Elysia rufescens (Pease, 1871)
|Maximum size: 48 mm.
This is a highly variable species with sedentary and migratory forms.
The sedentary form has tall, thin parapodia containing elaborate
ramifications of the digestive gland. The parapodia usually meet in a
more or less continuous line without obvious chimneys and are
olive-brown with closely spaced cream patches. There is a continuous
violet-black marginal line and an orange submarginal line, both
overridden by cream patches. The rhinophores are olive-brown with cream
patches and violet-black tips. Occasional animals may lack most dark
pigment, including the marginal black line (perhaps due to loss of
chloroplasts with senescence?).
(Note 1) The migratory form is usually smaller
(seldom exceeding 33 mm) with lower and proportionately thicker
parapodia that are typically held in three chimneys. Unlike in
the sedentary form, they contain relatively few branches of the
digestive gland but are otherwise similar in color. Some animals have a
few small orange flecks on the interior of their parapodia.
marginal line may be absent. (Note 2)
The sedentary form of Elysia
rufescens is commonly found in beds of the green
algae Bryopsis, often
occurring in mixed populations with the sedentary form of Elysia marginata. Such Bryopsis beds usually grow in
highly protected to moderately exposed back reef locations at depths of
< 1 m
(< 3 ft). Mature animals show little change in
or night. (Note 3) The migratory form is
common in moderately protected to moderately exposed rocky habitats. It
from the low intertidal to about 3 m (10 ft). Mature migratory animals are
active but rest in the open at night. We've observed the sedentary form
feeding on Bryopsis in
dishes. A migratory animal laid a
tightly coiled, white egg mass without extra-capsular yolk. Hatching
occurred in about five days in the laboratory. (Note 4)
Big Island, Maui, Oahu, Kauai, French Frigate Shoals and Midway (also Johnston Atoll): widely distributed in the
Elysia kushimotoensis Baba, 1938 may be a synonym.
As suggested in Kay (1979), it
seems likely that it was first reported from Hawaii in Pease, 1860 (as Pterogasteron
bellum). It's probably the animal illustrated as Elysia sp. in Edmondson, 1946. The name means "becoming red."
sedentary form: found by John Hoover; Oahu; Sept., 1998.
Observations and comments:
A pale animal collected along with normal animals at Black Point, Oahu
on Oct. 3, 1999 died when held overnight supporting the suggestion that
such animals may be senescent (or diseased?) individuals that have lost
their normal complement of chloroplasts and dietary pigments.
The marginal violet-black line largely disappeared over an 8-day period
when a 4 mm animal was held without food (collected on Oct. 23, 1998 at
Hekili Point, Maui). Similar fading has also been observed on other
occasions. That supports the suggestion that the occasional absence of
character in the field may be due to diet. (see photos).
Note 3: The tall
parapodia of sedentary animals observed on Bryopsis (at Black Point, Oahu)
were entangled in the algae
making the animals difficult to extract. When collected, they appeared
more difficulty crawling in a dish than most species. Also, there was
no obvious damage to the
algae in the field despite the relatively dense populations. That
suggests the sedentary form may be an adaptation to a preferred host
with the animals "sacrificing" mobility in order to increase the
surface area available for photosynthesis by retained chloroplasts.
An egg mass associated with a larger animal in the field had evenly
spaced "dashes" of opaque white pigment on its upper surface similar to
those in egg masses of Elysia marginata. Deposition of the pigment may be facultative. However, E. rufescens and E. marginata are often found in the same areas raising the possibility that the association was coincidental.