Author: Cuvier, 1804

Order: Nudibranchia  Family: Phyllidiidae

Maximum Size: 70 mm

Sightings: Sunshine Coast, Capricorn/Bunker Group GBR

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Phyllidia ocellata Cuvier, 1804

Most divers who look for nudibranchs will have seen Phyllidia ocellata. It is a common species and widespread in the Indo-Pacific region. Its conspicuous appearance of black and white ocellations on a yellow background with large tubercles on the dorsum makes it hard to overlook. However, if they only dive in a particular area they may not be aware of the high variability in colour and pattern that this species can exhibit on the dorsum. Some of the many variations are shown above.

Phyllidia ocellata belongs to the Phyllidiidae family of porostomata nudibranchs. The Phyllidiidae are colloquially referred to as the “lumpy” nudibranchs – a reference to the conspicuous tubercles most bear upon their dorsum. Porostomata (now considered by some to be a synonym of superfamily Phyllidioidea) is a term used to group together the nudibranchs that lack a radula, the toothed ribbon used to scrape, pierce or grip their prey. Instead, these radula-less nudibranchs secrete digestive enzymes directly onto their prey that partially breakdown the tissue externally so that it may be sucked up.

The Phyllidiidae lack the circle, arc or line of dorsal gills that serve to define the dorid nudibranchs. Instead they possess ventral secondary gill leaflets located in the groove between the body wall and the underside of the mantle and are the only dorids to do so. These secondary gill leaflets are triangular in shape and attached at right angles across the groove of the hyponotum. They have a rounded apex and alternate large and small around the entire periphery of the animal except for interruptions anteriorly at the mouth and on the right side at the reproductive openings. It is strange to discover that they are closely related to the Dendrodorididae, the other major family in the porostomata, with their large dorsal gill circle.

Phyllidia ocellata is a medium to large dorid nudibranch with an elongate-oval shape. It has a stiff body, the flesh being toughened with embedded spicules present in the mantle, foot and body wall. The mantle completely covers the foot. The most typical presentation of Phyllidia ocellata is that of a golden yellow background with large roundish black ocelli (from four to ten in number) each surrounded with a ring of white. Small to quite large, yellow or white hard tubercles are present irregularly over the dorsum and may or may not appear within those ocelli. There are many variations on this basic theme. The black spots may be incomplete or coalesce appearing to meander across the dorsum. Sometimes the black might cover almost the whole of the dorsum apart from the mantle border with the usual white ocelli being absent whilst at other times the majority of the dorsum may be white with or without black ocelli. An extreme form has also been recorded where the whole animal, including pustules, is completely golden yellow. There is no doubt that the colours and pattern are a warning to potential predators that it is toxic. The tubercles may be isolated or compound but never form proper ridges, however the midline pustules may line up quite regularly as a low longitudinal crest. The tubercles are capable of exuding a milky defensive secretion that the nudibranch obtains through its sponge diet. The lamellate rhinophores are always of a golden yellow colour and can withdraw into pockets for protection. The retractile anal papilla is sometimes visible on the dorsal midline posteriorly when extended out of the protective anal pocket. Ventrally the colour is grey in the hyponotum (with cross hatching), secondary gills and sole of the foot with the separate finger-like oral tentacles bearing gold tips.

Distribution is said to be very wide spread in the Indo-Pacific, as far as the Red Sea. Yonow however (2008, 2011 & 2012) disagrees and cites Phyllidia multituberculata and Phyllidia undula as separate species mistaken as P. ocellata in the Indian Ocean.

Often some of the variations may occur in the same locality making one wonder what a comprehensive molecular sequencing of all the variations might reveal.

References:
– Cuvier, G. L. C. F., (1804). Memoire sur la Phyllidie et sur le Pleurobranch, deux nouveaux genres de mollusques de l’ordre des gastropods, et voisin des patelles les des oscabrions, dont l’autre porte une coquille cachée. Annales du Museum National d’Historie Naturelle, Paris 5: 266-276.

– Eliot, C. N. E., (1910), Nudibranchs collected by Mr. Stanley Gardiner from the Indian Ocean
in H.M.S. `Sealark’, Transactions of the Linnean Society, London, 13, 411-438.

– Willan, R. C. & Coleman, N. (1984). Nudibranchs of Australia, Neville Coleman, AMPI: 42-43

– Brunckhorst, D. J. (1993). The systematics and phylogeny of Phyllidiid Nudibranchs (Doridoidea). Records of the Australian Museum, Suppl. 16, 1-107.

– Rudman, W. B., (1998). Family Phyllidiidae Pp.1000-1001 in Beesley, P. L., Ross, G. J. B. & Wells, A. (Eds) Mollusca: The Southern Synthesis. Fauna of Australia. Vol.5 CSIRO Publishing, Melbourne, Part B 565-1234 pp.

– Marshall, J. G. & Willan, R. C., (1999). Nudibranchs of Heron Island, Great Barrier Reef. A Survey of the Opisthobranchia (sea slugs) of Heron and Wistari Reefs. Backhuys Publishers, Leiden. 257 pp.

– Fahrner, A. & Beck, L. A., (2000). Identification key to the Indo-Pacific species of the nudibranch family Phyllidiidae Rafinesque, 1814, including the description of two new species. Archiv für Molluskenkunde 128 (1/2): 189-211.

– Yonow, N., (2008) Sea slugs of the Red Sea. Pensoft Publishers, Sofia

– Yonow, N., (2011). Results of the Rumphius Biohistorical Expedition to Ambon (1990). Part 15. The suborder Doridina (Mollusca, Gastropoda, Opisthobranchia, Nudibranchia).— Zoologische Mededelingen, Leiden 85 (17), 30.xi.2011: 905-956, figs 1-22, Plates 1-8.

– Yonow, N., (2012). Opisthobranchs from the western Indian Ocean, with descriptions of two new species and ten new records (Mollusca, Gastropoda). ZooKeys 197: 1-129.

– Stoffels, B. E. M. W., van der Meij, S. E. T., Hoeksema, B. W., van Alphen, J., van Alen, T., Meyers-Muñoz, M. A., de Voogd, N. J., Tuti, Y., van der Velde, G. (2016). Phylogenetic relationships within the Phyllidiidae (Opisthobranchia, Nudibranchia). ZooKeys 605: 1–35.

– Gosliner, T. M., Valdés, Á., and Behrens, D. W. (2018). Nudibranch & Sea Slug Identification: Indo-Pacific – 2nd Ed. New World Publications: Jacksonville, FL.

– This Species Profile has been modified from previously published article in Dive Log Magazine’s – Critter ID with NudiNotes Column, Issue: #381 (April 2020): 18 by David A. Mullins

David A. Mullins – April 2020