Subtidal Reefs Research Articles

Microorganisms and Detritus in the Water Column of a Subtidal Reef of Natal

The parameters measured in this study were light and dark-bottle uptake of NaHI4CO3, the chlorophyll content of seawater, direct counts of bacteria, microbial uptake of 14C glucose and a labelled algal extract, organic carbon in the seawater and the composition of debris washed onto the reef. The phytoplankton consisted largely of nannoplankton and primary production was low with an annual mean of P = 12.82 mg C n1r3h-', while the mean chlorophyll a content of 2.13 mg m-3 was equivalent to a dry biomass of 170.1 mg m-3 Microbial heterotrophic activity was relatively high with an annual mean glucose assimilation of V,,, = 0.612 @g l-'h-' and a total bacterial count of 2.02 X 106 ml-', equivalent to a dry biomass of 20.3 rng m-3. The largest proportion of this activity, measured by differential filtration and using antibiotic inhibitors, is performed by free bacteria as opposed to the smaller proportion conducted by bacteria attached to particles, or by microflagellates. Uptake of the labelled algal extract (V,,, 13.2-72.6 mg C m-3h-') indicates that microbial heterotrophic activity exceeds phytoplankton production. Carbon analyses gave an annual mean of 18.3 g m-3 total organic carbon, of which 20 % was particulate, the rest being dissolved organic carbon. Analysis of debris revealed that it consisted almost exclusively of seaweed, except when rivers were in flood, providing a large influx of terrestrial plant debris. These findings are compared with other published results, and their ecological significance is discussed in relation to the remainder of the environment. It is concluded that primary production by phytoplankton is not as important in supporting the biomass of filter feeders on the reef as is heterotrophic activity associated with organic detritus.

Competitive Interactions between California Reef Fishes of the Genus Embiotoca

Embiotoca lateralis and E. jacksoni are very similar morphologically and exhibit the greatest dietary overlap within a guild of microcarnivorous subtidal reef fishes off Santa Barbara, California. Within this zone of sympatry, they segregate by depth: E. lateralis numerically dominates shallow reef areas covered by various algae, from which it picks its prey; E. jacksoni dominates and forages over deeper areas covered by a relatively food—poor "turf" of sessile invertebrates and small algae. E. lateralis rarely fed from turf—covered substrates experimentally translocated from deep water, while E. jacksoni frequently fed from translocated shallow algae. In allopatry, however, each species occupies all reef depths and commonly forages over both food substrates. When E. lateralis was experimentally removed from a reef inhabited by both species, E jacksoni entered shallow areas and foraged over algae. However, removing E. jacksoni from another reef did not affect the distribution of E. lateralis. E. lateralis aggressively dominated E. jacksoni, and E. jacksoni avoided foraging near E. lateralis. E lateralis eventually entered and foraged over deep reef ares only when all shallow algae and E. jacksoni were removed. Sympatric population densities of E. lateralis, here at the southern margin of its geographical range, are much lower than those of allopatric populations elsewhere. Hence, shallow food sources may be sufficiently abundant that marginal E. lateralis populations occupy only this richest end of the bathymetric reef food gradient. This situation may provide a competitive refuge for E. jacksoni in less productive deep reef areas, thus maintaining the coexistence of these species within the same habitat.

Resource Partitioning by Predatory Gastropods of the Genus Conus on Subtidal Indo-Pacific Coral Reefs: The Significance of Prey Size

On subtidal reefs, most Conus species that feed on errant polychaetous annelids spe- cialize on various members of the family Eunicidae, whereas on intertidal benches, members of the family Nereidae dominate the diet. There are at least 2 groups of Conus species (termed complexes) that share major prey species on reefs in Micronesia and the Indian Ocean. Members of these respective groups feed primarily on the eunicids Palola siciliensis and Eunice afra. There are also 2 such groups on reefs in Hawaii, one feeding on Palola siciliensis and the other on Eunice australis, plus a Lysidice collaris specialist and a Eunice afra specialist. Mean prey sizes of Conus species within such feeding complexes tend to differ. Mean prey sizes, mean sizes of particular prey species, and mean body sizes of Conus species within feeding complexes differ more often on subtidal reefs than on intertidal benches. Furthermore, mean prey-size overlap between Conus populations on subtidal reefs is less than (but on intertidal benches not different from) a random expectation derived by a new technique described in this paper. It is suggested that divergence in food and body size of Conus, possibly in response to competition, has occurred on subtidal reefs where food may limit its abundance, but not on intertidal benches where food is probably superabundant.

