Radula of Okenia mija

December 21, 2004
From: Bill Rudman

To accompany the updated Fact Sheet on Okenia mija  here is some information on the radula. It is fairly typical for the Family Goniodorididae, with a large denticulate inner lateral tooth and a small outer tooth on each side of the midline, in each row.  The radular formulae of the illustrated radula is 22 x 1.1.0.1.1. The innermost tooth has a broad base and a long elongate pointed cusp with many (12 -14) broad pointed denticles along the cutting edge.  The much smaller outer teeth have a broad quadrangular base with a large recurved bicuspid tip. Below the bicuspid tip, on the outer edge, there appears to be a triangular projection.

PHOTO: SEM of section of right side of radula. Blackmans Bay, Hobart, Tasmania, Australia, 18 November 1986, AM C151776. Scale bar = 10 µm. Photo: A.C.Miller.

Okenia mija and the equally small Okenia zoobotryon, are very similar in colour and size, but as I have discussed on the Fact Sheet there are some consistent differences between the two species.  The radular morphology also shows some distinct differences. In Okenia mija there about approximately 12 short broad denticles on the inner teeth while in Okenia zoobotryon the denticles are longer, narrower, and more numerous. Both species have bifid cusps on the outer teeth but they are much smaller in O. mija, and there is a triangular pointed flange below the cusps, a feature absent in O. zoobotryon. The two species also seem to differ in their food bryozoan, O. mija being found on Amathia wilsoni, and O. zoobotryon always being reported from Zoobotryon verticillatum (see Rudman, 2004).

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

Bill Rudman

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