Scintilla larcombae, P. G. Oliver & Holmes, 2004

Oliver, P. Graham & Holmes, Anna, 2004, Cryptic bivalves with descriptions of new species from the Rodrigues lagoon, Journal of Natural History 38 (23), pp. 3175-3227 : 3199-3204

publication ID	https://doi.org/ 10.1080/00222930410001695123
persistent identifier	https://treatment.plazi.org/id/03B33A71-562D-FF99-36FE-DEC615C5FC10
treatment provided by	Felipe
scientific name	Scintilla larcombae
status	sp. nov.

Treatment

Scintilla larcombae n. sp.

( figures 83 View FIGS , 89 View FIGS )

HOLOTYPE: one specimen from Totor , north Rodrigues, 19 ‡ 40.234 ’ S, 63 ‡ 25.770 ’ E. NMW.Z.2001.061.00028.

PARATYPES: one specimen as holotype, NMW .Z.2001.061.00029, eight specimens from Grand Pate´ , off Port Mathurin, shell gravel, 17 m, NMW .Z.2001.061.00030.

Measurements (mm, L 6 H). Holotype: 3.29 6 2.60. Paratypes: 3.12 6 2.61 (Totor), 3.28 6 2.70, 2.95 6 2.38, 3.17 6 2.60, 2.98 6 2.61, 3.20 6 2.63, 3.28 6 2.65, 2.91 6 2.43, 3.63 6 2.94.

Description. Shell small to 4 mm in length. Fragile. Hyaline. Compressed. Equivalve. Slightly inequilateral with beaks just in front of midline. Outline subelliptical with posterior more expanded than anterior; dorsal margins short, dipping slightly into umbos which project slightly; posterior a broad curve with a slight straightening, ventral curved; anterior narrowly rounded and also straightening towards dorsal margin. Sculpture of closely spaced concentric lines, which are more developed around margins and crossed by very weak radial striae, which are best seen under transmitted light. Hinge ( figure 89 View FIGS ) weak; rv with a single large tooth below beaks, bounded by shallow sockets and a weak lateral ridge and depression posteriorly; lv with two teeth below the beak with a prominent socket between them and a single ridge-like lateral posteriorly. Anterior and posterior teeth separated by a wide space occupied by the internal ligament. Prodissoconch large, PdI smooth demarcated by a low rim, 160 M m in diameter; PdII with concentric lines, 250 M m in diameter. Adductor scars small and placed close to the dorsal margins.

Living animals were not observed but two preserved specimens were found following sorting of crevice samples. These have the overall structure of other Scintilla species observed here except that there is no apparent development of mantle papillae or tentacles except for a single anterior dorsal tentacle arising from the mantle over the anterior adductor muscle.

Derivation of name. Named for Juliet Larcombe (now Burnett), manager of the Shoals of Capricorn Programme.

Habitat. Living specimens were taken from crevices and holes in coral rubble blocks on the reef front. Dead shells were plentiful in shell gravel at 18 m.

Remarks. The weakly decussate sculpture is reminiscent of that found in some species of Divariscintilla ( Mikkelsen and Bieler, 1992) , in particular their D. luteocrinita and D. octotentaculata . These American taxa, however, have well-developed mantle decoration and all are commensal with mantis shrimps (Crustacea: Stomatopoda). We saw no association and the lack of mantle development may suggest that the shell similarities have no phylogenetic significance.

Comparison with the type material of all of Deshayes’ and Sowerby’s species revealed none with a weakly reticulate sculpture.

Scintilla vitrea Deshayes, 1856

( figures 82 View FIGS , 92 View FIGS , 98 View FIGS )

Scintilla vitrea Deshayes, 1856b: 177–178 .

Material examined. Thirteen specimens, Anse aux Anglais, under rocks lying on sand, NMW.Z.2000.086.00004; one specimen, Stn 15, Plaine Mapou, west Rodrigues , NMW .Z.2000.086.00006; one specimen, Grand Baie , NMW .Z.1999.073.00002; one specimen, Petite Butte , NMW .Z.19999.086.00003; one specimen (body in alcohol), Stn 38, Ile Hermitage , NMW .Z.2000.086.00029.

Measurements (mm, L 6 H). 7.37 6 6.10, 1.46 6 1.28, 2.11 6 1.90, 1.94 6 1.66, 3.59 6 3.21, 3.80 6 3.13, 4.69 6 4.13, 5.66 6 4.91, 7.23 6 6.05, 6.96 6 6.04, 7.18 6 5.96, 7.39 6 6.09, 2.30 6 2.15.

