Foragers versus farmers: contrasting effects of two behavioural groups of herbivores on coral reefs

Herbivorous fishes have been attributed a central role in structuring benthic communities on coral reefs. However, the relative importance of different behavioural groups of herbivores may differ and their interactions may be complex. This study focuses on an experiment that discriminates between two groups of herbivorous fish: (1) “Foragers” (relatively mobile, schooling grazers, including parrotfishes and surgeonfishes) and (2) “Farmers” (highly site-attached, territorial species, primarily damselfishes). Preliminary observations at Kimbe Bay (Papua New Guinea) showed that both groups were common, and that farmers defended areas from foragers and maintained algal communities that were distinct from the surrounding undefended substratum. An orthogonal combination of a farmer removal treatment and a forager exclusion treatment was applied to isolate their separate effects on algae and corals, and to determine whether farmer territory composition results from forager exclusion or algal cultivation. The experiment showed that foragers had quantitatively greater and qualitatively different effects on sessile benthic community structure than farmers. Where foragers were excluded, there were substantial increases in the cover and biomass of macro-algae and a decline in some corals, regardless of the presence of farmers. Where farmers were removed there was a moderate decline in the cover of some food algal species, regardless of whether foragers had access. No effect of the exclusion of foragers by farmers could be detected. Our results support prevailing views that foragers have a major impact on coral reefs and farmers cultivate selected algae, but challenge the hypothesis that damselfish influence habitat structure by moderating forager disturbance.     

Territorial behavior of threespot damselfish (Eupomacentrus planifrons) increases reef algal biomass and productivity

Synopsis Algal growth and damselfish (Eupomacentrus planifrons) territories were studied in two reef habitats at Discovery Bay, Jamaica. Damselfish territories were contiguous in the reef flat (0 to 2.5 m), where the algal composition and biomass varied from territory to territory. In contrast, on the lower reef terrace (22 m), damselfish territories were often spatially segregated. While the algal composition of the territories was more uniform on the reef terrace, the total algal biomass was lower than in the territories on the reef flat. Damselfish are largely herbivorous, and they defend their territories against most intruding fish, including a number of herbivorous species. Areas of the reef terrace outside of damselfish territories were heavily grazed by herbivorous fishes and contained only small quantities of non-crustose algae.The reef terrace territories were characterized by a multispecific turf of algae (greens, blue-greens, and reds) covering the Acropora cervicornis framework and by the leafy, brown alga, Lobophora variegata. A rapid reduction in the biomass of brown algae and filamentous algae was noted when damselfish were permanently removed from their territories. Only calcified, encrusting algae — plants apparently somewhat undesirable as fish food sources — would be common on the terrace zone of this reef if damselfish territories were absent. Damselfish territoriality may significantly influence the dynamics of some reefs by increasing the biomass of the algal turf thereby increasing; reef productivity. Since blue-green algae, potential nitrogen fixers, occur in these algal turfs, the fish may also be indirectly affecting reef nutrition.     

Comparative feeding ecology of two herbivorous damselfishes (Pomacentridae: Teleostei) from the Gulf of California,Mexico

Two species of benthic damselfishes from the Gulf of California, Mexico, use contrasting behaviors when feeding on benthic algal communities. The small (±70 g) Cortez damselfish, Eupomacentrus rectifraenum (Gill, 1862), feeds selectively from a multi-species algal mat, eats fleshy red and green algae and ignores brown and calcareous algae. The giant blue damselfish, Microspathodon dorsalis (Gill, 1862), is a large (±450 g), lethargic, nonselective feeder which grazes on a near monoculture of a fleshy red alga, Polysiphonia sp. Feeding activity for both species is low in the morning peaks during late afternoon, and drops sharply as night approaches. Based on feeding rates, gut-filling times, and weights of gut contents, Cortez damselfish process six to eight full guts of food and giant blue damselfish three full guts of food per day. The algal mat exhibits high standing crops (291–618 g dry wt · m^?2) and low productivity, but the preferred food of the Cortez damselfish (Ulva) appears to colonize the mat frequently and grow rapidly. The Polysiphonia dominated community on giant blue damselfish territories exhibits low standing crops (23 g · m^?2) and high productivity (34–47 times that of the mat per gram algae). Even though the feeding behaviors and resources used by the two damselfishes differ, both species eat similar food (delicate red and green fleshy algae, and depend on rapid colonization and/or high productivity to maintain their primary foods in the grazed algal community.     

