Contrasting feeding strategies of two freshwater gastropods, Radix peregra (Lymnaeidae) and Bithynia tentaculata (Bithyniidae)

Abstract

The two freshwater gastropods Radix peregra and Bithynia tentaculata were fed with unlimited quantities of diatoms, green algae, cyanobacteria, maple leaf detritus and natural sediment for 11 weeks. Shell growth rates ranged from 0.0 mm per week with sediment and maple leaf detritus as food to 0.29 mm per week (B. tentaculata) and 0.36 mm per week (R. peregra) when a combination of lettuce, Chlamydomonas and Tetramin was fed. An intermediate growth rate of 0.21-0.28 mm per week (B. tentaculata) and 0.09-0.16 mm per week (R. peregra) was found with diatoms, green algae and cyanobacteria. Measured growth rates depended on the combined effects of uptake efficiency, digestability and nutritional quality of the foods. Specific ingestion rates were rather similar among foods in Radix peregra, from 0.19 to 0.34 mg mg -1 d -1 . Bithynia tentaculata did not ingest sediment and maple detritus, and had specific ingestion rates from 0.22 to 0.36 mg mg -1 d -1 for diatoms, green algae and cyanobacteria. Assimilation efficiencies of Radix were from 35 % to 73 % for diatoms, cyanobacteria and green algae, but were negative for natural sediment which resulted in degrowth. Bithynia had assimilation efficiencies from zero for natural sediment, to between 33 % and 48 % for diatoms, maple detritus and cyanobacteria, up to a maximum of 74 % for Chlamydomonas. The animals' behaviour and soft-body dry mass at the end of the experimental period indicated that Radix and Bithynia have different feeding strategies. Radix feeds rather unselectively on whatever food is available, much of which can be broken into pieces by its jaws and radula, further triturated by the gizzard and efficiently hydrolysed by digestive enzymes. Bithynia, in contrast, feeds only on high-quality foods and stays inactive when these are not available. These behaviours can also be observed in the field.