Scott & Jeanette Johnson, Kwajalein Unterwater
Info
Hexabranchus aureomarginatus Ostergaard, 1955
Hexabranchus aureomarginatus is one of the largest Hawaiian nudibranchs. Adults are purple, mostly with opaque white spots. When the animal is at rest, the mantle edge is curled up. However, when the slug is disturbed, the mantle widens to reveal a yellow-white rim.
The mouth tentacles are unusual in that they resemble tiny hands. As with other Hexabranchus species, the slug undergoes complex color and shape changes as it matures. Very juveniles are translucent cream with faint red lateral spots on the notum and a yellow margin on the mantle. In mature animals, the lateral spots darken and a submarginal red band appears.
When Hexabranchus aureomarginatus is fully grown, the rest of the notum darkens to red, obscuring these features, and opaque white spots may appear, particularly in the areas between the original lateral red spots. However, the proportion of opaque white in adult animals is very different.
In very large animals, the mottling may become finer (but more extensive) and the white spots may appear less bright/well defined. In all stages, the yellow-white border is preserved.
It differs from Hexabranchus pulchelus in the presence of a yellow-white marginal band in all sizes and the absence of purple spots in juveniles.
Hexabranchus aureomarginatus is a common nocturnal species in moderately exposed to highly exposed rocky areas at depths of 1–8 m. It is also found in tidal pools in more exposed locations. Although the slug often hides under rocks or in crevices during the day, it sometimes rests outdoors and may even remain active at times.
Like many other slugs, it absorbs protective chemicals from its sponge diet. Hexabranchus aureomarginatus is said to feed on a yellow sponge. The nudibranch is able to leave the seabed and swim when threatened. When swimming, the mantle edge is unfolded to reveal a yellow-white band and vigorous dorsoventral flexion occurs, perhaps serving to elicit a startle response or to alert potential predators (in addition to swimming) of their toxicity.
The commensal shrimp, Zenopontonia rex, sometimes lives on its body, often between the gills.
The egg mass is large and conspicuous, consisting of 1–5 pale pink whorls, often deposited in an exposed location such as atop a coral boulder or rocky outcrop. The band is typically more loosely coiled, narrower, and lighter in color than in Hexabranchus pulchelus egg masses.
The eggs are very small and are laid in individual packets of a few dozen embedded in the ribbon. Antibodies are deposited in the egg mass in much higher concentrations than in the slug itself, so the masses are protected during their week-long development. However, as with Hexabranchus pulchelus, the egg masses are eaten by the predatory thread snail Favorinus japonicus, which feeds exclusively on other snail spawn, despite the toxic antibodies.
Taxonomic Notes: This species is presented in Bertsch and Johnson, 1981 as Hexabranchus sanguineus. It is also listed as Hexabranchus sanguineus in Hoover, 1998 and 2006 (corrected in 5th printing).
The species name "aureomarginatus" means "gold-edged" and is one of the species commonly referred to as "Spanish dancer" in Hawaii and elsewhere. In the 5th edition, Hoover 2006 referred to him as the "yellow-rimmed Spanish dancer". It was first reported in 1950 in Ostergaard, Hawaii (as Hexabranchus sp.).
Hexabranchus aureomarginatus is one of the largest Hawaiian nudibranchs. Adults are purple, mostly with opaque white spots. When the animal is at rest, the mantle edge is curled up. However, when the slug is disturbed, the mantle widens to reveal a yellow-white rim.
The mouth tentacles are unusual in that they resemble tiny hands. As with other Hexabranchus species, the slug undergoes complex color and shape changes as it matures. Very juveniles are translucent cream with faint red lateral spots on the notum and a yellow margin on the mantle. In mature animals, the lateral spots darken and a submarginal red band appears.
When Hexabranchus aureomarginatus is fully grown, the rest of the notum darkens to red, obscuring these features, and opaque white spots may appear, particularly in the areas between the original lateral red spots. However, the proportion of opaque white in adult animals is very different.
In very large animals, the mottling may become finer (but more extensive) and the white spots may appear less bright/well defined. In all stages, the yellow-white border is preserved.
It differs from Hexabranchus pulchelus in the presence of a yellow-white marginal band in all sizes and the absence of purple spots in juveniles.
Hexabranchus aureomarginatus is a common nocturnal species in moderately exposed to highly exposed rocky areas at depths of 1–8 m. It is also found in tidal pools in more exposed locations. Although the slug often hides under rocks or in crevices during the day, it sometimes rests outdoors and may even remain active at times.
Like many other slugs, it absorbs protective chemicals from its sponge diet. Hexabranchus aureomarginatus is said to feed on a yellow sponge. The nudibranch is able to leave the seabed and swim when threatened. When swimming, the mantle edge is unfolded to reveal a yellow-white band and vigorous dorsoventral flexion occurs, perhaps serving to elicit a startle response or to alert potential predators (in addition to swimming) of their toxicity.
The commensal shrimp, Zenopontonia rex, sometimes lives on its body, often between the gills.
The egg mass is large and conspicuous, consisting of 1–5 pale pink whorls, often deposited in an exposed location such as atop a coral boulder or rocky outcrop. The band is typically more loosely coiled, narrower, and lighter in color than in Hexabranchus pulchelus egg masses.
The eggs are very small and are laid in individual packets of a few dozen embedded in the ribbon. Antibodies are deposited in the egg mass in much higher concentrations than in the slug itself, so the masses are protected during their week-long development. However, as with Hexabranchus pulchelus, the egg masses are eaten by the predatory thread snail Favorinus japonicus, which feeds exclusively on other snail spawn, despite the toxic antibodies.
Taxonomic Notes: This species is presented in Bertsch and Johnson, 1981 as Hexabranchus sanguineus. It is also listed as Hexabranchus sanguineus in Hoover, 1998 and 2006 (corrected in 5th printing).
The species name "aureomarginatus" means "gold-edged" and is one of the species commonly referred to as "Spanish dancer" in Hawaii and elsewhere. In the 5th edition, Hoover 2006 referred to him as the "yellow-rimmed Spanish dancer". It was first reported in 1950 in Ostergaard, Hawaii (as Hexabranchus sp.).