Effects of territorial damselfish on an algal-dominated coastal coral reef

Territorial damselfish are important herbivores on coral reefs because they can occupy a large proportion of the substratum and modify the benthic community to promote the cover of food algae. However, on coastal coral reefs damselfish occupy habitats that are often dominated by unpalatable macroalgae. The aim of this study was to examine whether damselfish can maintain distinctive algal assemblages on a coastal reef that is seasonally dominated by Sargassum (Magnetic Island, Great Barrier Reef). Here, three abundant species (Pomacentrus tripunctatus, P. wardi and Stegastes apicalis) occupied up to 60% of the reef substrata. All three species promoted the abundance of food algae in their territories. The magnitudes of the effects varied among reef zones, but patterns were relatively stable over time. Damselfish appear to readily co-exist with large unpalatable macroalgae as they can use it as a substratum for promoting the growth of palatable epiphytes. Damselfish territories represent patches of increased epiphyte load on macroalgae, decreased sediment cover, and enhanced cover of palatable algal turf.     

Damselfish territories as a refuge for macroalgae on coral reefs

Herbivory is widely accepted as a key process determining the benthic community structure and resilience of coral reefs. Recent studies have mostly focused on the importance of roving herbivorous fishes in ecosystem processes. Here, we examine the role of territorial damselfish in shaping patterns of macroalgal distribution based on benthic surveys and macroalgal bioassays. The territory composition and effect of resident damselfish on the removal of Sargassum bioassays were quantified for six species of damselfish on Lizard Island, a mid-shelf reef in the northern Great Barrier Reef (GBR). The functional composition of algal communities within territories varied markedly among species. The territories of four species (Dischistodus perspicillatus, Dischistodus pseudochrysopoecilus, Plectroglyphidodon lacrymatus, and Stegastes nigricans) were characterized by algal turfs, while the territories of two species (Dischistodus prosopotaenia and Hemiglyphidodon plagiometopon) were characterized by foliose and leathery brown macroalgae. Sargassum, a generally rare alga on mid-shelf reefs, was a particularly common alga within D. prosopotaenia territories on the leeward side of the island but absent within their territories on the windward side of the island. D. prosopotaenia was the only species to retain the transplanted Sargassum, with only a minimal reduction in Sargassum biomass (1.1%) being recorded within their territories at both leeward and windward sites over a 24-h period. In contrast, reductions in Sargassum biomass were high in areas adjacent to D. prosopotaenia territories (83.8%), and within and adjacent to the territories of the five remaining damselfish species (76.2–92.5%). Overall, only one of the six damselfish species provided a refuge for leathery brown macroalgae and may facilitate the development of this macroalgae on mid-shelf reefs of the GBR.     

Effects of habitat-conditioning by the damselfish Stegastes nigricans (Lacepède) on the community structure of benthic algae

We compared the community structure of benthic algae inside and outside pomacentrid damselfish (Stegastes nigricans) territories in a moat at Sesoko Island, Okinawa, Japan. S. nigricans maintained “algae farms” that were dominated by the filamentous rhodophyte, Womersleyella setacea. Species richness and biomass were higher inside damselfish territories than outside, while species diversity and evenness were higher outside. Detrended correspondence analysis (DCA) based on species composition showed that the dominance of W. setacea was maintained throughout the year in all samples collected from inside damselfish territories. The observed strong dominance of filamentous rhodophytes was consistent with the findings of most studies on damselfish territories worldwide. However, the dominance of a single species of alga and low species diversity inside the territories was in contrast to the findings of previous studies, in which the reduction of grazing pressure caused intermediate disturbance and enhanced algal species diversity. This discrepancy in algal species diversity inside the damselfish territories seems to have been caused by unique characteristics of the alga and the fish. W. setacea traps sediment, which reduces the availability of firm substrata for attachment and inhibits the recruitment of some algae. Moreover, S. nigricans “weeds” indigestible calcareous and thicker algae. The algal assemblage outside damselfish territories varied among samples, and included mat-forming cyanophytes (Calothrix aeruginosa and Calothrix codicola), a prostrate laminar phaeophyte (Padina sp.), thin and small-scaled algae (Cladophora sp. and Feldmannia indica), finely branched filamentous rhodophytes (Taenioma perpusillum and Herposiphonia obscura), and a coarsely branched rhodophyte (Gelidiopsis variabilis).

