Isognomon (Isognomon) incisum ( Conrad, 1837 )

Raines, Bret & Huber, Markus, 2012, 3217, Zootaxa 3217, pp. 1-106 : 24-25

Isognomon (Isognomon) incisum ( Conrad, 1837)

Figures 11 A–B

Perna incisa Conrad, 1837: p. 245 , pl. 19, fig. 9.

Isognomon (Melina) incisa ( Conrad, 1837) — Dall et al., 1938: p. 64, pl. 13, figs. 1–8.

Isognomon incisum ( Conrad, 1837) — Kay, 1979: p. 520, figs. 167 E–F; Osorio, 1995: p. 200, figs. 1 & 4; Severns, 2011: p. 446, pl. 203, fig. 2.

Material examined. Illustrations of the two live collected specimens, which were provided to us for this study (Osorio, pers. comm., 2010).

Diagnosis. Shell medium in size, stout, and irregularly ovate. The LV larger and more inflated than the RV. Surface consisting of fine commarginal lamellae and low broad radial ribs. Well developed byssal notch. Hinge line straight with five to six elongate-transverse teeth. Color variable, with the margins often quite dark.

Remarks. This species as well as the next isognomonid were first recorded by Osorio (1995). Although additional examples have yet to be collected by other workers, these specimens were found at opposite ends of the island and collected during separate trips. This would infer that the two species are naturally occurring and not a by-product of human activities.

Habitat. One specimen (30.8 x 30.9 mm) was collected from the intertidal zone, byssally attached to the underside of a rock (Anakena), while another specimen (8.3 x 9 mm) was collected from a depth of 20 m, attached to a colony of Pocillopora Lamarck, 1816 (off Motu Kaokao).

Distribution. Isognomon incisum was originally described from the Hawaiian Islands, but also collected live from Easter Island. Recent reports from offshore Panamic Islands should be instead refer to as I. recognitus ( Mabille, 1895) (Coan, pers. comm., 2010)— E2.