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Hexabranchus pulchellus
Pease, 1860
 
Hexabranchus pulchellus
Maximum size:  300 mm.

Identification:  This species is 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 day. It lives in protected to moderately exposed rocky areas from < 1 to 55 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) In addition, it is 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 imperator, is 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 protected during their week-long development. Most predators are deterred by the chemicals (Note 3) but Favorinus japonicus, a species of aeolid nudibranch, is not.

Distribution:  Big Island, Maui, Kahoolawe, Lanai, Oahu, Kauai and Midway.

Taxonomic notes:  This species is 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 the identity 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."

Photo:  PF: about 250 mm: Molokini Islet, Maui; Nov. 20, 2008.

Observations and comments:

Note 1:  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 almost 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.
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