Fish Grazing Research Articles

Effects of grazing and damselfish territoriality on internal bioerosion of dead corals : indirect effects

An experiment was performed on Britomart Reef, Great Barrier Reef (central region), to determine the relationship between fish grazing, damselfish territoriality, and internal bioerosion of dead coral substratum. The damselfish Hemiglyphidodon plagiometopon Bleeker (Pomacentridae) was used for the study because it actively excludes herbivorous fish, particularly scarids and acanthurids, from its territories, creating undergrazed patches in the environment. Its territories simulated conditions of naturally reduced grazing. Freshly killed pieces of the plating coral Pachyseris speciosa Dana were placed under four experimental conditions: (1) within cages, excluding grazing fish; (2) within damselfish territories; (3) beneath shade tops to control for light; and (4) outside damselfish territories, fully exposed to grazers. Internal bioeroders were identified by pattern of substratum excavation and characterization of borings, and were quantified by digitizing x-ray radiographs of the substratum. Three major categories of borers were identified: Cliothosa hancocki Topsent, “other sponges” (of the Cliona viridis Schmidt species complex), and “worms” (including polychaetes and sipunculids). Variations in grazing pressure were found to significantly alter the taxonomic composition of the bioeroder community. Bioerosion by C. hancocki, a boring sponge with large exposed papillae, was found to increase significantly when grazing was reduced within damselfish territories. By contrast, other boring sponges of the C. viridis complex decreased in abundance; they were not affected by higher sedimentation in cages. The response of bioerosion by “worms” was less clear but increased slightly within damselfish territories. This was due primarily to a shift in taxonomic composition and dominance from polychaetes to sipunculids (particularly Cleosiphon aspergillum Quatrefages). The effects of grazing on the internal bioeroder community were often altered or obscured in the caged treatments; this was most likely due to caging artifacts such as increased sedimentation and decreased light. In general, bioerosion rates of the substratum P. speciosa were low in comparison to rates established or estimated for corals with less dense skeletons. Total internal bioerosion rates did not vary significantly with changes in grazing pressure. This study implies that, overall, reduced grazing pressure will lead to production of fine sediments derived from internal bioeroders. Under high grazing pressures, the addition of external bioerosion effected directly by grazers would also produce coarse sediment, resulting in an increase in total bioerosion rates (internal and external) and an increased contribution of both coarse and fine sediments to the reef environment.

Territoriality of the Dusky Damselfish: Influence on Algal Biomass and on the Relative Impacts of Grazing by Fishes and Diadema

Grazing by the sea urchin, Diadema antillarum, has a greater impact on algal biomass in the elkhorn field habitat on patch reefs in the Caribbean San Blas Islands of Panama than does grazing by fishes. The dusky damselfish, Stegastes dorsopunicans, is the predominant territorial fish in this habitat. Because observations demonstrated that this damselfish could defend its feeding areas against solitary fishes but not against D. antillarum, the minor impact of fish grazing on biomass in the elkhorn field could result from resource defense by this damselfish. To test this, 85% of the S. dorsopunicans were removed from a patch reef. Though the abundance of herbivorous fishes increased in the elkhorn field after the removal, algal biomass was indistinguishable from that on a control reef throughout the ensuing three months, despite major fluctuations in biomass. Thus, it is unlikely that resource defense by this damselfish is responsible for the relative importance of grazing by urchins and fishes in this habitat. Unlike its close congener, S. planifrons, which is capable of excluding urchins from its territories, S. dorsopunicans probably plays a relatively minor role in the regulation of coral reef community structure.

