Seasonality of asexual reproduction in Holothuria (Halodeima) atra, H. (H.) edulis and Stichopus chloronotus (Holothuroidea: Aspidochirotida) on the Great Barrier Reef

Asexual reproduction by fission was monitored for 18 mo in populations of Holothuria (Halodeima) atra, H. (H.) edulis and Stichopus chloronotus on three nearshore fringing reefs and one midshelf reef in the Great Barrier Reef. Fission in S. chloronotus occurred exclusively between March and October, with a peak value of 31% recently divided individuals in one population in July. H. atra showed a similar pattern, with maxima of between 16 and 26% from May to July. In H. edulis, asexual reproduction occurred only between March and July, with a maximum of 17% recently divided individuals in March. Fission rates of H. atra and S. chloronotus in winter were significantly higher than in all other seasons. For H. atra, at least 76% of all individuals at Fantome Island were estimated to undergo fission per year, whereas only 9% undergo fission on the midshelf reef. Highest annual fission rates (43%) for S. chloronotus were found in a dense population on Great Palm Island. The lower-density midshelf reef population exhibited comparatively lower annual fission rates (19%). About 24% of H. edulis undergo fission each year. Annual fission rate and population density were positively correlated in the four populations of S. chloronotus and H. atra studied. Received: 21 March 1997 / Accepted: 26 March 1997     

Genetic structure of fissiparous populations of Holothuria (Halodeima) atra on the Great Barrier Reef

Allozyme variation at five polymorphic loci was surveyed in a total of 311 individuals of the sea cucumber Holothuria (Halodeima) atra (Jäger, 1833) collected from two nearshore and two midshelf populations in the Great Barrier Reef in November 1996. Strong deviations in genotype frequencies from those expected under Hardy–Weinberg equilibrium, particularly a large number of heterozygote excesses, confirmed the occurrence of asexual reproduction. Females and males differed significantly in genotype frequencies as a result of differences in either the amount of fission in each sex or in the mortality of asexually produced recruits. The estimated maximum sexual input (number of sexually produced individuals: sample size = N*:N_i) to the two nearshore reefs (38 to 67%) was low relative to that to the two midshelf reefs (74 to 87%). The three ratios and G _o :G _e , N _go :N _i , N*:N _i , (where G _o = observed genotypic diversity, G _e = expected genotypic diversity, N _go = number of genotypes) considered to be indicators of the extent of asexual reproduction, showed a consistent trend in the degree of asexual reproduction similar to that derived from the number of regenerating individuals observed in the populations for which data were available. F-statistic analyses of clonal gene frequencies demonstrated that all populations received sexual recruits from the same gene pool. There was evidence of restricted sexual recruitment to the Fantome population, suggesting that asexual reproduction was dominant only in areas where sexual recruitment was limited by other factors.     

Zooplankton community structure and water flow in the lee of Helix Reef (Great Barrier Reef, Australia)

To investigate which physical processes contribute most in moulding zooplankton community structure in the waters close to coral reefs, light traps moored in a grid pattern were used to collect zooplankton from the sea surface at 16 stations on the downstream side of Helix Reef during three time periods (2100-2200, 2400-0100, and 0300-0400 hours) over three consecutive nights covering the new moon period in January 1992. Two distinct zooplankton communities were present: a community composed primarily of reef-resident, demersal plankton immediately to the south of the reef in an area of reduced flushing, and a community containing coastal and shelf-seas taxa at the more exposed sites in the open flow field. The fauna composition at a number of exposed stations was as rich as that at sheltered stations both in terms of number of taxa and diversity indices but was almost an order of magnitude less abundant. The reef-resident, demersal plankton community was dominated by gammarid amphipods, mysids, and polychaetes, whereas only transient, meroplanktonic forms such as echinoderm and echinopluteus larvae and shelf-seas, holoplanktonic forms such as doliolids and larvaceans were significantly more abundant in the exposed community. Zooplankton associations were apparently formed by a combination of hydrodynamic processes, spatial and temporal distribution patterns of individual taxa, specific behaviours of certain taxa, and the interactions among taxa at different trophic levels.     