We placed artificial slate plates inside and outside damselfish territories, and showed that the W. setacea inside territories gradually increased in biomass, reaching the same levels of biomass and dominance as W. setacea on natural substrata. Outside the territories, the algal assemblage underwent succession from early colonizers, i.e., thin and small-scaled algae, to grazing-resistant algae such as mat-forming cyanophytes and prostrate laminar Padina sp. Under heavy grazing, the flora outside the territories was composed of early colonizers, grazing-resistant algae, and scattered erect algae that had probably escaped grazing by chance.

Our findings suggest that sediment trapped by the turf of W. setacea inhibited recruitment of some algae, and that moderate cropping and selective weeding by S. nigricans excluded grazing-resistant algae and prevented early colonizers and competitively superior algae from out-competing W. setacea. Consequently, low species diversity and a high-biomass “farm” suitable for harvesting was maintained.     

Dual Roles of an Algal Farming Damselfish as a Cultivator and Opportunistic Browser of an Invasive Seaweed

Herbivory is a fundamental process determining reef resilience, and while algal farming damselfishes can help shape benthic assemblages, an understanding of their contribution to areas outside of defended territories is relatively unexplored. Here, we demonstrate how the farming damselfish Stegastes marginatus plays a dual role in benthic structuring by 1) contributing to persistence of the invasive macroalga Acanthophora spicifera within a Hawaiian marine protected area, where the macroalga occurred exclusively inside Stegastes territories, and 2) behaving as an opportunistic browser of the exotic alga outside their territorial borders. Greater than 50% of the biomass of tethered A. spicifera was consumed within one-hour when placed outside Stegastes territories, compared to <5% lost from tethers within territories or herbivore exclusion cages. In situ remote video revealed that tethered A. spicifera located outside territories was grazed primarily by the surgeonfish Acanthurus nigrofuscus (∼68% of total bites) and, surprisingly, by S. marginatus (∼27% of total bites) that left their territories to feed on this resource on 107 occasions during 400 min of filming. Further, for over half of those occurrences where S. marginatus grazed on the tethered macroalga outside of territories, they fed alongside conspecifics and other species, displaying little of the aggressiveness that characterizes this damselfish. These results show that S. marginatus plays a wider role in determining benthic assemblages than previously recognized, acting both as cultivators of a canopy-forming invasive macroalga within their territories, and as opportunistic browsers in neighboring sites. Consequently, S. marginatus can affect benthic species composition across their territory borders. These results provide a rare example of interspecific facilitation of an exotic alga by an indigenous marine fish. Accounting for fish behaviors more broadly is important to further our understanding of ecological processes that shape reef ecosystems to improve management of MPAs that often support extensive farming damselfish populations.     

Effects of territorial damselfish on cryptic bioeroding organisms on dead Acropora formosa

Many damselfishes exclude other grazers from their territories and “farm” filamentous algae within their territories. In this study the indirect effect of damselfish territories on faunal composition and abundance of internal bioeroders of dead Acropora formosa (Dana, 1846) was investigated in territories of two damselfish species, Stegastes nigricans (Lacepède, 1802) and Plectroglyphidodon lacrymatus (Quoy and Gaimard, 1825). S. nigricans tends to be more protective and defend their territories more aggressively than P. lacrymatus. Newly killed branches of A. formosa were placed inside and outside damselfish territories, for 1 or 2 years, at a coral reef near Zanzibar, Tanzania. As predicted, the coral branches became covered with more filamentous algae in the S. nigricans territories than in the controls, with intermediate levels in the P. lacrymatus territories. Among the internal bioeroding fauna, polychaetes were by far the most common group. In total, there were significantly more borers in the first year than the second, which was mainly due to a high abundance of sabellids. Furthermore, sabellids were significantly more abundant in control areas and in the P. lacrymatus territories compared to the S. nigricans territories. However, many other genera showed the opposite pattern, with more polychaetes in the fish territories compared to the controls. There was also a clear difference in assemblage structure between S. nigricans territories and controls. Thus, we found strong effects of whether a piece of coral was placed inside or outside a damselfish territory on the abundance of many of the bioeroding taxa. We discuss multiple reasons for these indirect effects of the territories, including that deposit feeding bioeroders may benefit from the dense algal turf found inside the territories whereas suspension feeding bioeroders may benefit from substrate with less filamentous algae found outside territories. Considering our results in the context of the large areas of coral reefs that typically are defended as territories by damselfishes, these fish are likely to have a considerable impact on the boring community of a coral reef.     

