young, about 4 mm
Elysia tomentosa Jensen, 1997
|Maximum size: 40 mm.
This is a highly variable species with sedentary and migratory forms as
well as various intergrades. The sedentary form has tall, thin,
lightly-papillate parapodia containing elaborate ramifications of the
digestive gland. The parapodia are usually held in three chimneys and
are bright to dull green with white flecks and small, solid-brown dots.
There is a pale orange marginal line and, sometimes, a faint brown
sub-marginal line. The migratory form is usually smaller (seldom
exceeding 24 mm) with lower parapodia and better developed papillae.
Its dark spots may be ocellate rather than solid and it is usually
light olive in color. The papillae may vary from fine and branching
with branches of the digestive gland in their centers to broad and
conical without branches of the digestive gland in their centers. (Note 1) In some animals, there may be a
violet-black band (sometimes interrupted) surrounding the posterior
extension of the
pericardium. (Note 2) Background color is
probably derived primarily from retained chloroplasts and usually
color of the animal's food algae. Occasional animals may have dark
pigment on their rhinophores.
The sedentary form of Elysia
tomentosa is commonly found in stands of large-form Caulerpa racemosa. (Note 3) Such stands usually occur in protected to
exposed locations at depths of < 1 m (< 3 ft) but are seldom
migratory form is moderately common and is found on shallow rocky
bottoms at depths of 1-2 m (3-6 ft) where it apparently feeds
small, widely scattered clumps of Caulerpa
serrulata. Mature animals are diurnally active and may jerk
their parapodia occasionally while crawling. They typically rest in the
open at night with the parapodia spread. (Note 4) We've observed feeding on C. racemosa
(both large and small forms) and C.
serrulata in dishes. However, individuals may accept or reject
pieces of apparently identical algal clumps. (Note 5)
Keoki Stender also reports finding it on Caulerpa taxifolia. (Sea Slug Forum)
egg mass is a flattened, 3/4 mm to 1 mm wide, yellow ribbon composed of
a smaller egg string laid down in a zig-zag pattern. It is deposited in
a loosely coiled, irregular spiral. An irregular band of bright
yellow extra-capsular yolk runs along the upper surface of the
string emphasizing the zig-zag pattern. The eggs hatch in about six
days in the laboratory. The extra-capsular yolk remains in the egg mass
after hatching. (Note 6)
Big Island, Maui, Lanai, Oahu, Kauai and Midway: widely distributed in
This is the species that Kay, 1979 lists as Elysia cf. cauze Marcus, 1957. It has also
been listed in some sources as Elysia
expansa (O'Donoghue, 1924). See the Sea Slug Forum
for further discussion. There's some possibility that the Indo-Pacific population may be a complex.
Severns: sedentary form; about 30 mm: found by PF & CP on an
of large-form C. racemosa;
Ko'ie'ie Fish Pond, Kalepolepo, Maui; Sept., 1990.
Observations and comments:
1: The suggestion that animals with
fine and conical
papillae are the same species is supported by three animals that were
raised in dishes for several weeks on a diet of Caulerpa racemosa and C. serrulata but in relatively low
light. In all three cases, the papillae changed from fine and branching
to short and conical during the course of the experiment. During this
process, visible branches of the digestive gland in the papillae were
2: The violet-black line around the posterior extension of
the pericardium largely
disappeared in an animal held between Oct. 10 and Dec. 11 in 2002 and
fed Caulerpa racemosa and C. serrulata. That suggests
that its presence or absence may be related to diet or age.
3: Elysia tomentosa
is often found in much higher densities on the large form of Caulerpa racemosa than on other
hosts with little apparent damage to the algae. This suggests that it
may be getting more of its nutrition from retained chloroplasts when
feeding on that form and supports the suggestion that morphology and
behavior may change depending on the host algae and its chloroplasts.
4: When several animals of the sedentary form were held in
a dish with a clump of large-form C.
racemosa, they spent the day tucked among its branches but left
the alga at night to rest on the nearby surface. The migratory form has
also been seen resting in the open at night. Perhaps, this is a
response to reduced oxygen concentrations between the tightly packed
5: In Oct. 2004, a captive animal had been fed pieces
from a large clump of Caulerpa
serrulata. Then, a second captive E. tomentosa was released on that
clump. Over several days, about 1/3 to 1/2 of the clump disappeared in
the field. Afterwards, the first animal was fed another sprig from the
same clump. It responded by turning dark green, becoming "quiescent"
and developing necrotic patches on its parapodia (with subsequent
tissue loss). Afterwards, it recovered and continued to feed on pieces
of C. serrulata from other
clumps. This suggests that the alga may produce defensive toxins in
response to grazing. Animals have also been observed accepting or
rejecting pieces from otherwise identical clumps of the small form of C. racemosa. Perhaps, that species
also produces toxins that vary in concentration due to either genetic
or environmental factors?
6: An animal held in isolation from Nov. 10 till Dec. 11
in 2002 laid six consecutive egg masses. The first three were fertile
but the second three were not, suggesting that sperm may be retained
at least 13 days (the third mass was laid on Nov. 23). The animal was
fed Caulerpa racemosa and C. serrulata.
7: A 4 mm juvenile fed Caulerpa
racemosa and C. serrulata
reached 18 mm in 12
days. This growth rate is much faster than that observed in animals
that were mature when captured suggesting an initial "spurt" followed
by a decline in growth rate after sexual maturity. (see photos)
Addendum: Subsequent to most of the observations detailed above, Caulerpa macrophysa was split out
of the large form of C. racemosa,
and Caulerpa nummularia was
out of the small form of C. racemosa
(Abbott & Hohn, 2004) (
Huisman, Abbott & Smith, 2007). So, the different
responses to the various forms of C.
mentioned above may represent different responses to
those species, instead.