Genetic structure of outbreaking and non-outbreaking crown-of-thorns starfish (Acanthaster planci) populations on the Great Barrier Reef

The study used population genetic data to test whether outbreaks of crown-of-thorns starfish (Acanthaster planci) on the Great Barrier Reef (GBR) are largely derived from a single outbreak or are independent events. The consequences of different modes of outbreak for population differentiation and gene flow were predicted and then compared with those estimated from a set of 8 outbreaking and a set of 5 non-outbreaking populations sampled in 1986 and 1987. The level of inter-population variation observed among outbreaking populations was less than that among non-outbreaking populations and gene flow was greater among outbreaking than among non-outbreaking populations. Greater population differentiation among non-outbreaking than outbreaking populations was not consistent with the hypothesis that outbreaks were independent events, but was consistent with a number of outbreaks being secondary. Estimates which took account of a number of aspects of sampling error demonstrated significant levels of genetic subdivision among non-outbreaking populations but not among outbreaking populations. The lack of significant genetic subdivision of outbreaking populations, given significant levels among non-outbreaking populations, was also inconsistent with outbreaks being indenpendent events, but was consistent with outbreaks being largely secondary. The allozyme data were insufficient to identify clearly individual populations that might have been the result of an independent outbreak. It is concluded that the majority of outbreaks are probably secondary derivatives from a single primary outbreak occurring in the northern part of the Central Section of the GBR, although the possibility that a small number of populations might outbreak independently of the main set cannot be excluded.Contribution No. 547 from the Australian Institute of Marine Science     

No genetic differentiation of giant clam (Tridacna gigas) populations in the Great Barrier Reef,Australia

Six Tridacna gigas populations were sampled in 1990 from locations throughout the central and northern Great Barrier Reef (GBR). Despite separations in excess of 1000 km, mean Nei's unbiased genetic distances among the populations was 0.0007. The complete lack of spatial variation observed among populations did not results from lack of genetic variability. Genetic variation within populations was high, with mean heterozygosities from 0.221 to 0.250. Gene frequencies were consistent with expectations under conditions of Hardy-Weinberg equilibrium. These data suggest panmixis, or random mating, throughout the highly connected reef system of the central and northern GBR. The large gene exchange among the giant clam populations has important implications for conservation management of one of the few large populations of T. gigas in the world. Small local effects are likely to be overcome in time by inputs from other sources. However, large genetic perturbations, particularly from up-current sources, may spread rapidly through the population.Contribution No. 561 from the Australian Institute of Marine Science     

Reproduction of the commercial sea cucumber Holothuria whitmaei [Holothuroidea: Aspidochirotida] in the Indian and Pacific Ocean regions of Australia

Seasonal trends in the gonad index (GI) of two widely separated populations of black teatfish, Holothuria whitmaei (formerly included in Holothuria nobilis), were investigated between Pacific (Great Barrier Reef) and Indian Ocean (Ningaloo Reef) coral reefs of Australia. Reproductive activity followed a similar annual trend, with the GI of both populations peaking typically between April and June. Macroscopic and histological analysis of Ningaloo Reef specimens revealed that large germinal tubules, positioned centrally on the gonad basis, progressed through four maturity stages: growing (II); mature (III); partly spawned (IV); and spent (V). Growing tubules dominated the central region of the gonad basis between January and March, followed by an increase in the number of mature tubules throughout the GI peak (April–June). The progressive appearance of partly spawned and spent tubules between June and October suggests that spawning in H. whitmaei continues intermittently over an extended period throughout the austral winter. The examination of the gonad structure of sexually mature male and female specimens identified five tubule size classes in total (C₁ to C₅), each of differing physical and gametogenic status. In females, smaller C₁ tubules located at the anterior edge of the gonad basis contained pre- to early vitellogenic oocytes. Larger C₂ and C₃ tubule cohorts, positioned centrally on the gonad basis, contained mid- to late-stage vitellogenic oocytes. Smaller C₄ and C₅ tubules, located at the posterior edge, contained only relict oocytes. Similar physical and gametogenic differences were apparent between tubule cohorts in male specimens. We propose that these results, together with evidence of incomplete gonad resorption over the austral summer, indicate that gonad development in H. whitmaei conforms to the predictions of the Tubule Recruitment Model (TRM). The TRM is reported rarely among tropical aspidochirotes, and results presented here (1) provide the first direct evidence of this model in H. whitmaei, and (2) confirm that this species is one of the few winter-spawning tropical invertebrates.     

