Microtopography and the organization of two assemblages of coral reef fishes in the west indies

J. Exp. Mar. Biol. Ecol., 1984, Vol. 78,

253

pp. 253-268

Elsevier

JEM 278

MICROTOPOGRAPHY AND THE ORGANIZATION OF TWO ASSEMBLAGES OF CORAL REEF FISHES IN THE WEST INDIES’

LESLIE S. KAUFMAN New England Aquarium, Boston, MA 02110. U.S.A. and JOHN P. EBERSOLE Department of Biology, University of Massachusetts - Boston, Boston, MA 02125, U.S.A. Abstract: Two walls of Salt River Canyon, St. Croix, U.S. Virgin Islands are at the same depth in the fore reef environment and are separated by only 100 m, but they differ in microtopography. Replicate visual censuses of the fish assemblages on these two walls, by four saturation divers, gave consistent estimates of the species assemblages on the two walls. Significant fauna1 differences between the two walls are best understood in terms of combined features of coloration, predator avoidance, and feeding habit displayed regularly by individuals occupying a given microhabitat. It appears that characteristic sets of environmental circumstances occur within a habitat with sufficient frequency to give a consistent direction to natural selection, producing the patterns we have observed. Thus, form and function is predictable in relation to microhabitat, although species occurrence is not. The “order” and “chaos” concepts of community structure in coral reef fish, with their narrow focus on the predictability of species occurrence, underemphasize the possibility for a simple and direct role for natural selection in shaping the functional characteristics of an assemblage on a small scale.

INTRODUCTION

Ecologists have long sought general principles governing the number of species and the patterns of coexistence in communities. Two highly polarized theories attempt to explain community structure of coral reef fishes. The adherents to one view see reef fish assemblages as orderly composites of ecological specialists with low niche overlap, molded by interspecific competition (Jones, 1968; Vivien & Peyrot-Clausade, 1973; Smith & Tyler, 1973, 1975; C. L. Smith, 1975; Clarke, 1977; Gladfelter et al., 1980; Anderson et al., 1981). Diametrically opposed are investigators who, sometimes interpreting the same data, see a chaotic structure of generalized species with high niche overlap (Sale, 1975, 1977, 1978a,b, 1979, 1980a,b; Sale & Dybdahl, 1975, 1978; Molles, 1978; Talbot et al., 1978). They attribute the chaos that they see to high larval predation and the stochastic nature of larval recruitment. It has become clear that community structure is influenced by both stochastic and deterministic factors, The “order” hypothesis predicts different species assemblages on the different microhabitats within a reef zone, because of habitat selection and differential survival. ’ Contribution No. 120 of the West Indies Marine Laboratory, Fairleigh Dickinson University 0022-0981/84/.$03.00 0 1984 Elsevier Science Publishers B.V.

