Sex change and female dimorphism in Calanus finmarchicus

Adult females were examined from November until spring in northern Norway ( 70 degrees N) to determine the fraction with the quadrithek and the fractio n with the trithek setal pattern on the antennule. The quadrithek (Q) patte rn is a male characteristic in which odd numbered, proximal segments carry 2 aesthetases 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 Mar ch. Higher proportions appeared early, consistently between years, i.e. hig her proportions were found among the first females emerging from the stock of resting copepodids. In light of the hypothesis of Fleminger (1985; Mar B iol 88:273-294) that females with the quadrithek pattern result from sex sw itching by genetic males, we tested whether %Q in newly moulted females cou ld be altered by separate exposure of the maturing CVs to adult males or fe males. The experiments were run with specimens from Grotsund during the las t 3 wk prior to moulting of CVs in the field. Of females not exposed to adu lts as CVs, 35 % had quadrithek patterns, while those exposed to adult male s or females produced 33 and 37 % quadritheks respectively. In these experi ments, presence of adult Calanus finmarchicus (Gunnerus) did not influence the proportion of females with male characteristics. No evidence for sex sp ecific pheromones was found and therefore the stimulus for sex change was n ot revealed. A possible influence of the environment on C. finmarchicus sex determination (ESD) is suggested and discussed.