On the ecology of theJaera albifronsgroup (Isopoda)

Abstract

In field investigations on the significance of the surface and colour of the substratum for the distribution of Jaera albifrons syei Bocquet and J. ischiosetosa Forsman on stony shores, the tendency was that light stones with a rough surface were preferred to dark smooth stones. The occurrence of an algal or bacterial film is of importance for the distribution of the two species. Stones with an algal film were preferred to clean stones. This was demonstrated by experiments in the field and in aquariums. In salinity tolerance expriments (0–75 ‰ S) J. albifrons Leach from the North Sea (34 ‰) and J. albifrons syei and J. ischiosetosa from the Baltic (6 ‰) had all a better resistance in salinities around 30 ‰ than in salinities below 15 ‰ and above 40 ‰. J. albifrons could withstand a salinity as high as 60–75 ‰ for at least 8 hours. When the animals were kept in environments with different amounts of oxygen, different salinities and temperatures, the shortage of oxygen soon became a limiting factor, especially at high temperatures (20° C) and low salinities (3.4 ‰). When the oxygen supply was good, the high temperature and low salinity did not have as great an importance. When subjected to light in an alternative chamber with light and dark sections the dark ones were significantly preferred. In aquarium experiments the animals chose the dark side of the stones irrespectively of the geotaxis.