Phyllodesmium sp. #1
|Maximum size: 25 mm.
an extremely slender body and very long cerata. (Note
1) The cerata usually appear straight when
the animal is crawling in the open at night but show "corkscrew" tips
when slightly contracted in response to disturbance. The rhinophores
are simple. The cream body and cerata are decorated with white blotches
and the cerata have white tips. The branches of the digestive gland
have short side pockets that are usually dark brown giving the cerata
and body a speckled appearance. A touch of blue iridescence is often
noticeable subapically on the cerata.
#1 is a moderately common aeolid found in the low intertidal and very
shallow subtidal at protected to semi-protected cobble beaches. Rarely, it's been found to 7 m (22 ft). It's
nocturnally active but may be found during the day on the undersides of
rocks, often in pairs. It has been observed feeding on a small,
unidentified octocoral (currently under study by Phill Alderslade, Sam
Kahng & Gary Williams) and is often closely associated with its
host in the field. (Note 2) It appears to feed
preferentially on the
stolons and retracted polyps. It may also get some nutrition by
retaining functional zooxanthellae in its digestive gland. (Note 3) (Note 4). The egg mass is a "kinked" white spiral
5-6 mm in diameter with a 1 mm high ribbon. Hatching occurs in about
four days in the laboratory.
Distribution: Big Island, Maui and Oahu:
from Enewetak Atoll and Heron Island (may be widely distributed in the Indo-Pacific).
Taxonomic notes: It was first
Hawaii from Hekili Point,
Maui by CP in August, 1983.
Photo: CP: Hekili Point, Maui; May 12, 2002.
Observations and comments:
1: A 17 mm animal collected in Sept., 1988 was held for
several days in a small dish with a basalt cobble having openings
only 1-2 mm in diameter. Nevertheless, it managed to
"disappear" completely during the day and "reappear" at dusk. Perhaps,
the ability to hide in extremely narrow crevices allows it to better
resist dehydration in the low intertidal?
Note 2: The host octocoral of this
species is also unusual in that it lives almost exclusively in the
intertidal. It grows on the undersides of cobbles,
but near their edges, at the boundary between the algal and sponge
communities. The polyps may be widely spaced and arise from a stolon
that is usually buried in the algal/invertebrate turf. They can fully
retract when disturbed.
3: An animal found on April 12, 2002 was offered a
cobble with a colony of its host octocoral on April 16. It was observed
feeding on the stolons and closed polyps of the octocoral over several
days. During feeding, the side branches of its digestive gland became
noticeably darker suggesting that it may be concentrating zooxanthellae
from its food in those pockets. (see
photos) The darker appearance persisted for several days and most
animals seen in the field have dark pockets when found. That may
indicate that the zooxanthellae remain functional long enough to
contribute to the nutrition of the animal. This possibility is also
supported by the fact that the species typically spends the day resting
near its host in the same "twilight zone" conditions that the coral's
zooxanthellae are presumably adapted to. The branches of the digestive
gland also appear more elaborate than those of species that feed on
azooxanthellate corals (although less elaborate than in most
species that are known to rely extensively on zooxanthellae).
A similar, but smaller, stoloniferous octocoral that lacks
zooxanthellae inhabits invertebrate turf in the "dark zone" on the
bottoms of rocks in the subtidal. It also occurs in invertebrate turf
on heavily shaded dock pilings. Perhaps, the occasional animal found
subtidally (see photo) eats that
species, instead? There's some chance that the subtidal octocorals found
under rocks and on pilings are different species...