Predation by the benthic isopod Saduria entomon on two Baltic Sea deposit-feeders, the amphipod Monoporeia affinis and the bivalve Macoma balthica

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This laboratory study demonstrates that Saduria entomon prefers the amphipod Monoporeia affinis over the bivalve Macoma balthica. When offered bivalves only, S. entomon selects the smaller individuals. Predation by S. entomon may thus alter the species composition and size distributions of its prey community, which in the field is often dominated by these two species. When offered one M. balthica size class at a time, 30-mm-long isopods ate only bivalves of the 2–3 and 6–7 mm size classes and rejected classes 10–11 and 15–16 mm, even though they can open them. More bivalves were eaten in the 2–3 mm size class than in the 6–7 mm class, but energy yield was slightly higher from the 6–7 mm class. When 10- to 45-mm-long S. entomon were offered a range (1–22 mm) of bivalve sizes, they consistently selected small prey. The length of the largest bivalve eaten increased with predator size, without the smallest bivalves being excluded from the diet. Energy consumed increased with increasing predator size. The size-selective predation in our experiment results in preferential survival of larger prey. When given a choice between ∼5-mm-long M. balthica and ∼7-mm-long M. affinis in ratios of 0:40, 10:30, 20:20, 30:10 and 40:0 (total density of 3600 m −2), S. entomon clearly preferred the amphipod (preference constant 69), and ate very few bivalves. In the treatment with 30 bivalves and 10 amphipods, bivalves were not as strongly selected against as in other two-prey treatments. In a 28-day prey depletion experiment with an initial 1:1 prey ratio, almost all amphipods were consumed in the first week, after which bivalves started to be eaten.