Sessile Epifauna Research Articles

Damselfish Territoriality: Influence on Diadema Distribution and Implications for Coral Community Structure

Experiments were performed in Discovery Bay (Jamaica, W I. ) to determine the relationship between behavior of threespot damselfish Eupornacentrus planifrons Cuvier, and distribution of the common shallow-water echinoid Diaderna antillarum Philippi. E planifrons actively excludes Diadema from its algal lawn, causing a distinct alteration in local distribution of Diaderna on the reef. Physical attacks on the urchin were usually sustained for 2 5 min. The damselfish's agonistic behavior occurred only dunng the day. Diadema densities were higher withln algal lawns at night than during the day but were always significantly lower than the mean density outside. The size of the damselfish's algal lawn was positively correlated with damselfish density, implying that damselfish which occur in aggregations are able to defend larger territories together than they would when occurring alone. The impact of interactions between fishes, urchins and sessile epibenthos are predictable from experimental data derived from a concurrent study. E. planifrons territories are probably sites of locally high mortality in coral spat, particularly in Agaricia and Porites spp. This would result from competition for space with algae and encrusting epifauna. The algal lawns, however, may also represent refuges for other corals such as Favia fragum which are well-adapted to survive competition with other sessile epifauna as well as with algae. Thus, the algal lawns could represent dlscrete patches In the environment within which mortality of dominant species is high and that of rarer species is relatively low, causing an increase in the latter's representation in the benthic community. The net effect of this would be an overall increase in coral diversity within the community. This interpretation is consistent with hypotheses concerning the role of patch-formation in determining con~munity structure.

The food and feeding of the deep-sea morid fish Lepidion eques (Gunther, 1887) in the Rockall Trough

The stomach contents of Lepidion eques trawled at depths ranging from 500 to 1250 m on the Hebridean Terrace (56°-57° N, 9°-11° W) are analysed relative to size of fish, depth of habitat and season of the year. Of the 1281 fish examined, 47% had everted stomachs, 29% empty stomachs, 2% had food that was totally unrecognizable while 21% had food whose constituents could be identified. The principal dietary components are the epibenthic and hyperbenthic decapod crustaceans, although a wide variety of other organisms also occur as minor components. Benthic infauna and sessile epifauna are absent from the diet. L. eques therefore feed predominantly on organisms swimming immediately above the sea-bed or actively moving over it.

Diadema and its relationship to coral spat mortality: Grazing, competition, and biological disturbance

The benthic grazer Diadema antillarum Philippi (Echinoidea) has been demonstrated experimentally to contribute to the control of coral community structure in shallow water. In Discovery Bay, Jamaica, West Indies, Diadema densities were manipulated over a range of 0-64/m2 with the aid of enclosures. Grazing by Diadema under primary and post-primary succession conditions were compared.Algal percent-cover decreased as Diadema density was increased. Despite the presence of high algal cover. highest coral recruitment and diversity occurred at lowest Diadema densities, with planular settlement occurring predominantly in openly exposed micro-habitats. However, since algal growth rates greatly exceeded those of corals, space was rapidly monopolized by the former, resulting in intense competition and high coral mortality. This was particularly evident in Agaricia and Porites spp. At high Diadema densities, coral recruitment was greatly depressed in at least a genus-specific manner by intense levels of biological disturbance resulting from the echinoid's abrasive grazing activities. Favia Fragum (Esper) was especially susceptible to this perturbation. The surviving coral spat were found generally in cryptic, protected areas. Here they suffered some competitive losses to other sessile epifauna and -flora, particularly coralline algae, polychaetes and forams, which were well adapted to these physical and biological conditions. Increased sedimentation also depressed coral recruitment, replacing grazing as a limiting factor for successful settlement.Optimal conditions for coral survival, competitive success, and possibly growth were found at intermediate densities due to a balance between competition for space and biological disturbance. Diadema antillarum plays an important role in controlling the distribution and abundance of coral spat in the shallow reef community.