Costasiella sp. #3
|Maximum size: 4 mm.
species has a translucent-cream body decorated with iridescent flecks.
There is a complex black patch behind the eye spots that surrounds a
rust-brown spot and has branches extending onto the sides of the head
and onto the bases of the rhinophores. An iridescent blue patch is
located between the two branches on each side of the head.
The front of
the head is variably flecked with rust-brown and there are three short,
off-set black lines on the distal third of each rhinophore. Very young
animals have a single black line extending the whole length of the
rhinophore. The cerata
contain dense green granules and iridescent flecks. However, the
central cerata have white granules in their distal portions. (Note 1) The lateral cerata
often have small, rust-brown subapical spots and the central cerata usually
have intensely blue patches on their outer faces. (Note 2) There are a few widely spaced brown flecks on the apexes of the central cerata.
Prior to 2020/2021, Costasiella sp. #3 was known from only two animals found at 1-2 m (3-6 ft)
on an exposed to
moderately exposed accretion platform. Recently, however, numerous animals were found at 15 m (49 ft) on Avrainvillea amadelpha (in association with Costasiella kuroshimae and/or Costasiella sp. #2).
(see photos) It's diurnally active and readily autotomizes its central cerata.
Distribution: Oahu: also known from Kwajalein Atoll.
was first recorded in Hawaii from Diamond Head State Park, Oahu by CP
Oct. 3, 1999. Since its food algae, Avrainvillea amadelpha, is
thought to be introduced there's some chance that this species is, as
well. However, it's also possible that it's an indigenous species that
originally ate something else but found A. amadelpha to its liking once it was introduced. (Note 3)
Photo: Roney Rodrigues: west shore, Oahu; March 7, 2021.
Observations and comments:
Note 1: The
white granules appear to be outpocketings of the digestive gland that
lack stored chloroplasts. Several other sacoglossids that readily
autotomize their central cerata also concentrate chloroplast storage in
their lateral cerata (that aren't shed). Presumably, this reduces the cost
Note 2: The intensely blue patches on the
cerata appear to be irregular, subcutaneous masses (in close-ups)
little variation in appearance with changes in the apparent angle of
the light. That suggests that they may be composed of an intrinsically
blue (or fluorescent?)
pigment rather than being the result of iridescence. The bright blue
spots may bring the species into the "periphery" of the mimicry complex
associated with the ocellate sea hares. Or, perhaps the color is
incidental and the masses contain toxins evolved to deter predators
Note 3: The original two animals from 1999 weren't found in a typical Avrainvillea amadelpha habitat. Meanwhile, the one animal from elsewhere
that appears to be a close match (under the assumption that it had
autotomized its central cerata before being photographed) was found
on Codium sp. at Kwajalein Atoll.