young, < 20 mm
young, > 30 mm
Hexabranchus aureomarginatus Ostergaard,
|Maximum size: about 200 mm.
largest Hawaiian nudibranchs. Mature animals
are crimson, usually with patches of opaque white. When the animal is
at rest, the mantle edge is rolled but, when disturbed, it's flared
revealing a yellow-white margin. The oral tentacles are unusual in that
they resemble tiny hands. As with other species of Hexabranchus, it
undergoes complex changes in color and form as it matures. Very young
animals are translucent-cream with faint red lateral patches on the
notum and a yellow mantle margin. In maturing animals, the lateral
patches darken and a submarginal red band appears. As the animals reach
maturity, the rest of the notum darkens to red obscuring those features
and opaque white patches may appear, particularly in the areas between
original lateral red patches. However, the amount of opaque white in
mature animals is highly variable. (Note 1) In
very large animals, the spotting may become finer (but more extensive)
and the white patches may become less bright/well-defined. At
all stages, the yellow-white margin
retained. It can be distinguished from Hexabranchus
pulchelus by the
presence of a yellow-white marginal band at all sizes and the lack of
spots in juveniles. See the article on this site
for further discussion.
aureomarginatus is a common nocturnal species in moderately
exposed to highly exposed rocky areas from 1-8 m (3-25 ft).
It may also be found in tide pools at more exposed sites. Although it
often conceals itself under rocks or in crevices during the
day, it sometimes rests in the open and may even remain active at times.
(Note 2) Like many dorids, it obtains
protective chemicals from its sponge food and Scott Johnson reports
it feeding on a yellow sponge.
(Note 3) It is one of
the few dorids that can leave the sea floor and swim when
"dance" referred to in its common name. When swimming, the mantle
margin is unrolled to reveal a yellow-white band and there is strenuous
dorso-ventral flexing, perhaps serving to elicit a startle response or
advertise its toxic nature to potential predators in addition to
swimming. The commensal imperial shrimp, Periclimenes imperator, is
sometimes found living on its body, often among the gills. Its egg mass
is large and conspicuous consisting of 1-5 light pink coils often laid
in an exposed location such as the top of a piece of coral rubble or
rocky spur. The ribbon is typically more loosely coiled, narrower and
more lightly colored than in Hexabranchus
pulchelus egg masses. Defensive chemicals are deposited in the
egg mass at much higher concentrations than in the nudibranch itself
(Scheuer, 1990) so the masses are protected during their week-long
Big Island, Maui, Lanai, Molokai, Oahu, Kauai and Midway.
and Johnson, 1981 as Hexabranchus
sanguineus. It's also
listed as H. sanguineus in
Hoover, 1998 & 2006 (left photo) (corrected in 5th
species name means "gold margined" and it is one of the species
referred to as the "Spanish dancer" in Hawaii and elsewhere. In
the 5th printing, Hoover, 2006 refers to is as the "yellowmargin
Spanish dancer." It
reported from Hawaii
in Ostergaard, 1950 (as Hexabranchus sp.).
Keopuka Rock, Maui; Sept. 2, 2007.
Observations and comments:
1: We've seen photos of animals
with extensive white pigment on the notum and no white pigment on the
notum copulating (supporting lumping the two extremes).
On Maui, we've only seen this species resting under shaded ledges
during the day or crawling actively at night. However, on May, 3, 2012
we observed at least seven animals crawling in the open by day in tide pools at
Po'ipu Beach Park, Kauai. The following night, when we returned to the
same pools, we found none. This suggests more flexible behavior than our
Maui observations with at least some diurnal (or crepuscular?)
activity. The daytime sightings were in the morning under cloudy
Note 3: On May 10, 2012 at Ho'ai Bay on Kauai, Darcy Kehler found a large convex crab, Carpilius convexus, feeding on a mature H. aureomarginatus in shallow water. The nudibranch showed extensive damage to its mantle and had secreted a large quantity of mucus. So, it may be preyed on by some crustaceans in spite of its chemical protection.