The Regulation of Morphological Plasticity in Tropical Reef Algae by Herbivory

Many tropical algal species exhibit considerable morphological variation associated with different coral reef habitats, but the factors contributing to such variation have not been identified. Two strikingly distinct morphologies are described here for the common Caribbean alga Padina jamaicensis. These distinct morphologies are characteristically found in different reef habitats, and are shown to represent phenotypic responses to different levels of herbivorous fish grazing. Experimental reduction of grazing intensity in a high—herbivory reef habitat resulted in a rapid (96 h) morphological shift from a prostrate, highly branched turf morphology to an erect foliose morphology. Transplant experiments indicated that foliose Padina plants were preferentially consumed by herbivorous parrotfishes. These results suggest that morphological plasticity may represent an important adaptive strategy in some tropical algal species, enabling plants to persist in intensely grazed reef habitats while maintaining the ability to respond rapidly to spatial or temporal fluctuations in herbivory.

Effects of fish grazing on nutrient release and succession of primary producers1

Macrophytes (Anacharis canaden sis) growing in aquaria were eliminated by the grazing activity of the omnivorous rudd (Scardinius erythrophthalmus). The phosphorus content in both the water column and periphyton was greater in those aquaria that contained fish. Grazing activity of rudd caused 33PO43− that was injected into the sediment and taken up by A. canadensis through their roots to be released into the water phase. In a pond experiment, grazing rudd reduced the biomass of A. canadensis, which in turn caused the biomass of epiphytic algae to increase. We suggest that nutrients, which are taken up by macrophytes from the sediments, are released by grazing fish. This nutrient pathway may thus promote a succession from submersed macrophytes to other primary producers and accelerate the eutrophication process.

The Role of Herbivorous Fishes in the Organization of a Caribbean Reef Community

Experimental manipulations of grazing intensity were used to examine the role of herbivorous fishes in the families Acanthuridae (surgeonfishes) and Scaridae (parrotfishes) in determining distributions and abundances of benthic species within and among shallow tropical reef habitats. A back reef habitat along the Belizean barrier reef was characterized by a diverse benthic assemblage of algal turfs, coralline algae, and the coral Porites astreoides, but by extremely low macroalgal abundance. In contrast, several nearby shallow habitats were dominated by dense stands of several macroalgal species. Experimental reduction of herbivorous fish grazing in the back reef (achieved by constructing exclosures) rapidly and dramatically altered existing patterns of benthic species composition and species abundances. After 10 wk of reduced herbivory, total macroalgal abundance increased significantly in herbivore exclusion areas relative to unmanipulated controls, and was correlated with decreased percent cover of available space, several algal turf species, crustose coralline algae, and Porites. Some macroalgal species were able to directly overgrow and kill portions of Porites colonies within herbivore exclusion treatments. Successful recruitment and growth of several algal species under experimentally reduced herbivory indicated that macroalgal species distributions may be limited by herbivory rather than by lack of spore availability or unsuitable physical conditions. Algal turfs characteristic of many reef habitats appear to represent herbivore—tolerant assemblages, persisting under high grazing intensity but responding rapidly to reduced herbivory with increased abundances, morphological changes, and altered reproductive status. These results suggest that herbivorous fish grazing profoundly influences benthic species distributions and abundances within some tropical reef habitats. Spatial variation in herbivory appears to be of fundamental importance in determining regional patterns of benthic community structure on tropical reefs. The spatial mosaic of benthic community composition among shallow reef habitats was associated with patterns of grazing intensity by herbivorous fishes. Several reef habitats supporting dense macroalgal stands represented spatial refuges from herbivory, with low herbivorous fish densities and reduced grazing intensities. Transplant experiments revealed that algal species characteristic of these low—herbivory habitats were highly susceptible to grazing by herbivorous fishes. Spatial heterogeneity in grazing intensity may contribute to high regional diversity among tropical reef habitats by maintaining different benthic species assemblages under fundamentally distinct selective regimes.

