Defaecation strategies of two freshwater gastropods, Ancylus fluviatilis M�ll. and Planorbis contortus Linn. (Pulmonata) with a comparsion of field and laboratory estimates of food absorption rate

Attached Algae: The Cryptic Base of Inverted Trophic Pyramids in Freshwaters

It seems improbable that a thin veneer of attached algae coating submerged surfaces in lakes and rivers could be the foundation of many freshwater food webs, but increasing evidence from chemical tracers supports this view. Attached algae grow on any submerged surface that receives enough light for photosynthesis, but animals often graze attached algae down to thin, barely perceptible biofilms. Algae in general are more nutritious and digestible than terrestrial plants or detritus, and attached algae are particularly harvestable, being concentrated on surfaces. Diatoms, a major component of attached algal assemblages, are especially nutritious and tolerant of heavy grazing. Algivores can track attached algal productivity over a range of spatial scales and consume a high proportion of new attached algal growth in high-light, low-nutrient ecosystems. The subsequent efficient conversion of the algae into consumer production in freshwater food webs can lead to low-producer, high-consumer biomass, patterns that Elton (1927) described as inverted trophic pyramids. Human perturbations of nutrient, sediment, and carbon loading into freshwaters and of thermal and hydrologic regimes can weaken consumer control of algae and promote nuisance attached algal blooms.

Elimination of the cyanobacterial hepatotoxin microcystin from the freshwater pulmonate snail Lymnaea stagnalis jugularis (say)

It has been suggested that little to no microcystin (MC), a cyanobacterial hepatotoxin, accumulates within freshwater pulmonate snails because the toxin is associated primarily with undigested gut contents that are eliminated from the animal via egestion. To test this, Lymnaea stagnalis exposed to MC-containing cyanobacteria were placed into toxin-free environments and sampled over short (24 h at 21 degrees C) and long (30 d at 22 and 10 degrees C) time periods. Within 8 h after being removed from exposure to microcystin-containing phytoplankton, the gizzard and cecal string fractions of the feces were eliminated, accounting for 57% of the initial MC concentration. However, detectable concentrations remained beyond 24 h, likely in association with the digestive-gland contents, which can be retained up to 100 h. Long-term MC loss was biphasic at two ambient temperatures. The greatest change (fast phase) occurred over the first 3 d after exposure. By 6 d, the cumulative MC loss from L. stagnalis was 80 and 95% at 10 and 22 degrees C, respectively. Toxin loss over this period was attributed to egestion of indigestible cells/colonies from gizzard and cecum, as well as elimination of unassimilated MC-laden fragments and vacuolate excretion of residues from the digestive gland. The fast-phase depuration rate constant was significantly higher at 22 than at 10 degrees C, indicating an influence of ambient temperature on the rate of toxin loss from pulmonate snails. Depuration continued at slower rates until 30 d, when most (97.5 and 99.5% at 10 and 22 degrees C, respectively) of the initial MC was eliminated.

The seasonality and role of the neurotoxin tetramine in the salivary glands of the red whelk Neptunea antiqua (L.)

The red whelk Neptunea antiqua (family Buccinidae) is a sublittoral species of marine prosobranch mollusc that occurs in the cold temperate waters of the Eastern Atlantic Boreal Region. A clearly defined seasonal cycle was revealed in the concentration of the whelk's salivary gland neurotoxin, tetramine, ranging from undetectable levels to 6530 microg/g over the course of an annual sampling period in the central western Irish Sea. Concentrations of this amine were indirectly related to feeding activity through the use of digestive gland indices. Additionally, laboratory observations confirmed the suspected ability of these whelks to actively predate on bivalves, supporting the use of this toxin in food procurement.

Postingestive selection in the sea scallop, Placopecten magellanicus (Gmelin): the role of particle size and density

Suspension-feeding bivalves can influence the energy value of their food supply by particle selection at various stages from particle clearance to production of feces. Previous workers have found that some bivalve species (Mercenaria mercenaria, Mytilus edulis) are capable of postingestive selection within the stomach. Few studies, however, have attempted to isolate the factors that influence postingestive selection. In this study, we examined the ability of the sea scallop Placopecten magellanicus to select particles within the stomach on the basis of physical properties. Scallops were presented with a mixture of three sizes of beads (5, 10 and 20 µm) or with a mixture of beads of different densities (1.05 g ml(-1) and 2.5 g ml(-1)). We demonstrate that P. magellanicus can distinguish between particles of different sizes and densities, retaining larger particles (20 µm) longer than smaller ones (5 µm) and lighter particles longer than denser ones. This ability to reject small, dense particles may benefit the scallop by reducing the amount of energy expended attempting to digest poor quality particles such as silt. This paper presents the first quantitative analysis of the effect of particle size and density on particle processing within intact bivalves.

A shortage of food for the terrestrial snail Helicella virgata in South Australia

Two field experiments were conducted to investigate the likelihood of snails experiencing a shortage of food. In the first experiment and in one by a previous worker, various measures of growth, reproduction, activity or survival decreased with increasing density of snails. These results appear inconsistent with the hypothesis of an absolute shortage of completely-accessible food. They could be explained by a modification of that hypothesis or by the hypothesis that snails at high density interfere with each other by means other than competition for food.The results of the second experiment suggest that the food naturally available on the study-area is of poor quality; the snails may experience a relative shortage of food, of the kind where food is abundant but much of it is low in nutrients.