Seasonal variation of male-type antennular setation in female Calanus finmarchicus

Abstract

Female calanoid copepods of the family Calanidae can mature with either the juvenile and most typical female pattern of setation, termed trithek (T), or with the male pattern, termed quadrithek (Q). It was suggested by Fleminger (1985), but not proved, that females with the Q pat- tern have switched from development as males to development as females after initiation of seta formation prior to the maturation molt. Such switching, if it occurs, has population dynamical con- sequences. Therefore, we determined the fractions of Q females in the population of Calanus fin- marchicus from Georges Bank and nearby deeper water during each month of the animals' growing season, January through July of 1995. Based on >780 individuals examined from each month, the fraction of Q individuals was ~20% in January, decreased to 8.5% in April, rose to 12.5% in June and was 9% in July. This suggests that the individuals of the several seasonal generations select repro- duction as females (or conversely as males) at different rates. Smaller samples from January, April and June of 1996 showed the same pattern. A similar study in a northern Norwegian fjord with 1 annual generation (Svensen & Tande 1999) showed 38% Q females in February, early in maturation, and 25% Q in March. If proportions of individuals selecting male or female reproduction are some- how optimized in response to habitat circumstances, then variation in fraction of Q females implies that the optimum is different in different seasons and latitudes. It remains to explain the implied differences in the reproductive value (in the sense established by Fisher 1930) of female versus male reproduction in Calanus.