Early detection of gender and sex ratio in Calanus finmarchicus fifth copepodites

J. Crain, C. Miller and M. Miller

We have examined Calanus finmarchicus fifth copepodites from samples taken throughout the year, distinguishing five gonadal development types. Two morphological facies are early stage gonads and are positively correlated with postmolt jaw facies. The first type, thin elongate rudiment (TER), is a long thin gonad lying along the dorsal surface of the oil sac in live animals. The second type, short wide rudiment (SWR) is a small rounded cell mass suspended by membranes well above the oil sac. Both have uniform cell size, and are distinguished from later, developing gonads by the anterior end being thinner than the posterior end. Gonads with the three remaining facies are beginning to mature. The least developed of these three types (DEV), unlike the two early gonad types, has an enlarged anterior end. It retains uniform cell size, but is beginning to take on the pear- or vase-shape typical of mature gonads. The last two facies, representing developing ovaries (DEVF) and testes (DEVM), are differentiated based on relative cell sizes and, in late stages, presence of oviducts or vas deferens.

All of the gonad types we have described were present in spring of 1998 in the Georges Bank region and in the emerging stock in December 1998. All types were present in the resting stock in October1998, although TER and SWR facies predominated.

Comparisons between gonad type, jaw phase and oil sac fullness were made for individual fifth copepodites. Results show that new jaws begin to form under the existing integument at a later point in the stage than gonad development. Seasonal comparisons of oil sac fullness for each gonad type from January through June, 1998 show that in June, individuals of all gonad types had fuller oil sacs than in January through May.