Effects of grazing and damselfish territoriality on bioerosion of dead corals: Direct effects

An experiment was performed on Britomart Reef, Great Barrier Reef (central region) to determine the relationship between fish grazing, damselfish territoriality, and external bioerosion of dead coral substratum. The damselfish, Hemiglyphidodon plagiometopon Bleeker (Pomacentridae), was used because it excludes herbivorous fish from its territories, creating distinct undergrazed patches in the environment. Its territories were used to simulate conditions of naturally reduced grazing. Pieces of the freshly killed plating coral Pachyseris speciosa Dana were placed under four conditions: (1) within cages designed to exclude grazing fish; (2) within the territories of the damselfish H. plagiometopon; (3) beneath shade-tops to control for a decrease in light; and (4) outside damselfish territories, fully exposed to fish grazing. Undeterred fish grazing was found to cause high levels of external bioerosion of dead foliose corals, causing a decrease in photographic area of plates by up to 50% within 18–24 months. H. plagiometopon effectively reduced rates of fish grazing and thus external bioerosion within their territorial borders. The degree of this effectiveness, however, varied with the individual. Fish grazing was found to greatly alter the microtopography of the substratum, almost completely removing the carinae and septa of the coral. This was concomitant with the loss of most microalgae and sessile invertebrates, suggesting that newly settled invertebrate larvae are probably disturbed by fish grazing, as has been found in related studies. Time-lapse photography revealed that, in addition to larger scarids and acanthurids, small fish ( Pomacentrus molluccensis Bleeker and blennies) graze diurnally and small invertebrates (pagurid and galatheid crabs) feed nocturnally on the coral substratum. These organisms were found to have negligible bioerosional effects. The guts of both P. molluccensis and the crab Galathea subsquamata Stimpson, however, were found to contain the spicules of boring sponges, suggesting that predation may influence internal bioerosion by affecting population dynamics within the boring cryptofaunal community.

Growth of a grazing phytoplanktivorous fish and growth enhancement of the grazed alga.

The blue tilapia, Tilapia aurea, consumes the green alga, Ankistrodesmus falcatus, with some algal cells passing through the gut in viable condition. Grazing fish grew and the grazed algal populations had enhanced densities compared to ungrazed populations. Algal growth enhancement was not produced by either bacteria or fish excreting an algal limiting nutrient. Possible ingestion and digestion of bacterial cells by the fish might release nutrients to the algal cells that were otherwise unavailable. Blue tilapia may be affecting algal communities in ways other than by passive size selection. Enhancement of algal growth by the blue tilapia could have important implications for understanding the phytoplankton dynamics in waters containing blue tilapia.

Herbivory on coral reefs: algal susceptibility to herbivorous fishes.

The susceptibility of several tropical algal species to fish grazing was studied on the Belizean barrier reef off the Caribbean coast of Central America. Short-term transplant experiments indicate that plant species vary markedly in their rates of biomass loss to grazing by a shallow-water guild of herbivorous fishes. Algal species transplanted from habitats with low grazing pressure are highly susceptible to grazing, while species occurring in habitats with high herbivore densities are highly resistant to grazing. Algal species show differential susceptibility to grazing by two major components of the tropical herbivore guild, Acanthurus (surgeonfishes) and Sparisoma (parrotfishes).Variability in plant susceptibility to grazing by herbivorous fishes was not clearly correlated with morphological or chemical characteristics that have been previously suggested as plant defenses against herbivory. Plants found to be highly resistant to fish grazing, such as Halimeda, exhibit both morphological characteristics and secondary chemical compounds which do appear to reduce herbivory. In contrast, species of Caulerpa, Sargassum, Turbinaria, and Padina, which also possess alleged morphological and/or chemical defenses, are nevertheless highly susceptible to fish grazing.

Patterns of Fish and Urchin Grazing on Caribbean Coral Reefs: Are Previous Results Typical?

