LUCINIDAE

Taylor, John D. & Glover, Emily A., 2006, Lucinidae (Bivalvia) - the most diverse group of chemosymbiotic molluscs, Zoological Journal of the Linnean Society 148 (3), pp. 421-438 : 423-425

THE LUCINIDAE

Shells of Lucinidae are usually white, subcircular in outline and range from discoidal to subspherical in profile. Living species range in height from around 3.0 − 140 mm. External sculpture mainly comprises variations of commarginal lamellae and radial ribbing is usually absent or subordinate. Posterior sulci are often present. However, many lucinids have smooth, unornamented shells. Ligaments range from long external to short internal. There are usually two or less cardinal teeth in each valve, with lateral teeth present in some taxa. In many lucinids hinge teeth are highly reduced or completely absent. One of the most distinctive features of lucinids is the anterior adductor muscle scar. The adductor muscles are usually unequal in size, and most (but not all) lucinids possess a distinctive elongate, anterior adductor muscle scar that diverges inwards from the pallial line. There is no posterior pallial sinus. Shell microstructure usually consists of three layers, an outer spherulitic prismatic layer, a middle layer of crossed-lamellar structure and within the pallial myostracum, an inner complex crossed lamellar layer often with intercalated prismatic sheets ( Taylor, Kennedy & Hall, 1973).

Anatomically, lucinids have a number of distinctive features ( Fig. 4 ). Ctenidia are usually large, comprising inner demibranchs only; the filaments are thick with a narrow, outer ciliated zone and an extended abfrontal zone comprising bacteriocytes, intercalary cells and mucocytes. Distal portions of gill filaments are often fused into cylindrical channels ( Distel & Felbeck, 1987). Labial palps are highly reduced, consisting of small folds at the edge of the lips. Posterior inhalant and exhalant apertures are present, the latter with an eversible tube. The foot is elongate, usually cylindrical and highly extensible with a differentiated, ciliated and glandular tip. A large pallial blood vessel runs diagonally from the auricle to near the ventral tip of the anterior adductor muscle, often leaving a deep impression in the shell interior. The inner mantle around the anterior adductor muscle is often thickened by blood space or in some species thrown into complex folds as mantle gills.

Figures

Figure 4. General anatomy of Anodontia philippiana, Dampier, Western Australia, with left valve and mantle removed. Abbreviations: aa, anterior adductor muscle; exa, exhalant aperture; f, foot; fm, fused mantle; ld, left demibranch of ctenidia; me, mantle edge; mg, mantle gills on septum; p, periostracum; pa, posterior adductor muscle. Shell length = 40 mm