Seasonal observations on moulting and maturation in Stage V copepodites of Calanus finmarchicus from the Firth of Clyde

Abstract

Stage V copepodites of C. finmarchicus collected from the Firth of Clyde during April-Aug. 1978 and Oct. 1979-Jan. 1980 molted to adults in the laboratory. The interval between capture and ecdysis was longer on average and less variable during the months July-Nov. (when overwintering stocks build up in the Firth) than April-June. Overwintering Stage V copepodites molted earlier in the laboratory than in the sea; this is not associated with differences in environmental factors known to affect developmental rate and may have been induced by stress associated with capture or handling. A similar induction of molting is not evident in spring and summer generations. Changes in the gonad are associated with the molt to the adult. Throughout the year most females observed during the 2 days following ecdysis had oocytes in anterior oviducal diverticula and most males had a spermatophore forming in the spermatophore sac. Comparative observations on gonads before ecdysis indicate an intrinsic coordination between the release of oocytes from the ovary, or spermatophore formation and the molt cycle in copepodite Stage V. These processes probably are under hormonal control. As in both sexes, precocious molting was accompanied by a corresponding precocious maturation of the gonad, overwintering Stage V copepodites probably were in a state of dormancy, which is under hormonal control. Observations on Stage V copepodites kept without food indicate the presence of energy reserves at all times of the year that can be mobilized for molting, associated morphological changes, maturation of the male reproductive system and early stages of ovary maturation. As is usual in Calanus, proximate feeding conditions influenced the rate of release from the ovary and the growth of oocytes in adult females that had molted precociously from overwintering Stage V copepodites. Dormancy in overwintering Stage V copepodites of Calanus in the Firth of Clyde probably is an adaptation that minimizes the untimely production of young in response to feeding conditions that may be unsuitable for early instars.