Sex change and female dimorphism in Calanus finmarchicus

Abstract

ABSTRACT. Adult females were examined from November until spring in northern Norway (30" N) to determine the fraction with the quadrithek and the fraction with the trithek setal pattern on the antennule. The quadrithek (Q) pattern is a male characteristic in which odd numbered, proximal segments carry 2 aesthetascs and 2 setae; the trithek (T) pattern has 1 aesthetics on all antennular segments. The frequency of quadrithek females (%Q = 100 X [Q/T]) varied with season. It peaked at 38% in February, declining to 25 % in March. Higher proportions appeared early, consistently between years, i.e. higher proportions were found among the first females emerging from the stock of resting copepodids. In light of the hypothesis of Fleminger (1985; Mar Biol 88:273-294) that females w t h the quadrithek pattern result from sex switching by genetic males, we tested whether %Q in newly moulted females could be altered by separate exposure of the maturing CVs to adult males or females. The experiments were run with specimens from Gratsund during the last 3 wk prior to moulting of CVs in the field. Of females not exposed to adults as CVs, 35% had quadrithek patterns, while those exposed to adult males or females produced 33 and 37% quadritheks respectively. In these experiments, presence of adult Calanus finmarchicus (Gunnerus) did not influence the proportion of females with male characteristics. No evidence for sex specific pheromones was found and therefore the stimulus for sex change was not revealed. A possible influence of the environment on C. finmarchicus sex determination (ESD) is suggested and discussed.

Sex, sex-ratios, and the dynamics of pelagic copepod populations

“…Other mechanisms of sexratio control may be invoked. There is circumstantial evidence that copepods of the family Calanidae may change sex in the course of development (Flemminger 1985;Svensen and Tande 1999;Irigoien et al 2000;Miller et al 2005), as has been demonstrated in other copepods (see review by Miller et al 2005), and that sex-determination may be under environmental control (reviewed by Mauchline 1998). The pronounced seasonal variation in sex-ratio, exceeding the expectation based on a simple population dynamics model, makes further examination of sex-determination in pelagic copepods potentially rewarding.…”

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

“…), where there are 2 strong generations each year, showed 66% Q females (N = 63) in March, 4%Q (N = 52) in May. The initial fraction exceeds Svensen & Tande's (1999) February result, and the seasonal change in fraction is in the same direction and greater than that at Georges Bank. Both observations strengthen the conclusion that sex determination varies with differences in habitat, suggesting that it is an adaptive mechanism.…”

“…Svensen & Tande (1999) determined the fraction of Q females of C. finmarchicus in Malangen Fjord in northern Norway near Tromsø, where more than 95% of the stock completes only a single life cycle in the year (Tande 1982). They found 38% Q in February (early maturers) and 25% in March.…”

A synoptic sampling method applied to Calanus finmarchicus population on the Norwegian mid-shelf in 1997

“…Due to the short period simulated, we also assumed zero mortality. The sexratio was set to 50%, indicating that moulting CVs split equally into male and females at this time of the year (Svensen & Tande 1999).…”