Anatomy of finfish and shellfish

Unit 13 - Mollusca

13.2.16. Reproductive system
The freshwater clams are dioecious, i.e., the sexes are sepa¬rate, but there is no sexual dimorphism. Reproduction involves a parasitic larval stage, called the glochidium in freshwater clams and the veliger in marine forms.
Gonads
The gonads, either testes or ovaries, are a pair of large, simple, racemously branched structures, lying among the intestinal coils in the visceral mass just above the foot. During breeding season, the gonads become greatly enlarged and conspicuous, when it is difficult to distinguish their paired nature. When mature, the testes are whitish and the ovaries reddish, and the two sexes can be distinguished though not very distinctly. The coelornic epithelium, lining the tubules of the gonads, gives rise to the spermatozoa in male and ova in female. The mature ovum is large, rounded, filled with a finely granular cytoplasm rich in yolk, and containing a vesicular nucleus with nucleolus.
The gonad of each side has a short duct, the vas deferens in male or the oviduct in female. It leaves the gonad from its upper side and opens into the supra-branchial chamber of the inner gill lamina by a genital aperture, just in front of the renal aperture of the ureter. There are no accessory reproductive organs.