Freshwater molluscs, snails (Gastropoda) and mussels (Bivalvia), are an excellent food for many species of freshwater fish and crustaceans. In rivers, lakes and reservoirs, molluscs form powerful bottom placers and foulings that are actively exploited by many cyprinid and other fish.

For example, it is shown that bream, Abramis brama, white bream, Blicca bjoerkna, common roach, Rutilus rutilus, and carp, Cyprinus carpio, can eat zebra mussel, Dreissena polymorpha, of size ranged from 1-2 to 45 mm depending on fish age (Prejs et al., 1990; Nagelkerke & Sibbing, 1996; Tucker et al., 1996). The real switch to zebra mussels would be expected in fish of 23–24 cm length (Prejs et al., 1990). Other abundant freshwater molluscs are Viviparus viviparus, V. ater, V. contectus and other viviparous snails. For example, newborn V. ater are eaten by barbel, Barbus barbus, roach, R. rutilus, rudd, Scardinius erythrophtalmus and tench, Tinca tinca (Keller & Ribi, 1993), while adult viviparids (with the relatively hard shell and shell size up to 45 mm) are eaten by all large cyprinid fish with the well developed pharyngeal teeth.

Composition

The proximate compositions and fatty acid profiles of the freshwater mussels Unio terminalis and Potamida littoralis are compared by Ersoy & Şereflişan (2010).  The crude protein (11,87-11,97 %), lipid (2,55-1,05 %), ash (1,68–1,61%) and moisture (80,36-81,69 %) contents of U. terminalis and P. littoralis are observed. Lipid content in U. terminalis is found to be significantly higher than in P. littoralis. The percentages of total saturated fatty acids and total monounsaturated fatty acids are higher in U. terminalis than in P. littoralis. At the same time, the corresponding content of total polyunsaturated fatty acids is lower. The n3-n6 ratio are 1,54-1,40 in U. terminalis and P. littoralis, respectively. Using these data, Ersoy & Şereflişan (2010) conclude that freshwater mussels U. terminalis and P. littoralis are suitable as the healthy food.

Freshwater and Saltwater Molluscs

There are strong differences in amino acids and other substances content between molluscs of freshwater and saltwater origin. Firstly, amino acids in general are found in the higher concentrations in marine species than in the freshwater forms (Allen, 1961). The second feature is associated with taurine. This acid is absent in terrestrial and freahwater molluscs, such as gastropods, Lymnaea palustris, Marisa cornuarietis, Pomacea bridgesi, and pelecypods, Anadonta grandis, Quadrula quadrula, Lampsilis sp., Eiliptio sp., but is present in large amounts in marine species (Allen, 1961; Carr et al., 1996). Taurine is considered as an osmoregulator (achieving in such marine mussel as Mytilus edulis 1,6 % of the dry weight) and feeding stimulant. The third feature is associated with betaine that is abundant in marine molluscs, abscent in freshwater ones and play similar roles.

In general, taurine,betaine, glycine and alanine are the most abundant substances (and feeding stimulants) in tissues of marine pelecypods (Carr et al., 1996).

Seasonal Variations

The effect of autumn and winter decrease in environmental temperature on the content of free amino acids in body fluids has been studied in freshwater snail Lymnaea stagnalis (Karanova, 2006). In autumn, when temperature drops to 4 and 0°C, the highest increase in alanine concentration is observed, its pool is almost three times that in summer. Less pronounced accumulation of glutamate, glycine, histidine and serine is observed in the same temperature range. Cysteine is detected at 0°C. The accumulation of essential amino acids methionine, leucine, isoleucine, tyrosine and phenylalanine takes place at 0°C. Only traces of these amino acids are detectable at 4°C. At the same time, free lysine undetectable in summer has been revealed in autumn, its concentration increases as temperature decreases to 0°C. During winter, when snails are hypermetabolic for 2.5 months, pools of all amino acids decrease 4-8 times, while essential amino acids, except lysine, are undetectable. The involvement of alanine and, possibly, lysine in the L. stagnalis adaptation to near-zero temperature is proposed (Karanova, 2006).

