The seagrass elysia, Elysia serca, is a small opisthobranch mollusc
of the Order Sacoglossa. The body color is variable, ranging from green to
dark brown. A characteristic pattern of dorsal whitish patches is also
usually present, with one patch on the body at about the heart and two more
located midway along the free edge of the parapodia. Other smaller white
spots may be present as well. Relative to other elysiids, the head is
proportionally larger and the parapodia are somewhat reduced (Rudman
Jensen (1983a) notes that the shape of the radular teeth of E. serca
is indistinguishable from that of another seagrass-feeding ascoglossan,
Elysia catulus. The author suggests that E. serca may be an
ecotype or subspecies of E. catulus. In light of this, relevant
information from the literature pertaining to E. catulus is included
here along with that for E. serca.
Potentially Misidentified Species
Elysia serca is one of 11 Elysia species reported from
shallow marine environments in Florida (Camp et al. 1998). At least eight
Elysia congeners are known to occur in the IRL (E. canguzua,
E. chlorotica, E. evelinae,E. ornata, E.
patina, E. serca, E. subornata, E. tuca).
The distinctive physical appearance of E. tuca (see above) combined
with the strong association of all members of the genus with their
respective preferred food sources (seagrass in the case of E.
serca), should be sufficient to allow differentiation of the seagrass
elysia from co-occurring Elysia species.
HABITAT AND DISTRIBUTION
Elysia serca is a subtropical to tropical species, occurring in the
western Atlantic from Florida to Brazil (Rudman 2004a). Elysia
catulus, a possible northern morph with older synonymy, is distributed
along the Atlantic coast of North America from Nova Scotia to South
Carolina (Rudman 2004b). Clark (1975) notes that the larvae of most
nudibranchiate molluscs from southern New England waters, including E.
catulus, are allochthonously produced, not the result of endemic
Although cryptic in its occurrence, Elysia serca may well be found throughout the seagrass beds of the Indian River Lagoon system.
LIFE HISTORY AND POPULATION BIOLOGY
Age, Size, Lifespan
Elysia serca is a small elysiid, attaining a total length of about 8
mm (Rudman 2004).
Information on the lifespan of Elysia serca in Florida is
lacking. The lifespan of Connecticut Elysia catulus individuals
appears to be just under one year. Newly settled individuals arriving
in late August and early September, growing to maturity over the next
several months, producing eggs in June and July, and disappearing
(apparently due too mortality) by late July just prior to the next
juvenile settlement event (Clark 1975).
Clark (1975) reports Elysia catulus is one of very few
nudibranchiate molluscs present in abundance in the Mystic River Estuary,
CT, throughout all or most of the year. For all months except late summer,
the author was able to collect more than 50 individuals per hour and noted
its presence virtually wherever Zostera marinaoccurred. As
with most nudibranchiate molluscs, the sudden appearance of large E.
catulus populations is due to the arrival of large numbers of
settling larvae followed by the rapid growth of newly-settled individuals
to visible size.
Sacoglossans are simultaneous hermaphrodites that cross-fertilize by means
of copulation and internal fertilization (Fox 2001).
In Connecticut E. catulus populations, individuals that settled out
of the plankton as juveniles in late summer begin producing eggs the
following June. Mean animal length decreases throughout the egg production
period, apparently due to tissue resorption to offset the energetic cost of
reproduction. Spiral E. catulus egg masses, containing as many as
1,000 eggs, are typically attached to the top 1/3 of a Zostera
seagrass blade. All adults disappear in late July after the period of egg
production has ended (Clark 1975).
Egg diameters in Connecticut E. catulus averaged approximately 75µm. Using the criteria of Thompson (1967), Clark (1975) describes E.
catulus as having Type I development, characterized as having
relatively small ova, short hatching times (2-28 days), and a long
planktonic larval phase.
Larval settlement in Connecticut E. catulus populations occurs in
late August and early September. Newly settled individuals typically
exploit the safety of (inrolled Is this a word?) Zostera leaf
margins, and older/larger animals typically occupy the exposed flat leaf
surfaces (Clark 1975).
The subtropical/tropical Elysia serca is restricted to warm waters
from Florida to Brazil. Low water temperature is likely the key factor
restricting the northern distributional limits of the species.
Elysia catulus exhibits significantly more thermal tolerance.
Animals in Connecticut's Mystic River Estuary persist in habitats that
exceed 27°C in the summer and are susceptible to ice formation in winter
(Clark 1975). Clark (1975) reports a behavioral response of E. catulus
to the onset of cold temperatures in which individuals seek the shelter
of the crevice within the fascicle binding adjacent seagrass leaves.
