Members of the genus Thuridilla are brightly colored but not cryptic and are often found crawling on substrates other than the algae they eat (Gosliner, 1995).

"Specimens with coloration typical of T. bayeri are dark brown to black with a series of 6-10 cream to yellow longitudinal lines on teh head and parapodia. The edge of the parapodia may be lined with an orange to red-orange line (a single specimen from Papua New Guinea), an absence of other pigment giving the impression of a row of black spots (specimens from Guam) or bluish green patches (specimens from the Marshall Islands). Other specimens lack any additional pigment along the margin of the parapodia. Red-orange pigment may be present at the posterior junction of the parapodia. Six to seven bright blue, ovoid spots are found submarginally along the length of each parapodium. The head may also have a mid-dorsal blue mark or paired spots between the rhinophores. Basally, the rhinophores are the same color as the general body, with cream longitudinal lines. The lines expand and unite into a large transverse white band. This band may continue to the apex of the rhinophores or may be followed by a red band or streaks. A single specimen from the Solomon Islands (Scott Johnson, pers. comm.) has an additional apical black ring distal to the red ring. In specimens from the Marshall Islands, there is considerable variation in the presence of the white and red pigment on the rhinophores. Only white pigment may be present or there may be longitudinal red streaks the marge with the white band. The anterior margin of the foot is red, often with opaque white. Blue spots may also be present on teh anterior portion of the foot. In specimens with heavy blue spotting, the inside of the parapodia has a narrow white marginal band, a broad orange band, and a broader black area. Inside the black band is an area of bright blue pigment. The coloration of the inside of the parapodia is not known from specimens with a little blue pigment.

In specimens with coloration typical of T. bayeri, the pericardium is ovoid (Fig. 2A-C). From its sides, are two lateral vessels which bifurcate terminally or remain undivided. From the posterior end of the pericardium are two other blood vessels, that are joined basally. The are bifurcate or slightly more highly branches.

The branching of the dorsal vessels was examined in four specimens with coloration typical of T. ratna (Fig. 2D-G). In all cases, there are both lateral and posterior vessels. In one individual, only the left lateral branch was present, but a secondary right branch was situated off the side of the right posterior branch. The lateral branches are undivided or contain bifurcate or more highly branched tips. The posterior vessels have a common origin from the pericardium. The may entirely lack branches or have a few secondary branches." (Gosliner, 1995, Proceedings of the California Academy of Sciences, 49(1), p. 3-4)

Other characteristics:

- Radula has 24-26 teeth (Gosliner, 1995)

- Penis is without a stylet

A colormorph resembling Thuridilla ratna is found in Palau, Guam, Marshall Islands, Australia, Thailand, Philippines, Indonesia, Malaysia, and Madagascar (Gosliner, 1995).

Length: 12-20 mm (Gosliner, 1995).

Gosliner (1995) compared 50 color photographs of T. ratna and T. bayeri and found their coloration and morphology highly variable. He concluded that the described differences between these species, "are simply variation within a single species that is extremely variable in its coloration." Therefore, he considers T. ratna a junior synonym of Thuridilla bayeri.

Thuridilla bayeri also looks similar to T. splendens except for yellow spots found on the parapodia and foot found of T. splendens (Gosliner, 1995).

Like all sacoglossans, this species is a simultaneous hermaphrodite. Copulation occurs via a triaulic system (the female reproductive system has two separate openings; the male reproductive system has one) (Gosliner, 1995).

Gosliner, T. 1995. The genus Thuridilla (Opisthobranchia: Elysiidae) from the tropical Indo-Pacific, with a revision of the phylogeny and systematics of the Elysiidae. Proceedings of the California Academy of Sciences 49(1): 1-54.

Jensen, K.R. 2003. Distributions, diets and reproduction of Hong Kong sacoglossa (Mollusca: Opisthobranchia): a summary of date, 1980-2001 in Perspectives on Marine Environment Change in Hong Kong and Southern China, 1977-2001 (ed. B. Morton). Proceedings of an International Workshop Reunion Conference, Hong Kong 21-26 October, 2001. Hong Kong University Press.