The distribution of herbivorous fishes on the Great Barrier Reef

The composition and functionality of ecologically important herbivorous fish assemblages were examined throughout much of Australia’s Great Barrier Reef (GBR). Diversity and abundance of surgeonfishes (Acanthuridae), parrotfishes (Labridae) and rabbitfishes (Siganidae) were strongly associated with position on the continental shelf, whilst effects of latitude were weaker and inconsistent. Species distributions varied considerably amongst taxonomic groups; parrotfishes were mostly widespread whilst distributions of surgeonfishes were often restricted. Most inshore environments supported depauperate herbivore assemblages dominated by different taxa and functional groups compared with assemblages in offshore environments. There were also strong cross-shelf transitions in the main taxa performing each functional role. Overall, this study show that the functional contributions of herbivorous fish assemblages to important ecosystem processes and the contributing taxa vary considerably amongst different GBR environments. Additionally, the two most numerically dominant herbivores actively select detritus, not algae, supporting increasing evidence for the importance of detritus in coral reef ecology.     

Reproduction of the commercial sea cucumber Holothuria scabra (Echinodermata: Holothuroidea) in the Solomon Islands

Over a 3-year period (1996-1998), reproduction of the commercial sea cucumber Holothuria scabra (Jaeger, 1833) was investigated in the Solomon Islands to determine the spawning pattern and whether gametogenesis is continuous or seasonal. The gonad consisted of a single cohort of tubules that developed uniformly. Macroscopic examination of the gonads revealed that mature gametes were present throughout the year. Individuals with gonads at different stages of maturity were present in most samples. Partly spawned gonads were prevalent in females, whereas mature gonads were prevalent in males. The time at which the peak gonad index was recorded differed among years. Although gametogenesis was continuous, with a potential for prolonged gamete release, a period of enhanced spawning occurred during the dry season, from September to December. Maximum gonad indices were reached prior to and during this period of enhanced spawning. Histology revealed that gametogenesis reinitiated in partly spawned gonads, resulting in the presence of gametes at different stages of development in the gonad. The uniform growth of gonad tubules indicated that H. scabra does not conform to the progressive tubule recruitment model described for holothurian oogenesis. Continuous reproduction in H. scabra and prolonged availability of mature gametes would facilitate use of this species for aquaculture.     

Fission and its effect on population structure of Holothuria atra (Echinodermata: Holothuroidea) in Taiwan

Populations of Holothuria atra Jaeger occurring at Wanlitung and Nanwan were sampled monthly at spring tide from March 1990 to February 1992 in southern Taiwan. At Wanlitung, small individuals with evidence of fission and regeneration were abundant in shallow tidepools on the wide reef flats (density: 98±40 ( ± SD) 100 m^-2, body weight: 6 to 182 g). At Nanwan, large individuals with no evidence of fission were sparse in deep tidepools and narrow flats [density: 0.24±0.07 ( ± SD) 100 m^{- 2}, body weight: 351 to 1400 g]. At Wanlitung, fission occurred throughout the year with an average monthly fission frequency of 4.5%, peaking at 18% in September 1990 and 16% in August 1991. Fission was accomplished by revolving, twisting, and stretching of the body, resulting in two unequal fragments. The minimal weight of 3007 individuals measured during 1 yr at Wanlitung was 6 g. All ten individuals weighing 6 to 9 g ( ± SD = 6.7 ± 1.1) 100 m^{- 2} showed evidence of regeneration from fission products. Nine large individuals (>500 g) transferred from Nanwan to Wanlitung and 50 small individuals (<100 g) transferred from Wanlitung to Nanwan in February 1991 showed no evidence of fission during the following year. Fission occurs in small individuals living in shallow tidepools, suggesting that fission probably is triggered by a stressful environment resulting from solar radiation at the unusually low water level of spring tide. Fission of H. atra at Wanlitung results in a population consisting of small individuals.     

Genetic relationships of the colour morphs of Acanthochromis polyacanthus (Pomacentridae) on the northern Great Barrier Reef