Strips of the seagrass Thalassia testudinum were used in a field bioassay to assess herbivory on 11 coral reefs scattered throughout the Caribbean. Patterns of herbivory on overfished reefs in Haiti and the United States Virgin Islands were compared to patterns on seven less fished reefs (United States Virgin Islands, Panama, Honduras, Belize, and the Bahamas). On the overfished reefs, the rate of Thalassia removal increased significantly with depth, urchin densities were high, and urchin grazing was equal to, or greater than, fish grazing in shallow (<10 m deep) habitats. On reefs subject to little fishing pressure, the rate of Thalassia removal decreased with depth, urchin densities were low, and herbivorous fishes were responsible for almost all Thalassia removal. Previous studies assessing the importance of urchin grazing in the Caribbean have been conducted on overfished reefs where urchin densities were unusually high and the density of grazing fish unusually low. It is doubtful that the intensity of urchin—algal and urchin—coral interactions observed on these heavily fished reefs occurs on reefs unaffected by humans.

Distribution and abundance of herbivorous grazing fishes in the central Great Barrier Reef. I. Levels of variability across the entire continental shelf

Visual censuses were used to study the distribution and abundance of herbivorous grazing fishes (Acanthundae, Scandae, Sigan~dae) on reefs in the central region of the Great Barrier Reef Three locations on the continental shelf (inshore, mid-shelf, outershelf), 3 reefs within each of these locations and, for the mid- and outershelf reefs, 5 zones within each of these reefs were examined The assemblages of herbivorous fishes on inshore reefs were distinctive from those on mid- and outershelf reefs with sianificantlv fewer numbers of species and individuals of acanthunds and scands nshoro A major cumponeni i^f thi varidbiliiy in lhr- data I, withm-n PI (holwof-n-/one, coriiporn~iii. The wnhin-rwf conip<.ncnt of variiihil~ly ussoci~itcd largely wit" diftc-ronces in the ds'semhldgfs with depth, is greater than that between mid- and outershelf locations and between reefs within these locations When zones are examined individually and compared with reefs inshore the majority of species display significant cross-shelf change in abundance irrespective of the zone examined, with cross-shelf change accounting for often 80 to 90 % of the variability in abundance For most species much of this variability is due to an absence or low abundance on inshore reefs

Distribution and abundance of herbivorous grazing fishes in the central Great Barrier Reef. II. Patterns of zonation of mid-shelf and outhershelf reefs

This study describes the patterns of species richness and abundance of herbivorous grazing fishes (Acanthuridae, Scandae, Siganidae) in 5 zones (reef slope, reef crest, reef flat, lagoon, back reef) of 3 reefs on the mid-continental shelf and of 3 reefs on the outer continental shelf in the central region of the Great Barrier Reef. Herbivorous fishes (all 3 families considered together) occur in assemblages which are characteristic of several major zones and this pattern of zonation is consistent on the 6 reefs. Assemblages in deep, outer-slope zones are distinctive from those in shallow zones, There are distinctive assemblages of herbivorous fishes on reef flats, reef crests and over sandy areas in both lagoon and back reef zones. Similarity in these patterns of zonation between reefs suggests that they will be maintained through time. Families display distinctive patterns of zonation also, but patterns differ between families. Acanthurids and scarids generally have higher numbers of species and individuals on reef crests and in lagoons than on reef flats or reef slopes; siganids have higher numbers of species and individuals in lagoons and back reefs than in the other 3 zones studied, Variation in species richness and abundance of individual families between zones was, however, not consistent among reefs at the same shelf location. Different guilds of herbivorous fishes were distributed differently between zones. Suckers feeding on fine sediments were most abundant near windward and leeward edges of reefs; suckers feeding over sand were most abundant in back reefs and lagoons; large and small croppers and scrapers were more abundant in shallow zones (reef crest, reef flat, lagoon) than in deep zones (reef slope, back reef).