254

LESLIE

S. KAUFMAN

AND JOHN P. EBERSOLE

According to the “chaos” view, extensive areas of reef should exhibit the same assemblages of fish species regardless of microtopography, so long as they recruit from the same larval pool. In the past these hypotheses have been tested in three ways. One technique was to study correlations between species abundance patterns and habitat gradients on a large scale (Talbot & Goldman, 1969; Chave & Eckert, 1974; Adey et al., 1977; Gladfelter & Gladfelter, 1978; Luckhurst & Luckhurst, 1978; Gladfelter et al., 1980; Anderson et al., 1981). Another was to set up small artificial reefs of varied structure, surrounded by sand, and follow patterns of recruitment (Russell et al., 1974; Molles, 1978; Talbot et al., 1978; Bohnsack & Talbot, 1980; Shulman et al., 1983). The third approach was to defaunate small natural patch reefs and monitor recruitment to them (Sale & Dybdabl, 1975, 1978; Brock et al., 1979; G. B. Smith, 1979). These empirical studies have generated a series of compromise views. Ogden & Ebersole (1981), Sale (1980b), and Brock et al., (1979) have suggested that the apparent degree of order or chaos in a reef fish community may be dependent on the temporal or spatial scales of examination. C. L. Smith (1978), Brock et al. (1979) and Shulman et al. (1983) have argued that community structure of coral reef fishes arises from a combination of random processes affecting the planktonic larvae and deterministic processes affecting postlarval settlement and adult interactions on the reef itself. Grossman (1982) and Grossman et al. (1982) have claimed that fish communities can be either deterministic or stochastic, and “it is currently important to quantify the relative frequencies of these two types of assemblages” (Grossman et al., 1982). Sale (1980a) and Sale & Dybdahl (1975, 1978) have asserted that reef fish communities are organized into deterministic trophic guilds, but that species composition within such guilds is random. Although these hypotheses may appear to represent a convergence of views, a strong polarity remains. The salient feature of all these perspectives is that they incorporate more biological reality than is possible in the simple and naive order-chaos dichotomy. In this study, we compared the fish assemblages on two extensive sites (z 350 m2 each) that are parts of a single extensive reef system but that are separated by a narrow sand channel (30-100 m wide) and different in microtopography, with two objectives in mind. First, we intended to find whether the two assemblages were different in species composition, as predicted from the order view, or the same, as predicted from the chaos standpoint. Secondly, we intended to look beyond the typical comparison of species composition by examining the proportional representation of fishes at the two sites in terms of functional characteristics, such as trophic habit, behavioral defense, and coloration. METHODS

All field work was performed by four divers living in the National Underwater Laboratory System- 1 habitat “Hydrolab” operated by the National Oceanographic and Atmospheric Administration. The Hydrolab is located at a depth of 15.2 m (50 ft) in

MICROHABITAT AND ORGANIZATION

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255

Salt River Canyon, St. Croix, U.S. Virgin Islands (Fig. 1). Four observers censused the fishes at two sites on opposite sides of the canyon. These two canyon walls, only 30 to 100 m distant from each other, experience similar wave energy and water chemistry

15’

Fig. 1. Location and topography of Salt River Canyon, site of the NULS-I habitat “Hydrolab”.

regimes but differ in microtopography (NULS-1 operation manual). The West Wall census area is characterized by steep coral buttresses, labyrinthine caves, talus slopes and sand chutes; coral and sponge growth forms are chiefly foliaceous and encrusting. At the same depth, the East Wall census area consists of low-relief coralline limestone pavement with interspersed patches of sand < 1 m in diameter, the whole sloping at roughly a 45” angle; arborescent gorgonians form an open “woodland” (estimated densities 0-10/m2) peppered by coral mounds and large barrel sponge Xe~t~~~~~giu rnuta (Schmidt). Both sites were established at the same depth range of 15.2-18.3 m (50-60 fsw) as determined by a submersible depth gauge, and ran x 50 m along the Canyon wall. As described by Kaufman (1983), the census technique was designed to determine which fish species were present at each site and the pattern of their relative abundances, rather than the population densities of each species, so the exact dimensions of the census area were not needed and not measured. Upon arrival at a census area, an observer set his stopwatch to zero, stopped moving, and closed his eyes. The census was begun by simultaneously opening the eyes and starting the stopwatch. Each time a new species was encountered, the watch was stopped and the species and running time in minutes and seconds were recorded. After