Acanthochromis polyacanthus is an unusual tropical damselfish because it lacks pelagic larvae and has lost the capacity for broad-scale dispersal among coral reefs. On the modern Great Barrier Reef, this fish has multiple colour morphs and we have previously described steep genetic clines in the central region between three colour morphs. A similar transition of colour morphs has been found in the far northern region (Latitude 11°S), where at least five colour morphs and intergrading hybrids were found during a survey of 14 locations on nine adjacent reefs. On two of these reefs, monochromatic white and bicoloured morphs coexist, partitioning the reef slope vertically when sympatric, but occupying the full depth range elsewhere. In areas of overlap, they interbreed, producing hybrids of intermediate colour. Although the discriminatory power of our genetic analysis was weakened by the low level of variability in these populations (only four polymorphic loci), allozyme electrophoresis revealed significant genetic differentiation within and among reefs. There was neither simple linkage between colour and protein variation, nor correlation between genetic similarity and proximity of sampled populations. We suggest that the observed chaotic structure is the result of differences among the founding populations, although the two colour-based stocks have been homogenized through a long history of introgression during multiple episodes of isolation and secondary contact. Despite genetic mixing, phenotypic diversity (i.e. colour morphs) seems to have been preserved in these populations by sexual selection, perhaps reinforced by natural selection. The long-term stability of this outcome, however, is suggested by the presence of unique morphs on two reefs that are detached from the continental shelf and surrounded by very deep water. The strong identity of these populations can be attributed to long occupation of these isolated sites compared with those on the continental shelf which have only been established in the last 15 000 yr and still retain differences arising from the initial founder events. Received: 15 January 1997 / Accepted: 15 August 1997     

Sperm ultrastructure of the giant clam Tridacna maxima (Tridacnidae: Bivalvia: Mollusca) from the Great Barrier Reef

Using transmission electron microscopy, spermatozoa from a member of the Tridacnidae, or giant clams, are described for the first time and compared with spermatozoa of other bivalves, especially other heterodonts. The acrosomal vesicle of Tridacna maxima (Röding, 1798) is short (0.37?μm), blunt-conical, and exhibits a prominent basal ring. A narrow apical elaboration of the nucleus, the nuclear peg, projects deep into the basal invagination of the acrosomal vesicle. Aside from this specialization, the nucleus is a solid elongate-cylindrical structure (7.66?μm) that exhibits several small irregular lacunae. Four or occasionally three round-ovate mitochondria surround a pair of orthogonally-arranged, triplet-substructure centrioles. The proximal centriole is connected to a small indentation of the nuclear base by a thin layer of granular pericentriolar material, whereas the distal centriole is anchored to the plasma membrane by nine terminally-forked satellite fibres. The 9?+?2 pattern axoneme of the tail is continuous with the distal centriole. Comparison with other bivalves indicates a very close relationship between tridacnids and cardiids based on sperm ultrastructure. Specifically, the presence of a nuclear peg links Tridacna spp. with the cardiid genus Cerastoderma, but further information on the many unstudied genera is required to test the exact nature of this relationship. The sperm ultrastructure provides additional support for the recently proposed hypothesis that the Tridacnidae may be no more than a specialized subfamily of the Cardiidae.     

Production and reworking of sediment by parrotfishes (family Scaridae) on the Great Barrier Reef,Australia

Sediment produced by parrotfishes (family Scaridae) may comprise new bioeroded material and/or reworked sediment. The relative contribution of the two components was examined in two bioeroding Chlorurus species, C. gibbus and C. sordidus, from Lizard Island in the Northern Great Barrier Reef. The relative importance of reworked sediment was determined based on direct estimates of sediment ingestion. In C. gibbus, 2.4% of the sediment produced is reworked. In C. sordidus, reworking contributes 27.2%. Comparisons of sediment size-distributions in epilithic algal communities, gut contents and defaecation sites suggest that both C. gibbus and C. sordidus markedly decrease the particle size of sediment as a result of ingestion and trituration in the pharyngeal apparatus.     

Genetic differentiation among populations of a broadcast spawning soft coral, Sinularia flexibilis, on the Great Barrier Reef