Zonation of corals (Scleractinia: Coelenterata) on intertidal reef flats at Ko Phuket, Eastern Indian Ocean

The intertidal zonation on sheltered reef flats at Ko Phuket is described, with the emphasis on the vertical zonation in relation to tolerance to emergence. Zonation is compared to that on reef flats described in the literature. Factors limiting vertical growth are discussed. A terminology is proposed for the description of intertidal reef flats, following the system of associations developed for subtidal reefs. It is shown that intertidal coral growth is best developed on sheltered places around larger, high islands and on continental coasts; that coincidence of extreme low tide and maximum insolation is a major limiting factor; and that rain during emersion is not necessarily fatal for coral growth.

Ecology of Conus on eastern Indian Ocean fringing reefs: Diversity of species and resource utilization

The most diverse assemblages of the genus Conus known occur on fringing coral reefs in Thailand and Indonesia. As many as 27 congeneric species of these gastropods inhabit a single reef; in all, we examined 1,350 individuals of 48 species. Several attributes of the populations we observed conform to expectations of a model of ecological characteristics of bench and reef Conus proposed by Kohn (1971a). Number of species (S) averaged 15, and species diversity (H″) averaged 2.3 in the most heterogeneous habitat type — topographically complex, subtidal reef platforms (Type III habitat). Both species richness and evenness of distribution of individuals among species contribute strongly to H″. Fewer congeners and greater numerical dominance by single species characterize more homogeneous habitats. On subtidal reef platforms with large areas of sand substrate and less coral limestone (Type I–III habitat), mean values were S=10, and H″=1.6. In the one intensively studied, truncated reef-limestone platform (Type II–III intermediate habitat), S=13 and H″=1.4. Summed population density of all Conus species in Type III and I–III habitats is similar (0.02 to 0.05 individuals /m2) and comparable to estimates from similar habitats elsewhere in the Indo-West Pacific region. Mean density (0.7/m2) and other population attributes in Type II–III habitat more closely resemble those of Type II than Type III habitats in general. We combined analysis of species diversity and other attributes of assemblages in habitats of different environmental complexity with analysis of microhabitat and food-resource utilization, in order to demonstrate the extent to which specialization on different resources occurs in assemblages differing in diversity and habitat type. In the habitats studied, co-occurring species of Conus specialized to a greater extent on different prey species than on different microhabitat patches, but degree of microhabitat specialization was greater than in similarly complex habitats with assemblages of lower diversity elsewhere in the Indo-West Pacific region. While most Conus species preyed primarily on a different species or higher taxon of polychaetes, diets are not more specialized or dissimilar than in similar habitats elsewhere. Degree of specialization on different prey is not correlated with Conus species diversity in the different types of habitats studied. The data lead to the conclusion that differential predation is as important — and differential microhabitat utilization is more important — in permitting coexistence of potentially competing congeners, compared with conditions in habitats of comparable heterogeneity that support fewer congeners farther from the center of the Indo-West Pacific region. Pairwise comparisons of congeners indicate that many species pairs have low or no overlap in both microhabitat and food utilization. Members of species pairs with high overlap in microhabitat utilization typically eat different prey organisms, and those with similar diets typically occupy different habitats or microhabitats. This applies to molluscivorous as well as vermivorous species. Information on the diets of 11 species is reported here for the first time. Of 48 Indo-West Pacific Conus species whose food is now known, 35 prey on polychaetes, 2 on enteropneusts, 6 on gastropods, and 5 on fishes. Vermivorous Conus prey on relatively few of the polychaete species present in the environments. Species eaten represent only 12% of a total estimated polychaete population density of 27,000 individuals /m2. Certain very abundant polychaetes may be protected from predation by Conus by their small size, others by their long tubes. Two new aspects of size-selective predation by Conus are reported: (1) Although comparisons of predation rate with prey standing-crop suggest that food is plentiful, selective predation on the largest prey individuals present suggests that only small proportions of prey-species populations may have large enough body size to repay foraging effort by the Conus present; (2) composition of the diet changes qualitatively with increase in body size in several vermivorous Conus species; shifting by larger individuals to larger prey species could be documented in C. ebraeus.

A Note on Trophic Complexity and Community Stability

A Note on Trophic Complexity and Community Stability