THE FUNCTIONAL MORPHOLOGY OF THE ANTARCTIC BIVALVE THRACIA MERIDIONALIS SMITH, 1885 (ANOMALODESMATA: THRACIIDAE)

The functional morphology of the Thraciidae is poorly understood. Although some morphological aspects of several members have been described, only Trigonothracia jinxingae from Chinese waters is known in detail. Thracia meridionalis is the only representative of the family in Antarctic waters, and is common in Admiralty Bay, King George Island, where it inhabits muddy sediments. Thracia meridionalis shares many features with Tr. jinxingae that are typical of most Anomalodesmata, i.e. a secondary ‘ligament’ of thickened periostracum, extensively fused mantle margins, ctenidia of type E, a ctenidial-labial palp junction of category III, a stomach of type IV and simultaneous hermaphroditism. Thracia meridionalis is, however, strikingly different from Tr. jinxingae in a number of ways, such as the presence of a fourth pallial aperture, statocysts of type B3, heterorhabdic ctenidia, direct communication between the mantle chambers, a deep-burrowing habit (individuals lying on the left shell valve), siphons that retract into mucus-lined burrows, a stomach with extensive sorting areas, a rectum which passes over the kidneys and separate male and female gonadial apertures. There is, therefore, a greater range of morphological diversity within the Thraciidae than previously suspected.