young, < 5 mm
young, about 20 mm
young, 30 mm
Hexabranchus pulchellus Pease, 1860
|Maximum size: 300 mm.
the largest and, perhaps, most widely recognized
Hawaiian nudibranch. Mature animals are crimson
mottled with pink and sometimes flecked with opaque white. When the
animal is at rest, the mantle
edge is rolled but, when disturbed, it's flared revealing a
dark red 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 cream,
decorated with violet spots, and lack broad lateral extensions of the
mantle. Maturing animals gradually lose the violet spots and develop
cream mottling. The background darkens to golden yellow-orange, then
crimson. Opaque white flecks appear on the notum in very old animals.
It can be distinguished from Hexabranchus
aureomarginatus by the lack of a yellow marginal band at all
sizes. See the article
on this site for further discussion.
Natural history: Hexabranchus pulchellus is a common
nocturnal species that shelters under rocks or in crevices during the
It lives in protected to moderately exposed rocky areas from < 1 to
m (< 3 to 180
ft) and has also been photographed
from HURL submersibles at 63
m (207 ft). Rarely, it may be seen in the open during the day. (Note 1) Like many dorids, it obtains protective chemicals from its
sponge food and Kay (1979) reports that it feeds on the calcareous sponge Leucetta solida. (Note 2)
one of the few that can leave the sea floor and swim
when threatened--the "dance" referred to in its common name. When
swimming, the mantle
margin is unrolled to reveal a dark red band and there is strenuous
dorso-ventral flexing, perhaps serving to elicit a startle response or
advertise its toxic nature to potential predators. The commensal
imperial shrimp, Periclimenes
sometimes found living on its body, often among the gills. Its egg mass
is large and conspicuous consisting of 1-5 bright pink coils often laid
in an exposed location such as the top of a piece of coral rubble or
rocky spur (see
photo). As the eggs develop, the egg mass fades to a pale pink and
may become worn and tattered-looking. Defensive
chemicals are deposited in the egg mass at much higher concentrations
than in the nudibranch itself (Scheuer, 1990) so the masses are
week-long development. Most predators are deterred by the chemicals (Note 3) but Favorinus japonicus, a species
of aeolid nudibranch, is not.
Big Island, Maui, Kahoolawe, Lanai, Oahu, Kauai, Niihau and Midway.
listed as Hexabranchus
sanguineus (Rupell and Leuckart, 1831) in Kay, 1979.
It's listed as both Hexabranchus marginatus and Hexabranchus cf. pulchellus in Kay & Young, 1969. Animals referred to as possible
Hexabranchus pulchellus Pease, 1860 in Kay, 1979 and illustrated
as such in Bertsch and Johnson, 1981 are probably juveniles as
suggested by Kay. It is also listed as H. sanguineus in Hoover, 1998 &
2006 (right photo) (corrected in 5th printing). Other
names may have priority once
of the Hawaiian material is fully resolved (both Doris cardinalis Gould, 1852 and Doris sandwichensis Eydoux &
Souleyet, 1852 are probably this species and were described from Hawaiian material). The drawing labelled Hexabranchus sp. in Edmondson, 1946 is probably this species and Hexabranchus tinkeri
Ostergaard, 1955 is also a synonym (Kay, 1979). It was probably first
reported from Hawaii in Eydoux & Souleyet, 1852. The species name
means "beautiful," and it is one of the species commonly referred to as
the "Spanish dancer" in Hawaii and elsewhere. In the 5th printing, Hoover, 2006 refers to is as the "redmargin Spanish dancer."
about 250 mm: Molokini Islet, Maui; Nov. 20, 2008.
Observations and comments:
On Maui, we've only seen this species resting in the open by day on one
or two occasions. However, in May, 2012 several animals were observed
crawling in the open by day in tide pools and the shallow subtidal on
Kauai. 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 conditions.
Note 2: In spite of its chemical
defense, we have seen it preyed upon by sponge crabs (Dromia dormia), the splendid pebble
crab (Etisus splendidus) and
the banded spiny lobster (Panulirus
marginatus), all nocturnal hunters.
Note 3: Fish
that have not learned of their toxicity may bite the egg masses but
always let go without completely removing a piece of the ribbon, leaving
it dangling. We
have also seen green sea turtles (Chelonia
mydas) mouthing egg masses but not eating them.