ULTRASTRUCTURAL-STUDY OF MATURE SPERM OF POMPHORHYNCHUS-LAEVIS MULLER(ACANTHOCEPHALA, PALAEACANTHOCEPHALA), A FISH PARASITE

Ultrastructural observations on mature sperm of Pomphorhynchus laevis (Muller, 1776), the dominant acanthocephalan parasite of fish from the River Brenta, were carried out by scanning (SEM) and transmission ele ctron microscopy (TEM). Adult males of P. laevis were obtained from th e digestive tract of Leuciscus cephalus (pisces: Cyprinidae). The sper matozoon of P. laevis is inverted with respect to conventional sperm m odels; it is filiform and consists of an anterior flagellar apparatus and a posterior nucleocytoplasmic derivative. The latter includes a la yer of condensed chromatin beneath the plasma membrane opposite to the flagellum, numerous large subspherical electron dense inclusions, gly cogen deposits and a pentalaminar structure next to the flagellum. The nucleocytoplasmic derivative terminates caudally with a flat lamina d evoid of the dense inclusions and glycogen deposits. Proceeding throug h the posterior lamina, the 9+2 axoneme progressively disorganizes. Th e ultrastructural organization resembles that reported in other acanth ocephalan species, although the number of central axonemal microtubule s does not vary. Hence the atypical 9+n axonemal pattern with 0<n<5 is still only found in the eoacanthocephalan sperm.