Studies on gametogenesis and fecundity in the lampreys Mordacia praecox and M. mordax (Petromyzonidae)

Abstract

The pattern of histological changes occurring during gametogenesis in the non-parasitic lamprey, M. praecox, is described and compared with that of the closely related parasitic species, M. mordax. The gonads of M. praecox undergo extensive development during, and immediately after, metamorphosis whereas only relatively slight changes occur at the same stage in M. mordax. Oogenesis in M. praecox is similar to that found in northern-hemisphere lampreys except that the advanced oocytes are surrounded by a single, rather than a double, vitelline membrane. Spermatogenesis in M. mordax differs in that spermatids are produced continually over a period of several months. The ultrastructure of the advanced spermatid of M. praecox reveals the presence of an intranuclear filament and a parallel orientation of the centrioles similar to that described for the northern-hemisphere species, Lampetra planeri. Histological evidence indicates that M. praecox breeds in late winter or early summer. The limited data available suggest a similar breeding season for M. mordax. Oocyte counts from transverse sections of ammocoetes are greater for M. mordax than M. praecox, although some overlap occurs. Massive atresia of oocytes, after the onset of metamorphosis, results in the absolute fecundity of M. praecox being substantially less than that of adult M. mordax.