Okenia zoobotryon
(Smallwood, 1910)

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

DISTRIBUTION

Western Atlantic (Caribbean region). Possible records also from Canary Islands, Brazil, and Australia, but as discussed below,  our knowledge of this species needs to be improved before identifications can be confirmed

PHOTO

Unlocalised photo provided by the late Kerry B. Clark.

Also known by various authors as Polycerella zoobotryon, Bermudella zoobotryon, Bermudella polycerelloides, this small brown goniodorid was originally described from Bermuda (Smallwood, 1910) where it was found in large numbers on the ctenostome bryozoan Zoobotryon verticillatum (= Zoobotryon pellucidum). The rhinophores were described as having 3-6 'cup-like, equidistant folds on the posterior surface of its distal two-thirds', and the 16 - 19 dorsal and lateral papillae were described as 'clavate… the tips of which are translucent'. The dorsal papillae were arranged in a distinct pattern, with two in the midline between the gills and the rhinophores, two on either side of the midline in front of the rhinophores, two just in front of the gills and two behind the gills. There are also 4 - 6 lateral papillae on each side. From Smallwood's drawing I would consider the anterior pair of dorsal papillae to be the most anterior of the lateral papillae and the posterior pair to be the most posterior of the lateral papillae. Determining the nature of these papillae is important because the arrangement of the dorsal papillae is often a species-specific character. In a later anatomical description (Smallwood, 1912), he describes the reproductive system in some detail. Unfortunately his description, based on dissection and histological sections, doesn't seem to make sense, which is unfortunate because it means we have no knowledge of the reproductive system of this species. Clark (1984) redescribed specimens from Bermuda and reported populations from Florida. Externally his description follows Smallwood's except for the report of two dorsal papillae on each side of the pericardium, instead of a single pair. He also corrected the radula description. As I discuss in a separate message, Clark's suggestion that Okenia evelinae Marcus, 1957 is a synonym of O. zoobotryon is most unlikely, as they differ in colour, rhinophore shape and host bryozoan.. Redfern (2001) describes and illustrates the external features of animals from Bermuda and confirms they match Valdes & Ortea's (1995) illustration of a Cuban specimen which shows 2 pairs of secondary dorsal papillae as described by Clark.

I have recently reported specimens, which I tentatively identify as O. zoobotryon, from South Australia [see message #12656, Rudman, 2004] but until further information is available on the anatomy of Atlantic specimens of Okenia zoobotryon, the identification must remain tentative. In general external shape and colour they agree, and they have the characteristic cup-shape lamellae on the rhinophores,  but they lack the secondary dorsal papillae which are characteristic of Atlantic specimens of O. zoobotryon. Drawings of the radular teeth (Clark, 1984; Valdes & Ortea, 1995) suggest they are very similar to the radula of Australian specimens, but it would be valuable to have information on the reproductive system. By coincidence, there is another small brown species, Okenia mija, found in Australian waters which has a similar arrangement of dorsal papillae to that of Atlantic O. zoobotryon. It differs in having papillae with pointed, rather than swollen, tips, and feeds on a related bryozoan Amathia wilsoni.

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.

Based on the literature, here is a brief external description of Atlantic animals: The ground colour is translucent white with small irregularly sized and shaped spots scattered over the mantle. The foot and sides of body lack the brown spotting but may have a few small scattered opaque white spots. There can be up to 9 papillae in front of the gills and about 6 on each side along the mantle ridge. They are characteristically shaped with a relatively thin stalk and a rounded terminal bulb, compared by Valdes & Ortea (1995) with a chess pawn. The rhinophores are also uniquely shaped with 3 to 6 cup-like folds on the posterior face. It grows to about 6mm in length and in all reports in which a host has been named it as been found on the arborescent bryozoan Zoobotryon verticillatum.

• Clark, K. B. (984) New records and synonymies of Bermuda opisthobranchs (Gastropoda). The Nautilus, 98(2): 85-97.
• Edmunds, M. & Just, H. (1985) Dorid, dendronotoid and arminid nudibranchiate Mollusca from Barbados. Journal of molluscan Studies, 51(1): 52-63.
• Redfern. C., (2001) Bahamian Seashells: a Thousand Species from Abaco, Bahamas. Bahamianseashells.com Inc: Boca Raton. 1-280.
• Rudman, W.B. (2004) Further species of the opisthobranch genus Okenia (Nudibranchia: Goniodorididae) from the Indo-West Pacific. Zootaxa, 695: 1-70.
• Smallwood, W.M. (1910). Notes on the Hydroids and Nudibranchs of Bermuda. Proc. Zool. Soc. London. 1: 137-145
• Smallwood, W. M. (1913) Polycerella zoobotryon (Smallwood). Proceedings of the American Academy of Arts & Sciences, 47(16): 609-630.
• Valdes, A. & Ortea, J. (1995) Revised taxonomy of some species of the genus Okenia Menke, 1830 (Mollusca: Nudibranchia) from the Atlantic Ocean, with the description of a new species. The Veliger, 38(3): 223-234.

ARCHIVE NOTE:  An earlier edition of this Fact Sheet [27 January 2004] is available on request.

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