Competition between the introduced polychaete Marenzelleria sp. and the native amphipod Monoporeia affinis in Baltic soft bottoms

Abstract

The North American spionid polychaete Marenzelleria cf. viridis was reported from the Baltic Sea for the first time in 1985. The species has spread rapidly and now dominates many soft- bottom communities, where it dwells in burrows down to 30 cm deep in the sediment. In the Baltic Sea, below 10 m water depth, the macrobenthic community is composed of only a handful of species, among which the amphipod Monoporeia affinis is a key member. This species is highly mobile; it swims actively at night but remains burrowed in the sediment during the day. Due to similarities in feeding mode and sympatric occurrence, the polychaete may compete with the amphipod for both food and space. One plausible outcome of interactions between the 2 species is that the amphipod, the more mobile species, would avoid areas where the more sessile polychaete is present in high numbers. In the laboratory, we tested the hypothesis that, if given the choice, the amphipod avoids burrowing in sediment with high polychaete abundances. The amphipod burrowed in significantly lower numbers in patches with high polychaete abundance compared to those with lower abun- dance. Also, plastic tubes mimicking polychaete body structure were not avoided by the amphipods, indicating that the physical tube structure of the polychaetes does not explain the amphipod's choice of burrowing site. Furthermore, the amphipod did not increase or prolong swimming activities as a function of higher worm density, indicating that increased swimming activity over dense polychaete patches and subsequent migration away from them is not a likely explaination of why this amphipod burrow less frequently in patches of high polychaete abundance.