A histopathological survey was performed to search for the cause of high mortality of the clam Venerupis rhomboides (Pennant) in exploited beds of the Ensenada de Riveira (Ria de Arousa, Galicia, NW Spain). V rhomboides from 2 beds affected by high mortality, Airos and Coroso, and a 3rd non-affected bed. Agudos, were sampled in spring and autumn of 1996. In addition, clams of the species Venerupis pullastra, with unnoticeable mortality, were taken from Airos during autumn sampling. According to prevalence, infection intensity and associated histopathological signs, a branchial rickettsia-like organism was the only pathogen that could be tentatively blamed for the mortality. Spherical to elongated intracytoplasmic rickettsia-like colonies up to 25 pm in length were observed at the base of gill filaments of the clams. Transmission electron microscopy study permitted identification of the micro-organisms in the colonies as rickettsia-like. Individual prokaryotes measured about 0.5 to 0.8 pm in diameter and up to 3 pm in length. The infection process resulted in extreme hypertrophy and lysis of host epithelia1 cells Infection intensity was rated for each clam and comparison among highmortality-affected and non-affected populations indicated the branchial rickettsia-like Infection as the probable cause of the high mortality.

The ultrastructure of the mature spermatozoa and spermatogenesis of the bivalve Scrobicularia plana are described. Support cells extend from the basal lamina to the lumen of the testis and are laterally connected to the germinal epithelium. Germ cells present intercellular bridges and flagella since the spermatogonial stage. While spermatogonia and spermatocytes appear connected to support cells by desmosome-like junctions, elongated spermatids are held at the acrosomal region by support cell finger-like processes. During spermiogenesis, the acrosomal vesicle differentiates from a golgian saccule and then migrates to the nuclear apex. A microtubular manchette arising from centrioles surrounds the acrosomal vesicle, the nucleus, and the mitochondria at the time these three organelles start their elongation, disappearing after that. The mature spermatozoon of S. plana lacks a distinct midpiece because the mitochondria extend from the region of the pericentriolar complex along the nucleus anteriorly for approximately 1.4 microns. The features of this bivalve type of modified spermatozoon are compared with those of other animal groups having similar modifications.

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Light and transmission electron microscopy were used to study different stages of Perkinsus atlanticus (Apicomplexa) during induced zoosporulation, with fluid thioglycollate medium and seawater. Cytokinesis and nucleokinesis of different developmental stages were studied every 12 h during the incubation period of 72 h, at which time the zoospores became free. Uninucleated and flagellated zoospores present the apical complex formed by conoid, polar ring, micronemes, rhoptries and subpellicular microtubules observed at different sections. Ultrastructural details were compared with the other two species of the genus Perkinsus.

Haplosporidian spores from Ostrea edulis, previously described as Minchinia armoricana, were reexamined by light and electron microscopy. These spores were either free among host cells or enclosed in sporocysts. They contained two long epispore cytoplasm extensions (ECE), each possessing cytoskeletal structures corresponding to the filaments. After lysis and degradation of the ECE, these extensions disappeared and the spores became devoid of membrane-bound extensions. However, 2 long filaments (approximately 130 microm long) persisted that were closely attached, in opposition to the spore wall, by a bundle of 9-13 fibers each. Thus, we propose a revised description of M. armoricana (= H. armoricana) to confirm its placement in the genus Haplosporidium as H. armoricanum.

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