young, about 30 mm
with egg mass
Pteraeolidia semperi (Bergh, 1870)
|Maximum size: about 216 mm
color of this species is translucent tan and the cerata,
in Hawaii vary from dark purple to lavender to golden brown, give the
most of its apparent color. This is an extremely elongate species with
large, curved arches of cerata along the length of the body. The
cephalic tentacles have two distinctive dark purple bands. Young
animals, however, are much shorter with fewer cerata and are
often mistaken for other species.
Due to its large
size and diurnal habits, Pteraeolidia semperi is the most commonly-sighted aeolid in
Hawaii. It inhabits
moderately protected to exposed rocky locations at 2-55 m (6-180
ft), often near dropoffs. However, it's rare at < 10 m (< 33 ft).
observed in its resting posture with cerata laid back exposing the
surface area to the sun for photosynthesis by symbiotic
zooxanthellae living in its cerata. Protected there, the algae
manufacture food and pass a significant amount on to the nudibranch.
Individuals vary in color partly due to the number of zooxanthellae in
their bodies. It also
obtains food by preying on several species of hydroids, including Pennaria disticha, a relatively
conspicuous non-native species. During digestion, some of the hydroid's
nematocysts are retained in the cerata of the nudibranch for
use in defense.
When touched, the nudibranch will "flare" its cerata and the
discharge on contact--it is one of the few nudibranchs with a sting
to be felt by humans (though usually not in areas with thicker skin such
as the palm of the hand). It is also able to autotomize the
posterior part of its body in order to distract, or free itself from,
a potential predator. (Note 1)
Later, the missing portion can
be regenerated (see photo).
It lays a spiral egg mass of convoluted white strands.
Big Island, Maui, Lanai, Molokai, Oahu, Kauai, Niihau, French Frigate
widely distributed in the Indo-Pacific.
Hawaii and elsewhere it is
commonly referred to as the "blue dragon nudibranch." It was also
referred to as the "blue dragon sea slug" in Hoover, 1997. It was first
reported from Hawaii in Zahl (1959) (as Pteraeolidia ianthina). It's also listed as P. ianthina in Kay (1979), Bertsch & Johnson (1981), Hoover (1998) and Hoover, (2006)(corrected in 2019 printing). (Note 2)
Molokini Islet, Maui; Mar. 8, 2008.
Observations and comments:
1: On July 12, 1994 a Hawaiian
swimming crab (Charybdis hawaiiensis)
with the posterior
of a P. semperi in its claw.
Note 2: Wilson & Burghardt, 2015 found Pteraeolidia ianthina (Angas, 1864) to be a species complex with P. ianthina restricted to New South Wales, Australia. The name Pteraeolidia semperi
(Bergh, 1870) was reinstated for the remaining Indo-Pacific populations
pending further work to clarify the relationships in the complex.