Opisthobranch of the Week Data
Aldisa pikokai is considered to be very rare in Okinawan waters; I've not seen the animal here on Okinawa's main island and Atsushi Ono mentions (pers. comm.) having seen only two members of the species in the Keramas*, the above featured animal and an uncollected 5mm individual. The mantle, which is considerably wider than the foot, is moderately elongate in outline and flattened. The animal is orange in color and there are four indistinct transverse lines of white speckles radiating toward the margin of the dorsum. The above animal was collected by Atsushi during May of 2000 from the underside of an overturned rock.
I've taken the following information concerning the species from John Hoover (1999):
This tiny orange-red nudibranch resembles a small encrusting sponge, the three pits on its back mimicking excurrent pores. The gills are white. Active only at night, it remains hidden under stones or in crevices during the day, most often between 6 and 30 ft. and sometimes as deep as 80 ft. . . . The three pits reminded the zoologists who described this animal of pits pounded into pahoehoe lava in ancient times, into which children's umbilical cords were deposited. A rare and much larger nudibranch of the genus Sclerodoris closely resembles this species but has only one pit. A small orange-red lamellariid with a smooth rather than pitted surface also resembles it. To about 1/2 in. Known only from the Hawaiian Islands. **
Prior to Ono's discovery, A. pikokai was known only from the Hawaiian Islands (Bertsch & Johnson, 1982; Johnson, 2002) and the Marshall Islands (Johnson, 2002). With an approximate 7500km distance between Okinawa and Hawaii this would be quite a range extension for the species. I've collected and photographed from Okinawa's main island a nearly identical appearing 10mm animal from 59m; this 10mm animal had identical color but only possessed two of the three dorsal pits. Neither the similar 10mm animal nor Ono's specimen has the characteristic, often sharp, longitudinal and transverse ridges that crisscross the dorsum as reported by Scott Johnson in the Sea Slug Forum (Johnson, 2002); there is a possibility that the above featured animal is an unnamed Aldisa species.
The only described species of Aldisa I've found here on Okinawa's main island is A. albatrossae. However, I've collected quite a few unknown species, mostly having a dark red color.
* Keramas = the Kerama Islands Group, located 30 ~ 40 kilometers west of the Okinawa capitol, Naha.
** Reported on the Sea Slug Forum (Johnson, 2002) as known from Hawaii, and Kwajalein in the Marshall Islands.