Idas washingtonia (Bernard, 1978)
Diagnosis: A modioliform shell with slightly recurved ventral margins. The periostracum is polished with fine concentric lirae. The ligament is weak, opisthodetic and external. The hinge line has minute vertical striations.
Description: The shell is thin, elongated and modioliform. The anterior and posterior margins are rounded, with the ventral margin slightly incurved. The surface is ornamented by fine concentric lirae and occasional growth-check marks. There are two obscure radial riblets present on the anterior third of the shell. The periostracum is thin, adherent and light yelow to brown in colour with an iridescent sheen. The umbones are prominent with prosogyrous beaks. The ligament is opisthodetic, sunk in a shallow channel. The interior is polished and subnacreous. The hinge margins are slightly thickened with numerous fine vertical striations. The adductor muscle scars are subequal and indistinct. The pallial line is not visible, and there is no pallial sinus.
Anatomy: The mantle is fused only by an inner fold to form a small separate exhalant aperture. The mantle margins are muscularised and strongly attached to the periostracum which forms a band on the inner shell margins. There are no tentacles or papillae present and the marginal folds are reduced, except in the region of the inhalant aperture where an extensible hood is present. The foot is small, laterally compressed with a small planar sole. A deep byssal groove is present. The pedal retractor and extensor muscles are well developed. The gills are rather small and thick. The labial palps are small and narrow. The lips are small and simple. The stomach is small; the crystalline-style pouch and midgut are conjoined. The intestine passes over the adductor muscle and terminates in an extensible rectum.
Ecology and Distribution
I. washingtonia demonstrates strong evidence of protandric hermaphroditism. Developing males were recognised in individuals as small as 1.7 mm shell length and spermatogenesis continued until 7 mm length. At >6.5 mm, males were generally spent and the first previtellogenic oocytes were observed. Although developing females were found as small as 4.5 mm shell length, most well-developed females were >6 mm shell length. Overall, females only formed 12% of the population. As with other modiolid bivalves, fecundity was high and the egg size <50 μm, indicative of planktotrophic development. The occurrence of protandric hermaphroditism, high fecundity and planktotrophic dispersal may be an adaptation to the ephemeral nature of their habitat (Tyler et al., 2009).
- Habepegris washigtonia Bernard, 1978 (synonym)
- Idasola washingtonia (synonym)
Synonyms - Habepegris washingtonia Bernard, 1978 Idasola washingtonia