Invasion History

First Non-native North American Tidal Record: 1980
First Non-native West Coast Tidal Record:
First Non-native East/Gulf Coast Tidal Record: 1980

General Invasion History:

Thecacera pennigera is a cosmopolitan species, described from southern England, but known from locations around the globe. It occurs in the Eastern Atlantic from Scotland to South Africa (Dekker 1986; Valles et al. 2000; Mead et al. 2011b); Brazil (Marcus 1957); the Arabian Sea (Eliot 1905, cited by Willan 1976); and the Western Pacific, from the Sea of Japan (Baba 1960) to Australia and New Zealand (Willan 1976; Burn 2006). It has not been reported from the Eastern Pacific, to our knowledge. At many of the sites where it has been collected, it appears to be rare and/or sporadic, often known from single specimens, or briefly abundant and then disappearing (Willan 1976; Dekker 1986; Clark 1995; Valles et al. 2000; Australian Museum 1998-2014). Thecacera pennigera feeds on bryozoans of the genus Bugula, which are abundant in ship fouling, and so, has great potential for ship transport. Although this sea slug was described from Europe, its origin is unknown. William Rudman (Australian Museum 1997) suggests the Western Pacific as a possibility. Occurrences of this conspicuous nudibranch in fairly well-studied areas of Australia in 1951 (Allan 1957, cited by Willan 1976), New Zealand in 1973 (Willan 1976), the eastern Mediterranean (Barchana, in Australian Museum 2006), Florida in 1972-1980 (Clark 1995; Goddard, in Australian Museum 2005), and Woods Hole, Massachusetts (Shepard, in Australian Museum 2003) are indicative of modern introductions.

North American Invasion History:

Invasion History on the East Coast:

Willan (1976) stated that there were no records of T. pennigera from North America. However, Kerry Clark collected this nudibranch at Sebastian Inlet, in the Indian River Lagoon, Florida between 1972 and 1980, but did not find it in later sampling (1985-1994) (Clark 1995). Thecacera pennigera was collected again at this location in 2005 (Goddard, in Australian Museum 2005). In 1981, two specimens were collected off Georgia and South Carolina, well offshore at depths of 28 and 33 m, unusual for this species (USNM 848877 and 848880, U.S. National Museum of Natural History 2014). In November 2003, T. pennigera appeared much farther north, in Woods Hole, Massachusetts (Alan Shepard, in Australian Museum 2003). It soon disappeared, but was found again 'in great numbers' in November 2006 (Alan Shepard, in Australian Museum 2006).

Invasion History Elsewhere in the World:

As described above, T. pennigera appears to be cryptogenic in much of its world wide range. Many of its occurrences are single specimens, or brief population 'booms', often near harbors, and very spotty geographically. In Australia, the first collection was in 1951, in Sydney Harbour (Allan 1957, cited by NIMPIS 2014), and subsequent collections have been in harbor areas [Adelaide, Port Philip Bay, and Port Stephens (Burn 2006; Australian Museum 1998-2014)]. New Zealand records are from two harbors on the North Island [Waitemata and Whangarei (Willan 1976; Australian Museum 1999)]. Scattered records are known from the Central and Eastern Mediterranean, from Italy (Vayssiere 1913, cited by Willan 1976; Cattaneo-Vietti & Chemello 1987, cited by Kapiris et al. 2014; Cattaneo-Vietti & Giovine 2008, cited by Kapiris et al. 2014) and Israel (in 2008, Dani Barchana, in Australian Museum 2008). These records are collections or sightings of single specimens, so the establishment of this species in the Mediterranean is unknown. The absence of records in the Western Mediterranean suggests that the central and eastern records are due to ship transport, either from the Eastern Atlantic, or by the Suez Canal.


