Spawning and management of gametes, fertilized eggs and embryos in Siluroidei

Abstract

The structure of gonads and gametes is highly diversilied in siluroid fishes; in some groups the testes are composite with an anterior part formed of spermatogenetic tissues and the posterior part of seminal vesicles which may or may not store spermatozoa. For catfish species of aquacultural interest, ovaries present the same general morphology as in other teleosts with oviducts. Spermatozoa are biflagellated in the Channel catfish (Ictalurus punctutus) and monoflagellated in the European catfish (Silurus glanis) and African catfishes (Clarias gariepinus, Heterobranchus lingijilis). The egg size shows considerable intergroup differences, from 0.8-1.2 mm in some Pimelodidae and Clariidae up to 15-20 mm in Anidae. In numerous cases, the eggs are adhesive and develop either a surrounding sticky layer (Ictulurus, Chrysichthys, Silurus) or an adhesive attachment disc (Clarias, Heterobranchus). Fertilization is in general extemal but intemal fertilization is reported in some species. The annual sperm production was measured in European catfish and Channel catfish as 1.7 x 10 and 1.8 x 10 spermatozoa.kg-' body weight, respectively. In females, the fecundity ranges from about 50 eggs per spawn in ariids up to more than 100000 eggs.kg-' body weight, e.g. in pimelodids and clariids. A large variety of controlled reproduction systems are found in the siluroids. In some cases (Ictulurus, Chrysichthys, Hoplosternuin), spawning oceurs naturally in ponds or tanks provided that adequate spawning substrates are available; fertilized eggs are collected immediately after spawning and placed into incubators until hatching. But for most species, although natural or seminatural spawning could be achieved in captivity, the tendency at the present time is to develop techniques using hormonal-induced ovulation and artificial insemination in order to control the various steps of reproduction and to allow gamete preservation and manipulation. Oocyte maturation and ovulation or spermiation can be induced in many species by a large variety of hormones (GnRHs, fish gonadotropins, hCG, vanous steroids etc). Some information is available on gamete biology and preservation. Sperm motility is short-lived, not exceeding 70-80 s of fonvard movement as in most other freshwater teleosts. A peculiarity of the European catfish sperm is the activation by urine during sampling which could be prevented by direct collection in an immobilizing solution (NaCI 200 mM, Tris HCl 30 mM, pH 7.0). Spermatozoa cryopreservation was successfully attempted in several species. Ova generally survive only a few hours after ovulation and fertilization must be camed out soon after. Methods for gamete collection, insemination and incubation of eggs are descnbed for the most widely cultured siluroid species.