Okenia hiroi
(Baba, 1938)

Order: NUDIBRANCHIA
Suborder: DORIDINA
Superfamily: ANADORIDOIDEA
Family: Goniodorididae

DISTRIBUTION

Known from Pacific coast of Japan, Korea, China (Hong Kong).

PHOTO

Upper:Mirs Bay, Hong Kong, April 1982, AM C.133758. Photo: B.W. Darvell. Lower: SEM of radula showing (upper) inner teeth on right side. Note groove along the upper edge of the blade. Close-up (lower) shows tip of inner tooth with bifid cusp. Wai Kap Pai, Rocky Harbour, Hong Kong, 5–6m, 23 January 1983, AM C138136. Scale bar = 10 µm. Photo: G.
Avern.

NOTE: This species has previously been known on the Forum as Hopkinsia hiroi.

The animal has a low flattened profile and from above, the mantle, foot and head are merged into one, although ventrally a groove indicates the junction of the head and the foot. There are no oral tentacles. There are about 20 elongate lateral papillae around the body, indicating the edge of the mantle. There is no indication of symmetrical pairing of papillae as in some other species of Okenia. There are a few dorsal papillae, one in the midline, halfway between the gills and the rhinophores, and there is one on each side just behind this central one, forming a staggered transverse  row. There are also a pair of papillae just behind the gills, one on each side. The arrangement of dorsal papillae is quite variable and there can be a few more or a few less than the basic 5. The rhinophores are similar in size and shape to the lateral papillae. There are 4 or 5 small gills forming an arch around the anal papilla. The whole of the animal is pink, matching the colour of the bryozoan on which it is feeding. The tips of all the dorsal processes, papillae and rhinophores are white. There is some variation in the intensity of the pink, and in some animals there is a considerable amount of white on the papillae.  It grows to 10 mm in length.

The radula is fairly typical for the Goniodorididae but the inner lateral teeth have some unique features. They have a wide base and a long rod-like blade which narrows to a bicuspid tip. The cutting edge is lined with large bluntly rounded denticles, and there is a prominent shallow groove along the length of the blade.  The much smaller outer teeth have a broad rounded base and up to 4 recurved pointed cusps along the outer edge of the tooth

Gosliner (2004) recently proposed synonymising the genera Hopkinsia and Hopkinsiella with Okenia. Like many of the 'Hopkinsia-group' of species, O. hiroi has a relatively broad flattened body in which the head and foot are fused, the lateral papillae are relatively large and crowded, and the body, like its food, is reddish pink. The inner radular teeth of this species have two interesting features that link it with both 'typical' species of Okenia and some more extreme forms. The teeth have a distinctive rounded groove along the upper edge of the blade which is also found in the unusually shaped Okenia stellata, and they also have a bicuspid tip, an unusual feature also found in O. pellucida, and O. purpureolineata. This grooved and bifid tooth may also indicate the origin of the strange tooth found in Okenia hallucigenia and Okenia kondoi in which the groove, the bifurcation, and the denticulation appear to have become greatly accentuated.

This species is often found associated with a similarly coloured encrusting bryozoan (Rudman & Darvell, 2001). This has recently been identified by Dr D.P. Gordon as a new species of Integripelta Gordon, Mawatari & Kajihara, 2002 (Eurystomellidae; Cheilostomata) (Rudman, 2004).

• Baba,K. (1938) Opisthobranchia of Kii, Middle Japan. Journal of the Department of Agriculture, Kyusyu Imperial University, 6(1): 1-19.
• Gosliner, T. M. (2004) Phylogenetic Systematics of Okenia, Sakishimaia, Hopkinsiella and Hopkinsia (Nudibranchia: Goniodorididae) with descriptions of new species from the tropical Indo-Pacific. Proceedings of the California Academy of Sciences, 55: 125-161.
• Rudman, W.B. & Darvell, B.W. (1990) Opisthobranch molluscs of Hong Kong. Part 1. Goniodorididae, Onchidorididae, Triophidae, Gymnodorididae, Chromodorididae, (Nudibranchia). Asian Marine Biology, 7: 31-79.
• Rudman, W.B. (2004) Further species of the opisthobranch genus Okenia (Nudibranchia: Goniodorididae) from the Indo-West Pacific. Zootaxa, 695: 1-70.

ARCHIVE NOTE:  An earlier edition of this Fact Sheet [5 July 2001] as Hopkinsia hiroi is available on request.

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