limiting damage caused by C. muricinum is a program of manually removing all visible snails and obviously infested clams from the nursery system. Clams infested by large snails are quite easy to spot when the mantle begins to retract from the shell lip (Fig. 2B). Since specimens of C. muricinum become increasingly destructive as they grow, it is particularly important to remove the large snails as rapidly as possible. At the Yap study site, snails were removed from the clam trays at roughly twice-weekly intervals. Given this moderate level of attention, 16.3% of the clams had died by the end of the 5-month trial. It must be remembered that this figure includes all sources of mortality since it was not usually possible to determine cause of death in the absence of a snail being caught in the act. The effectiveness of the snail removal program at Yap is reflected in the decreasing clam mortality rate during the final month of the trial (Fig. 4), and by the fact that only 10 snails were found on the clams from the middle of June until the end of the trial. Infestations of C. muricinum appear to be episodic or seasonal. At the Yap study site, small juveniles appeared on the clams in mid April 1984. Since the average size of the snails at that time was approximately 10 mm, the cohort probably settled from the plankton 2-3 weeks earlier (Fig. 6). By June 1984, the average size of the snails had increased considerably and no new juveniles were being found. These results were paralleled by observations made at the MMDC nursery system in Palau. Early postlarval snails were common during the late winter and spring of 1984, but were no longer in evidence by the summer of that year. Since Palau and Yap are 300 miles apart, the roughly simultaneous appearance of C. muricinum postlarvae at both locations could indicate a late winter or early spring settlement peak for this species. If this suggestion of seasonality is borne out by further observations, it may prove advantageous to schedule outplanting of giant clam juveniles for seasons during which the probability of C. muricinum settlement is low. Because infestations of C. muricinum are episodic and can be controlled through simple manual removal of snails from clam trays, this predator is
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more likely to be a minor annoyance than a major constraint to the development of commercial giant clam farms in the tropics.
REFERENCES Fabens, A.J., 1965. Properties and fitting of the Von Bertalanffy growth curve. Growth, 28: 265-289. Heslinga, G.A. and Perron, F.E., 1983. The status of giant clam mariculture technology in the Indo-Pacific. South Pacific Commission Fisheries Newsletter, No. 24: l-5. Heslinga, G.A., Perron, F.E. and Orak, O., 1984. Mass culture of giant clams (f. Tridacnidae) in Palau. Aquaculture, 39: 197-215. Heslinga, G.A., Ngiramengior, M. and Perron, F.E. An ocean-based nursery system for tridacnid clams. Aquacultural Engineering. In press. Houbrick, J.R. and Fretter, V., 1969. Some aspects of the functional anatomy and biology of Cymatium and Burma. Proc. Malacological Sot. Lond., 38: 415-429. Kay, E.A., 1979. Hawaiian Marine Shells. Bishop Museum Press, Honolulu, HI, 653 pp. Munro, J.L. and Heslinga, G.A., 1983. Prospects for the commercial cultivation of giant clams (Bivalvia: Tridacnidae). Proceedings of the Gulf and Caribbean Fisheries Institute, 35: 122-134. Scheltema, R.S., 1971. Larval dispersal as a means of genetic exchange between geographically separated populations of shallow-water benthic marine gastropods. Biol. Bull., 140 (2): 284-322.