Distribution, abundance and size-structure of cerithiid gastropods in sediments at One Tree Reef, southern Great Barrier Reef

The patterns of abundance of molluscs in sediments at One Tree Reef, southern Great Barrier Reef were determined for three adjacent habitats in October 1985: a shallow subtidal sandflat, a deep channel, and the sloping, unstable edge separating the two. Deposit-feeding cerithiid gastropods, especially Rhinoclavis aspera (Linnaeus, 1758), R. fasciata (Bruguière, 1792) and R. vertagus (Linnaeus, 1758), were the most abundant molluscs present but these species were predominantly found in the coarse sandy sediments of the sandflat. The abundances and sizes of these three species were monitored seasonally from October 1985 to January 1988 on the sandflat. Densities fluctuated during the three years of the study and varied at a number of spatial scales. The density of R. aspera was generally greater at the beginning of the year, after recruitment, then gradually declined until the next period of recruitment. This pattern was not as evident for R. fasciata or R. vertagus. Changes in size-frequency distributions through time suggest that movement of animals, either by migration and/or passive transport in the shifting sediments may contribute to the observed patterns. Comparisons are made between temperate and tropical soft-sediments habitats, especially in relation to the distribution of deposit-feeding organisms

Recruitment of cerithiid gastropods (Rhinoclavis spp.) in sediments at

The abundance of three species of deposit-feeding cerithiid gastropods, Rhinoclavis aspera (Linnaeus, 1758), R. fasciata (Bruguiere, 1792) and R. vertagus (Linnaeus, 1758), was monitored seasonally from May 1986 to January 1987 in sediments at One Tree Reef, southern Great Barrier Reef. All three species were most abundant in the coarse sandy sediments of a shallow sandflat, rather than in the finer sediments found in deeper channels within the lagoon. In contrast deposit-feeding temperate zone infauna are typically more abundant in finer sediments. To evaluate whether this pattern of greater abundance in sandy sediments was a result of greater recruitment to the sandflat compared with the channels, recruitment of each species was determined in each of two successive summers, January 1987 and January 1988. R. aspera recruited mostly to the sandflat compared with other habitats in both of 1987 and 1988, but for R. fasciata and R. vertagus this was only the case in 1988. These results are consistent with a model of larval depletion in the water column because larvae are always carried across the sandflat by water currents before reaching the lagoonal habitats so there are fewer larvae left in the water to settle into the finer sediments of the lagoon. The effect of removing all adult macrofaunal molluscs from the sediment on the recruitment of each species was also experimentally determined at 10 locations. The results varied among locations supporting alternative models of adult enhancement of recruitment, reduction of recruitment due to adults, or no effect of adults on recruitment. The results of this experiment, however, provide evidence against the previous larval depletion model because in some cases recruitment was increased in the lagoonal habitats where adults were removed indicating that there were still larvae available in the water over the fine sediments

An experimental test of artifacts from repeated sampling in soft- sediments

Experimental and monitoring studies done in soft-sediments are made difficult by the need to disrupt physically the structure of the habitat in order to sample the biota, which often lives buried beneath the surface. Collection of a sample causes some disturbance as sediment moves to fill the hole left by the removed core or grab, possibly exposing animals in the surrounding sediment to predators. Physical disturbance from sampling may also disrupt microbial assemblages in the sediments or breakdown biotic structures which help bind the sediments leg. animals tubes, roots and rhizomes or microbial mars). An experiment was done specifically to test the hypothesis that disturbance from repeated sampling in soft-sediments leads to artifacts which affect subsequent estimates of the structure of populations and assemblages. Repeated sampling of the same patches is often necessary due to the logistical constraints of establishing experiments in soft-sediments

Effects of Habitat Composition On Recruitment of Cerithid Gastropods in Sediments At One Tree Reef, Great-Barrier-Reef

Cerithiid gastropods (Rhinoclavis Spp.) are common in sediments at One Tree Reef (southern Great Barrier Reef, Australia). They are more abundant in and recruit more abundantly to coarse sandy sediments than in finer sands in the lagoon. Recruitment in different habitats could differ because, in some places, fewer larvae arrive or because fewer larvae are able to settle. In this study, experimental transplants of sediments were used to determine the influence of small-scale variations in substratum on the recruitment of the snails. We predicted that if different substrata influence recruitment and were available in one place (where similar numbers of larvae arrive) recruitment should vary among substrata, Sandy substrata of different composition from 4 locations were tranSplanted in plastic tubs into 2 locations in and 2 locations away from the lagoon, with replicated sites in each location. Recruitment of 3 species of Rhinoclavis and of Cerithium zonatum was significantly affected by the available sediments. Generally, where there were differences, numbers of recruits were greater in coarse sediments, but results varied from location to location and species to species. The experiments were capable of demonstrating variations in recruitment in response to the type of sediment. Recruitment was, however, not greater in coarser sediments placed in the lagoon (except for Rhinoclavis aspera). Thus, the smaller recruitment in the lagoon cannot be explained by lack of suitable substrata. Further work is needed to determine the patterns of availability of the larvae in different habitats and the processes that influence choice of substratum

