Potentially Misidentified Species:
Although Bursatella is a monophyletic genus containing only B. leachii,
a wide range of color and morphological variations has prompted come authors to
suggest that the global distribution comprises several distinct subspecies
(Eales and Engel 1935, Bebbington 1969). If this convention is followed, the
subspecies occurring in Florida is Bursatella leachii pleii Rang, 1828.
Sea hares of the genus Aplysia co-occur with B. leachii
throughout much of its range. Bursatella and Aplysia are easily
distinguished from one another, as Aplysia are larger, lack the frilled,
ragged appearance of Bursatella and possess large swimming flaps that
are absent in B. leachii.
II. HABITAT AND DISTRIBUTION
Bursatella leachii is a circumtropical species found nearly worldwide in
warm temperate to tropical marine environments (Rudman 1998). Kruczynski and
Porter (1969) list North Carolina as the northern limit of the species on the
US east coast.
Bursatella leachii can be found throughout the IRL system, although much
of the time it apparently persists at relatively low population densities (see
III. LIFE HISTORY AND POPULATION BIOLOGY
Age, Size, Lifespan:
Adult ragged sea hares are capable of attaining lengths of 25 cm, although
7.5-10 cm is more typical of Florida populations (Voss 1980, Kaplan 1988).
Published field and laboratory information is lacking, but the experience of
aquarists suggests that the maximum life expectancy of this and other species
of sea hares is just over one year (van Horn 2005).
Although Bursatella leachii is spatially and temporally highly sporadic
in occurrence, it is periodically encountered at high densities in the
environment (Rudloe 1971, Lowe and Turner 1976). Localized populations of
B. leachii have been reported at densities of 660 individuals per square
meter. Such irruptive population outbreaks are probably attributable to
vagaries of the environment and the relative rarity of situations in which
environmental conditions are near-optimal. Population explosions most likely
occur when larval supply is good, dietary resources are abundant, and tides,
currents, and weather conditions are favorable (Rudman undated).
Just as mass-settlement events are a common aspect of the life history, so too
are mass mortality events. Hundreds to thousands of dead and dying animals are
sometimes encountered washed ashore, and storms, tides, and extreme high
temperatures often serve as exacerbating factors.
As with most sea hares, Bursatella leachii is a cross-fertilizing
simultaneous hermaphrodite (Kaplan 1988). Fertilization is internal, with one
individual transferring sperm via an eversible penis located on the right side
of the head to the dorsally located gonopore (genital opening) of a second
individual (Van Horn 2005). Thus inseminated, individuals lay large, tangled,
spaghetti-like benthic egg masses that are usually orange, yellow, green, or
brown in color. The string-like egg masses are comprised of numerous separate
capsules, each of which contains 1-20 eggs that are approximately 87 Ám in
diameter (Paige 1988).
Ragged sea hares attain sexual maturity at 2-3 months of age (Paige 1988). At
least in some parts of its range, the species appears to undergo continuous
recruitment with no well-defined reproductive season (Clarke 2004).
The larval biology of Bursatella leachii had previously been poorly
studied, largely due to lack of success at rearing specimens in the laboratory
(Henry 1952, Bebbington 1969). More recently, research findings by Paige
(1988) have provided more detail on larval and postlarval development. This
work is summarized here.
Under laboratory culture conditions at 25°C, animals hatch out as
planktotrophic larvae approximately a week after egg masses have been laid.
Newly hatched veligers lack eyes and possess a pair of sensory statocysts at
the base of the foot. Individuals are negatively geotropic and they maintain
themselves in the water column by swimming using a well-developed velum.
Growth of the planktonic larvae after hatching is rapid. Veligers attain
maximum size approximately 15 days after hatching, and metamorphic competency
is reached at 19 days posthatch. Paige (1988) notes that this is the shortest
known larval duration for aplysiids possessing planktonic larvae.
Competent B. leachii metamorphose to take up a juvenile existence on the
species of cyanobacteria that they will utilize as food, such as Microcoleus
lyngbyaceus (now Lyngbya majuscula). Individuals that are competent
to metamorphose are capable of delaying settlement for as much as 2.5 months
until a suitable settlement substratum (i.e., blue-green algae from the Family
Oscillatoriaceae) is encountered.
