Burying-depth and trematode infection in macoma balthica

Abstract

In the Corophium-zone about 20% of Macoma balthica present makes feeding tracks indicating they were deposit feeding; no feeding tracks are found outside the Corophium-zone. The infection rate in summer of Macoma by a gymnophallid trematode was studied. Large specimens are infected to a larger percentage than small ones. In the course of the summer an increasing mortality occurred amongst large infected specimens. Infected Macoma was mostly found in the higher part of the littoral zone. At the level of 30 cm below N.A.P. all size classes (11 to 18 mm) were buried equally deep, and much deeper than at the highest level (60 to 40 cm above N.A.P.) where the small specimens were buried deeper than the large ones. These differences in depth are thought to be influenced by differences in oxygen content of the sediment and in the risk of being washed out. Macoma (11 to 21 mm) transplanted within the highest zone bury down to original depth. Macoma (of 15 mm) transplanted from the highest zone to a lower zone in the littoral, bury just as deep as the specimens already present in that lower zone. This is deeper than in the higher zone. Infected Macoma buries just as deep as non-infected Macoma. Macoma did not perform horizontal movements in the substrate in the highest zone of the littoral. In the highest littoral zone at low water in daytime 80% of Macoma is vertically oriented in the mud, 20% non-vertically. At a lower level the percentage of non-vertically oriented Macoma increases (mean 50%).