256

LESLIE S. KAUFMAN

AND JOHN P. EBERSOLE

a record was made, the watch was started again, with census time accumulating. The observer traced a haphazard path through the census area, continuing to census until the rate of new sightings fell to two species per lo-min interval or below. A completed census produced a list of the fish species sighted and the times at which they were first encountered. Each census dive lasted NN180 min, including swimming to and from the study site and a single change of SCUBA tanks, with an accumulation of 40-50 min of “census time” on the stopwatch. Each observer made two censuses per day, one in the morning and one in the afternoon. By the end of the Hydrolab mission, each of the four observers had made two censuses on each of the two sites, one in the morning and one in the afternoon. Thus, comparisons between walls can be checked against comparisons between observers and between times (morning vs afternoon). As a first step in data analysis, all species sighted were categorized as to: (1) feeding behavior (trophic categories), (2) response to predators (predator defense categories), and (3) coloration and pattern (aspect categories). We assigned fishes among the five trophic categories (piscivore, invertebrate specialist, opportunist, planktivore, herbivore) on the basis of published accounts of stomach contents, personal observations of their feeding behavior, and morphological characteristics (as in Kaufman & Liem, 1982). Fishes were distributed among three behavioral defense categories (dodge-and-run, shelter-seeking, stay-put) according to personal observation. For some species this consisted of observations of encounters with natural predators, but often the defense category indicates the response of the fish to harassment by a diver. Dodge-and-run species are those that head for open water or dodge among coral heads when threatened, while shelter-seeking species head for crevices and cavities within the reef structure. Stay-put species are generally armored, spiny, poisonous, cryptic, or inquiline. We also classified fishes into four aspect categories (drab, bold, barred, striped) based on personal observation. Bold species are typically marked with solid primary colors or adjacent complementary colors, while drab species are typically mottled in shades of brown, gray, and olive. Barred fishes bear vertical lines, and striped fishes bear horizontal lines. Fishes were assigned to categories on the basis of what we agreed are typical diurnal ch~acteristics of adults. However, juvenile grunts are a prominent part of these assemblages and differ markedly from adults in trophic category, behavioral defense, and aspect. Since the different species of juvenile grunts are practically indistin~ishab~e in the field, we lumped then together to be treated as a single “species” separate from the adults. We can assess interdependence between functional categories and compare walls in terms of functional categories in two different ways. First, it is possible to examine these relationships with respect to the number of species in each functional category. Secondly, our census technique allows us to count the number of sightings within categories, with each species represented by a maximum of 16 sightings. The sum

257

MICROHABITATAND ORGANIZATIONOF CORALREEF FISHES

of the sightings for a given functional category roughly indicates the abundance of that portion of the fish assemblage. RESULTS PATTERNSOF SPECIES RICHNESS A total of 108 species were recorded during the 16 censuses in Salt River Canyon, with species richness running consistently higher on the West Wall (86 species total) than on the East Wall (65 species tota1) (West Wall S = 44.6 f 4.350 SD; East Wall S = 34.2 2 4.150 SD). A list of all species seen (nomenclature as in Bohlke & Chaplin, 1968) is in Table I, including their classifications into trophic, coloration, and defense categories, and the number of sightings made on each wall. Since each observer eensused each wall once in the morning and once in the afternoon it is possible to compare variation in sampling characteristics among microhabitats, time of day, and observers. Comparisons were made by means of species-time accumulation curves (Fig. 2). The asymptotic nature of the species-time accumulation curves indicates that the fish fauna was adequately sampled on both walls. We compared rates of species accumulation by arbitrarily dividing accumulated census time into four intervals and applying a chi-square test for heterogeneity to the number of species recorded per interval. As might be expected from the difTerence in species richness, the two walls accumulated species at different rates (x2 = 27.15, P = 0.04). Rates of species accumulation were independent of time of day (x2 = 24.18, P = 0.09), but varied among observers (x2 = 17.82, P = 0.02); some added species monotonically while others tended to observe new species in spurts. This variation apparently has no biological signifi-

WESTWALL AM EASTWALLAM

EAST

WALL

PM

Fig. 2. A representative set of species accumulation curves, taken by a single observer: the ordinate indicates the total number of species seen by an observer during the time interval on the abscissa.

LESLIE S. KAUFMAN AND JOHN P. EBERSOLE

258

TABLEI A list of all species encountered, indicating trophic, color, and behavioral defense categories, and the number of sightings on the wall.