Thecacera pennigera has a slender body, tapering to a long tail. Its foot is narrow, but projects slightly beyond the body. Below the head are two broad, pointed extensions. There are no oral tentacles. The rhinophores are club-like structures, each consisting of 14 lamellae, and are flanked anteriorly by a larger, leaf-like sheath. There are 3-5 gills on the dorsal midline, each consisting of smaller feathery structures. Behind the gills is a pair of cerata (simple, tapering structures). The tail is about half the body length. The whole animal reaches about 30 mm. Its body is translucent, but marked with conspicuous orange and black spots, which extend to the rhinophores, gills, and cerata. The color patterns are variable between individuals, but at least for European specimens, the orange sots are 'always' larger than the black ones, and the tops of the rhinophores are yellow, with black speckles. Description based on Willan (1976), Thompson and Brown (1984), Rudman (1997), Valles et al. (2000), and Jung et al. (2013).


Taxonomic Tree

Kingdom:   Animalia
Phylum:   Mollusca
Class:   Gastropoda
Subclass:   Opisthobranchia
Order:   Nudibranchia
Family:   Polyceratidae
Genus:   Thecacera
Species:   pennigera


Doris pennigera (Montagu, 1815)
Thecacera lamellata (Barnard, 1933)
Thecacera maculata (Eliot, 1905)

Potentially Misidentified Species



Thecacera pennigera is a nudibranch which inhabits harbors, inlets, and open coasts. Nudibranchs are simultaneous hermaphrodites and copulate reciprocally or unilaterally. Eggs are laid in masses forming a white, buff, or pink band (Thompson and Brown 1984; Shepard, in Australian Museum 2003). We have not found information on larval development of T. pennigera. Its reproductive mode is unknown. Dramatic population fluctuations seem to be common in nudibranchs, and T. pennigera seems to have brief blooms, followed by disappearances, and then reappearances years later at the same location (Sebastian Inlet, e.g. Clark 1995, Goddard, in Rudman 2014; Woods Hole, Shepard 2003, 2006, in Rudman 2014). This may indicate an ability to persist at very low densities.

Thecacera pennigera inhabits rocky areas, pilings, and soft bottoms with bryozoans. Its wide range indicates a broad tolerance of temperature from cold-temperate to tropical regions, but not much penetration into low salinities (Baba 1960c; Willan 1976; Dekker 1986; Thompson and Brown 1984; Rudman 1998-2014; Kapiris 2013). It feeds on bryozoans of the genus Bugula, on colonies attached to hard surfaces and algae. The association with Bugula has probably led to wide dispersal in ship fouling (Willan 1976; Thompson and Brown 1984; Mead et al. 2011b).


Bryozoans (Bugula spp.)

Trophic Status:




General HabitatCoarse Woody DebrisNone
General HabitatMarinas & DocksNone
General HabitatRockyNone
Salinity RangePolyhaline18-30 PSU
Salinity RangeEuhaline30-40 PSU
Tidal RangeSubtidalNone
Vertical HabitatEpibenthicNone

Tolerances and Life History Parameters

Maximum Length (mm)30Willan 1976; Thompson and Brown 1984
Broad Temperature RangeNoneCold Temperate-Tropical
Broad Salinity RangeNonePolyhaline-Euhaline

General Impacts

No impacts have been reported for this species.

Regional Distribution Map

Bioregion Region Name Year Invasion Status Population Status
NEA-III None 1815 Crypto Estab
NEA-II None 1815 Crypto Estab
WA-I None 1995 Crypto Estab
WA-IV None 1933 Crypto Estab
NWP-3b None 1960 Crypto Estab
EAS-III None 0 Crypto Estab
NWP-3a None 0 Crypto Estab
NZ-IV None 1973 Def Estab
AUS-X None 1951 Def Estab
NEA-IV None 0 Crypto Estab
SA-II None 1957 Crypto Estab
CAR-I Northern Yucatan, Gulf of Mexico, Florida Straits, to Middle Eastern Florida 1980 Def Estab
NA-ET3 Cape Cod to Cape Hatteras 2003 Def Estab
M010 Buzzards Bay 2003 Def Estab
S190 Indian River 1980 Def Estab
MED-V None 2008 Def Unk
IP-1 None 1901 Crypto Estab
NEA-V None 0 Crypto Estab
AUS-VII None 2005 Def Estab
AUS-IV None 0 Crypto Estab
EAS-VI None 0 Crypto Estab
CAR-VII Cape Hatteras to Mid-East Florida 1981 Def Unk
MED-III None 1913 Def Unk
MED-IV None 0 Def Unk
EAS-I None 0 Crypto Estab
NWP-2 None 0 Crypto Estab
AUS-VIII None 2003 Def Estab
WA-V None 0 Crypto Estab
NWP-4a None 0 Crypto Estab
CIO-I None 0 Crypto Estab
MED-VII None 2017 Def Unk