Description. Shell small to 8 mm in length. Fragile. Semi-transparent. Equivalve. Compressed with narrow anterior and posterior gapes. Equilateral. Outline subcircular but posterior broader than anterior; dorsal margins dipping into umbos which do not project beyond the summits of the dorsal margins; posterior margin broadly curved, anterior margin slightly narrower; ventral margin curved. External surface highly polished, sculpture with growth lines and weak concentric lines. Hinge plate short, teeth weak ( figure 92 View FIGS ). Right valve with two cardinals just anterior of the beak, the uppermost is minute and sits close to the dorsal margin, the lower is large and projects out of the hinge plate. Posterior teeth as two oblique, diverging ridges bounding a deep socket. Left valve with a similar arrangement but posterior teeth are smaller and the upper anterior tooth is not set so close to the dorsal margin. Ligament mostly internal, as an oblique, posteriorly directed resilium separating the anterior and posterior teeth. External ligament as a thin layer over hinge plate. Prodissoconch, probably of a PdI and a PdII but demarcation obscure, PdII 276 M m in diameter. Shell colour restricted to opaque marginal areas where it is a pale greyish tan.

The extended mantle displays an array of sparse but robust tentacles, all of which are white in colour. The most prominent are the single medially placed anal and cephalic tentacles, which have a wrinkled appearance in contrast to the smooth form of the other tentacles. Of the latter the longest are two pairs which are adjacent to the inhalant aperture, the remainder are short and blunt. The anterior mantle can extend well beyond the shell margin and the posterior exhalant aperture takes the form of a short siphon.

Habitat. Small colonies were found under coral rubble blocks and volcanic rocks lying on sand in the lagoon at very shallow depths of a few centimetres to 1 m. There was no obvious association except that large fireworms ( Polychaeta: Amphinomidae ) were frequently found.

Remarks. Very few of those species described can be considered as subcircular in outline but the syntypes of S. vitrea ( BM (NH) 196 779) were so similar that we have applied this name here. The syntypes, from the Philippines, appear very slightly more expanded posteriorly but they are larger.

Scintilla anomala Deshayes, 1856

( figures 84 View FIGS , 91 View FIGS , 96 View FIGS )

Scintilla anomala Dehayes, 1856b: 181 .

Material examined. One specimen, Anse aux Anglais, Stn 28, NMW. Z.2000.086.00007; two specimens, Anse aux Anglais, NMW . Z.1999.00001, under rocks lying on sand; one specimen, Stn 31, Anse aux Anglais, NMW . Z.2000.00009; one specimen, Stn 38, Ile Hermitage, NMW .Z.2000.00008.

Measurements (mm, L 6 H). 9.21 6 6.06, 9.56 6 7.07, 8.43 6 5.94, 10.82 6 7.57, 8.78 6 6.12.

Description. Shell small to 10 mm in length. Fragile. Semi-transparent. Equivalve. Compressed with narrow anterior and posterior gapes. Equilateral. Outline subelliptical but posterior broader than anterior; dorsal margins dipping into umbos which do not project beyond the summits of the dorsal margins; posterior margin broadly curved, anterior margin slightly narrower; ventral margin gently curved, almost straight. External surface highly polished, sculpture with growth lines, weak concentric lines and areas of dense opaque dots within the shell structure. Dorsal margins finely crenulated, these fading towards the lateral margins. Hinge plate short, teeth weak ( figure 91 View FIGS ). Right valve with an anterior, oblique, low ridge and above it a laminar projection of the dorsal margin. Posterior tooth also an oblique ridge with a laminar extension of the dorsal margin. Left valve with a similar arrangement but the anterior ridge has a projecting peg-like extension immediately below the beak. Ligament mostly internal as an oblique, posteriorly directed resilium separating the anterior and posterior teeth. External ligament as a thin layer attached to the posterior and anterior shelf-like projections of the dorsal margins. Prodissoconchs I and II weakly defined; PdI minutely reticulate and 160 M m in diameter; PdII with concentric lines and 256 M m in diameter.

The extended mantle displays an array of slender tentacles that have white, yellow or orange tips ( figure 96 View FIGS ). The most prominent is the single medially placed anal tentacle, which is wrinkled with the tip coloured a vivid scarlet-red. The next largest is the cephalic tentacle which is also wrinkled but has a yellow tip. There are two pairs of slender tentacles adjacent to the inhalant aperture and a similar pair below the exhalant siphon, the remainder are short and may be all white or with yellow tips. The anterior mantle can extend well beyond the shell margin, is speckled yellow, and the posterior exhalant aperture takes the form of a short siphon.

Habitat. Individuals and pairs were found under coral rubble blocks and volcanic rocks lying on sand in the lagoon at very shallow depths of a few centimetres to 1 m. There was no obvious association except that large fireworms ( Polychaeta: Amphinomidae ) were frequently found.

Remarks. The denticulate shell margin is a distinctive character and was found only in S. anomala . The outline of the Rodrigues shells equate exactly with the syntypes of S. anomala ( BM (NH) 196 793) and this Philippine taxon has been applied here.