Family and species

Trophic category

Coloration category

Defense category

East wall

West wall

Inv. Spec.

drab

stay

2

0

drab

run

8

6

Pisc. Pisc.

drab drab

shelter shelter

1 4

0

Pisc.

drab

run

0

5

striped striped striped striped striped striped

shelter shelter shelter shelter shelter shelter

0 8 0 0 0 0

6 5 I 1 8

Pisc.

drab

run

0

1

Pisc. Pisc. OPP. Pisc. Pisc. Pisc. Inv. Spec. Inv. Spec. Inv. Spec. Plank.

drab drab drab drab drab barred bold drab bold striped

run shelter run shelter run run shelter run shelter shelter

4 1 8 6 0 0 8 8 6 0

6 4 8 0 3 7 3 0 3

Plank.

bold

shelter

0

8

Pisc.

drab

stay

0

8

Pisc.

drab

shelter

0

I

Plank. Plank. Plank. Plank.

bold barred barred drab

shelter shelter shelter shelter

0 3 0 0

1 8 4 2

striped

run

0

1

Plank.

striped

run

0

I

GPP.

drab

run

6

8

Dasyatidae Dasyatis americana

Synodontidae Synodus intermedius

Muraenidae Gymnothorax moringa G. vicinus

I

Aulostomidae Aulostomus macuIatus

Holocentridae Holocentrus ascensionus H. rufus Flammeo marianus Adioryx coruscus A. vex~~~a~.us Myripristisjacobus

Inv. Inv. Inv. Inv. Inv. Inv.

Spec. Spec. Spec. Spec. Spec. Spec.

I

Sphyraenidae Sphyraena barracuda

Serranidae Epinephelus gutratus E.adscensionis E. fulvus E. cruentatus E. iterajara Hypoplectus unicofor Serranus ti~‘n~ S. tabacarius S. baldwini Lipropoma rubre

I

Grammidae Gramma foreto

Grammistidae Rypticus saponaceus

Priacanthidae Priacanthus cruentatus

Apogonidae Apogon lachneri A. townsendi A. binotatus Phaeoptyx canklini

Echeneidae Echeneis sp.

Inermidae Inermia vittata

Carangidae Caranx ruber

MICROHABITAT

Family

and species

Scombridae Scomberomerus regalis Lutjanidae Lutjanus apodus L. mohogoni L. analis Ocyurus chrysurus

AND ORGANIZATION

OF CORAL

REEF

FISHES

259

Trophic category

Coloration category

Defense category

East wall

West wall

Pisc.

drab

run

0

1

Pisc. Pisc. Pisc.

run run run run

GPP.

striped drab drab striped

Pomadasyidae Haemulon spp. t&v) H. sciurus H. aurolineatum H. chtysargyreum H. jlavolineatum H. plumieri

Plank. Inv. Spec. Inv. Spec. Inv. Spec. Inv. Spec. Inv. Spec.

striped striped striped striped striped striped

shelter run run run run run

1 8 0 1 1 8

2 0 5 0 0 2

Sparidae Calamus sp.

Inv. Spec.

drab

run

0

I

Sciaenidae Odontoscion dentex

Inv. Spec.

drab

shelter

0

3

Mullidae Mulloidichthys martinicus Pseudupeneus maculatus

Inv. Spec. Inv. Spec.

striped drab

run run

3 5

5 6

Gerreidae Gerres cinereus

Inv. Spec.

drab

run

3

1

Bothidae Bothus lunatus B. ocellatus

Inv. Spec. Inv. Spec.

drab drab

stay stay

2 1

0 3

Opistognathidae Opistognathus aurttrons

Plank.

drab

stay

I

0

Scorpaenidae Scorpaena plumieri

Pisc.

drab

stay

3

1

Dactylopteridae Dactyloptenrs volitans

Inv. Spec.

drab

run

2

0

Chaetodontidae Chaetodon capistratus C. shiatus C. aculeatus

Inv. Spec. Inv. Spec. Inv. Spec.

barred barred bold

run run shelter

0 1 0

8 0 8

Pomacanthidae Pomacanthus paru P. arcuatus Holacanthus tricolor H. ciliaris

Inv. Inv. Inv. Inv.

barred drab bold barred

run run shelter shelter

Herb.

drab

shelter

0

2

Herb. Plank. Plank. Plank. Plank.

drab drab barred bold drab

shelter shelter run shelter shelter

0 8 0 8 0

6 8 8 8 8

Pomacentridae Eupomacentrus spp. (dorsopunicans & diencaeus) E. planifrons E. partitus Abudefduf saxatihs Chromis cyaneus C. multilineata

Spec. Spec. Spec. Spec.

