Species Description: Bursatella leachii, the ragged sea hare, is a medium- to large-sized benthic opisthobranch mollusc within the Order Anaspide, the sea hares. The body is variably colored, grayish-green to white-tan with dark brown blotches and spots, compact and rounded, with distinct head and neck regions evident. The body is also covered with numerous long, branching fleshy papillae that give the animal its ragged appearance. The gill is covered by a pair of fleshy parapodia. Two long, retractile olfactory tentacles called rhinophores occur on the head, and also two fleshy enrolled oral tentacles occur at each side of the mouth. Adults completely lack a shell (Voss 1980, Kaplan 1988, Rupert and Fox 1988).
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.
Regional Occurrence: 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.
IRL Distribution: Bursatella leachii can be found throughout the IRL system, although much of the time it apparently persists at relatively low population densities (see below).
Abundance: 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.
Reproduction: 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 µm 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).
Embryology: 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).
Temperature: 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 temperatures.
Paige (1988) observed that embryonic development proceeded normally at temperatures ranging from 20-30°C, but development ceased at 15°C.
Salinity: The occurrence of ragged sea hares in a variety of oceanic and estuarine habitats suggests a moderate tolerance for salinity fluctuations.
Trophic Mode: 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).
Competitors: 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).
Predators: 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.
Habitats: 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).
Activity Time: Although nocturnalism has been documented for several sea hares, including Aplysia brasiliana, a nocturnal habit appears not to be typical for Bursatella leachii.
Special Status: None
Economic Importance: 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).
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