Studies on some intracellular parasites of the marine bivalve, Tellina tenuis (Da Costa)

Abstract

This study is divided into four sections beginning with a consideration of the effects of an hitherto undescribed coccidian parasite of the ovary of this bivalve on the dynamics of a particular population from Kames Bay, Millport, Isle of Cumbrae, Scotland. The parasite was found to bring about complete or partial castration of female Tellina tenuis but had no effect on males. The general biology of Tellina-tenuis is reviewed and discussed in relation to observations that were carried out over one year on the age structure, growth, density, reproduction and degree of parasitization of this bivalve. The population parameters were found to have changed little over the last fifty years. There was not found to be any significant change in the condition index that could be related to the destruction of the gonad by the parasite. This is the first description of a coccidian parasite of the ovaries of any marine mollusc. The second section describes the life cycle and developmental stages of the coccidian parasite. The parasite is believed to be monoxenous with sporogony and anisogamy orcurring in the ovary of the host. Endogenous stages were observed in the primary germ cells of the gonadal follicles. A diagnosis is presented based on the number of sprozoites per sporocyst and sporocysts per oocyst. The name Mcrocystis tellinovum (sp. nov. ) is proposed for this coccidian. The genus Merocystis belongs to the family Aggregatidae within the sub-order Eimeriorina of the order Eucoccidiorida. An ultrastructural study of both sexual and asexual stages is presented in support of this diagnosis. The third section of this study is an investigation of the observation that a large proportion of the Tellina tenuis examined histologically contained inclusion bodies within the secretory cells of the digestive gland. These inclusions were comprised of dense masses of pleomorphic mycoplasma-like organisms. The first part of this section describes the morphology and ultrastructure of these organisms and the histopathological effects on the host digestive gland. A description of the normal digestive gland iss-given including observations of the cyclic changes in appearance that take place in response to the influence of tidal rhythms. This is believed to be the first description of a mycoplasma-like organism from a marine invertebrate. The fourth section is concerned with a series of experiments to determine the nature of a virus seen in association with the mycoplasma-like organism. The virus was isolated from the host cells by density gradient centrifugation and its morphology was compared with a second virus isolated by Hill (1975) through the medium of a fish cell culture. It was found that these two viruses were quite distinct from each other. Attempts were made to propagate both viruses in an established cell line from the Atlantic salmon and the results are described and discussed.