Algal resistance to herbivory on a Caribbean barrier reef

Field and laboratory research at Carrie Bow Cay, Belize showed that macroalgae, grouped in functional-form units resisted fish and urchin herbivory in the following order (from high to low resistance): Crustose-Group, Jointed Calcareous-Group, Thick Leathery-Group, Coarsely Branched-Group, Filamentous-Group and Sheet-Group; thereby supporting the hypothesis that crustose, calcareous and thick algae have evolved antipredator defenses and should show the greatest resistance to herbivory with a gradation of increasing palatability towards filaments and sheets. Of the 21 species examined, several (e.g.,Dictyota cervicornis on grids,Laurencia obtusa andStypopodium zonale) had exceptionally low losses to fish grazing, probably due to chemical defences. The sea urchin,Diadema antillarum, was more inclined to feed on algae with known toxic secondary metabolites than were herbivorous fishes; hypothetically related to the differences in mobility and concomitant modes of feeding. Tough leathery forms such asSargassum polyceratium andTurbinaria turbinata resisted grazing by bottom feeding parrotfishes (Scaridae) and surgeonfishes (Acanthuridae) but were susceptible when suspended midway in the water column, possibly due to the presence of rudderfishes (Kyphosidae) which readily consume drift Sargassaceae. The overall tendencies support our predicted relationship between grazer-resistance and algal morphology. In conjunction with our previously reported findings concerning primary productivity, toughness and calorimetry for many of the same species, these results lend credence to generalizations relating form with function in marine macroalgae.

Grazing responses of tropical freshwater fishes to different scales of variation in their food

Grazing fishes in neotropical streams confront variation in their attached algal food that ranges in scale from differences in quality among algal cells to differences in the primary productivity of habitats available to the fishes. Fishes may respond to this variation on some scales but not others. For example, loricariid catfish in a Panamanian stream tracked variation in algal productivity among pool habitats very closely. In sunny pools where algae grew about seven times faster than in shaded pools, loricariids were six to seven times denser. Consequently, growth rates of pre-reproductiveAncistrus spinosus (the most common species in pools) were similar in pools of different canopies, corresponding to predictions from the ‘ideal free distribution’ hypothesis. But on a smaller scale, within pools, avoidance of avian and terrestrial predators outweighed foraging considerations. Larger species and size classes avoided water shallower than 20 cm, where (as a result) the only standing crops of attached algae large enough to be measurable by scraping occurred. During the dry season when food was. most limiting, loricariids overlapped more in their substrate use as different species sought cover in common refuges such as logs and root tangles in pools. Seasonal variation in growth rates of pool-dwelling loricariids reflect these constraints.

FISH PRODUCTION AND COMMUNITY METABOLISM IN AN ORGANICALLY FERTILIZED FISH POND

ABSTRACTFish production and community metabolism were monitored from March 1 to October 12, 1979, in an organically fertilized fish pond. Six hundred l/ha/min of warm water, simulating condenser cooling water from a steam‐electric power plant, was diverted into the pond to optimize fish production and lengthen the growing season. Liquid swine manure was added daily to the pond at a mean dry matter loading rate of 61 kg/ha/day. Two species of tilapia and 3 species of Asiatic carp were stocked at a combined density of 15,400/ha. In September, fish density was reduced to 6,625/ha.Net fish production after 213 days of culture was equivalent to 7,200 kg/ha. Community metabolism values (minimum‐maximum, mean) expressed as gO2/m2/day were: gross primary production (9.2–45.5, 22.1), net primary production (5.9–17.4, 10.4), and community respiration (8.2–46.6, 22.1). From June to September, a negative correlation existed between fish standing crop and primary production. Net primary production during afternoon hours averaged 32% less than production during morning hours. Seasonal and daily changes in primary production are discussed in relation to fish standing crop and fish grazing pressure.

