Pholadomya candida
P. candida is one of the rarest living mollusks, having even been mistakenly declared extinct (Runnegar, 1979; Diaz et al., 2009). However, the species is not currently the target of any conservation effort.
P. candida is one of the rarest living mollusks, having even been mistakenly declared extinct (Runnegar, 1979; Diaz et al., 2009). However, the species is not currently the target of any conservation effort.
Shell: thin, white, ovate and elongate in general shape, equivalve. Large anterior and posterior gapes present. Umbones anterior, perforated. Sculpture of commarginal undulations and broad radial ribs, the latter running along the anterior and mid portions of the shell flank only. Broad nodules present at the intersections of radial and commarginal elements of sculpture. Periostracum olive green to light brown, with radial, fine arenophilic threads secreted by specialized glands in the mantle margin (Sartori and Harper, 2009). Internal surface of the shell matt to sub-nacreous. Pallial sinus deep. Anterior adductor scar elongate; posterior rounded. Scars left by a specialized muscle of the pedal aperture are recognized as (1) a separate scar anterior to the anterior adductor scar, and (2) forming a small dorsal projection of the pallial line approximately at its anterior 1/4. Hinge without functional teeth but sometimes with one or a few protuberaces of the hinge plate positioned under the beaks. Ligament external, supported by nymphs.
The anatomy of P. candida was studied in most detail by Morton (1980). Below is a summary of his observations.
Mantle: Extensively fused, with inhalant and exhalant apertures at the tips of the siphons, a small antero-ventral pedal aperture and a small fourth pallial aperture ventral to the inhalant siphon. In the vicinity of the pedal aperture, P. candida displays a muscle in the form of a cross which is not known in any other animal. However, its function remains unknown. Multicelular glands along the mantle margin deposit threads of secretion on the external surface of the periostracum, which may bind sediment particles to the shell (Sartori and Harper, 2009).
Siphons: Long, fused to one another along most of their length. Distal tips devoid of tentacles or papilae.
Foot: Small, with a ventral groove but no functional byssal gland. Posterior to the foot a tentacular projection of presumed sensory function (the opisthopodium) is present.
Gills: Plicated, with differentiated filaments at the crests and grooves of each plica. Inner demibranch with a food groove in between ascending and descending lamellae; outer demibranch comprising only the descending lamella, reflected to the dorsum.
Labial palps: Long and straplike.
Digestive tract: Lips of the mouth fused laterally, forming a globular sac on each side of the mouth. Crystalline style sac fused to the mid gut. Hind gut penetrates the pericardial cavity, but passes below the heart.
Gonads: Male and female gonads discharge through the same aperture in this hermaphroditic species.
Reports of organisms living in association with P. candida are available from dissections of only two specimens of the species (Morton, 1980). The following organisms were found: gravid female pea crab (Pinnotheres) and an amphipod of the family Leucothoidae, both residing in the mantle cavity; larval digenetic trematodes parasitising the kidneys.
Simultaneous hermaphrodite; produces relatively large (up to 90 micrometers in diameter), individually encapsulated (capsule up to 15 micrometers thick) eggs (Morton, 1980).