THE DETERMINANTS OF PRODUCTION IN NORTHERN SEAS: A STUDY OF THE BIOLOGY OF THEMISTO LIBELLULA MANDT.

Abstract

The level of production in the sea is determined not only by the extrinsic factors of the environment, but also by the intrinsic response of the organism to those factors, temperature in particular. In cold seas, the degree of adaptation to low temperature as regards activity, metabolism, growth rate, and the onset of maturity is therefore of great importance. This matter is reviewed and discussed here, and illustrated by a study of the growth rate, breeding cycle, and longevity of a dominant arctic pelagic amphipod, Themisto libellula Mandt.Themisto libellula, in Hudson Bay, Hudson Strait, and the waters of southeast Baffin Island, is shown to have a normal length of life of 2 years, although a 1-year or 15-month life cycle is a possibility in a small proportion of the population. Material from the months of November and December is lacking, but it appears that spawning extends from September through the winter until some time in May or June. If a temperature stimulus is involved in the onset of breeding, it appears to be a fall below, not a rise above, a critical point, since the temperatures during most of the breeding period are below −1.5 °C. The spawning in the autumn, however, suggests that temperature is in fact not involved.The alternating, or two-phase, breeding cycle is confirmed in this species, but the evidence shows that dislocations in the cycle (owing to the possibility of maturation after approximately one year in some individuals) may destroy the supposed reproductive isolation between adjacent year-classes.