Competition for food between the introduced polychaete Marenzelleria viridis (Verrill) and the native amphipod Monoporeia affinis Lindstrِm in the Baltic Sea

Since 1985 the spionid polychaete Marenzelleria viridis (Verrill) has invaded large parts of the Baltic Sea. In deeper soft-bottom habitats (>10 m) a marked long-term decrease of the native amphipod Monoporeia affinis has been noted and is presently associated with the establishment of the polychaete. One plausible explanation is that the polychaetes and the amphipods are competing for food resources as both are deposit feeding animals and likely to share similar food resources. Interspecific competition for food between the polychaete and the amphipod was studied in a laboratory experiment. Two year classes (0y+,1y+) of the amphipods were kept at various densities, with and without added food resources, with and without the polychaete, in microcosms with sediment and continuous supply of cooled water for 2 months. The polychaetes did not have any effect on mortality in the amphipods. 4-way ANOVA showed that food addition, density of amphipods and presence of the polychaete had a significant effect on the growth of amphipods of different age classes. 1y+ amphipods showed increased growth with added food and this increase was density-dependent in the absence of the polychaetes but not in their presence. The polychaetes reduced the growth of 1y+ amphipods at natural densities (2000 ind m−2) by 60%, but had no clear effects on the growth of juveniles. It is concluded that lower amphipod growth in the presence of M. viridis was caused by competition for food and is likely to affect the population of M. affinis in deep soft-bottom habitats of the northern Baltic Sea.