LESLIE S. KAUFMAN AND JOHN P. EBERSOLE

260

T~BI F

Family and species

Trophic category

I.

continued

Coloration category

Defense category

West

East wall

Wall --

Labridae Bodianus rufus Halicheres bivittatus H. pictus H. radintus H. garnoti H. mnculipinna H. poeyi Thalassoma bifasciatum Clepticus parrae Hemipteronotus sptendens

Plank. Inv. Spec.

bold striped striped bold bold striped drab bold drab drab

run run run run run run run run run shelter

Herb. Herb. Herb. Herb. Herb. Herb. Herb. Herb. Herb.

drab bold bold striped bold striped striped bold striped

run run run run run run run run run

2 5 5 0 0

Plank.

drab

stay

3

0

Plank.

striped

shelter

0

6

Plank. Plank. Inv. Spec. Inv. Spec. Plank. Inv. Spec. Plank. GPP.

bold bold barred drab bold striped drab barred

run run shelter shelter shelter run shelter shelter

Herb. Herb. Herb.

bold drab drab

run run run

8 1

8 7

5

5

Inv. Spec.

drab

shelter

1

0

Inv. Spec. Inv. Spec. Inv. Spec.

bold bold drab

stay stay stay

3 1 0

3 1

Inv. Spec. Inv. Spec.

drab bold

stay stay

6

0 8

Inv. Spec. Inv. Spec.

drab drab

stay stay

0 1

1 0

GPP. GPP. Plank. GPP. GPP. OPP. OPP. OPP.

I 0 0 2 8

3 1 S 0

2

I 5 1

0 8 I 0 I 5 0

Scaridae Sparisoma ch~sopte~m S. viride S. aurofrenatum S. automarium Scams vetula SC. croicensis SC. taeniopterus SC. guacamaia Cryptotomus roseus

t 3 3

1

0 4 8 0 0 6 4

1 4

Clinidae Emblemaria pandionis

Blenniidae Luea.vablennius zingaro

Gobiidae Coryphopterus lipemes C. personatus C. glaucofraenum Gnatholepis thompsoni Gobiosama tenox G. genie fogloss~ hefenae Quisquilius hipoliti

Acanthuridae Acanthurus coeruteus A. bahianus A. chirurgus

Monac~thidae ~onacanthus tucker‘

Ostraciidae Lactophrys triqueter

L bicaudalis L. potygonia

1

Tetraodontidae Sphoeroides spengferf Canthigaster rostrata

1

Diodontidae Diodon hystrix Chylomycterus sp.

MICROHABITAT

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261

came, and merely reflects the slightly different routine employed by each observer for encountering the more cryptic species. PATTERNS

OF SPECIES

COMPOSITION

Species composition differed substantially between the two walls, with 43 species found only on the West Wall and 22 species found only on the East Wall. We calculated Jaccard Coefficients of Concordance (number of species shared/total number of species), a measure of similarity in species composition, for comparisons between walls (within observers and times of day). The similarity values we obtained are low, ranging from 0.283 to 0.390 (n = 8). By contrast, similarity values between observers (within walls and times of day) ranged from 0.413 to 0.708 (n = 24), and similarity values between times of day (within observers and walls) ranged from 0.456 to 0.720 (n = 8). Following an arcsine transformation, we applied an ANOVA-SNK test to these values, which showed that the mean similarity in species composition between walls is significantly lower than between observers and between times of day. The statistical test merely reinforces what is obvious from inspection of these data: the similarities obtained by comparisons between times and observers fall in the same range, which is not overlapped by the lower range of similarity values obtained from comparisons between walls. A real and substantial difference is indicated in species composition between walls. We proceed to examine the nature of this difference. COMPARISONS

