Tylodina fungina Gabb, 1865

What is the yellow umbrella slug?

The yellow umbrella slug or mushroom sidegill (Tylodina fungina) is a bright sulphur-yellow sea slug of the eastern Pacific that still carries an external, cap-shaped shell on its back and feeds only on yellow Aplysina sponges. It is not a nudibranch: its gill sits on the right side of the body, tucked under the shell.

Description

The body is a vivid sulphur yellow. The shell is white to pale pinkish brown, covered by a reddish brown periostracum whose edge projects beyond the shell margin; the apex is near the centre and blunt, and the inside of the shell is straw yellow shading to bluish white towards the rim. Juveniles have flatter, more translucent shells, which grow taller as the animal grows. The rhinophores are rolled, with an eye at the base of each. Living animals are usually about 30 mm long, though an animal of some 75 mm has been reported.

Distribution

Northeastern to tropical eastern Pacific, recorded from Cayucos, San Luis Obispo County, California, south to Bahía de Banderas, Jalisco, Mexico, and from the Galápagos Islands. The type locality is Santa Barbara Island, California. It lives from the low intertidal onto shallow rocky reef and is most often seen at around 5-6 m.

Etymology

The specific epithet fungina is formed from the Latin fungus, "mushroom".

Remarks

A specialist on yellow sponges. Animals have been photographed feeding on Aplysina fistularis in California and observed in situ on Aplysina gerardogreeni on the Pacific coast of Mexico. Every species of Tylodina is thought to depend on sponges of this group, and this one is not known to take anything else.

Despite the "slug" in its common names it is not a nudibranch. Nudibranchs carry a plume of gills on the back; here the single gill lies on the right side of the body beneath the shell, and the shell itself is retained externally throughout life.

The original description rests on a single empty shell. The living animal was unknown at the time, and only the shape and colour of the shell were recorded.

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References

• T. fungina, G., Gabb W.M. (1865). Description of new species of marine shells from the coast of California. Proceedings of the California Academy of Sciences. 3(3): 182-190.
• Tylodina fungina (Rudman 2001), Rudman W.B. (1998-2010). Sea Slug Forum. Australian Museum, Sydney. http://www.seaslugforum.net/
• Tylodina fungina, Verdín Padilla C.J., Carballo J.L. & Camacho M.L. (2010). A qualitative assessment of sponge-feeding organisms from the Mexican Pacific Coast. The Open Marine Biology Journal. 4: 39-46.
• Tylodina fungina, Behrens D.W., Hermosillo A., Fletcher K. & Jensen G.C. (2022). Nudibranchs & Sea Slugs of the Eastern Pacific. Molamarine.