Effects of the Caribbean threespot damselfish,Stegastes Planifrons (Cuvier), on algal lawn composition

Algal lawn communities within territories of herbivorous threespot damselfish, Stegastes planifrons (Cuvier) were studied in a shallow back-reef environment at La Parguera, Puerto Rico. Caging exclusion experiments, monitoring of non-manipulated lawns and gut content analyses were used to determine the effect of S. planifrons on algal species composition of the lawn. An average of 40 algal species per sampling period were found in naturally occuring lawns, with a total of 53 species being recorded throughout the course of study. Examination of algal species found in damselfish guts and comparison with natural lawns indicate that damselfish feed primarily on their algal lawns and graze the algae present in proportion to abundance. Caged algal lawns showed significant decline in algal species number and abundance over time. Non-selective feeding by Stegastes planifrons within their territorial lawns appears to result in an “intermediate disturbance” which maintains a greater algal species richness than is found in their absence.     

Microhabitat characteristics of <Emphasis Type="Italic">Stegastes planifrons</Emphasis> and <Emphasis Type="Italic">S. adustus</Emphasis> territories

Stegastes adustus and Stegastes planifrons are two species of damselfishes commonly found in the Caribbean. These territorial fishes have been widely studied due to their major ecological role on coral reef in controlling the growth of macroalgae that compete with corals for space and, inversely, on their deleterious role in destroying coral tissues to impulse the development of algae. However, few studies were conducted on the biotic and abiotic components of their territories. In the present study, territory size and surfaces of benthic components (macroalgae, algal turf, massive corals, branching corals, Milleporidae, sponges, sand and rubbles) were estimated for the two species at two contrasted sites. At Ilet Pigeon site (IP), the two damselfishes were found at different depth and exhibited different territory sizes. S. adustus defended a larger territory characterized by massive corals, sand and Milleporidae, while S. planifrons territories were smaller, deeper and characterized by branching corals, sponges and rubble. At Passe-à-Colas site (PC), the two fish species coexisted in the same depth range and defended territories of similar size. Their territories presented higher proportions of macroalgae, but smaller surfaces of Milleporidae than at IP. At PC, the main difference between the two species was a higher surface of massive corals inside S. planifrons territories than S. adustus territories. Differences in microhabitat characteristics between the two Stegastes seemed mostly site related. This resulted from the high plasticity of two species, allowing them to persist on Caribbean coral reefs after the decline of most branching acroporids, their former favorite habitats.     

Farming behaviour of reef fishes increases the prevalence of coral disease associated microbes and black band disease

Microbial community structure on coral reefs is strongly influenced by coral–algae interactions; however, the extent to which this influence is mediated by fishes is unknown. By excluding fleshy macroalgae, cultivating palatable filamentous algae and engaging in frequent aggression to protect resources, territorial damselfish (f. Pomacentridae), such as Stegastes, mediate macro-benthic dynamics on coral reefs and may significantly influence microbial communities. To elucidate how Stegastes apicalis and Stegastes nigricans may alter benthic microbial assemblages and coral health, we determined the benthic community composition (epilithic algal matrix and prokaryotes) and coral disease prevalence inside and outside of damselfish territories in the Great Barrier Reef, Australia. 16S rDNA sequencing revealed distinct bacterial communities associated with turf algae and a two to three times greater relative abundance of phylotypes with high sequence similarity to potential coral pathogens inside Stegastes''s territories. These potentially pathogenic phylotypes (totalling 30.04% of the community) were found to have high sequence similarity to those amplified from black band disease (BBD) and disease affected corals worldwide. Disease surveys further revealed a significantly higher occurrence of BBD inside S. nigricans''s territories. These findings demonstrate the first link between fish behaviour, reservoirs of potential coral disease pathogens and the prevalence of coral disease.     