OF TROPHIC

STRUCTURE

When the two walls were compared in terms of the number of species in each of the five trophic categories, the patterns proved virtually identical (between-walls, withinobservers and times : x 2 = 4.11, P > 0.997). Similar results were obtained from comparisons between times of day (within observers, within wall: x2 = 1.48, P > 0.999) and observers (within times, within wall: x2 = 0.11, P > 0.999). However, when the two wails were compared in terms of the number of sightings (a maximum of eight per species per wall) in each category (regardless of species), differences emerged. Planktivores were more common on the West Wall, while oppo~unistic feeders were more prevalent on the East Wall (x2 = 14.77, P < 0.01). MICROHABITAT

AND ANTI-PREDATOR

DEFENSES

As their first line of defense reef fishes can either seek shelter, run and dodge to another location, or remain stationary so as to blend in which the surroundings or to prepare for the secondary use of weapons. The proportion of species employing these defensive modes differed between the two walls (Table IIA), with more shelter-seekers on the West Wall (x2 = 4.28, P < 0.05). Even more pronounced differences were revealed by contingency analysis of the number of sightings (regardless of species) in each of the three defensive categories (Table IIB), further demonstrating that fishes

262

LESLIE

S. KAUFMAN

AND JOHN

P. EBERSOLE

which run and dodge and fishes that stay put were more prevalent on the East Wall (x2 = 21.03, P < 0.001). TABLE II Behavioral

defenses

of (A) fish species,

and (B) sightings Canyon.

of fish species on opposite

Number Behavioral A Seek shelter Dodge-and-run

35 51

65 86 x2 = 4.28, P = 0.04 Number

B Seek shelter Dodge-and-run Stay put

of sightings

15 175 29 Total

AND

West wall

16 49

and Stay-put Total

ASPECT

of species

East wall

defenses

walls of Salt River

154 180 19

279 353 x2 = 21.03, P < 0.001

MICROHABITAT

The proportion of species in each of the four aspect categories did not differ between the two walls, but the representation of these categories did vary when the numbers of sightings in each group were compared between walls (2 = 20.25, P < 0.001). Among fishes sighted on the East Wall there was a preponderance of horizontally striped fishes and drab fishes; on the West Wall bold and barred fishes were disproportionately represented (Table III). TABLE III Color patterns

of fishes sighted

on opposite

walls of Salt River Canyon. Wall

Color pattern

East

West

Bold Barred Striped Drab

104 14 68 93

136 55 69 93

219

353

Total

x2 = 20.25, P < 0.001

MICROHABITAT

AND ORGANIZATION

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OF CORAL REEF FISHES

INTERACTIONBETWEENTROPHIC AND BEHAVIORALDEFENSE CATEGORIES The vast majority of species classified as herbivores fell into the dodge-~d-~n defense category, most phmktivores were shelter-seekers, and most invertebrate specialists were stay-put species (x2 = 36.52, P < 0.001 for species; x2 = 219.09, P < 0.001 for sightings). Sightings on the East Wall exhibited a disproportionately high frequency of dodge-and-run invertebrate specialists, opportunists, and stay-put planktivores. The West Wall, on the other hand, had a higher prevalence of shelter-seeking invertebrate specialists, shelter-seeking or dodge-and-run planktivores, and shelter-seeking herbivores (x” = 62.91, P-c0.001). INTERACTIONBETWEENTROPHIC AND ASPECTCATEGORIES Oppo~~ists and herbivores tended to be boldly colored, invertebrate specialists tended to be horizontally striped, and piscivores tended to be drab (x2 = 51.75, P < 0.001 for species; x2 = 285.15, P -=I 0.001 for sightings). Sightings of barred or striped planktivores were disproportionately low (Table IV). TABLE

