Bembicium Auratum Research Articles

Experiments on the influence of oysters on the distribution, abundance and sizes of the gastropod Bembicium auratum in a mangrove swamp in New South Wales, Australia

Mean density of the littorinid snail Bembicium auratum varied with height above low water in a mangrove swamp (Avicennia marina) in New South Wales, Australia. Snails were more abundant on oysters (Crassostrea commercialis) than on other substrata (pneumatophores, mud and on the trunks of trees). On all substrata, the densities of snails were also greatest at the heights where clumps of oysters were found naturally. At these, relatively low levels on the shore, there was a greater proportion of smaller snails than found elsewhere. Smaller snails also occurred more frequently on oysters than on other substrata. Experimental removal of oysters from areas where they occurred naturally (with associated controls for disturbance of the substratum, the oysters and the snails) rapidly resulted in decreased densities and increased mean size of snails. More snails emigrated or disappeared from plots without than with oysters. More juveniles than adults emigrated or vanished. When snails were transplanted to higher levels in the swamp with oysters, they remained in experimental plots in larger densities than in plots where no oysters were provided. Again, greater proportions of juveniles than adults emigrated or disappeared. Virtually identical results were reproduced in three series of experiments in autumn, 1 month and 1 yr apart, but, in summer (December), no snails moved or disappeared. Oysters are an important influence on distribution, abundance and sizes of snails in mangrove swamps. Previously proposed causes of observed distribution of B. auratum include influences of availability of food. This was not supported by measurements of chlorophyll as an index of microalgae. We propose that oysters provide refuges from predation (by crustaceans or fish) during high tide.

A midden excavation - Royal Tasmanian Botanical Gardens, Hobart

Radiocarbon analysis of charcoal taken from the base of a midden located within the Royal Tasmanian Botanical Gardens, Hobart, indicates that first occupancy of this site occurred some 5 000 years ago. The mussel Mytilus edulis planulatus provided the bulk of the shellfish diet which may have been supplemented with a number of small species including the gastropods Bembicium auratum and Austrocochlea constricta zebra. A feature of this site is the relatively rich supply of petrologically diverse stone artefacts. This is at variance with the general pattern for middens previously excavated in the Derwent River estuary. Also present are small, round, hard pebbles, a feature not previously reported from similar sites on the Derwent River. Scanning electron microscopic analysis of charcoal indicates that the she oak Casuarina stricta provided some of the wood fuel for fires at this site.

Competition in Bembicium auratum (Gastropoda) and its effect on microalgal standing stock in mangrove muds.

Field experiments were undertaken on the intertidal gastropod Bembicium auratum, which is one of the dominant organisms in sheltered bays and mangroves in New South Wales. Animals were caged at various densities ranging from natural levels to about four times normal density. Increased density resulted in increased mortality (particularly of juveniles), reduced body weight, and (less conclusively) a decline in growth rate. Chlorophyll levels of the mangrove mud within the cages were monitored as an index of food availability, and increased rapidly in the control cages in the absence of Bembicium, remained steady at normal Bembicium densities, and declined at higher densities. Parallel experiments were conducted on the effects of substratum on Bembicium. One series of animals was allowed access to a hard substratum and the other only to mud. Body weights and survivorship were proportionally lower in animals deprived of a hard substratum. Juveniles were particularly susceptible to a shortage of food and more dependent on a hard substratum than adults. The population structure also suggests a high juvenile mortality but high adult survival and longevity. Bembicium is very abundant low on the shore but numbers decline and body weights increase higher up. Chlorophyll levels and amounts of hard substratum also decline up the shore. Possibly juveniles settle selectively (or survive better) low on the shore where oysters provide a dense hard substratum. Adults are less dependent on the hard substratum and may disperse up the shore, explaining the patterns of abundance and size.As Bembicium limits its own food supply and seemingly is limited by it, predation is unlikely to play an important role in restricting population density.

Relationships between intertidal zonation and circatidal rhythmicity in littoral gastropods

The presence and phase of circatidal rhythmicity was correlated with vertical zonation and other ecological factors. Ten species were studied in the field and in aktographs under controlled conditions in the laboratory. Retina plicata, Melanerita atramentosa, Bembicium nanum, Austrocochlea obtusa and Morula marginalba occupied mid- or upper-littoral zonations, and were subjected to regular tidal influence. They were active at high water and for a period after the ebb, possibly as these were the times of least desiccation. Each possessed a circatidal activity rhythm under non-tidal conditions. Midlittoral Amphinerita polita and lower-littoral Theliostyla albicilla were active at low water, possibly because their prefered habitats reduced desiccation and other selective forces, for example wave action and predation, determined the phase of the rhythmicity. Supra- and upper-littoral Nodilittorina pyramidalis and Melarapha unifasciata and lower- and infralittoral specimens of Bembicium auratum displayed no overt circatidal rhythmicity, possibly because they were not subjected to regular tidal action.