DEVELOPMENT OF THE TESTIS DURING POST-METAMORPHIC LIFE IN THE SOUTHERN-HEMISPHERE LAMPREY GEOTRIA-AUSTRALIS GRAY

Spermatogenesis during post-metamorphic life in the anadromous parasitic lamprey Geotria australis was examined in histological sections of tests of animals collected in the field or maintained in the laboratory. The fully metamorphosed downstream migrants of G. australis possessed a very small tests in which the germ cells consisted of irregularly arranged prospermatogonia. By the end of the subsequent protracted marine trophic phase, the testis had increased in cross-sectional area by about 20 times and become organised into well-defined cysts. Since approximately 40% of these cysts possessed spermatocytes and 12% contained spermatids or early sperm, meiosis is initiated prior to the non-feeding upstream migration, which in G. australis lasts for the unusually long period of 15-16 months. The incidence of cysts with meiotic and post-meiotic stages decreased during the first 3 months of the spawning run before starting to rise during subsequent months. Spermiogenesis was never completed in any animals caught in the field during the first 9 months of the upstream migration or in those subsequently held in the laboratory up to and beyond November, the month when spawning naturally takes place. The phasing and asynchrony of spermatogenesis during post-metamorphic life in G. australis differs markedly from the situation in Northern Hemisphere lampreys.