Sex change and female dimorphism in Calanus finmarchicus

Abstract

Adult females were examined from November until spring in northern Norway (70°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 with 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 Grotsund 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.