Okenia atkinsonorum Rudman, 2007, sp. nov.

Okenia atkinsonorum sp. nov. (Figures 1A–C, 2, 3, 4 C) Okenia hallucigenia: Rudman, 1998, 1999, 2003a, b, 2005b not O. hallucigenia Rudman, 2004 Material: The Pipeline, west side of Nelson Bay, Port Stephens, New South Wales, Australia, on Pleurotoichus clathratus, 11 m, 11 June 2005, 13– 15 mm long alive, 1 specimen, 9 mm long preserved, AM C445670 [Holotype], 5 specimens and radula SEM stub, AM C445669 [Paratypes], coll: L & D. Atkinson. Bare Island, Botany Bay, Sydney, New South Wales, Australia. 6–15 m, 4 February 2005, 5– 10 mm long alive [Photo only: Roxanne Fea]. Etymology: This species is named after Leanne & David Atkinson in recognition of the valuable observations they have made on opisthobranch biology in New South Wales and the many contributions they have made over many years to the Sea Slug Forum. Geographic distribution: This species is known only from New South Wales, Australia. External morphology: Animal elongate, with relatively low profile (Figs 1A,B, 4 C); mantle ridge absent but position indicated by presence of up to seven large mantle papillae on each side. Anterior-most papilla on each side sits just in front of adjacent rhinophore and points anteriorly; similar in size and shape to other papillae. Other lateral papillae on each side point outwards and somewhat upwards; length approximately one and one half times width of body, some with bluntly rounded, others with pointed tip. Papillae usually paired with those on opposite side of body, but not always. Usually two dorsal mantle papillae, in midline, one in front of gills, one behind. Gills, usually five, arranged in arc around anal papilla; sometimes two smaller posterior gills absent; each gill inserted separately. Gills simple, with broad rounded outside edge. Rhinophores long and tapering; club with up to 14 lamellae, occupies upper four-fifths of rhinophore. From above, head indistinguishable from foot; no oral tentacles; ventrally, distinct V-shaped groove differentiates anterior edge of foot from head (Fig. 2 A). Edge of foot extends out on each side some distance from body, anterior corners extended out into triangular processes. Colour: Animal bright pink (Fig. 1A, B) except for tips of mantle processes, gills and rhinophores, which are a deeper, redder shade of same colour. Foregut anatomy: Anterior end of buccal bulb lined with cuticle, but no distinct thickened regions or areas with cuticular rodlets. Buccal bulb with relatively large buccal pump, ring of oral glands around mouth (Fig. 2 B). Radula formula 24 x 1.1.0.1.1. (AM C445669). Innermost tooth with broad base and extremely long, twisted, rod-like blade with bluntly pointed tip and single flattened denticle (Fig 3). Outer teeth, much smaller, with broadly quadrangular shape, recurved hook-like tip sometimes with two or three remnant cusps. Reproductive system: Reproductive opening on right side of the body, below and just posterior to rhinophores (Fig. 2 C). Vaginal duct narrow, leading back to large spherical bursa copulatrix. Exogenous sperm sac small, elongate, opening separately into bursa copulatrix alongside openings to vaginal duct and narrow exogenous sperm duct. From ovotestis, spermoviduct expands into large ampullar region, usually distended with endogenous sperm. Inside female gland mass, spermoviduct divides into two short ducts; the oviduct, opening into fertilisation chamber; the sperm duct, opening into large recurved prostate gland. Vas deferens, short, narrow, running from prostate gland to muscular elongate penial bulb. Remarks: Two species of pink Okenia have recently been described from Australian waters, O. stellata Rudman, 2004 and O. hallucigenia Rudman, 2004. Both species are found feeding on the bryozoan Pleurotoichus clathratus, sometimes together on the same colony. Both nudibranchs are almost identical in colour to the bryozoan. Okenia atkinsonorum is a third species, also of similar colour, found feeding on the same bryozoan. In fact the only records we have of this species are from colonies of Pleurotoichus. At present it is known only from central New South Wales, where its distribution overlaps with that of Okenia hallucigenia. The radular