Author: (Tate, 1889)

Order: Umbraculida  Family: Tylodinidae

Maximum Size: 50 mm

Sightings: Sunshine Coast

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Tylodina corticalis  (Tate, 1889)

The evolutionary paths of the sea slugs have seen a gradual loss of the molluscan shell to the point where in the adult of all species of the higher order, Nudibranchia, it is completely absent. The more primitive orders however have members that exhibit a wide range of shell variety whether it be a shell into which the animal can completely withdraw for protection or just an internal or external vestige or even in some cases the shell has been lost altogether. The order Umbraculoidea (containing only a very small number of species) however, only has members that bear a characteristic external limpet-like shell upon the dorsum.

Tylodina corticalis is classified in the Umbraculoidea within Family Tylodinidae and is often referred to as the “Umbrella Shell Slug”. Its limpet-like shell is of a flattened conical shape, somewhat oval and possessing radial ribs that are at times delineated by brown markings. These ribs can be quite prominent in some specimens. The yellow shell has a protective periostracum that can vary in presentation from thin and transparent to thick, dark and fibrous and extends beyond the edge of the shell. It is often to be seen covered with a brown algal growth obscuring the protoconch (the remnant shell of its larval stage) that is located towards the centre of the shell at the apex. It has been said that the animal is able to completely withdraw beneath the shell, however, although I have seen it come close to that state of contraction I have never personally observed it be completely retracted and, looking at how large the animal appears when fully stretched out crawling it is somewhat difficult to imagine that this is possible. At such times the foot is considerably extended posteriorly with a rounded end. The anterior margin is also enlarged and carries a deep transverse groove.

The other observable features it has in common with its near relatives are a bipinnate, plume-like gill located down along the right side between the mantle and foot, mostly protected by the shell, and two pairs of head tentacles. The gill is attached for half of its length and bears alternating pinnae along each side of the smooth rhachis. The anterior-most pair of head tentacles are the oral tentacles that are enrolled and joined at their bases by a flap of tissue to form a broad oral veil. The larger, rear-most pair are the rhinophores with eye spots located anterio-medially at each base. The rhinophores although enrolled at the distal end are solid below that but continue the groove on the lateral face down to the base. Not so observable however is the external ciliated seminal groove, considered a particularly primitive feature, that carries sperm from the genital pore to the non-retractile penis that is located just below the right oral tentacle. The location of the copulatory organs requires these simultaneous hermaphrodites to mate head to head, offset slightly. Distributed over the yellow (may be brown or cream) body are small white glands that can produce a yellow secretion believed to contain defensive chemicals derived from their sponge diet. The spawn is laid as a flat, irregular yellow spiral often upon browsed sections of their food sponge.

It is most often found (sometimes in groups) upon its food, sponges of the genus Pseudoceratina (family Aplysinellidae). These sponges often have an external layer or at least patches that are coloured dark brown. Where the slug has been feeding is often conspicuous by the scars that reveal the yellow colour of the sponge tissue beneath.

Tylodina corticalis normally grows to a maximum size of 50 mm for the animal and 30 mm for the shell, however an extremely large specimen has been recorded at 120 mm long from Jervis Bay, New South Wales. It is a temperate dweller and of the four, maybe five, known (and a couple of undescribed) Tylodina species it is the only one from Australia. Distribution has been recorded from southern Queensland down and around the southern coastline of Australia to the south coast of Western Australia and also from Lord Howe Island.

Originally described as Umbrella corticalis.

Also has been known as Umbraculum corticalis.

David A. Mullins – January 2021

References:

– Tate, R. (1889). Descriptions of some new species of marine Mollusca from South Australia and Victoria. Transactions of the Royal Society of South Australia, 11, 60-66, pl.11.

– Burn, R. (1959). Comments on the Australian Umbraculacean Mollusca, Journal of the Malacological Society of Australia, 1:3, 28-30.

– Burn, R. (1960). On Tylodina corticalis (Tate), A Rare Opisthobranch From South-Eastern Australia, Journal of the Malacological Society of Australia, 1:4, 64-69.

– Thompson, T. E. (1970). Eastern Australian Pleurobranchomorpha (Gastropoda, Opisthobranchia). Journal of Zoology, London 160: 2, 173-198

– Willan, R. C. (1984). A review of diets in the Notaspidea (Mollusca: Opisthobranchia), Journal of the Malacological Society of Australia, 6:3-4, 125-142.

– Willan, R. C. (1998). Order Notaspidea Pp. 977-980 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.

– Rudman, W. B. (1999 March 7). Tylodina corticalis (Tate, 1889). [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/factsheet/tylocort and associated messages.

– Wells, F. E., & Bryce, W. (2000). Sea Slugs of Western Australia. Western Australian Museum.

– Burn, R. (2006). A checklist and bibliography of the Opisthobranchia (Mollusca: Gastropoda) of Victoria and the Bass Strait area, south-eastern Australia. Museum Victoria Science Reports 10: 1-42.

– Coleman, N. (2008) Nudibranchs Encyclopedia. Neville Coleman’s Underwater Geographic Pty Ltd, Springwood, Qld.

– Burn, R. (2015). Nudibranchs and related molluscs. Museum Victoria.

– Nimbs, M. J., Smith, S. D. A. (2017). An illustrated inventory of the sea slugs of New South Wales, Australia (Gastropoda: Heterobranchia). Proceedings of the Royal Society of Victoria 128(2): 44-113.

– Nimbs, M. J., Hutton, I., Davis, T. R., Larkin, M. F. & Smith, S. D. A. (2020). The heterobranch sea slugs of Lord Howe Island, NSW, Australia (Mollusca: Gastropoda). Proceedings of the Royal Society of Victoria, 131, 12-41.

– This Species Profile has been modified from a previously published article in Dive Log Magazine’s – NudiNotes Column, Issue: #385 (December 2020): 16 by David A. Mullins.