Thecacera pennigera (Montagu, 1815)
Family: Polyceridae
Common names: Winged Thecacera
Synonyms: Doris pennigera Montagu, 1813,  more...
Thecacera pennigera image
Thecacera pennigera  

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.

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 its presence (Goddard 2005, in Sea Slug Forum). Whether a cryptic population persists here remains unknown.

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).

Temperature: Thecacera pennigera is a temperate species with a broad thermal tolerance, inhabiting waters as thermally dissimilar as Brazil and Massachusetts.

Salinity: Most species records come from coastal and oceanic salinity regimes, although limited occurrence in (low) intertidal habitats and in the vicinity of tidal estuarine inlets suggest at least some tolerance for salinity fluctuation.

Trophic Mode: Thecacera pennigera is a specialist carnivore that feeds perhaps exclusively on certain species of bryozoans such as the arborescent Bugula bryozoan species on which the slug and its egg masses are often found (Willan and Coleman 1984, Dekker 1986, NIMPIS 2002). In the IRL, the bryozoans Bugula neritina and B. stolinifera would constitute the principal prey species.

Associated Species: As noted above, bryozoans of the genus Bugula are the organisms with which Thecacera pennigera is most strongly associated. The bryozoans function as habitat and refuge, substratum for egg deposition, and food source for the slug. T. pennigera can also be found with other members of the fouling community like sea squirts, sponges and hydroids (NIMPIS 2002).

Little information is available on potential predators of T. pennigera but it is possible that some exist. Likewise, little information on potential T. pennigera habitat competitors is currently available (NIMPIS 2002).

Invasion History: Close association with their bryozoan food source/habitat has likely aided human-facilitated spread of the species, e.g., possibly as egg masses deposited on ship hull fouling Bugula species (Willan 1976).

Although the original records and description of this species are from the Atlantic coast of Europe (e.g., Montagu 1815), the true native range of this temperate and now cosmopolitan species remains unknown. Rudman (2005) suggests the possibility of a Pacific native range based in part on the wide color variation in Pacific T. pennigera populations compared to the uniformly small-spotted North Atlantic populations. The North Atlantic populations may have been based on a small founder population of small-spotted individuals or eggs transported to the North Atlantic 200 years ago.

Potential to Compete with Natives: The potential to compete with native species is unknown but is assumed to be minimal.

Possible Economic Consequences of Invasion: There are no reported economic impacts of this species.

Clark K.B. 1995. Rheophilic/oligotrophic lagoonal communities: Through the eyes of slugs (Mollusca: Opisthobranchia). Bulletin of Marine Science 57:242-251.

Dekker R. 1986. Second record of Thecacera pennigera (Gastropoda, Opisthobranchia) from the Dutch coast. Basteria 50:45-46.

Gosliner T. 1987. Nudibranchs of southern Africa, a guide to opisthobranch molluscs of southern Africa. Sea Challengers, California, USA. 136p.

NIMPIS. 2002. Thecacera pennigera species summary. National Introduced Marine Pest Information System.

Hewitt C.L., Martin R.B., Sliwa C., McEnnulty, F.R., Murphy, N.E., Jones T. and S. Cooper (eds.). Web publication available online at

Picton B.E. and Morrow, C.C. 1994. A field guide to the nudibranchs of the British Isles. Immel Publishing Ltd., 20 Berkeley Street, Berkeley Square, London W1X 5AE. ISBN 1-898162-05-0 Available online as part of the Encyclopedia of Marine Life of Britain and Ireland.

Sweenen C. 1961. Data on distribution, reproduction and ecology of the nudibranchiate molluscs occurring in the Netherlands. Netherlands Journal of Sea Research 1:191-240.

Valles Y., Valdes A., Ortea J., and M. Tavares, M. 2000. On the phanerobranch dorids of Angola (Mollusca, Nudibranchia); a crossroads of temperate and tropical species. Zoosystema 221:15-31.

Willan R.C. 1976. The opisthobranch Thecacera pennigera (Montagu) in New Zealand, with a discussion of the genus. Veliger 18:347-352.

Willan R.C. and N. Coleman. 1984. Nudibranchs of Australasia. Australasian Marine Photographic Index, Sydney. 56p.

Thecacera pennigera image
Thecacera pennigera  
Thecacera pennigera image
Thecacera pennigera  
Thecacera pennigera image
Thecacera pennigera  
Thecacera pennigera image
Thecacera pennigera