The genetic structure of 12 reef populations of the soft coral Sinularia flexibilis (Octocorallia, Alcyoniidae) was studied along the Great Barrier Reef (GBR) at a maximum separation of 1,300 km to investigate the relative importance of sexual and asexual reproduction, genetic differentiation and gene flow among these populations. S. flexibilis is a widely distributed Indo-Pacific species and a gamete broadcaster that can form large aggregations of colonies on near-shore reefs of the GBR. Up to 60 individuals per reef were collected at a minimum sampling scale of 5 m at two sites per reef, from December 1998 to February 2000. Electrophoretic analyses of nine polymorphic allozymes indicated that genotypic frequencies in most populations and loci did not differ significantly from those expected from Hardy–Weinberg predictions. Analysis of multi-locus genotypes indicated a high number of unique genotypes (N _go) relative to the number of individuals sampled (N) in each reef population (range of 0.69–0.95). The maximum number of individuals likely to have been produced sexually (N*) was similar to the number of individuals sampled (i.e. N*:N ˜ 1), suggesting that even repeated genotypes may have been produced sexually. These results demonstrated a dominant role of sexual reproduction in these populations at the scale sampled. Significant genetic differentiation between some populations indicated that gene flow is restricted between some reefs (F _ST=0.026, 95% CI= 0.011 − 0.045) and even between sites within reefs (F _ST=0.041, 95% CI=0.027 − 0.055). Nevertheless, there was no relationship between geographic separation and genetic differentiation. Analyses comparing groups of populations showed no significant differentiation on a north-south gradient in the GBR. The pattern in the number of significant differences in gene frequencies in pairwise population comparisons, however, suggested that gene flow may be more restricted among inner-shelf reef populations near to the coast than among mid/outer-shelf populations further from the coast. Received: 10 July 2000 / Accepted: 5 October 2000     

Genetic consequences of long larval life in the starfish Linckia laevigata (Echinodermata: Asteroidea) on the Great Barrier Reef

Gene flow between populations of the asteroid Linckia laevigata (Linnaeus) was investigated by examining over 1000 individuals collected from ten reefs throughout the Great Barrier Reef (GBR), Australia, for genetic variation at seven polymorphic enzyme loci. Despite geographic separations in excess of 1000 km, Nei's unbiased genetic distance (0 to 0.003) and standardised genetic variation between populations (F _ST) values (mean 0.0011) were small and not significant. Genetic homogeneity among L. laevigata populations is consistent with the long-distance dispersal capability of its 28 d planktonic larval phase, and is greater than that observed for other asteroid species, including another high-dispersal species, Acanthaster planci, which has a 14 d larval phase. Variation within populations was also higher than previously recorded for asteroids (mean heterozygosity=0.384; number of alleles per locus ranged from 5.1 to 6.0 in each population). Among asteroids, dispersal ability is positively correlated with gene flow and levels of variation, and negatively correlated with levels of differentiation.     

Synchronous spawnings of 105 scleractinian coral species on the Great Barrier Reef

Following observations of mass spawning of hermatypic corals on the Great Barrier Reef in 1981 and 1982, spawning dates were successfully predicted and documented at five reefs on the Central and Northern Great Barrier Reef in 1983. During the predicted times, 105 species from 36 genera and 11 families were observed to spawn. Of these, 15 species were shown to have an annual gametogenic cycle. All but two of the species observed during mass spawnings shed gametes which underwent external fertilization and development. Synchronous spawning was observed both within and between the five reefs studied, which were separated by as much as 5° of latitude (500 km) or almost a quarter of the length of the Great Barrier Reef. The mass spawning of corals took place on only a few nights of the year, between the full and lastquarter moon in late spring. Maturation of gametes coincided with rapidly rising spring sea temperatures. Lunar and diel cycles may provide cues for the synchronization of gamete release in these species. The hour and night on which the greatest number of species and individuals spawned coincided with low-amplitude tides. Multispecific synchronous spawning, or mass spawning, of scleractinian and some alcyonacean corals represents a phenomenon which is, so far, unique in both marine and terrestrial communities.     

Bacterial diversity of the sediments transiting through the gut of Holothuria scabra (Holothuroidea; Echinodermata)

This work analyzes bacterial diversity of sediments transiting through the gut of Holothuria scabra which is an important bioturbator in tropical shallow waters. This edible holothurian species has a social and economic importance for coastal populations in many developing countries. Bacterial biodiversity was analyzed by sequencing the 16S rRNA of bacterial cultures and clones. DAPI and FISH methods were used to determine and compare the number of bacteria found in the various gut compartments. A total of 116 phylotypes belonging to the γ-Proteobacteria (60.5 %), α-Proteobacteria (24.5 %), Bacteroidetes (6 %), Actinobacteria (2.75 %), Fusobacteria (1.75 %), Firmicutes (1.75 %), Cyanobacteria (1.75 %) and δ-Proteobacteria (1 %) were identified. The number of bacteria is significantly greater (1.5×) in the foregut than in the ambient sediments. The number of bacteria significantly decreases in the midgut and remains stable until defecation. Some γ-Proteobacteria, especially Vibrio, are less affected by digestion than other bacterial taxa. The season has an impact on the bacterial diversity found in the sediments transiting through the gut: in the dry season, γ-Proteobacteria are the most abundant taxon, while α-Proteobacteria dominate in the rainy season. Vibrio is the most frequent genus with some well-known opportunistic pathogens like V. harveyi, V. alginolyticus and V. proteolyticus. Findings show that sediment-associated microbial communities are significantly modified by H. scabra during their transit through the gut which supports the view that holothurians play a substantial role in the structuring of bacterial communities at the sediment–seawater interface.     

