Serotonin (5-Hydroxytryptamine) in the Male Reproductive Tract of the Spiny Dogfish

Abstract

IN elasmobranch fishes, in which fertilization is internal, the caudal portion of the pelvic fin of the male is prolonged to form a stout rod-like copulatory organ or ‘clasper’. The clasper is rolled up in a scroll-like fashion, enclosing on its medial side the so-called clasper groove or tube, along which at copulation the spermatozoa pass from the urogenital papilla of the male to the oviducts of the female. The clasper groove communicates with an accessory organ, located under the skin of the abdomen, which in the shark and dogfish usually takes the form of a muscular sac, the so-called clasper siphon, whereas in the skate it assumes the form of a solid glandular organ, the so-called clasper gland. Leigh-Sharpe¹, who studied in detail the comparative anatomy of secondary sexual characters of elasmobranch fishes, came to the conclusion that the clasper siphon of the Selachii acts as a “reservoir of sea-water” which is emptied at copulation by muscular contractions of the siphon wall, and in this way helps to pump the spermatozoa from the clasper groove into the oviducts. Gilbert and Heath², on the other hand, expressed the opinion that the siphons may actually have some secretory function, and contribute a, part of the seminal plasma.