The Pelagic Phase in the Life History of the Amphipod Genus Bathyporeia

Abstract

The zonation of four species of the genus Bathyporeia, from high-water mark to low-water mark, in Kames Bay, Millport is given. B. pilosa occurs above the high-water mark of neap tides, B. pelagica is a mid-tidal form and B. elegans a low-water form. The latter, together with B. guilliamsoniana, extend beyond low-water mark. Of the population, as it occurs in the sand, 15 % are adult males, 26 % adult females, giving a proportion of adult to immature forms of 4 1 : 59.The analysis of forty-five tow-net samples taken across the bay at night shows that the same zonation of the species is maintained; the proportion of adults to immature forms in these samples, however, is as 57: 43, and of the 57 % of adults, 42 % are adult males, 15 % adult females.When the samples are related to the day of the lunar month it is shown that the numbers increase in the tidal waters in the periods immediately preceding the full moon and preceding and partly overlapping the new moon. A nocturnal vertical movement appears to be a marked feature of a few benthic amphipdd families, but the factors which cause this migration are problematical and require much further investigation.