Patterns of genetic subdivision in populations of a clonal cnidarian,Zoanthus coppingeri,from the Great Barrier Reef

Samples of an intertidal zoanthid, Zoanthus coppingeri, Haddon and Shackelton, 1891, were collected from three localities in the Great Barrier Reef region during 1992–1993, and subjected to allozyme electrophoretic analysis at seven polymorphic loci. The reduced ratio of observed to expected genotypic diversity indicated that populations were partly clonal, but they were not dominated by a few clones as occurs in some other cnidarians. Regular disturbance by wave action is postulated to prevent the formation of large stands of particular clones by clearing space and mixing genotypes over small scales. The sexual origin of clonal genotypes was confirmed by conformance to Hardy-Weinberg predictions of genotype frequencies at all but one locus. Values of the standardised genetic variance among populations, F _ST , were highly significant between localities and between replicate sites within localities separated by only 50 m. Strong genetic structure has not previously been described in a Great Barrier Reef invertebrate species, and is considered to be the consequence of stochastic changes in gene frequencies as a result of low levels of gene flow. High clonal longevity and low recruitment rates may maintain genetic differences over long periods. Similar effects may be seen in other Great Barrier Reef invertebrate species with comparable reproductive patterns.     

Reproduction and growth of Holothuria atra (Echinodermata: Holothuroidea) at two contrasting sites in southern Taiwan

Reproductive periods and growth of two populations of Holothuria atra Jaeger distinctly different in body size at two sites of southern Taiwan were determined. Individuals examined in the present study were collected between march 1990 and February 1992. At Nanwan (21° 57N, 120° 45E), large individuals (351 to 1400 g wet wt) spawned from June to September. At Wanlitung (22°N, 120° 42E), a small proportion of frequently dividing individuals (<190 g wet wt) have mature gonads in May, June and September, but histological examination revealed no sign of spawning. Sexual recruits, defined as small individuals <5 g wet wt without sign of regeneration, were not found at either site during this 2-yr study. After the peak of fission at Wanlitung, 40% of the population showed signs of external regeneration. At Nanwan, small individuals transferred from Wanlitung grew from 6 g (n=6) to 166±8 g within 8 mo, and from 48±4 g (n=50) to 324±16 g within 1 yr, with a 6.8-fold biomass increase in 1 yr. At Wanlitung, the monthly average body weight of H. atra is between 33 and 62 g, apparently due to frequent fission, and the biomass increased only 2.9-fold in 1 yr. In southern Taiwan, sexual reproduction of H. atra occurs in large individuals. Asexual reproduction, occurring in small individuals, is the chief mechanism for population maintenance and increase, but it may decrease sexual reproductive potential.     

Occurrence and characteristics of unusual protistan symbionts from surgeonfishes (Acanthuridae) of the Great Barrier Reef,Australia

The occurrence of unusual symbiotic microorganisms was examined in the intestines of a range of fish from the Great Barrier Reef, Australia. The fish taxa examined included 26 species of the family Acanthuridae, as well as representatives of phylogenetically related and herbivorous taxa. The microorganisms, referred to as protists, were only found in herbivorous and detritivorous members of the Acanthuridae. Protists were not found in planktivorous acanthurids, nor in any members of the families Kyphosidae, Pomacentridae, Scaridae, Zanclidae, Siganidae and Bleniidae we examined. In addition, protists were absent from the herbivorous acanthurids A. xanthopterus and A. nigricans. A range of protist forms, characterized by differences in size (8 to 417 m), shape and mode of cell division (daughter-cell production and binary fission), was observed. The occurrence of these forms appeared to be correlated with host feedingecology. Large forms (>100 m) of the protists were only found in acanthurids which fed over hard-reef substrata. Smaller forms were found in sand-grazing and detritivorous species. One of the protist forms appears identical to protists previously reported from Red Sea acanthurids.