Monoculture and mixed-species algal farms on a coral reef are maintained through intensive and extensive management by damselfishes

Algal farms maintained by different species of territorial herbivorous damselfishes vary in size, algal biomass, and species composition. To determine the factors that affect the structure of these farms, we compared farming behavior and intensity between two sympatric damselfish species, Stegastes nigricans (Lacepède) and S. obreptus (Whitley), which maintain two different types of farms in the Okinawa Islands, southern Japan. By weeding of indigestible algae and prompt exclusion of herbivorous fishes and grazing sea urchins, S. nigricans manage relatively small farms largely dominated by one algal crop species, Polysiphonia sp. In contrast, by delayed exclusion of herbivores, S. obreptus maintain larger farms invaded by diverse indigestible algae, such as Chondria sp. and Padina sp. As a result of intensive management, S. nigricans attains a higher algal biomass per area than does S. obreptus. This study provides a new perspective, as it suggests that damselfishes practise both intensive and extensive farming. The former results in small and highly productive monoculture farms of one palatable alga, while the latter leads to large, species-rich, mixed-culture farms.     

Evidence of a geographically variable competitive mimicry relationship in coral reef fishes

In Kimbe Bay, Papua New Guinea, juvenile surgeonfish Acanthurus pyroferus have been shown to gain access to food resources defended by the damselfish Plectroglyphidodon lacrymatus by mimicking a pygmy angelfish, Centropyge vrolikii , that does not compete with the damsel for food. I tested whether A. pyroferus juveniles gain the same competitive advantage from mimicking a different pygmy angelfish, Centropyge flavissima , in Moorea, French Polynesia. Through abundance and substrate surveys, behavioral observations and stomach content analyses, I demonstrate that in Moorea, mimicry of Ce. flavissima does not provide A. pyroferus with access to damselfish Stegastes nigricans territories; Ce. flavissima models are always attacked upon territory entry and A. pyroferus mimics avoid damsel territories. Damselfish aggression toward the model angelfish cannot be attributed to overabundance of the deceptive mimic; instead, aggression can best be explained by the fact that Ce. flavissima competes with damsels in Moorea by consuming their algal turfs, making them inappropriate models for competitive mimics. Juveniles of many Indo-Pacific surgeonfishes appear to mimic pygmy angelfishes; I suggest that these mimics' success in gaining access to damselfish territories is geographically variable and may be determined by the extent to which mimics, models and receivers overlap in resource use at a given site. This mimicry complex may thus present an excellent illustration of the geographic mosaic model of coevolution.     

Effects of herbivory and the season of disturbance on algal succession in a tropical intertidal shore,Phuket, Thailand

The effects of herbivory and the season of disturbance on species composition and algal succession were experimentally tested at a tropical intertidal shore, Phuket Island, Thailand. Dead coral patches were cleared of all organisms during both the dry and rainy seasons in order to study the effects of season on algal succession and cages were set up to exclude fish herbivory. Algal succession in this intertidal habitat showed a simple pattern and took a year from the early Ulva paradoxa C. Agardh stage to the late Polysiphonia sphaerocarpa Børgesen stage. The abundance of algae during succession was under the influence of seasonal change. U. paradoxa reproduced and recruited throughout the year. Caging effects did not apparently influence algal abundance, perhaps because resident herbivorous damselfishes excluded other herbivores from their territories and maintained their algal “farms”. Unexpectedly, the percent cover of Ulva in the caged plots was lower than in uncaged plots. This pattern may indicate that caging excluded damselfishes only, but allowed small herbivores that consumed substantial amounts of soft filamentous algae in the cages.     

Geographic variation in the damselfish-red alga cultivation mutualism in the Indo-West Pacific

Background  

On coral reefs, damselfish defend their territories from invading herbivores and maintain algal turfs, from which they harvest filamentous algae. In southern Japan, intensive weeding of indigestible algae by Stegastes nigricans results in overgrowth by one filamentous alga, Polysiphonia sp. 1. Because this alga is highly susceptible to grazing and is competitively inferior to other algae, it survives only within the protective territories of this fish species, suggesting an obligate mutualism between damselfish and their cultivated alga. The wide distribution of damselfish species through the Indo-Central Pacific raises the question of whether this species-specific mutualism is maintained throughout the geographic range of the fish. To address this question, from all 18 damselfish species we conducted comprehensive surveys of algal flora within their territories throughout the Indo-West Pacific, and identified species of Polysiphonia using morphological examination and gene sequencing data.     