Upon settlement, an individual will attach to the substratum via mucus threads
and retract into the shell to metamorphose. The process takes 1-2 days. Within
a day of metamorphosis, postlarvae are seen crawling across the substratum
ingesting food with a radula that is already well developed.
Postlarval growth is rapid. The shell stops growing when individuals reach
2.5-3 mm length, and between 15-20 days post-settlement the vestigial shell is
discarded. By this time, the juvenile begins to resemble the adult in
appearance, including the presence of rudiments along the body that will become
the fleshy papillae. At this time, the juvenile also has the ability to
discharge a small cloud of ink if irritated (Paige 1988).
IV. PHYSICAL TOLERANCES
Bursatella leachii is considered to be a circumtropical species whose
distribution extends into warm temperate waters. The northernmost limit
reported for this species on the US east coast is North Carolina (Kruczynski
and Porter 1969), and this limit appears to be dictated by seasonal low water
Paige (1988) observed that embryonic development proceeded normally at
temperatures ranging from 20-30°C, but development ceased at 15°C.
The occurrence of ragged sea hares in a variety of oceanic and estuarine
habitats suggests a moderate tolerance for salinity fluctuations.
V. COMMUNITY ECOLOGY
Bursatella leachii is a grazing benthic detritivore/herbivore that feeds
primarily on cyanophytes and diatom mats and films found on sand, mud and other
benthic substrata. It can also facultatively consume some macrophyte material
such as Ectocarpus and Enteromorpha (Paige 1988, Rudman undated).
Wu (1980) and Clarke (2004) report a possible dietary preference for
Enteromorpha over cyanobacteria in the Pacific populations, but such
preference appears not to be universal.
Ragged sea hares are known to consume Lyngbya majuscula (=Microcoleus
lyngbyaceus), a cyanobacterial species abundant in the IRL and other
shallow marine systems of Florida. Sea hares are likely to derive a dietary
benefit from sequestering toxic metabolites (e.g., lyngbyatoxin-a) in the
digestive gland and in bodily secretions (Capper et al. 2005).
Dietary resource competition is unlikely to be severe in most habitats
inhabited by Bursatella leachii, owing to the abundance of benthic
cyanobacteria mats and films. Interspecific competition with co-occurring sea
hares of genus Aplysia is unlikely since these animals typically consume
various macroalgal species rather than cyanobacteria and filamentous algae
eaten by B. leachii (Rudman undated).
Like other sea slugs, Bursatella leachii is chemically protected from
most would-be predators by the presence of skin glands which secrete noxious or
unpalatable compounds (Rudman undated). Kamiya et al. (2006) note that a
number of antimicrobial or cytotoxic proteins have been reported from B.
leachii and other sea hares. Appleton et al. (2002) isolated a novel
bioactive malyngamide from New Zealand B. leachii and speculates that
the metabolite is the result of feeding on chemically defended algae.
The purple ink-like secretion produced from the purple gland of B.
leachii and other sea hares is commonly speculated to be a defensive decoy
similar to that produced by many cephalopods. Unlike cephalopods, however, sea
hares are not capable of a rapid escape response so this substance may not be
primarily defensive in nature. An alternate possibility is that the ink is a
metabolic byproduct produced in response to eating algae, particularly red
algae (Chapman and Fox 1969). The fact that Aplysia spp. consume
substantially more red algae and are better known for ink release than
Bursatella seems to support this position.
Bursatella leachii are common in intertidal and subtidal sheltered bay
and estuarine habitats with sand or muddy bottoms, and are a frequently
encountered component of tropical and subtropical seagrass and mangrove
communities (Lowe and Turner 1976). Clarke (2004) suggests that the species
may at times exert a strong influence on seagrass habitats because of their
sporadic high densities and their feeding specialization on cyanobacteria.
There has been speculation that adults of some populations (e.g., on the Gulf
coast of Florida) migrate offshore, especially during the summer (Henry 1952).
Although nocturnalism has been documented for several sea hares, including
Aplysia brasiliana, a nocturnal habit appears not to be typical for
VI. SPECIAL STATUS
In general, Bursatella leachii are currently of little economic
importance in Florida. There is a limited amount of commercial utilization of
the species in the marine pet trade, although the individuals sold in pet
stores are often the more colorful Pacific varieties/subspecies. Rudloe (1971)
indicated that ragged sea hare densities can at times become so great as to
negatively impact commercial shrimping operations.
