Burrowing and crawling behaviour of three species of unionidae in Finland

The aim of this study was to examine the burrowing and crawling behaviour of three unionid clam species in the field. Of the four study lakes in southern Finland, Anodonta piscinalis occurred in all, Pseudanodonta complanata in three and Unio pictorum in only one. In all three species, there was a positive correlation between the height of the above-sediment part of the shell and the burrowing angle (deviation from the horizontal position). When surfacing, the position of the clam became more vertical. Anodonta piscinalis was more visible in June than in August in one population, while in other populations monthly differences were not found. The result suggests that A. piscinalis may be more visible during its spawning period in early summer (June) than in late summer. This behaviour could have a reproductive function, ensuring external fertilization. Monthly differences in crawling activity of A. piscinalis were also found in one population: in Lake Alvajarvi, A. piscinalis crawled more in August than in June. No monthly differences in the burrowing or crawling of P. complanatawere found. In Unio pictorum, no differences were observed in crawling, but the clams were more visible in August than in June. When compared with P. complanata, A. piscinalis and U. pictorum were more visible and in a more vertical position. In A. piscinalis, the bigger individuals were more visible than the smaller individuals, but this was not the case in P. complanata and U. pictorum.