Food habits of the farmer damselfish <Emphasis Type="Italic">Stegastes nigricans</Emphasis> inferred by stomach content,stable isotope,and fatty acid composition analyses

The territorial damselfish, Stegastes nigricans, maintains algal farms by excluding invading herbivores and weeding unpalatable algae from its territories. In Okinawa, Japan, S. nigricans farms are exclusively dominated by Polysiphonia sp., a highly digestible filamentous rhodophyte. This study was aimed at determining the diet of S. nigricans in Okinawa and its dependency on these almost-monoculture algal farms based on stomach content and chemical analyses. Stomach content analyses revealed that all available food items in the algal farms (i.e., algae, benthic animal inhabitants, trapped detritus) were contained in fish stomachs, but amorphous organic matter accounted for 68% of the contents. Therefore, carbon and nitrogen stable isotope ratios and fatty acid (FA) compositions were analyzed to trace items actually assimilated in their bodies. Stable isotope analyses showed that benthic animals were an important food source even for this farmer fish. Two essential fatty acids (EFAs), 20:4n6 and 20:5n3, which are produced only by rhodophytes among available food items, were rich in the muscle tissue of S. nigricans as well as in algal mats and detritus, suggesting that algal mats contribute EFAs to S. nigricans directly and indirectly through the food web. In conclusion, S. nigricans ingested algal mats, detritus, and benthic animals maintained within its farm. Algae and detritus were original sources of EFAs, and benthic animals, which were much more abundant in the farms than in outside territories, provided a nitrogen-rich dietary source for the fish.     

Territorial damselfish enhances multi-species co-existence of foraminifera mediated by biotic habitat structuring

The structures of benthic foraminiferal communities inside and outside the territory of the pomacentrid damselfish Stegastes nigricans on coral rocks in a moat of Sesoko Is. (26°38′N, 127°52′E) in Okinawa, Japan were compared. Inside the territory, an algal farm, i.e., a dense stand of a filamentous rhodophyte, Womersleyella setacea, was maintained throughout the year. Outside the territory, in areas subjected to intensive and continuous grazing by various grazers, the flora was composed of mat-like cyanophytes, a prostrate turf-form Padina sp., microscopically thin filaments and scattered filamentous rhodophytes. Algal biomass was greater inside the territory than outside the territory of the damselfish. These differences were reflected by the structure of the associated foraminiferal communities. We classified foraminiferal species based on their microhabitat use around algal communities into four life types: free-living type, crawling type, sedentary type, and sessile type. The abundance of foraminifera, especially the free-living type (e.g., Peneroplis pertusus and Quinqueloculina seminulum) and sedentary type (Rosalina globularis and Cymbaloporetta squammosa), was greater inside the territory of the damselfish than outside the territory. Species richness was also higher inside the territory, mainly due to an increase in species richness of the free-living and sedentary types. These increases in abundance and species richness were caused by habitat structuring. W. setacea is a relatively tall and complicated alga with tangled rhizoids. Its dense stand inside the territory trapped a larger amount of sediment, which provided free-living foraminifera with heterogeneous and stable habitats. In addition, the dense stand of W. setacea provided sedentary-type foraminifera with a large, complex substratum, providing refuge and food inside the territory. This assumption was confirmed by the results of plate experiments showing that foraminiferal community structures were controlled by associations between foraminifera and algae. We showed that habitat-conditioning is an important process in biotic habitat-structuring and that habitat-conditioning by territorial damselfish maintains and enhances multi-species coexistence of foraminifera on coral rocks in a coral reef.     