teeth of O. atkinsorum have some similarities to those of O. stellata in having a long elongate first lateral tooth but the teeth differ in shape and size. In O. stellata these teeth are simple rod-like structures, 800 microns in length, with a groove along one edge, while in O. atkinsonorum the teeth have a wide base, and taper to a tip which bears a subapical harpoon-like flange. Although more elongate than the teeth of most species of Okenia, they are only about 300 microns in length, so much shorter than those of O. stellata. In colour, O. atkinsonorum and O. hallucigenia are very similar, being a uniform pink except for the tips of the dorsal tentacles, papillae and rhinophores, which are a deeper reddish pink. Neither species have the red lines which form part of the colour pattern of O. stellata. In body shape, O. hallucigenia differs from the other two species in its elongate and high body profile and the distinct and separate oral veil. In shape, O. atkinsonorum is quite similar to O. hiroi (Baba, 1938) from the NW Pacific and O. rosacea (MacFarland, 1905) from the NE Pacific, but it has far fewer lateral processes, and differs in radular morphology and other anatomical features (Rudman 2004, Gosliner 2004). Recently photographs of a similar looking species have been published from northern New Zealand (Gordon & Rudman 2006, Rudman 2005a, Skipworth 2005), but as it was found associated with an encrusting eurystomellid bryozoan, it is more likely to be related to O. hiroi than to O. atkinsonorum. In the two other species the number of lateral mantle papillae is constant, four pairs in O. hallucigenia and five pairs in O. stellata (Table 1). In Okenia atkinsonorum however, the number is variable and like Okenia hiroi and O. rosacea they are not always arranged in matching pairs. Of seven individuals, all about 14 mm in length, two had five papillae on each side, one had six and one had seven. Of those with unequal numbers on each side, two had seven and five, and one had six and five. All had a median papilla in front of the gills, but one animal lacked the median papilla behind the gills. Distribution Lateral Papillae Groove Radula Bryozoan papillae colour on sole tooth 1 food O. hallucigenia Tropical, subtrop. 4 pairs, pink, no v. small Pleurotoichus sp. Australia tapering tips darker tips denticulate Euthyrisellidae 30 µm In an earlier publication (Rudman 2004), O. stellata and O. hallucigenia were compared with a pair of species from the tropical Pacific (O. nakamotoensis (Hamatani, 2001), O. kondoi (Hamatani, 2001)) which matched them in shape and colour and also fed on a similar euthyrisellid bryozoan, and a pair from the northern Pacific (O. rosacea, O. hiroi) which feed on similarly coloured eurystomellid bryozoans. In each pair the radular morphology is quite different, raising the question of whether the species are linked by food choice or radular morphology, or whether the radular differences indicate closely related species evolving to feed in a different way on the same food. We now have a case of three species, with distinctly different shaped radular teeth, feeding on the same bryozoan. Okenia atkinsonorum has been observed continuously by Leanne & David Atkinson in the Port Stephens region from the west side of Nelson Bay to Halifax Park, for over 15 years. They report that it is “not seasonal, nor a casual visitor, almost always present in reasonable numbers and very specific to their food bryozoan”. Diagnosis: Okenia atkinsonorum can be distinguished from other pink species of Okenia feeding on the bryozoan Pleurotoichus clathratus by its shape, arrangement of lateral papillae and radular morphology. Biology: It has only been observed in association with its food, the bryozoan Pleurotoichus clathratus (Fig. 1B). The pinkish egg ribbon is semicircular in cross-section, the flat surface being attached to the substrate. On a flat surface it is laid in a spiral of about two whorls and the whole ribbon is approximately 9 mm in diameter (Fig. 1C), but on rough and branching surfaces the ribbon can be deposited in a less orderly manner, sometimes two or three ribbons being tangled together.

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