First record of Okenia zoobotryon from Australia

December 21, 2004
From: Bill Rudman

The animal illustrated here was previously identified on the Forum as Okenia mija but I recently showed that its differs anatomically (Rudman, 2004) and is most probably Okenia zoobotryon (Smallwood, 1910) which was originally described from the Caribbean region of the western Atlantic and has since been reported from Brazil and the Canary Islands.

PHOTOS: Upper:  on bryozoan Zoobotryon verticillatum, Point Turton Jetty, Yorke Peninsula, SA, , 3m, 19 February 1985, many hundreds seen, approx 50 specimens collected, 4-6mm long, AM C145165. Photo: Bill Rudman. Lower: SEM photos of radula. showing section of  left side of radula, and close-up 3 outer lateral teeth. Point Turton, Yorke Peninsula, South Australia, 19 February 1985, on wharf piles and rocks, AM
C145165. Scale bar = 10 µm. Photo: G. Avern.

The animal were small and elongate with a distinct mantle edge bearing a series of lateral papillae which are slightly swollen and rounded at the tip. There are approximately 6 papillae on each side, including an anterior pair in front of the rhinophores. There are also a pair of dorsal papillae in the midline, one just in front of the gills and one just behind the rhinophores. I could find no sign of other papillae on the dorsum. The rhinophores are about twice the length of the lateral papillae and have 3 characteristic cup shape structures on the posterior side of the upper half. The body is translucent straw in colour with a speckling all over of opaque white. There are microscopic brown spots scattered all over the body and also a few larger darker brown spots as well. The lateral papillae are translucent clear with scattered opaque white patches. The rhinophores are also translucent clear but the white patches are mainly confined to the edge of the cup shaped 'lamellae' and the tip.

The radula is fairly typical for the Family Gonidorididae with a large denticulate inner lateral tooth and a small outer tooth on each side of each row. The formula was 23-5 x 1.1.0.1.1.  The innermost tooth has a broad base and a long elongate pointed cusp with many (22+) narrow, relatively long, pointed denticles along the cutting edge.  The much smaller outer teeth have a broad quadrangular base with a large recurved bicuspid tip. 

As I discuss on the species Fact Sheet, it is very difficult to be sure that these animals from southern Australia are the same as O. zoobotryon from the Atlantic, because there are many features of the Atlantric animal which have not yet been clealry described. In general external shape and colour they agree, and they have the characteristic cup-shape lamellae on the rhinophores,  but they lack secondary dorsal papillae which are characteristic of Atlantic specimens of O. zoobotryon. The obvious zoogeographic anomaly is not a major obstacle because its food bryozoan, the ctenostome Zoobotryon verticillatum (= Z. pellucidum) is a well-known 'tramp' species, with a worldwide distribution considered to be the result of shipping and perhaps drifting on pieces of brown algae. Okenia pellucida, also a feeder on Zoobotryon verticillatum, has a wide distribution which is also considered, at least partially, to be the result of the 'unnatural' distribution of Zoobotryon.

• Rudman, W.B. (2004) Further species of the opisthobranch genus Okenia (Nudibranchia: Goniodorididae) from the Indo-West Pacific. Zootaxa, 695: 1-70.

Bill Rudman

Rudman, W.B., 2004 (Dec 21) First record of Okenia zoobotryon from Australia. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/12656

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