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young, 9 mm


missing "horn"


egg mass

Ceratosoma tenue
Abraham, 1876
Ceratosoma tenue
Maximum size:  97 mm.

Identification:  This species ranges from yellow-orange to deep maroon blotched with cream and overlain with violet spots that become larger marginally. The branchia and rhinophores are orange lined with white. It is easily recognized by its unusual shape. The rigid body is high and elongate with three characteristic mantle lobes along each side. Even more conspicuous, the posterior end of the mantle occurs about midway along the body forming a prominent, curved "horn" just behind the gills.

Natural history:  A moderately common species, Ceratosoma tenue is occasionally seen in moderately exposed to exposed rocky habitats in as little as 0.5 m (1.5 ft). However, it is most common in rocky habitats at depths of 15-40 m (49-131 ft). It has also been recorded from rocks in Halimeda kanaloana beds. Mature animals are diurnally active and feed on a blue sponge in the genus Dysidea. The mantle glands, which contain chemicals obtained from sponges and distasteful to fish, are concentrated in the large "horn." A fish nipping off the protuberance may "expect" the entire animal to taste this way and leave the rest of it intact. Animals with damaged or missing "horns" and intact bodies support this hypothesis. The defense is not foolproof, however, as some C. tenue have been found in the stomachs of large fish that swallow their prey whole (Rudman 1988). It is one of the dorids that occasionally hosts the commensal shrimp, Zenopontonia rex (= Periclimenes imperator). (Hoover, 1998) It lays a frilly, orange egg mass of three or four whorls.

Distribution:  Big Island, Maui, Oahu, Kauai, Niihau and French Frigate Shoals: widely distributed in the Indo-Pacific.

Taxonomic notes:  This is the species that Kay, 1979 as well as Kay & Young, 1969 list as Ceratosoma cornigerum Adams and Reeve, 1848. The name means slender. It is referred to as the "kangaroo nudibranch" in Hoover, 1998 & 2006. It was first reported in Hawaii from off Barber's Point, Oahu, in Jan., 1965 (Kay & Young, 1969).

Photo:  PF: 62 mm: Makena, Maui; August 11, 2008.

Observations and comments:

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