While a review of the relevant literature suggests a lack of extraordinary
tolerance to salinity fluctuation, the presence of Elysia serca in
habitats of seasonally varying salinity suggest the species is at least
Elysia serca from Florida have been reported feeding on several
species of seagrass, including Halophila engelmanni, Halodule
beaudettei, (formerly Halodule wrightii), Syringodium
filiforme, and Thalassia testudinum (Jensen 1983b). Young
plants with fully developed blades and no epiphytes or epifauna are
preferentially consumed, and animals collected from Halophila
engelmanni were larger than those collected from Thalassia
testudinum (Jensen 1983a).
Earlier references reported E. serca inhabiting and feeding on
Ulva, sp. (Marcus 1955, Hosoe 1956), but more recent information
suggests a near-exclusive association with seagrasses.
The feeding strategy is typical of sacoglossans, with plant cell walls
punctured by means of the radular tooth and liquid cell contents withdrawn
via suction and ingested (Clark 1975). Plant cells which have been pierced
and sucked dry are visible as darker areas on leaf surfaces.
Food preference and growth experiments conducted by Jensen (1983b) suggest
that Halophila engelmanni is a preferred food source and also the
Florida seagrass species that allowed maximal growth. The author
postulates that the preference for and marked growth on H.
engelmanni is the result of the large epidermal cells of this seagrass
that allow exploitation of this food type by the size and shape of the
leading radular tooth in E. serca. Preference for H.
engelmanni may also be due to the absence of tannins, which are present
in some other seagrasses and offer a degree of chemical defense against
Clark (1975) confirms that Mystic River Estuary E. catulus
populations lived on and consumed the seagrass Zostera marina.
The author notes that this is the first ever report of a sacoglossan which
feeds on a spermatophyte plant.
Although several sacoglossan molluscs are known to be capable of
kleptoplasty (the retention of functional chloroplasts derived from
typically macroalgal food sources), functional chloroplasts have thus far
not been found in either E. serca or E. catulus (Clark et
al. 1990). This may be due to the unsuitability of seagrass chloroplasts
for kleptoplastic retention compared to those from macroalgae such as
Caulerpa spp., Halimeda spp., and other forms that apparently
lend themselves to kleptoplasty.
Despite the co-occurrence of several Elysia congeners in the IRL and
elsewhere in Florida, the dietary specializations particular to each
species very likely preclude severe interspecific competition for food
resources. The persistence of the species at relatively low overall
density relative to the availablility of seagrass suggests that food
availability is not a likely limiting resource.
No major predators on the seagrass elysia are reported in the literature.
Elysia serca is strictly associated with subtropical to tropical
seagrass habitats. The seagrasses act as both living space/refuge and food
source for this animal.
Detailed information is lacking, but an abundance of daytime encounters by
investigators suggests much of the foraging activity of the species is
carried out during the day (Rudman 2004a).
Clark (1975) suggests that the feeding strategy (cell piercing) of
Elysia catulus may have allowed the animal to serve as a vector in
the transmission of the Zostera wasting disease that decimated
Zostera seagrass populations in the early 1930s. The author notes,
however, that the actual act of feeding causes negligible damage to
seagrasses, due to the relatively small size of E. catulus
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marine invertebrates of Florida. Florida Marine Research Institute
Technical Report TR-3. 238 pp.
Clark KB. 1975 Nudibranch life cycles in the northwest Atlantic and their
relationship to the ecology of fouling communities. Helgolander wiss.
Clark KB, Jensen KR, and HM Stirts. 1990. Survey for functional
kleptoplasty among West Atlantic Ascoglossa (=Sacoglossa) (Mollusca:
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Fox RS. Invertebrate Zoology OnLine. Laboratory Exercises to Accompany Fox
REE and Barnes RB. 2004. Invertebrate Zoology, A Functional Evolutionary
Approach (7th edition). Brooks Cole Thomson, Belmont, CA. 963 p.
Hosoe K. 1956. Notas biologicas sobre Elysia serca Marcus, 1955.
Universidade de Sao Paulo Contribuicoes Avulsas do Instituto Oceanografico
Jensen KR. 1982. Occurrence of Elysia serca Marcus in Florida, with
notes on the synonymy and biology of the species. Journal of Conchology
Jensen, K.R. 1983. Further Notes on the Ecology and Systematics of
Elysia serca Marcus (Opisthobranchia, Ascoglossa). Journal of
Molluscan Studies 49:69-72.
Jensen KR 1983. Factors affecting feeding selectivity in herbivorous
Ascoglossa (Mollusca: Opisthobranchia). Journal of Experimental Marine
Biology and Ecology 66:135-148.
Marcus Er. 1955. Opisthobranchia from Brazil. Boletim da Faculdade de
Filosofia, Ciencias e Letras. Universidade de Sao Paulo, Zoologia
Rudman WB. 2004. Elysia serca Marcus, 1955. Species profile from
Sea Slug Forum. Australian Museum, Sydney. Available online.
Rudman WB. 2004. Elysia catulus Gould 1870. Species profile from Sea
Slug Forum. Australian Museum, Sydney. Available online.