Aspects of the biology of Thyasira Gouldi (Philippi) and its copepod parasite Axinophilus Thyasirae (Bresciani and Ockelmann).

Abstract

Taxonomic differences exist between Thyasira gould! and T. flexuosa both in conchological features and in the soft parts, perhaps the best of these differences are those concerning the sperm which are easily seen even in badly curated specimens. The occurrence of a population of T. gouldi within Loch Etive on the west coast of Scotland extends the known distribution of this arctic bivalve considerably further south. The development is nonpelagic and this, associated with the non-synchronized reproductive activity , means that independence is gained from plankton blooms and other external triggers allowing the characteristic dense but patchy populations to be built up. Salinity and sediment particle size both limit the distribution of T. souidi within L. Etive, the VI naturally occurring sediment is altered by the inhabi tants of the area into a sediment vlhich is better suited to the needs of T. gouldi. Asynchronized breeding, slow growth rate, longevi ty of life and 10\'1 adul t mortality all combine to give a characteristic bimodal population, the modes of which do not appear to shift with time. A method is described whereby an indication of both the mean state of the population and the state of the individual can be obtained for any period of the year. The biochemical composition varies with size and time of the year as well as with gonad state. Dry weight fluctuations are, however, not entirely explained by variations in the stage of gonad development as seasonal variations in the amount of somatic tissue do exist. Thyasira gouldi is one of the Lucinaceans infected by the much modified copepod parasite Axinophilus thyasirae. The life cycle and infective stage of this parasite are described as is the external morphology of the nauplius copopedite and adult. The reproductive system of both the male and the female are also described. The first infected parasite is always female and only rarely does more than one female reach maturity within anyone host even though up to five parasites may be present. The parasite has been shown to affect the biochemical composition of the host, reducing the amount of each of the components present, but tending to reduce the amount of nitrogenous material in preference to the amount of carbohydrate. VII The parasite affects the gonad of the host resulting in castration probably as an indirect effect of 'food robbing'. The interference to the host's food supply by the parasite is thought to be so great as to reduce the host to starvation levels of food,