Effects of “Reduced” and “Business-As-Usual” CO2 Emission Scenarios on the Algal Territories of the Damselfish Pomacentrus wardi (Pomacentridae)

Turf algae are a very important component of coral reefs, featuring high growth and turnover rates, whilst covering large areas of substrate. As food for many organisms, turf algae have an important role in the ecosystem. Farming damselfish can modify the species composition and productivity of such algal assemblages, while defending them against intruders. Like all organisms however, turf algae and damselfishes have the potential to be affected by future changes in seawater (SW) temperature and pCO₂. In this study, algal assemblages, in the presence and absence of farming Pomacentrus wardi were exposed to two combinations of SW temperature and pCO₂ levels projected for the austral spring of 2100 (the B1 “reduced” and the A1FI “business-as-usual” CO₂ emission scenarios) at Heron Island (GBR, Australia). These assemblages were dominated by the presence of red algae and non-epiphytic cyanobacteria, i.e. cyanobacteria that grow attached to the substrate rather than on filamentous algae. The endpoint algal composition was mostly controlled by the presence/absence of farming damselfish, despite a large variability found between the algal assemblages of individual fish. Different scenarios appeared to be responsible for a mild, species specific change in community composition, observable in some brown and green algae, but only in the absence of farming fish. Farming fish appeared unaffected by the conditions to which they were exposed. Algal biomass reductions were found under “reduced” CO₂ emission, but not “business-as-usual” scenarios. This suggests that action taken to limit CO₂ emissions may, if the majority of algae behave similarly across all seasons, reduce the potential for phase shifts that lead to algal dominated communities. At the same time the availability of food resources to damselfish and other herbivores would be smaller under “reduced” emission scenarios.     

The social organization of adult blue tangs, <Emphasis Type="Italic">Acanthurus coeruleus</Emphasis>, on a fringing reef,Barbados, West Indies

Blue tangs in Barbados exhibit three distinct social modes: territorial, schooling and wandering. We compared the mobility, foraging, aggression performed and received and the use of cleaning stations of adult blue tangs among modes and among habitats within a single fringing reef in Barbados. Evidence from observed switches during focal observations and multiple observations of tagged individuals indicate that fish are either territorial or non-territorial. Non-territorial fish formed schools and wandered. However, wandering can be used during solitary movements by fish in either type. Fish in the territorial mode, not previously described in adults of this species, restricted activity to a small area overlapping the territories of other tangs. They actively chased conspecifics and were chased mostly by damselfishes. They swam more slowly and fed at higher rates than other modes. Fish in the schooling mode ranged widely in compact, polarized groups of conspecifics, congeners and other species. They were not aggressive and were attacked mostly by damselfishes. They swam rapidly and fed at intermediate rates. Fish in the wanderer mode showed neither aggression nor association with other individuals. They swam rapidly, well above the substrate, fed little, were chased by conspecifics, ocean surgeonfish, A. bahianus, and damselfish and visited cleaning stations more often then other modes. All three modes were observed in all four main zones of the reef, and their behaviour changed quantitatively with habitat type. We suggest that territoriality reduces competition for algal food, schooling allows fish to overcome the food defence by damselfish, tangs and ocean surgeonfish, and wandering permits solitary movement over the reef to cleaning stations, feeding sites and other resources.     

Size-dependent territory defense by a damselfish

Summary Three adult size classes of the territorial Caribbean dusky damselfish,Stegastes dorsopunicans, are differently distributed with respect to habitat, and with respect to the biomass of filamentous algal turfs in the areas they defend. The density of large individuals is positively correlated with the decalcified dry biomass of these turfs, whereas the densities of medium and small individuals are inversely related to algal biomass. Density of the urchin,Diadema, is also inversely correlated with algal biomass. The high density of large dusky damselfish in sites with algal turfs of relatively high biomass probably results from preferences of dusky damselfish for sites in which algal turfs are thick, and superior abilities of large individuals to defend these sites.Because both rate of attacks and the effectiveness of attacks on territory invaders by dusky damselfish increases with increasing size, sites with relatively high biomass algal turfs are typically better defended than those with lower biomass turfs. Apparently as a result of this, small foraging groups of the blue tang surgeonfish,Acanthurus coeruleus, feed less on high biomass algal turfs than do larger foraging groups, the members of which experience attacks by defending damselfishes less frequently. The relatively low proportionate use of high biomass feeding sites by solitary blue tangs and members of small foraging groups is caused by dusky damselfish. When the density of this damselfish was reduced artificially, use of high biomass algal turfs by solitary blue tangs increased to a level indistinguishable from that of participants in large foraging groups.