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Diagnostic physical features of Thecacera pennigera. Modified from Willan 1976.

Species Name: 
Thecacera pennigera Montagu, 1815

Common Name:
Winged Thecacera

Synonymy:
Doris pennigera Montagu, 1815
Thecacera lamellata Barnard, 1933
Thecacera maculata Eliot, 1905
Thecacera pennigera Fleming, 1828
Thecacera pennigera nigrescens Labbe, 1931

Species Description:
Winged thecacera, Thecacera pennigera, is a small, attractive sea slug that is typically translucent white with small orange, yellow and black to blue-black spots or flecks. It has a blunt and broadened head with two lateral extensions and paired rhinophores (olfactory organs) located inside flared, incomplete rhinophore sheaths. The anterior of the body is broad and it tapers posteriorly to a long, slender foot. A u-shaped tuft of gill is located dorsally just forward from a pair of finger-like white-tipped, postbranchial processes. These processes have coloration similar to the rest of the body (NIMPIS 2002). Picton and Morrow (1994) report that the glandular postbranchial processes are defensive in function.

The color form of the Atlantic T. pennigera specimens is somewhat different than that of Pacific specimens, with the spots in Atlantic specimens being notably smaller than Pacific variants (Sea Slug Forum 2006).

Potentially Misidentified Species:
The Indian River Lagoon is home to several infrequently encountered sea slugs, but most of these are easily distinguishable from T. pennigera. Even other polycerids such as fellow IRL inhabitant Polycera hummi would be hard to confuse for T. pennigera if the diagnostics listed above (e.g., the presence of a rhinophore sheath) are employed in identification.

II.  HABITAT AND DISTRIBUTION 

Regional Occurrence:
Thecacera pennigera has been recorded from depths ranging from the low intertidal down to 36 m (Swennen 1961, Willan and Coleman 1984).

This is a temperate species with a now cosmopolitan distribution that includes the European Atlantic coast, the Mediterranean, South Africa, West Africa, Brazil, the eastern United States from Massachussetts to Florida (possibly discontinuous), Brazil, Pakistan, Japan, Korea, eastern and southern Australia, and New Zealand (Willan 1976, Gosliner 1987, Picton and Morrow 1994, Valles et al. 2000, NIMPIS 2002, MIT Sea Grant, Sea Slug Forum).

IRL Distribution:
In the IRL Thecacera pennigera has only been collected in the vicinity of Sebastian Inlet at the Brevard and Indian River county border. Clark (1995) reported rare to sporadic occurrence of the species at this location between 1972 and 1980, while later surveys conducted between 1985 and 1994 revealed T. pennigera to be apparently absent. The author suggested T. pennigera occurred at well-flushed Sebastian Inlet but appeared to be absent elsewhere in the IRL because of the mollusc's requirement for an oligotrophic environment.

T. pennigera was rediscovered at Sebastian Inlet in April 2005, 25 years after the last confirmed records of it's presence (Goddard 2005, in Sea Slug Forum). Whether a cryptic population persists here remains unknown.

III. LIFE HISTORY AND POPULATION BIOLOGY

Age, Size, Lifespan:
Authors from a number of different locations where T. pennigera occurs report a similar size range of 15 mm to 30 mm for this species (Dekker 1986, Picton and Morrow 1994, NIMPIS 2002).

Abundance:
Although cosmopolitan in distribution, Thecacera pennigera is often rare or cryptic where it occurs, persisting undetected at low densities until environmental conditions (e.g., food availability) favor an irruptive event or perhaps a large influx of new recruits occurs.

When food is available and conditions are right Thecacera pennigera can occur in some abundance. In a 2.5-hour dive at Woods Hole, MA, during a 2003 irruptive event, a lone diver located 40-50 individual specimens (Shepard 2003, in sea Slug Forum).

Reproduction:
Details on the reproductive biology of Thecacera pennigera are lacking but it is believed to be similar to other sea slugs. Like all sea slugs, T. pennigera is believed to be hermaphroditic (possessing male and female reproductive structures). Mating in sea slugs typically involves internal fertilization of eggs using sperm from sperm packets swapped by a mating pair. External deposition of ribbon-like egg masses on a suitable substrate follows. In the case of T. pennigera the substratum is typically arborescent (upright branching, tree-shaped) bryozoans of the genus Bugula which are also the slug's principle food (NIMPIS 2002).

Embryology:
As with other aspects of Thecacera pennigera reproduction, information regarding embryology is lacking. The embryos of most sea slugs hatch into a short-lived planktonic larval stage before settling out as benthic juveniles, but some species exhibit direct development where young bypass a planktonic phase and instead emerge as crawl-away juveniles. Which strategy is seen in T. pennigera remains unknown (NIMPIS 2002).