Gunilla Ejdung

Comparing rate and depth of feeding in benthic deposit-feeders: a test on two amphipods, Monoporeia affinis (Lindström) and Pontoporeia femorata Kröyer

Comparing rate and depth of feeding in benthic deposit-feeders: a test on two amphipods, Monoporeia affinis (Lindstrom) and Pontoporeia femorata Kroyer

Benthic predator-prey interactions: evidence that adult Monoporeia affinis (Amphipoda) eat postlarval Macoma balthica (Bivalvia).

Predation by adults of the amphipod Monoporeia affinis on the plantigrade postlarval stage of the bivalve Macoma balthica was studied in the laboratory. We confirmed that M. affinis consumes small M. balthica. Amphipods offered 14C-labelled postlarvae took up the radioactive tracer, while those presented Rhodamine B-stained postlarvae acquired gut contents fluorescing strongly in orange, whereas control amphipods did not. Both labelling methods proved convenient to use in laboratory experiments, and are particularly useful when organisms lack structures that can be easily identified after being ingested, or when cross-over reactions may bias the results of immunoassays. The results reported here support the conclusion from earlier studies that predation by M. affinis on M. balthica can affect population dynamics of M. balthica and is likely to be an important structuring factor in the low-diversity benthic macrofauna community of the Baltic Sea.

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

Abstract 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.