IV

Sightings of fish species in Salt River Canyon crosstabulated

in terms of Aspect and Trophic habit. Aspect

Trophic category

Bold

Piscivore Invertebrate specialist Opportunist Planktivore Herbivore Total

Barred

Striped

Drab

0

9

1

66

54 71 61 54

36 1 23 0

81 19 12 24

63

69 137 ;62= 285.15, P < 0.001

186

240

1 31 25

INTERACTIONBETWEENASPECTAND BEHAVIORALDEFENSECATEGORIES Data for both species and sightings revealed strong dependence between coloration and behavioral defense categories. Dodge-~d-~n fishes tended to be striped, shelterseeking fishes were mostly barred or brilliantly colored, and stay-put species tended to be drab (2” = 18.80, P < 0.005 for species; x2 = 63.02, P-c0.001 for sightings). DISCUSSION The results of this study in Salt River Canyon appear to contravene the chaos theory of community structure. The sites studied on the East and West walls of the Canyon

264

LESLIE

S. KAUFMAN

AND JOHN P. EBERSOLE

fall in the same depth range, share the same substratum life forms (corals, gorgonians, sponges) and, being separated by < 100 m, their fish faunas are presumably drawn from the same pool of larval recruits. However, distinct differences in the species composition of the two fish communities, apparently related to the microtopographic difference between walls, contradict the prediction of Sale (1978a, p. 100; 1980a, p. 249) that sufficient sampling of a single small site will accumulate all the species found in the habitat. The order view of community structure also falls short as an explanation of the situation in Salt River Canyon. The conventional order theory is based on an equilibrium between resource densities and population densities that forces a partitioning of resources among coexisting species. Thus, the conventional order theory predicts that the number of species partitioning a given trophic level is closely bound to the number of individuals the trophic category can support; assemblages with parallel representation of species in trophic categories should have parallel representation of individuals in trophic categories. The two walls of Salt River Canyon do not differ in the proportions of species in our five broad trophic categories, but they do differ significantly in the representation of individuals in these categories, with more opportunistic fishes on the East Wall, and many more planktivorous fishes on the West Wall. Our results from Salt River Canyon indicate that much about the organization of fish assemblages can be understood by abandoning the limitations inherent in the order-chaos dichotomy to focus instead on the morphological and behavioral features of fish species in relation to the microtopographic features of their environment. What we find most interesting in our data are not the associations of particular species with microhabitats, but rather the association of particular suites of traits with microhabitats. For example, in Salt River Canyon it is not very useful, or even appropriate, to speak of a planktivore guild. In fact, species that share the habit of eating plankton actually comprise a number of otherwise functionally distinct groups. The planktivores of the East Wall were predominantly drab species living in close association with the bottom, and they retreated deeper into their shelters when threatened (e.g., pearly jawtish Opistognathus aurifrns, emblemariid blennies, garden eels), whereas on the West Wall, the planktivores were boldly colored, ranged farther from the reef surface, and fled for shelter when threatened (e.g., the blue chromis Chromis cyanea, Creole wrasse Clepticus parrae). Similarly, the herbivores on the East Wall were mostly large, aggregating species that run and dodge when disturbed (e.g., parrot- and surgeonfishes), while on the West Wall the herbivores were mostly small, solitary species that sought shelter when threatened (e.g. pomacentrids of the genus Eupomacentnts). Thus, the groups of fishes found on the two sides of Salt River Canyon can be better compared on the basis of trait combinations that bear a logical relationship to each other and to microtopography, than on the basis of exclusively trophic distinctions employed by most investigators. To some degree, the patterns of trait association which we observed on a community level are already familiar to ethologists with respect to individual fishes. For example,

