double gill spirals
Hypselodoris imperialis (Pease, 1860)
|Maximum size: about 64 mm
high, white body with numerous lemon-yellow spots. The
mantle margin is dark blue-black with blue-black “scallops” extending
onto the notum. These scallops usually contain one or more gold spots
though they are absent in younger animals.
The pattern on the margin is quite variable and distinctive allowing
individuals to be recognized. Dusky "staining" may be present on the
notum in some animals and the size and spacing of the yellow spots is
variable. The rhinophore club is dark blue-black
with white flecks on the lamellae and longitudinal white lines along
anterior and posterior faces. The gills are white with dark blue-black
edges. Very young animals have a more "faded" coloration and develop
their yellow spots gradually as they grow. Some animals may develop more elaborate, double gill spirals with growth.
is a common, diurnal species found on protected to exposed
rocky bottoms, sometimes under silty conditions. (Note
1) It is often found in
pairs at depths of 1-33.5 m (3-110 ft) and is one of several
chromodorids that vibrate their gills. Individuals of the genus Hypselodoris are often found "trailing"
one another, a behavior where one animal follows another so closely
that the head of the follower actually touches, continuously or
intermittently, the tail of the other as they crawl. The reason for
this behavior is not known but may be an adaptation to insure
repeated mating between two individuals. It is known to feed on the
blue-gray sponge, Dysidea fragilis
(Bertsch & Johnson, 1981). (Note 2) The egg mass is pink.
Big Island, Maui, Molokai, Oahu, Kauai, French Frigate Shoals, Pearl & Hermes Reef, Midway
Kure (also Johnston Atoll): widely distributed in the western & central Pacific.
listed as Chromodoris
1877) in Bertsch and Johnson, 1981 and
Risbecia godeffroyana is now considered a
synonym (Terry Gosliner, pers. com.). It was first reported from
Hawaii by Pease in 1860 (as Doris prismatica, var. imperialis). It's listed as Risbecia imperialis and referred to as the "imperial
nudibranch" in Hoover, 1998 & 2006 (corrected in 2019 printing). The name means “mighty.”
Makena, Maui; July 31, 2007.
Observations and comments:
1: This species has had noticeable
population swings, sometimes
with years passing between sightings. It was present in high
numbers on Oahu in the late 1970s and early 1980s during which Scott
Johnson reported seeing as many as 50 on some dives in the Ala Wai
Yacht Harbor. Sometime in 1983, the species largely disappeared on
Oahu. From 1987 to
we didn't observe any H. imperialis
on Maui. We recorded our first in Sept. of 1996 and have seen them
regularly up to the present. From spring 2009 to spring 2019, they were
one of the most
commonly seen species on Maui.
Note 2: We've seen mature animals feeding on the
blue-gray sponge, Dysidea fragilis. However, we've also photographed a young animal apparently feeding on a brown sponge. Whether the latter is a color form of Dysidea fragilis or G. imperialis also feeds on related species (perhaps, only when young?) is unknown.