Yellow Umbrella Slug, Tylodina fungina

Yellow Umbrella Slug, Tylodina fungina. Underwater photograph taken in coastal waters off San Diego, California, September 2025. Photograph and identification of Bob Hillis, Ivins, Utah.

Phylogeny: The Yellow Umbrella Slug, Tylodina fungina (Gabb, 1865), is a gastropod mollusk in the Tylodinadae Family of False Limpets. The Tylodina Genus is the only genus in the Tylodinadae Family and there are seven species in the Tylodina Genus. They are also known as the Mushroom Sidegill Slug and in Mexico as Babosa Paraguas Amarillo and Concha Sombrilla. The common name Yellow Umbrella Slug is also the common name for the Atlantic Ocean species Tylodina perversa, and has lead to confusion.

Morphology: This bright yellow species is hard to miss. The dorsal mantle is smooth, well-developed, and completely covers the body. The head is marked by a pair of rolled rhinophores, a flat frontal veil, and a pair of eyes at the top of the veil. The dorsal surface carries a slightly calcified patelliform (cap-like) shell. The shell is transparent and marked with varying numbers of dark brown specks or small blobs or rectangles. The shell is covered with a thin, bristly, periostracum. The shell does not cover the entire animal. Younger animals have a flatter shell, which grows taller with age. The foot is broad. On the right side of the animal, between the mantle and the foot is the mantle cavity, which houses the gill, hence the name side-gill slug. Yellow Umbrella Slugs are fairly large, reaching  7.5 cm (3.0 inches) in length. These animals are distinguished from true limpets by their thin transparent shells that don’t cover the whole body and don’t clamp tightly to the substrate.

Habitat and Distribution:  Yellow Umbrella Slugs are found on, and under, rocks. They live in the intertidal zone, and to depths of around 18 m (60 feet). The  Yellow Umbrella Slug is a subtropical to tropical Eastern Pacific species that is found along the entire west coast of Mexico with the exception that is absent from north of Isla Ángel de la Guarda, Baja California, in the Sea of Cortez.

Diet: Yellow Umbrella Slugs are spongivores, which feed on yellow sponges from the Aplysina Genus. They are well camouflaged while feeding because the slug is very similar in color to the sponge.

Predators:  Yellow Umbrella Slugs sequester toxic compounds from the sponges that they eat. This makes them unpalatable or toxic to many predators. However, they are eaten by several fish species and the Discodorid Sea Slug, Tayuva lilacina.

Reproduction:  Yellow Umbrella Slugs are simultaneous hermaphrodites (having both male and female reproductive organs). They reproduce sexually with internal fertilization. Fertilization is reciprocal. The eggs are laid as an irregular ribbon-like egg mass.

Ecosystem Interactions: Any type of commensal, parasitic or symbiotic relationships between Yellow Umbrella Slugs and other species has not  been formally documented.

Human Interactions:  From a conservation perspective the Yellow Umbrella Slug has not been formally evaluated however they are fairly common with a relatively wide distribution and should be consider to be of Least Concern. They have no direct impact on human activities.

Synonyms:  None