Effects of fish grazing and damselfish territoriality on coral reef algae. II. Nitrogen fixation

Effects of fish grazing and damselfish territoriality on coral reef algae. II. Nitrogen fixation

Effects of fish grazing and damselfish territoriality on coral reef algae. I. Algal community structure

Effects of fish grazing and damselfish territoriality on coral reef algae. I. Algal community structure

A preliminary study of two representative seagrass communities in Palau, western Caroline islands (Micronesia)

Two distinctly different but representative seagrass communities were studied in the vicinity of Koror, Palau, Western Caroline Islands from May 21 to June 25 1978. The first community, a 30 m wide band adjacent to a beach, consisted primarily of Halodule uninervis (Forsk.) Aschers. and Cymodocea rotundata Ehrenb. and Hempr. ex Aschers. near the shore (Zone 1), then Thalassia hemprichii (Ehrenb.) Aschers. and Syringodium isoetifolium (Aschers.) Dandy (Zone 2) with T. hemprichii, C. rotundata and Halophila ovalis (R. Br.) Hook.f. at the seaward margin (Zone 3). The second site was an extensive tidal flat dominated by Enhalus acoroides (L.f.) Royle and T. hemprichii (Zone A). On the deeper seaward slope of this flat (Zone B), T. hemprichii disappeared while C. serrulata (R.Br.) Aschers. and Magnus appeared as depth increased. Enhalus density remained about the same in Zones A and B although the blades were larger (averaging 76 cm long and 2.1 cm wide) in Zone B than in Zone A (averaging 45 cm long and 1.5 cm wide). Blade elongation rates ranged from 1.2 to 3.0 cm week −1 for T. hemprichii with high variance and approximately 7.0 cm week −1 for Enhalus. Enhalus had a maximum blade elongation rate of over 2 cm day −1. All the broad-bladed seagrasses were grazed by fishes, presumably scarids and siganids, but grazers did not consume a significant amount. Fish grazing was almost entirely concentrated at the heavily epiphytized blade tips especially those of Enhalus. Infauna samples at both sites were dominated by polychaetes and bivalves and the total number of individuals increased as the seagrass biomass increased. Epifaunal macroinvertebrates (especially sponges, scleractinians, prosobranchs and ascidians) were particularly abundant in shallow Enhalus beds and thick subtidal Thalassia beds. The common macroalgae present at the two sites differed considerably except for the almost ubiquitous Amphiroa fragilissima (L.) Lamour.

The Role of Biological Disturbance in Temperate Subtidal Encrusting Communities

Biological disturbance was investigated in three subtidal encrusting communities in the warm temperature region of northeastern New Zealand. The mode of operation of the major disturbance agents was established and their effect on encrusting organisms qualified where possible. The three communities were all dominated by crustose coralline algae, sponges, and ascidians. The urchin Evechinus chloroticus was the most widely influential of the grazers in these encrusting communities. At densities >6—7 urchins/m2 this species grazed over the entire surface area in any locality, modifying community structure and reducing diversity to form crustose coralline flats (often referred to as barren grounds). At densities below this level urchins usually confined their grazing to patches within the encrusting community. The mean rate of free space release by an individual urchin was different in the three communities; the rate was four times higher where the urchins occurred at high densities on a coralline flat community than in the other two areas. Larger urchins appeared to clear space at a rate 1.5 times higher than that of smaller urchins. A 2—yr exclusion experiment in a community heavily grazed by Evechinus resulted in an increase in cover of ephemeral algae and coralline turf algae, but no basic change in species composition. Large numbers of five species of herbivorous gastropods, Cellana stellifera, Cantharidus purpureus, Trochus viridis, Micrelenchus sanguineus, and Cookia sulcata were positively associated with high densities of Evechinus. Removal of high densities of Cellana resulted in an increase in the cover of ephemeral algae and coralline turf algae in spite of continued Evechinus grazing. At high densities the grazing fish Parika scaber occasionally fed in loose schools that removed large sponges and ascidiams and released free space at a rate of °.02% of the surface per fish per year. This species also grazed selectively on small encrusting organisms in which case free space was not released. Exclusion of high densities of Parika for 2 yr resulted in a major shift in community structure from sponge/coralline domination to ascidian/sponge domination. Episodic outbreaks of a bacterial/fungal disease occasionally affected populations of long—lived sponges. In the summer of 1975, 18% of the population of Ancorina alata was destroyed and in 1976, 45% of Polymastia fusca degraded completely. These two episodes were the only extensive outbreaks of this disease in 10 yr of observation. The abundant grazing species investigated are generalist feeders that are capable of feeding on the entire size range of most encrusting species. Hence there is not escape from biological disturbance either in small or large size for encrusting organisms in these areas. The populations of the common grazers frequently fluctuate in numbers in a local area and their effect on the encrusting communities is not constant through time. The selective consequences of these features for long—lived encrusting organisms are discussed. It is suggested that the spreading colonial habit of many encrusting organisms is a response to these features of subtidal biological disturbance. Many spreading, encrusting sponges respond to grazing damage by rapid growth, up to 20 times the undisturbed growth rate, and this is thought to be a further mechanism to cope with Evechinus and Parika grazing. Finally, the influence of this disturbance on general community features such as diversity, dominance, and stability is discussed. Diversity was highest at intermediate levels of disturbance. A possible competitive dominant was identified but the disturbance regime did not allow monopolization of space by this species (the sponge Stylopus sp.), and it is suggested that this system is normally in a state of nonequilibrium.

