Opalia (Opalia) sumatrensis ( Thiele, 1925 )

Brown, Leonard & Raines, Bret, 2004, Two new records of Epitoniidae from Easter Island, Chile, Zootaxa 402 (1), pp. 1-7 : 4-6

Opalia (Opalia) sumatrensis ( Thiele, 1925)

( Figure 2 )

Scala (Nodiscala) sumatrensis Thiele, 1925: 307 , pl. XXIII [XI], fig. 26.

Nodiscala sumatrensis ( Thiele, 1925) : Kilburn, 1985: 274.

Opalia (Opalia) sumatrensis ( Thiele, 1925) : Weil et al., 1999: 108, fig. 346. Type material: ZMB type collection: 109.992 (two syntypes).

Type locality: Padang , Sumatra (by original designation) .

Distribution: Previously only known from Padang, Sumatra, Indonesia. Now, first reported from Easter Island.

Habitat: In sand and rubble collected at 100 m, off the western coastline near Orongo.

Description: White; 4 smooth white protoconch whorls, about 6 teleoconch whorls; the 12–14 non­lamellar costae are angular in the center of the teleoconch whorls; costae and interspaces are sculptured with raised spiral lines separating rows of small pits; a rounded cord is present on the base; outer lip is thickened; maximum size of 4.0 mm.

Remarks: Although the authors were graciously offered the opportunity to examine the type material via loan, we elected not to take any chances since one of the syntypes had already been degraded by Byne's disease (Köhler, 2003, personal communication). However, we did compare our material with the illustration in Thiele (1925), along with a photograph of the other syntype ( fig. 3 ), and note that it matches up very well with both Thiele’s illustration and description, as well as with the photograph. The minor differences seen between our material and the syntype are well within species variation, or are attributable to the fact that the illustrated syntype was a fresher specimen when collected.

The presence of the basal ridge sets this species apart from most members of the genus Opalia and lead us to assign this species to the subgenus Opalia rather than the subgenus Nodiscala as Thiele did in his original description of this species. This is consistent with Kilburn (1985), who used the subgenus Nodiscala for Opalia species with a body whorl with a basal angle but no distinct cord, as well as Clench & Turner (1950), who used the subgenus Nodiscala for species without a basal ridge. Furthermore, Clench and Turner (1950) used the subgenus Opalia for species with strong ribs and a well developed basal cord, shell characters present in this species.

It is easy to distinguish this species from O. (N.) bicarinata (Sowerby, 1844) and Opalia (N.) crassilabrum (Sowerby, 1844), two Indo­Pacific species lacking a basal ridge. Opalia (O.) burchorum DuShane, 1988 from Hawaii is similar to O. (O.) sumatrensis , but differs in having a dark brown protoconch and the costae are not angular in the center of the teleoconch whorls. Opalia (N.) minervae ( Thiele, 1925) from Padang is similar in that it has angular costae and spiral lines, but it lacks the basal ridge that is present in O. sumatrensis . Since there are similarities between the molluscan fauna of Easter Island and the fauna of northern New Zealand, we compared the Opalia species collected from Easter Island with O. (N.) ahiparana ( Powell, 1930) and O. (Pliciscala) zelandica ( Finlay, 1930) , two Opalia species found off the north end of North Island , New Zealand. Opalia (N.) ahiparana ( Powell, 1930) not only has nodules on the noticeably biangular body whorl, it lacks a basal ridge. Opalia (P.) zelandica ( Finlay, 1930) , a species illustrated in Powell (1979), lacks a basal ridge and the angular costae on the teleoconch whorls. Thus , the two Opalia species known from northern New Zealand are easily distinguishable from O. sumatrensis . Opalia sumatrensis is represented from Easter Island by two complete specimens. Outside of the type lot at the ZMB (2 specimens), this is probably the only other known record of this species.

Figures

FIGURE 2. Opalia (Opalia) sumatrensis (Thiele, 1925), length: 2.98 mm. A) Full apertural view. B) Close up of postnuclear whorls. C) Close up of suture, costae and sculpture. Scale bars: B & C = 500 µm. Photographs by B. Raines.

FIGURE 3. Opalia (Opalia) sumatrensis Thiele, 1925, syntype: ZMB 109.992, length: 4.0 mm. Photo­ graph by F. Köhler.