Attractiveness

Three species of crayfish (Orconectes virilis, O. rusticus and Cambarus robustus) are tested for feeding responses to odors of potential food molluscs (invasive zebra mussel, Dreissena polymorpha, and native gastropods, Stegnicola elodes and Aplexa elongata) by Hazlett (1994). Feeding responses to odors are shown only by individuals experienced with feeding on the corresponding prey. Like hermit crabs, such as Clibanarius vittatus and other species feeding on marine gastropods or use their shells (e.g., Rittschof, 1980), freshwater O. virilis respond to peptides produced during enzymatic degradation of proteins in gastropod flesh (Hazlett, 1994).

According to data obtained by Carr et al. (1977) for pigfish, Orthopristis chrysopterus, synthetic mixtures of betaine and amino acids could account for most of the stimulatory capacity of extracts of oyster, Crassostrea virginica (Mollusca). Flatfish Dover sole, Solea solea, eat the diet flavoured with the mussel, Mytilus edulis, extract or with mixtures of pure chemicals (glycine betaine plus amino acids) matched with the composition of mussel flesh (Mackie et al., 1980).

Basic References

Allen K. 1961. Amino acids in the mollusca. American Zoologist 1, 253-261

Carr W.E.S., Blumenthal K.M., Netherton III J.C. 1977. Chemoreception in the pigfish, Orthopristis chrysopterus: The contribution of amino acids and betaine to stimulation of feeding behavior by various extracts. Comparative Biochemistry and Physiology Part A: Physiology 58, 69-73

Carr W.E.S., Netherton III J.C., Gleeson R.A., Derby C.D. 1996. Stimulants of feeding behavior in fish: analyses of tissues of diverse marine organisms. The Biological Bulletin 190, 149-160

Ersoy B., Şereflişan H. 2010. The proximate composition and fatty acid profiles of edible parts of two freshwater mussels. Turkish Journal of Fisheries and Aquatic Sciences 10, 71-74

Hazlett B.A. 1994. Crayfish feeding responses to zebra mussels depend on microorganisms and learning. Journal of Chemical Ecology 20, 2623-2630

Karanova M. 2006. Variation in the content of free amino acids in body fluids of freshwater mollusk Lymnaea stagnalis during seasonal adaptation to low positive temperatures. Biology Bulletin of the Russian Academy of Sciences 33, 587-591

Keller G., Ribi G. 1993. Fish predation and offspring survival in the prosobranch snail Viviparus ater. Oecologia 93, 493-500

Mackie A.M., Adron J.W.,  Grant P.T. 1980. Chemical nature of feeding stimulants for the juvenile Dover sole, Solea solea (L.). Journal of Fish Biology 16, 701-708

Nagelkerke L.A.J., Sibbing F.A. 1996. Efficiency of feeding on zebra mussel (Dreissena polymorpha) by common bream (Abramis brama), white bream (Blicca bjoerkna), and roach (Rutilus rutilus): the effects of morphology and behavior. Canadian Journal of Fisheries and Aquatic Sciences 53, 2847-2861

Prejs A, Lewandowski K., Stańczykowska-Piotrowska A. 1990. Size-selective predation by roach (Rutilus rutilus) on zebra mussel (Dreissena polymorpha): field stuides. Oecologia 83, 378-384

Rittschof D. 1980. Chemical attraction of hermit crabs and other attendants to simulated gastropod predation sites. Journal of Chemical Ecology 6, 103-118

Tucker J.K., Cronin F.A., Soergel D.W., Theiling C.H. 1996. Predation on zebra mussels (Dreissena polymorpha) by common carp (Cyprinus carpio). Journal of Freshwater Ecology 11, 363-372