An anti-HIV protein, bursatellanin-P, has been isolated from the purple ink
secretion of the species, although it remains to be seen whether there will be
any tangible biomedical of economic benefits derived from the discovery
(Rajaganapathi et al. 2002, Avilla 2006).
Appleton DR, Sewell MA, Berridge MV, and BR Copp. 2002. A new
biologically-active malyngamide from a New Zealand collection of the sea hare
Bursatella leachii. Journal of Natural Products 65:630-631.
Avila C. 2006. Molluscan natural products as biological models: Chemical
ecology, histology, and laboratory culture. Progress in Molecular and
Subcellular Biology 43:1-23.
Bebbington, A. 1969. Bursatella leachi guineensis subsp. nov.
(Gastropoda, Opisthobranchia) from Ghana. Proceedings of the Malacological
Society of London 38: 323-341.
Capper A, Tibbetts IR, O'Neil JM, and GR Shaw. 2005. The fate of Lyngbya
majuscula toxins in three potential consumers. Journal of Chemical Ecology
Chapman DJ and DL Fox. 1969. Bile pigment metabolism in the sea-hare
Aplysia. Experimental Marine Biolgy and Ecology 4:71-78.
Clarke CL. 2004. The Ecological Role of Sea hares (Opisthobranchia: Anaspidea)
Within Tropical Intertidal Habitats. Unpublished thesis, James Cook University
School of marine Biology and Aquaculture. 142 p.
Eales NB and Engel, H. (1935) The genus Bursatella de Blainville.
Proceedings of the Malacological Society, 21: 279-303.
Henry LM. 1952. Observations on the sea hare Bursatella leachii plei
Rang. Florida State University Studies 7:8-14.
Kamiya H, Sakai R, and M Jimbo. 2006. Bioactive molecules from sea hares.
Progress in Molecular and Subcellular Biology 43:215-239.
Kaplan EH. 1988. A Field Guide to Southeastern and Caribbean Seashores: Cape
Hattaras to the Gulf Coast, Florida, and the Caribbean. Peterson Field Guide
Series. Houghton Mifflin Company, NY. 425 p.
Kruczynski WL and HJ Porter. 1969. A new northern record for Bursatella
leachii pleii Rang (Opisthobranchia), with notes on its biology. The
Lowe EF and RL Turner. 1976. Aggregation and trail-following in juvenile
Bursatella leachii plei (gastropoda: Opisthobranchia. Veliger
Marcus E and EdB-R Marcus. 1972. On the Anaspidea (Gastropoda: Opisthobranchia)
of the warm waters of the western Atlantic. Bulletin of Marine Science
Paige JA. 1988. Biology, metamorphosis and postlarval development of
Bursatella leachii plei Rang (Gastropoda: Opisthobranchia). Bulletin of
Marine Science 42:65-75.
Rajaganapathi J, Kathiresan K, and TP Singh. 2002. Purification of anti-HIV
protein from purple fluid of the sea hare Bursatella leachii de
Blainville. Marine Biotechnology 4:447-453.
Rudloe J. 1971. The Erotic Ocean. Thomas Y. Crowell Co., NY. 448 p.
Rudman WB. undated. Bursatella leachii de Blainville, 1817. In: Sea Slug
Forum. Australian Museum, Sydney. Available online.
Rupert EE and RS Fox. 1988. Seashore Animals of the Southeast. A Guide to
Common Shallow-Water Invertebrates of the Southeastern Atlantic Coast.
University of South Carolina Press. 429 p.
Van Horn J. 2005. Sea hares in the aquarium. SeaScope 22:1-4.
Voss GL. 1980. Seashore Life of Florida and the Caribbean. Dover Publications,
Inc., NY. 199 p.
Wu RS. 1980. Foraging strategy in the sea hare Bursatella leachii
(Mollusca: Opisthobranchia). In: Morton BS and CK Tseng (eds). Proceedings of
the First International Marine Biological Workshop: The Marine Flora and Fauna
of Hong Kong and southern China. Hong Kong University Press.
J. Masterson, Smithsonian Marine Station
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