The biology, functional morphology and taxonomic status of Fluviolanatus Subtorta (Bivalvia: Trapeziidae), a heteromyarian bivalve possessing “Zooxanthellae”.

Abstract

biology and functional morphology of Fluviolanatus subtorta is described. The species occupies coastal lagoons along the eastern coast of Australia and is widespread, in estuafies, throughout the continent. The shell is heteromyarian in form with a reduced anterior slope and a long, thin, internal ligament overlain by fused periostracum. The hinge plate possesses no hinge teeth and there is no byssal notch. The thin shellcomprises two layers: an outer crossed lamellar layer and an inner complex crossed lamellar layer. Ventral mantle fusion involves the inner folds only and the siphons are short with few papillae. The large ctenidia comprise both lamellae and the ciliation is of Type C (1) (Atkins 1937). The labial palps are small. The style sac and mid gut are initially conjoined but soon separate. The stomach is of Type V (Purchon, 1960) and adapted for dealing with large amounts of fine particles. The life style is discussed.

Hitherto, Fluviolanatus has been located in the Mytilacea. This study shows that this species is a typical eulamellibranch and after comparison with heteromyarian representatives of known heterodont families it is concluded that the genus should be more correctly located in the Trapeziidae (Arcticacea), its closest relative being the coralassociated nestler Coralliophaga.

Major interest centres around the greatly enlarged inner and outer folds of the vental mantle margin. These possess large numbers of what appear to be vegetative algal cells. It has proven to be impossible to obtain further specimens for more detailed study but the cells are some 8u in diameter and have all the characteristics of “zooxanthellae” (e.g. Symbiodinium microadriaticum) typically symbionts of, among others, hermatypic scleractinian corals, gians clams of the genera Tridacna and Hippopus and the cockle Corculum. The possible nature of this symbiosis between Fluviolanatus and its “zooxanthellae" is discussed. More research is called for on this unusual bivalve.