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