Control of foraging behavior of individuals within an ecosystem context: the clam Macoma balthica, flow environment, and siphon-cropping fishes

Macoma balthica (L.), an abundant clam, ubiquitous in temperate estuaries across the North Atlantic, is known to practice both alternative basic modes of feeding available to seafloor invertebrates. It either holds its feeding organ, the siphon, at a fixed position just above the sediment surface to filter out food particles suspended in the overlying water or else extends and moves its siphon around to vacuum up deposited food particles on the sediment surface. Previous laboratory experiments have established an understanding of the role of current flow in dictating the choice of whether suspension or deposit feeding will be used by marine invertebrates with the facultative flexibility to choose. Faster flows imply greater fluxes of suspended particles so that the energetic rewards of suspension feeding are enhanced. Slower flows imply reduced renewal rates of suspended foods in the bottom boundary layers and enhanced deposition of food particles on the seafloor so that a switch to deposit feeding is favored. Like early optimal foraging theory, this understanding is based on energetic considerations alone without incorporation of broader implications of how population interactions such as predation and competition influence individual foraging behavior. Feeding behavior of Macoma balthica is influenced in the Neuse River estuary by both hydrodynamics and siphon-cropping by juvenile demersal fishes. Under conditions of identical concentrations of suspended particulates in the water column and organic contents of surface sediments, Macoma exhibited much higher levels of deposit feeding where currents were slower. In addition, exclosure and fish inclosure experiments demonstrated that juvenile demersal fishes influence feeding behavior of Macoma by cropping exposed siphons and inducing reduction in deposit-feeding activity. Effects of croppers were substantial in early to midsummer, when juvenile fish abundances were greatest in trawl samples from this estuarine nursery and before the growing fish exhibited ontogenetic changes in diet away from early concentration on bivalve siphons. Field experiments in which siphon-cropping fish were caged at varying distances off the bottom failed to detect any effective behavioral avoidance by Macoma of cropping in response to proximity of fish. One might have hypothesized that under high risk of cropping, Macoma would switch to suspension feeding and away from deposit feeding, the feeding method entailing more risk of losses to croppers because of greater siphon activity and greater extension of siphons on the sediment surface. Consequently, partial predation by siphon-cropping fishes greatly reduces deposit-feeding activity by Macoma balthica during summer as an apparent direct effect of disfigurement and reduction of siphons, the organ required for efficient deposit feeding. Information on current flows alone would not suffice to predict feeding behavior of this marine invertebrate: the influence of partial predation must also be included

Effects of patch-size on the structure of assemblages in rock pools

Rock pools represent patches or islands of habitat different from the surrounding shore. They contain diverse assemblages of invertebrates and algae. The shape, size, height on the shore and exposure to waves are considered important influences on the abundances of organisms. In studies of natural pools, these factors are confounded because pools are rarely comparable; they differ in size, position on the shore and history of colonization. To solve this, experimental pools of different diameters (15, 30 and 50 cm) and depths (5, 15 and 30 cm) were drilled in sandstone rocky shores in Botany Bay, New South Wales, Australia. Replicate pools of each size were made in each of 24 randomly-chosen sites, six sites representing each combination of two heights (mid and low) on the shore and sheltered versus wave-exposed habitats. The influence of diameter of the pool is examined here, using data from a total of 360 experimental pools. Despite a 2-3-fold increase in surface-area of pools with increased diameter, there was no difference in densities of most taxa examined (gastropods, a starfish, sessile invertebrates, algae). An exception was foliose algae, the cover of which was generally greater low on the shore and in 50 cm than 30 cm diameter pools. There were also consistently more species in any stratum (i.e., depth in a pool) in the wider pools. There were approximately 25-50% more species in the widest than the narrowest pools, consistently from season to season and year to year. There were, however, significantly fewer species per unit area of a given stratum in Pools of 50 versus 30 versus 15 cm diameter. These results require further investigation in relation to different theories to explain species-area relationships. Multivariate analyses revealed no influence of diameter on the structure of assemblages of organisms in pools of different diameter. This was true for different strata in pools and at mid- or low-shore positions on exposed or sheltered shores. Diameter of pools thus had remarkably little influence on the organisms; those effects found were consistent. Experimental pools were shown to be effective for testing hypotheses about proposed influences on the ecology of inhabitants. The findings about diameter of pools greatly simplify analyses of influences of depth and physical features of the surrounding habitat