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Favorinus japonicus
Baba, 1949
 
Favorinus japonicus
Maximum size:  17 mm (Kay, 1979).

Identification:  This aeolid has a translucent white body with opaque white patches on the head and along the length of the notum. The color of its cerata varies depending on what type of eggs it has been eating--generally in Hawaii they range from white to pink. The rhinophores are translucent at the base, white at the tips and have two characteristic swellings on the distal half. They may be distinguished from the similar appearing Favorinus sp. #1 by the lack of violet spots in the tips of the cerata.

Natural history:  Favorinus japonicus is a common species that can be more easily found than many other aeolids due to its habit of eating (and laying eggs on) the egg masses of other opisthobranchs, particularly those of Hexabranchus spp. However, it's primarily nocturnal and may also be found crawling in the open at night, presumably seeking mates or new egg masses to feed on. It inhabits moderately protected to highly exposed rocky environments as well as Halimeda kanaloana beds from < 1 to 27 m (< 3 to 90 ft). It eats a wider variety of opisthobranch egg masses than other Favorinus spp. We've seen them on Dolabrifera dolabrifera, Hexabranchus and pleurobranch egg masses while Kay, (1979) also reports them eating Aplysia and Melibe eggs. Retention of pigments from its food produces its highly variable coloration. It lays a white egg mass of many low whorls, often on the egg mass it's feeding on. In the latter case, in order to survive, its eggs must hatch before or just after the food egg mass, probably in less than a week.

Distribution:  Big Island, Maui, Oahu, Kauai and Midway: widely distributed in the Indo-Pacific.

Taxonomic notes:   It's referred to as the "egg-eating nudibranch" in Hoover, 1998 & 2006 and was first reported from Hawaii in Gosliner, 1980.

Photo:  PF: 11 mm: off Lualailua, Maui; March, 1990.

Observations and comments:

Note 1:  ( )
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