Plicatula aff. plicata ( Linnaeus, 1767 )

Raines, Bret & Huber, Markus, 2012, 3217, Zootaxa 3217, pp. 1-106 : 42-44

Plicatula aff. plicata ( Linnaeus, 1767)

Figures 21 A–F

Spondylus plicatus Linnaeus, 1767: p. 1136 .

Spondylus plicatus Forsskål, 1775 — Yaron et al., 1986: p. 179, figs. 19–20.

Plicatula plicata ( Linnaeus, 1767) — Huber, 2010: p. 192, figs. 1-3.

Material examined. A single damaged valve, ca. 3.4 mm (BK).

Diagnosis. Shell small (ca. 3.4 mm in height), ovate, equilateral, thin, and very fragile. Prodissoconch P 1 somewhat eroded, small and round, ca. 150 µm length and 95 µm height, P2 not present. Nepioconch with a microsculpture of fine elongate pitting, ca. 1.4 mm length and 1.3 mm height. Primary adult sculpture begins rather abruptly with pronounced commarginal lamellae which are widely spaced, and starts about mid-valve to ventral margin. Interior consisting of eleven primary radial ribs with intercostate and rudimentary secondary riblets along margin. Auricles equal, weakly developed. Hinge straight , with two crurae flanking the resilifer. Color creamy white to pale yellow .

Remarks. This extremely fragile valve was already damaged upon discovery and unfortunately received further damage while in transit. A few sculptural elements of this small species are shared with Propeamussiidae . However, the hinge construction contradicts that placement. Without any doubt it is neither Propeamussiidae nor Pectinidae . This opinion was confirmed by H. Dijkstra, (pers. comm., 2010). The crurae-hinge suggests either Spondylidae or Plicatulidae . However, there is no known spondylid (recent or fossil) with this combination of exterior sculptural elements and internal riblets.

This type of hinge configuration with two crurae on both sides of the resilifer in combination with internal riblets does support a plicatulid placement. Despite the valve's initial exterior appearance, the interior morphology and hinge suggests this to be a left (=non attached) valve. Moreover, in smaller Plicatula plicata (e.g. Huber 2010: 192, fig. 1) such characteristic as commarginal lamellae adjacent to the extended nepioconch are still visible. The color and the number of radial ribs in adults would equally match. There remains little doubt that this species represents a very juvenile plicatulid.

However, due to the scarcity of material along with missing records from Marquesas Islands and the Hawaiian Islands, we recommend further collecting. Until an identity with the widely distributed P. plicata is proven this EI species is treated as eventually being endemic.

Material recently studied by the junior author from Tahiti indicates that an additional undescribed Plicatula species lives there, previously misidentified as P. plicata . However, conspecifity with the EI species is not obvious.

Habitat. Dredged at 50–80 m in fine sand, off the western coastline near Tahai , Easter Island, 27°07’20” S, 109°26’30” W .

Distribution. Although widely distributed throughout the Indo-Pacific, including the Red Sea, eastern Africa, Chagos Islands, Japan, China, Philippines, Indonesia, and Australia, Plicatula plicata has never been recorded from the Hawaiian Islands, New Zealand or the Kermadec Islands — E4.