BIOMASS EXPORTATION FROM OFFSHORE PRODUCTION PLATFORMS IN EAST TEXAS

ABSTRACT Studies of the structure and development of the fouling community on active offshore production platforms in the Buccaneer Field, Texas, indicated that energy accumulated within this community was exported to components of the adjacent marine ecosystem along three major pathways. The most apparent pathway was through grazing fish. Gut content analyses of 191 fish representing 27 species indicated that the spadefish (Chaetodipterus faber), sheepshead (Archosargus probatocephalus), crested blenny (Hypleurochilus geminatus), cocoa damselfish (Pomacentrus variabilis), and cubbyu (Equetus acuminatus) feed extensively on fouling organisms. A second pathway linked the fouling community to the benthic fauna living beneath the platforms, which was apparently enriched by consumption of barnacles and other invertebrates dislodged from the platform legs by wave and current stresses, abrasion by boats, and foraging fish. Several fouling species, which did not occur on the structures themselves, were found on dislodged barnacle shells and gastropod shells occupied by hermit crabs. Thus, accumulation of biomass beneath the structures is apparently increased by the presence of these shells. The third pathway linked the fouling community to planktivorous fish and invertebrates through the release of planktonic larvae by fouling organisms. A significant difference (p &amp;lt; 0.05) was found in the proportion of these larvae in the total plankton between stations within Buccaneer Field and those at a control site 8 kilometers east of the field.

The effect of micrograzers on algal community structure in a coral reef microcosm

The effect of amphipod grazing on algal community structure was studied within a 75 l refuge tank connected to a 6500 l closed-system, coral reef microcosm. When amphipods (Ampithoe ramondi) were absent or present in low numbers, a high biomass of mostly filamentous algal species resulted, including Bryopsis hypnoides, Centroceras clavulatum, Ceramium flaccidum, Derbesia vaucheriaeformis, Enteromorpha prolifera, Giffordia rallsiae, and Polysiphonia havanensis. These microalgae disappeared when amphipod density increase beyond approximately 1 individual cm-2 of tank surface. The macroalga Hypnea spinella germinated in the system in association with amphipod tube sites. H. spinella plants remained rare until filamentous species were eliminated by amphipod grazing. Feeding trials confirmed that H. spinella was protected from grazing by its size rather than a chemical defense strategy. The H. spinella community we observed is similar to the flora described on algal ridges where physical conditions exclude fish grazing. We suggest that amphipods and similar micrograzers are responsible for the algal community structure of these ridges. Caging experiments may be subject to similar effects from increased amphipod grazing on the algae. Introduction of fish that are amphipod predators into the refuge tank caused an increase in algal species diversity but total H. spinella growth rates fell from 25 g dry wt month-1 to less than 8 g dry wt month-1. We describe amphipod behavior in relation to changes in population density and food supply, and we stress the potential for increasing the productivity of commercial seaweeds through maintenance of appropriate amphipod species in mariculture facilities.