Seasonal variability in gonad and jaw
development in Calanus finmarchicus fifth
copepodite: comparisons with oil sac fullness
J. Crain, C. Miller and M. Miller
The ability to determine sex ratios from developing copepodites
collected in the field has been limited in the past to observations
of overall gonad shape and size. This approach has been useful in
distinguishing developing male and female fifth copepodites, but is
limited to individuals in the final stages of gonad development. We
have refined the techniques by categorizing the early stages of
sexual differentiation in the gonad based on gonad shape and cell
size. In Calanus, the first meiotic division of cells in
the gonad occurs well before the gonad has begun to take on the
characteristic shape of an ovary or testis. Cell size can be easily
seen after staining the gonads and clearing the integument and
somatic tissues. We have been able to recognize developing testes
and ovaries at an early stage based on observations of relative cell
size anterior and posterior to the zone where the first meiotic
division takes place.
We have used this technique to make preliminary determinations of
the sex ratio in developing C. finmarchicus fifth
copepodites on and around Georges Bank during the 1998 GLOBEC
Broadscale Survey sampling period. These data are compared with
monthly sex ratios calculated from male and female abundances from
the same sample set. Comparisons between sex ratios generated from
early versus late stages of gonad development and insights into the
timing of sex determination in Calanus are briefly
discussed.