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Halgerda albocristata Gosliner & Fahey, 1998
(fig. 9a-c, pl. 4 fig. 6)
Halgerda albocristata Gosliner & Fahey, 1998: 352, figs 1B, 7, 8; Marshall & Willan 1999: 77, fig. 132; Gosliner et al., 2008: 180.
Material.— Hatuhuran, Piru Bay, Ceram, 2 m depth, 28-30.xi.1997, under coral rubble, two specimens 10 × 6 mm (RMNH.MOL.131699) and 5 × 2 mm (lost), both pres., leg. and photos H. Strack.
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Fig. 9. Halgerda albocristata. a, 10 mm specimen, ventral view; b, 5 mm specimen, ventral view; c, last few radular teeth of 10 mm specimen from old edge of radula, showing brush-like denticles, scale bar 25 µm. |
Description.— The centre of the dorsum bears three large polygonal areas surrounded by ridges which were marked with short white lines aligned in various directions but not crossing the polygons (pl. 4 fig. 6). Outside the three polygonal areas was a ring of yellow-orange tubercles, also with white lines. The band of mantle surrounding the tubercles was translucent white with many black brown spots and dashes. The foot projected beyond the mantle posteriorly and was translucent white with brown dots and dashes. The upright rhinophores had a translucent stalk and terminal knob, but the lamellae were brown-black. The six simply pinnate gills were translucent, but the anterior two had brown-black pigment for much of their lengths which was absent from the tips.
The preserved specimens are both relaxed, although the gills and rhinophores are retracted into their pockets: the three central polygons are clearly visible, and the skin is translucent enough that the orange and brown gut is visible. The area outside the polygons still bears its dots and dashes of brown but not the tubercles. Ventrally the hyponotum of each specimen is marked with a ring of large brown spots and several smaller randomly scattered spots and tiny dots around its margin; the dorsal pigment is visible through the hyponotum in both specimens. The upper surface of the foot in both is covered in small dots of brown pigment but the sole is unmarked (fig. 9a, b).
Dissection of the radula showed the oral tube of the larger specimen to bear 22 brown spots on its dorsal surface. The radular formula is 60 × approx. 35.0.35, with hooked laterals and the innermost teeth arranged in a deep V-shape. The innermost teeth are small with a short sharp cusp; the mid-lateral teeth are hamate with a curved cusp longer that the broad base; the outermost teeth are small, hooked, and fimbriate, with approximately twelve long brush-like denticles (fig. 9c).
Remarks.— Halgerda albocristata was described and illustrated by Marshall & Willan (1999) from the Great Barrier Reef, similar to the original description; the rhinophores are similar to those of these Ambon specimens, with a translucent stalk and black clavus. Marshall & Willan (op. cit.) also describe the spots on the foot and hyponotum, which was not mentioned in the original description but is present in these specimens. This is the only pale species of Halgerda with white markings on the ridges (H. dalanghita Fahey & Gosliner, 1999, is orange with a crystalline pattern of white on the ridges and H. xishaensis Lin, 1975, is brown with white ridges) but all other descriptions and illustrations of H. albocristata differ in having single white lines along the ridges and meeting at the ridge intersections, short black lines at right angles around the mantle margin, and translucent gills and rhinophores with a black band near the tips. These specimens differ in several respects: the white markings are multiple and irregularly arranged, there are no perpendicular brown lines around the margin, and tubercles are present on the mantle skirt.
The radula in species of Halgerda is not particularly characteristic, but when taken into account with other features such as body, gill, and rhinophore morphology and colour pattern, they are useful. The shapes of the laterals in the Ambon material are similar to those described for H. albocristata, “3-4 outer laterals modified with a series of fimbriate denticles;” however, the illustration of the type shows a hand-like tooth with five or six short denticles.