The biology, functional morphology and taxonomic status of Gaimardia (Neogaimardia) finlayi (BivaMa: Gaimardiidae)

Gaimardia ( Neogaimardia ) finlayi Powell 1933 is a member of the little known family Gaimardiidae which collectively have an Australian, New Zealand and sub-Antarctic distribution. N. finlayi nestles amongst algal holdfasts at the bottom of wave-exposed beaches in New Zealand. It is a small, weakly heteromyarian dioecious bivalve attached by a relatively stout byssus. The mantle and the siphons are exceedingly simple; mantle fusions, other than anterior and posterior to the internal ligament, are of the inner folds only. The siphons are non-extensible wide openings with few sensory papillae. Each ctenidium comprises an inner demibranch only with no inter-lamellar and but a few inter-filamentar junctions. Within the ctenidium large telolecithal eggs are incubated to a crawling, shelled stage of approximately 0–75 mm in length. Sperm were found in the ctenidium of two mature males. The possible storage of sperm and eggs and the incubation of fertilized eggs have survival value for a small bivalve living in a difficult environment.
The modifications which suit N. finlayi to its mode of life are discussed, but major interest centres around the taxonomic position of the Gaimardiidae. It is concluded that the Cyamiidae and Gaimardiidae are very closely related, but more comprehensive studies on representatives of both families are needed.