Author: Gosliner, 2004

Order: Nudibranchia  Family: Goniodorididae

Maximum Size: 10 mm

Sightings: Sunshine Coast

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Okenia brunneomaculata Gosliner, 2004

To the naked eye Okenia brunneomaculata could be mistaken for a small anemone when all the papillae and rhinophores are erect. It is a small species to 10 mm in length but most usually seen at 5-6 mm.

Okenia brunneomaculata has a elongate-oval shaped body with a tail (posterior extension of the foot) that is devoid of appendages and approximately half the body length proper. The body bears a substantial notal brim from which extend, from each side, four elongate lateral papillae that are usually carried horizontally. Two further elongate papillae are situated in front of the rhinophores that extend anteriorly forming a vee configuration. The posterior of the body, just anterior to the commencement of the tail produces another two elongate papillae that are carried posteriorly and semi-upright. A pair of large oral tentacles can be seen below the two anterior-most papillae, being at least half of their length, projecting well past the foot. These are of a consistent thickness for their length and terminate bluntly compared to the pointed papillae. The non-retractile rhinophores are long and directed anteriorly and carry approximately ten distinctive, well-spaced lamellae on the posterior face except for the smooth apex. Between rhinophores and gills are a further 4 to 7 mid-dorsal upright elongate papillae with, usually, another one posterior to the gill. The gill consists of 3 to 5 simple branches and being a phanerobranch the gills are unable to retract below the mantle surface. Those dorsal papillae surrounding the gill branches afford some measure of protection. The essentially narrow foot widens anteriorly where it is well-developed into two lobes, elongate triangular in shape and joining medially.

The colour of the body is translucent white with a very fine speckling of white pigment, the white pigment sometimes being patchy on the tail. A band of dark brown extends from between the rhinophores laterally down each side to eventually enclose the gills but may be broken and often transforms into dashes posteriorly instead. Additional random dark brown spots are evident mid-dorsally and sometimes on the tail and sides of the foot. The papillae, rhinophores and gill are the same colouration as the body but are not adorned with any of the dark brown colouration.

Okenia brunneomaculata has a diet of ctenostome bryozoans (Gosliner, 2004). These are bryozoans of the order Ctenstomata, the defining feature being that they are uncalcified, i.e. soft tissued, allowing the exposed body walls to deform when contracted. The nudibranch is often found on the underside of broad-leafed algae or foliose sponges where the bryozoans grow epizoicly.

The spawn consists of white egg capsules fixed in a clear matrix, extruded as a tubular ribbon and laid upon the substrate as an untidy coil of three to four whorls.

Distribution has been recorded from the Philippines, Indonesia, Timor Leste, PNG and by us here on Flinders Reef off Cape Moreton, Queensland, Australia. It is considered uncommon through its range.

The specific epithet of brunneomaculata refers to the brown spots on the dorsum of the animal.

Two other, similar-looking species have been delineated in NSSI (2nd Ed. 2018), their Okenia sp. 1 –  yellowish mid-dorsum and dark brown lines rather than thicker bands, and Okenia sp. 2. – burnt orange rhinophores with black tips.

The recent paper by Paz-Sedano et al, 2021 identified, through molecular sequencing, that Okenia brunneomaculata clusters in a clade with O. vena, O. pellucida, O. longiductis and O. polycerelloides. (Not all of the known Okenia were successfully sequenced.)

David A. Mullins – October, 2021

References:
– 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., (2004, December 21). Okenia brunneomaculata Gosliner, 2004. [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/factsheet/okenbrun and associated messages.

– Coleman, N. (2008). Nudibranchs – Encyclopedia – Catalogue of Asia/Indo-Pacific Sea Slugs. Neville Coleman’s Underwater Geographic Pty Ltd.

– Gosliner, T. M., Valde ́s, A ́. & Behrens, D. W. (2018). Nudibranch & Sea Slug Identification – Indo-Pacific, 2nd Edition. New World Publications, Jacksonville, Florida, USA.

– Cook, P. L., Bock, P. E., Gordon, D. P. And Weaver, H. J., (eds) (2018). Australian Bryozoa Vol. 1: Biology, Ecology and Natural History. CSIRO Publishing, Melbourne.

– Paz-Sedano, S., Wilson, N. G., Carmona, L., Gosliner, T. G. & Pola, M. (2021). An ocean yet to be discovered: increasing systematic knowledge of Info-Pacific Okenia Menke, 1830 (Nudibranchia; Goniodorididae). Invertebrate Systematics, 35, 797–825.