Risbecia bullockii (Collingwood, 1881)

Yonow, Nathalie, 2012, Opisthobranchs from the western Indian Ocean, with descriptions of two new species and ten new records (Mollusca, Gastropoda), ZooKeys 197, pp. 1-130 : 36-38

Risbecia bullockii (Collingwood, 1881) Fig. 12Plates 515253

Chromodoris bullockii Collingwood, 1881: 128, pl. 9, figs. 15-17 (South China Sea); Yonow 2001: 9, pl. 1, fig. 6 (Indonesia).

Hypselodoris bullockii . - Marshall & Willan 1999: 106, figs. 191, 19 (Great Barrier Reef); Debelius and Kuiter 2007: 116 (South Africa + W Pacific); Yogesh Kumar et al. 2011: 109, fig. 2b (India).

Material.

La Réunion: 30 mm (24 × 10 mm pres.) on wall of cave, Etang Salé les Bains, 38 m depth, 21 December 2010, leg. H Flodrops; 35 mm (23 × 10 mm pres.), Sec Jaune, Saint-Leu, 12 m depth (night dive), 30 December 2010, leg. H Flodrops. - Mauritius: photographs only of three individuals, 1990's, M. Parmantier pers. comm. and http://seaslugs.free.fr/nudibranche/a_intro.htm. - Sri Lanka: three specimens 11 × 5 mm, 13 × 4 mm, and 16 × 7 mm all pres., Unawatuna, S of Galle, 29 December 2010, and Negombo, N of Colombo, 16 January 2011, leg. S Kahlbrock; photo of one individual, Pigeon Island, Trincomalee, 16 m depth, April 1995, RC Anderson.

Description.

All five specimens are of the original colour form as described and illustrated by Collingwood (1881) and reproduced on Plate 51: dorsum mauve with central orange tinge and distinct narrow white margin; gills and rhinophores bright orange with magenta bases, gills raised high on a peduncle. La Réunion specimens pale violet with white margin, bicoloured gills and rhinophores (Plate 52). Smaller Sri Lanka specimens with rosier orange central dorsum (Plate 53). Seven gills in La Réunion specimens and nine in Sri Lanka specimens. In some photographs, the peculiar arrangement described and illustrated by Collingwood (Plate 51, "fig. 17") is visible, in which the gills are connected: the last two or three gills on each side are joined at the base and there are three single central ones (Fig. 12A). Anal papilla protruded as a tube in most images showing open gills, same colour as mantle. Gills bright orange but deep magenta where connected basally. Pockets of both rhinophores and gills uniformly coloured with mantle; raised rim translucent. White marginal line present on both surfaces as well as edge of mantle.

The specimens are well preserved, solid with a high profile, the metapodium extending beyond the posterior mantle margin. All five specimens retain the white line around the margin but no mantle glands are evident. The gills are extended in the La Réunion specimens, and each gill is almost round in section. Those of the 35 mm specimen are still pink, especially at the base (April 2011). The head is rounded with small oral tentacles identical to that figured for Risbecia apolegma Yonow (2002, fig. 11). The rosy hue remains on the dorsum, rhinophores, and gills of the Sri Lanka material several months after collection. The radula of the 16 mm Sri Lankan specimen examined had a formula of 72 × 100.0.100. All teeth are denticulate: there is no median thickening; the first 10 laterals in each row are recurved: the main cusp has a strong "brow ridge" with 1 or 2 very small denticles on each side (Fig. 12B). The majority of the laterals rapidly become straighter with a large cusp bearing 5 or 6 small denticles along its edge; the largest cusp measures at least 100 μm. In the last 5-7 teeth of this group, the first denticle next to the main cusp is doubled (Fig. 12C, D), even in the newest row (Fig. 12E). The final 10 teeth of each row rapidly reduce in size of cusp and denticles to become almost rounded; however, the double first denticle persists (Fig. 12F). The jaw rodlets of Risbecia bullockii have a slightly curved point with a swollen base.

Remarks.

Much controversy surrounds the identity of this species, here recorded from the Indian Ocean for the second time: photographs from La Réunion and Mauritius are also amethystine violet, intensifying anteriorly and posteriorly, a thin, bright white line is present around the mantle but not around the foot, and the gills and rhinophores are orange with magenta at the base (http://seaslugs.free.fr/nudibranche/a_intro.htm); these are identical to Collingwood’s illustration. Controversy also surrounded the description of Risbecia apolegma Yonow, 2002: Rudman stated on Sea Slug Forum that he could "find no anatomical grounds to place this ‘species’ in the genus Risbecia " while also querying its distinction from Hypselodoris “bullocki” (sic). However, in the numerous pages showing variations in colour pattern he stated that "unpublished studies of the anatomy show that it should probably be placed in the genus Risbecia ." Examination of the radula of the La Réunion specimen, re-examination of all micrographs of the radulae of Risbecia apolegma and of Risbecia pulchella (see below) show that they are all very similar and bear the diagnostic features as described by Rudman (1984). None of these teeth has the bicuspid laterals typical of the genus Hypselodoris ; another character they have in common is that there are fewer rows than teeth per half row, in contrast to a statement by Gosliner and Johnson (1999) to the opposite effect. The teeth of Risbecia apolegma have a single large cusp with 5-6 denticles, the first of which faintly shows the division that is so obvious in bullockii and pulchella. Whether one assigns apolegma to Risbecia or Hypselodoris , it is clearly different from bullockii and a valid species based on radular morphology and consistent colour pattern.

Coincidentally, a recent paper by Johnson (2010) presented a phylogenetic tree placing Hypselodoris bullockii with Risbecia imperialis and Risbecia tryoni . Several species of Hypselodoris have a high body profile and an elevated gill peduncle, of which Hypselodoris regina Ev. Marcus & Er. Marcus is found along the East African coastline: the gills in this species nearly always form a goblet shape unlike those of Risbecia bullockii , which are similar to those of Risbecia apolegma .