Search

We studied the effect of crustacean predators on native/non-native Macoma balthica bivalves in aquarium experiments. North Sea M. balthica (NS Macoma) were recently observed in the southern Baltic Sea. They differ genetically and in terms of morphology, behaviour and evolution- ary history from Baltic Sea M. balthica (BS Macoma), and this may affect predation pressure and community structure. We hypothesised that predators consume more of the prey they co-exist with. NS Macoma and BS Macoma were exposed to crustacean predators common in the North Sea (Carci- nus maenas and Crangon crangon) and in the Baltic Sea (C. crangon and Saduria entomon). Contrary to our hypotheses, the North Sea predators ate more BS Macoma, and S. entomon ate more NS Macoma. The crush-limited C. maenas preyed more on globular BS Macoma, whereas S. entomon, which do not crush but pry open the bivalve shell, ate more NS Macoma, which have a lighter (thus probably thinner) shell than BS Macoma. When NS and BS Macoma were offered together, BS Cran- gon ate more NS Macoma. We also studied BS Crangon consumption of M. balthica to assess whether sizes offered fall within the size spectrum that C. crangon can eat. Small (20 to 40 mm long), medium (40 to 50 mm) and large (50 to 60 mm) C. crangon especially ate small M. balthica. Differences in shape, size and meat/shell weight ratio between the BS and NS Macoma partly explained the differences in the susceptibility to predation by native and non-native predators.

The effect of severe and moderate hypoxia on food uptake of benthic macrofauna was studied in the laboratory. The hypothesis was that low oxygen concentrations negatively affect feed- ing at oxygen levels that have little effect on the studied animals' survival. The bivalve Macoma balthica, the priapulid Halicryptus spinulosus, the amphipods Monoporeia affinis (subadult & juve- nile) and Pontoporeia femorata (subadult) were offered the 14 C-labelled diatom Skeletonema costa- tum in 0.8 to 10.6 mg O2 l -1 . Feeding was measured as radioactivity uptake. Subadult amphipods were studied one species at a time (single) or together (mixed). Feeding changed in all amphipods at the lowest oxygen concentrations, but no effect was found for M. balthica and H. spinulosus. At the lowest concentration (0.8 mg O2 l -1 ) feeding by subadult M. affinis (single) was only 17% of the full oxygen saturation (10.6 mg O2 l -1 ), and, at 1.6 mg O2 l -1 , 14% of the feeding at 8.9 mg O2 l -1 . Juvenile M. affinis consumed more labelled algae at 3 mg O2 l -1 than at higher oxygen concentrations. M. balthica feeding was not affected. Little radioactivity uptake was registered for H. spinulosus at any oxygen concentration, showing that H. spinulosus is not a surface deposit feeder. The amphipods were the most sensitive to week-long oxygen deficiency. Survival decreased significantly in the lowest oxygen concentrations (0.8; 1.6 mg O2 l -1 ). Of the subadult M. affinis 15 and 65%, respectively, survived, compared with 0 and 58% for P. femorata. Juvenile M. affinis mortality was high in all oxygen concentrations, whereas most M. balthica and H. spinulosus survived.

Some benthic deposit-feeders mainly eat freshly deposited phytodetritus, while others feed more on older material that has been mixed with the sediment and modified by diagenetic processes before being ingested. We studied the uptake of sedimentary foods of different ages by the Baltic amphipods Monoporeia affinis (Lindstrom) and Pontoporeia femorata Kroyer in laboratory experiments using 3 isotopic tracers. The amphipods were offered fresh 14 C-labelled diatoms spread on top of a thin unlabelled sediment layer, underlain by 1 yr old sediment to which 13 C- and 15 N-labelled diatoms had been added. Thus, 14 C uptake represented surface feeding on fresh organic material, and 13 C: 15 N uptake subsurface feeding on aged phytodetritus. Experiments using a single species only or mixed species were conducted in spring with 1 yr old adults and in summer with 3 mo old juveniles. Adult M. affinis (initial dry mass 1.6 mg) took up ∼5 times more 14 C than P. femorata (initial dry mass 1.7 mg), indicating that M. affinis depended more on fresh phytodetritus, while P. femorata had significantly higher 13 C: 15 N uptake, showing a greater reliance of this species on aged organic matter from the deep sediment. In experiments, adult P. femorata consistently fed at depth in the sediment, whereas adult M. affinis modified feeding depth depending on food quality. Juveniles (0.1 mg initial dry mass) of both species had similar tracer uptake and fed both on surface and subsurface sediment, suggesting greater potential for interspecific food competition in juveniles than in adults. Juveniles of both species had higher mass-specific 14 C uptake than adults. Single species treatments had higher 14 C uptake than mixed treatments in both adults and juveniles, indicating food competition at the higher density of the mixed treatments.

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

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.