Okenia echinata
Baba, 1949

Order: NUDIBRANCHIA
Suborder: DORIDINA
Superfamily: ANADORIDOIDEA
Family: Goniodorididae

DISTRIBUTION

Known from Japan and eastern Australia

PHOTO

Osezaki, Suruga Bay, Japan,
Upper: 30 Apr 2002. Approx: 5mm long, Depth: 5m.
Lower: 27 Jul 2002. Approx: 5mm long, Depth: 2m. Photos: Jun Imamoto

Known previously only from Japan, the original description of Okenia echinata was of a uniformly brown animal with scattered white spots and triangular papillae. Baba likened it to a small holothurian. That colour pattern is rather different from that of animals from eastern Australia, [see message #12455  ] but recent records from Japan (see Jun Imamoto's message #8858; Tomohiko Kurihara's message #9146) show this species is very variable in colour with animals ranging in colour from pale brown, as originally described by Baba, to those with a bright red background colour, and others with a dark brown background colour and bright red papillae.

The body is low and elongate with the mantle edge forming a distinct overlap around the body. There are many short, bluntly pointed tubercles attached around the mantle edge, one specimen having 14 on each side. In some specimens, the most anterior pair of tubercles appear compound, and are in fact a cluster of 2 or 3 individual tubercles. Similarly, the most posterior pair of tubercles can also be compound in some cases. Scattered all over the dorsum are slightly smaller, but similarly shaped tubercles. They do not seem to form a regular pattern. The head is separate from the foot and could best be described as a large bilobed oral veil. There are at least 8 bipinnate gills arrange in a large arch around the anal papilla. It grows to at least 10mm long alive.

The general shape, large bilobed head and many conical dorsal papillae all characterise this species externally. Okenia opuntia Baba 1960 and Okenia babai Hamatani 1961, both reported from Japan, are similar in general body shape but not enough is known of their colour and anatomy to make a proper comparison.

• Baba, K. (1949) Opisthobranchia of Sagami Bay collected by His Majesty The Emperor of Japan. Iwanami Shoten, Tokyo. 194pp., 50 Pls.
• Baba, K. (1960) The genera Okenia, Goniodoridiella and Goniodoris from Japan (Nudibranchia - Goniodorididae). Publications of the Seto Marine Biological Laboratory, 8(1), 79-83, pls 7-8.
• Hamatani, I. (1961) Preliminary account of a new species of Okenia from Osaka Bay, Japan (Nudibranchia - Goniodorididae). Publications of the Seto Marine Biological Laboratory, 9: 363-365.
• Rudman, W.B. (2004) Further species of the opisthobranch genus Okenia (Nudibranchia: Goniodorididae) from the Indo-West Pacific. Zootaxa, 695: 1-70.

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