MICROHABITAT

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the preponder~ce of planktivores that we observed on the West Wall is to be expected from Randall’s (1967) observation that planktivorous reef fishes tend to mass above steep drop-offs. Davis & Birdsong (1973) distinguished between planktivores that hover above specific refuge sites on the reef, a group corresponding to our common East Wall ptanktivores, and those that feed relatively high in the water column, like most of the common West Wall species. When we considered the types of color patterns observed in East versus West Wall fishes, it was evident that striped fishes that dodge and run from predators were predominant on the open ground of the East Wall, whiie vertically barred, shelterseeking species were more common on the West Wall (Table III). Barlow (1972) and IIailman (1977, 1982) postulated that horizontally striped color patterns on fishes dodging or fleeing in open water tend to confuse pursuing predators, and that vertical bars make it difficult for a predator to fix on fishes moving amongst vertical structures. They noted a general tendency for striped patterns to occur among fishes that flee in open water, and barred patterns in those that head immediately for shelter. Fishes living amongst rigid structures (e.g., scleractinian corals on the West Wall) are boldly colored, while those living amongst flexible structures (e.g., gorgonians on the East Wall) tend to be drab (Table IV). We are reminded that the fishes of seagrass beds, reefs with abundant growths of fleshy algae, and pelagic sargassum mats are all cryptically drab in coloration (Bohlke & Chaplin, 1968; Adey et al., 1977). In the clear waters of the African rift lakes, boldly colored cichlid fishes are associated with rocky reefs and headlands, while cryptically colored cichlid species are associated with plant-choked swamps and grassbeds (Fryer & Iles, 1972). An unexpected association of traits was the combination of dodge-and-run predator avoidance and striped color pattern with opportunistic feeding habits. In retrospect this makes sense, since in order to take advantage of spatially unpredictable trophic resources, a fish must have the speed and maneuverability that are also required of any species that dodges and runs to escape its predators. The East Wall also included a group of species that were typically encountered over open sand and rubble, and which stood their ground when confronted. As a group, these species were either heavily armed or able to dig instantly into the loose substratum, and they shared a characteristic color pattern consisting of vermiform blue lines or ocelh superimposed upon beige mottling: e.g., the lizardfish Synodus intermedius, peacock flounder Bothus lunatus, lancer dragonet Cullionymus bairdi, flying gumard Dactylopteris volitans, scrawled file&h Alutera sctipta, and scrawled cowfish Acanthostracion quadrico~~. We would not speculate on the functional significance of this trait association, but its occurrence is clearly substratumrelated. In Salt River Canyon, we have found that particular features of coloration, predator avoidance and feeding habit occur in combination in particular microhabitats with a regularity that allows for their prediction, though we can not predict precisely what particular species will be present even in a completely described microhabitat. Our variables of aspect, predator avoidance, and trophic category are counterparts of the

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selective filters proposed by C. L. Smith (1978), although some of our filtering factors stand apart from those usually considered in the order-chaos argument. It appears that within a given type of microhabitat, sets of environmental circumstances occur with sufficient frequency to give a consistent direction to natural selection, producing the patterns we have observed. It would be tempting to suggest that with detailed knowledge of all the selective factors operating in a system it would be possible to predict precisely the species that will be present in any microhabitat within that system. However, it must be emphasized that although natural selection can for convenience be modelled deterministically, it is really a probabilistic process, setting an ultimate limit to the degree of determinism possible, and hence, to our power of prediction. We believe that the debate over the role of deterministic processes in organizing ecological communities has become sterile; in the future it will be more productive to focus research on the relationship between the characteristics of organisms and the probability of success for the organisms in different environmental circumstances.

ACKNOWLEDGEMENTS

We are especially grateful for the technical assistance and sound scientific advice received from the three divers who shared the Hydrolab mission with Les Kaufman: Ray Clarke, Rod Katanach, and Bill Mahan. We also thank Ted Maney of UMass/Boston and all members of the NULS-1 team for their support of the Hydrolab and its aquanauts during the mission. Dr. William Schane, supervisor of the NULS-1 program, was helpful in all phases of training, planning, and execution of the mission. M. Ebersole, J. Hatch, J. Pederson, and M. Rex made helpful comments on the manuscript. Financial support was provided by a NULS-1 grant from NOAA, and by NSF Award No. OCE-7909577 to J. P. E.

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