Okenia rhinorma, Rudman, William B., 2007

Rudman, William B., 2007, Two new species of Okenia (Gastropoda: Nudibranchia: Goniodorididae) from eastern Australia and Tanzania, Zootaxa 1657, pp. 57-67 : 62-66

Okenia rhinorma sp. nov.

( Figures 1 D, 4A,B, 5, 6.)

Material: Bongoyo Island, off Msasani Bay, Dar es Salaam, Tanzania. Coll: G.H. Brown, 7 November 1976. 11 mm long alive, AM C443912 [Holotype]. Yatush slope, Eilat, Israel, Gulf of Eilat, Red Sea, 29 m, on sea grass and patch reef, 16 March 2005, 1 specimen approx 15 mm long alive, [Photo only: Binyamin Koretz, Shulamit Koretz].

Etymology: ‘ rhinorma’ is an invented word based on rhinophore and enormous to reflect the unusually large rhinophores in this species.

Geographic distribution: Western Indian Ocean ( Tanzania, Red Sea)

External morphology: Animal elongate and relatively high ( Figs 1 D, 4A,B), with mantle edge defined by ridge running between six large lateral papillae. Lateral papillae held out at right angles to edge of body; anteriormost pair point forward, relatively long, approximately equal to body width, tapering to a rounded point; second pair situated alongside rhinophores, of similar size and shape to anterior pair. Posterior pair of papillae situated just behind gills, half length of anterior papillae. Rhinophores relatively long, at least half the body length, held erect when fully extended, with lamellae on posterior side. Gills, long, six, held erect and arranged in arch around anus. Head region extends some distance forward of the mantle, anterior corners enlarged and rounded; ventrally, head and mouth are separate from anterior edge of foot ( Fig. 5 A).

Colour: Body opaque white with pattern of translucent regions, and orange and purple patches or spots ( Fig.1 D, 4A,B). Size and proportion of orange and purple patching variable. Lateral papillae white with orange tips and subapical purple band. Rhinophores with translucent white base, white anterior spots, mottled purple pigmentation down each side, and orange tip. Gills translucent with brown and white pigmentation.

Foregut anatomy: Buccal pump relatively large, non-pedunculate ( Fig. 5 B). Radular formula 23(+4) x 1.1.0.1.1. Inner teeth with wide base and long recurved pointed cusp; between five and seven large pointed denticles present at inside base of cusp ( Fig. 6 ). Outer teeth only slightly smaller, with broad triangular base and long recurved, non-denticulate pointed cusp. Jaw plates or cuticular thickenings not found in SEM preparation.

Reproductive system: Reproductive opening on the right side of body, below and slightly posterior to level of the rhinophores ( Fig. 5 C). Vaginal duct elongate, leading back to large spherical bursa copulatrix. Exogenous sperm duct narrow, running from junction of vaginal duct and bursa copulatrix to fertilisation chamber within female gland mass. Small, rounded, exogenous sperm sac opening about one-third of the way along exogenous sperm duct. From ovotestis, spermoviduct expands into ampullar region, distended with endogenous sperm. Inside female gland mass, spermoviduct divides into two short ducts; the oviduct, opening into the fertilisation chamber; the sperm duct, opening into relatively short prostate gland. Vas deferens joins prostate gland to bulbous penial sac.

Remarks: There is some variability in colour between the holotype from Tanzania and an animal from the Red Sea. In the holotype the body is opaque white except for a pair of translucent bands on the head running forward from beneath the anterior lateral papilla on each side. There are some large orange patches on the head, some of which are irregularly edged in brownish purple. An orange band, mottled with purple and white, runs back between the rhinophores expanding to form large patch over much of the mantle, anterior to the gills. An orange band, irregularly bordered with purple, runs along each side of the body, beneath the mantle, to the tip of the posterior foot, which has brownish purple patches. The lateral papillae are white with an orange tip and subapical purple band. The rhinophores have a translucent whitish base, an orange-brown tip and the rest is mottled with patches of translucent purple, pigmented purple, and orange-brown, with two or three white spots down the anterior face. The gills are translucent with brown and white pigmentation. In the Red Sea specimen, the orange-brown bands described for the Tanzanian specimen are replaced with translucent colourless bands, except along the leading edge of the head, and the purple patches on the body are reduced to the anterior head and the posterior tip of the foot. Scattered over the body are small rounded orange spots. Small purple spots are also present on the lateral papillae, rhinophores and posterior tip of the foot.

This species is similar in body shape to the ascidian-feeding species from the North Atlantic which has been identified as Okenia aspersa ( Alder & Hancock, 1845) , O. quadricornis ( Montagu, 1815) and O. ascidicola Morse, 1972 ( Cervera et al. 1991, Morse 1972, Picton & Morrow 1994, Thompson & Brown 1984, Thompson 1988, Valdés & Ortea 1995) but which may all be the same species. This, or these, species have a high profile and are very similar in external shape to ascidian-feeding species of the related genus Goniodoris , except for the absence of lateral mantle papillae in Goniodoris , and the presence of oral tentacles. The morphology of the radular teeth of O. aspersa , O. quadricornis and O. ascidicola as described in papers listed above, with the sole exception of Thompson & Brown, mention a large inner tooth with large denticles, and a slightly smaller non-denticulate outer tooth. This is very similar to the morphology described here for Okenia rhinorma and links this species to most other species of Okenia , where the outer teeth are small and reduced, usually six or more times smaller than the inner teeth. Although I have no information on the feeding habits of this new species, its general shape and radular morphology suggest it is probably an ascidian-feeding species as well.

Diagnosis: Okenia rhinorma can be distinguished by its colour pattern of orange and purple patches on a white background, its high body profile, long lateral papillae and very long rhinophores. It can also be distinguished by the morphology of the radular teeth with large pointed denticles on the inner tooth and outer teeth which are only slightly smaller than the inner teeth.

Figures

FIGURE 5. Okenia rhinorma sp. nov. (AM C 443912). A, ventral view showing head separated from sole of foot. B, buccal bulb. C, reproductive system. amp, ampullar region of spermoviduct; bc, bursa copulatrix (gametolytic gland); es, exogenous sperm sac (receptaculum seminis); ps, penial sac; pro, prostate gland. Scale bars = 1 mm.

FIGURE 6. Okenia rhinorma sp. nov. (AM C 443912). Scanning electron micrographs of radula. A, section of whole ribbon. B, central region showing variability of denticulation on inner teeth. Scale bars = 20 µm. SEM Photos: A. Miller.

FIGURE 1. A – C, Okenia atkinsonorum sp. nov. Nelson Bay, Port Stephens, New South Wales, Australia, 11 m, 11 June 2005, 13 – 15 mm long alive, AM C 445669. B, feeding on Pleurotoichus clathratus. C, egg ribbon, diameter = 9 mm. A, C, Photos: D. Beechey; B, Photo: L & D. Atkinson. D, Okenia rhinorma sp. nov. Bongoyo Island, off Msasani Bay, Dar es Salaam, Tanzania. 7 November 1976. 11 mm long alive, AM C 443912. Photo: W. B. Rudman