Occurrence Map

OCC_ID Author Year Date Locality Status Latitude Longitude


Orth, Donald J. (2010) Socrates Opens a Pandora’s Box of Northern Snakehead Issues, American Fisheries Society Symposiums 89: 203-221

Baba, Kikutaro (1960c) The genera Polycera, Palio, Greilada, and Thecacera, from Japan (Nudibranchia-Polyceridae), Publications of the Seto Marine Biological Laboratory 13: 75-79

Burn, Robert (2006) A checklist and bibliography of the Opisthobranchia (Mollusca: Gastropoda) of Victoria and the Bass Strait area, south-eastern Australia, Museum Victoria Science Reports 10: 1-42

Clark, Kerry B. (1995) Rheophilic/oligotrophic lagoonal communities through the eyes of slugs (Mollusca: Opisthobranchia), Bulletin of Marine Science 57(1): 242-251

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

Edmunds, Malcolm (1977) Larval development, oceanic currents, and origins of the opisthobranch fauna of Ghana, Journal of Molluscan Studies 43: 301-308

Fernando, Anthony V.; Kemp, Philip S., Jr. (2007) A key to the benthic shell-less opisthobranch gastropods of North Carolina, Journal of the North Carolina Academy of Science 123(4): 233-241

Fraser. C. McClean (1936) Some Japanese hydroids: Mostly new, Transactions of the Royal Society of Canada- Section V Biological Sciences 30: 49-53

Jung, Daewui; Lee, Jongrak; Kim, Chang-Bae (2013) A report on species of phyllidiid and polycerid nudibranch including two species new to Korea, Journal of Species Research 2(1): 7-14

Kapiris, K. and 25 authors (2014) New Mediterranean marine biodiversity records (April, 2014), Mediterranean Marine Science 15(1): 198-212

Marcus, Ernesto (1957) On Opisthobranchia from Brazil., Journal of the Linnean Society of London 43(292): 390-486

Mead, A.; Carlton, J. T.; Griffiths, C. L. Rius, M. (2011b) Introduced and cryptogenic marine and estuarine species of South Africa, Journal of Natural History 39-40: 2463-2524

Medrano, Emmanuel Merselis, Daniel G. Bellantuono, Anthony J. Rodriguez-Lanetty, Mauricio (2019) Proteomic basis of symbiosis: a heterologous partner fails to duplicate homologous colonization in a novel cnidarian– Symbiodiniaceae mutualism, Frontiers in Microbiology 10: Published online

NIMPIS (National Introduced Marine Pest Information System). 1998-2015 NIMPIS (National Introduced Marine Pest Information System). <missing URL>

Rudman, W. B. 1997-2016 Sea Slug Forum.

Smithsonian Marine Station at Fort Pierce 2007-2022 Indian River Species Inventory.

Thompson, T. E.; Brown, G. H. (1984) <missing title>, Ray Society, London. Pp. <missing location>

U.S. National Museum of Natural History 2002-2021 Invertebrate Zoology Collections Database.

Valles, Yvonne; Valles, Angel, Ortea, Jesus (2000) On the phaneroibranch dorids of Angola: A crossroads of temperate and tropical species., Zoosystema 22(1): 15-32

Willan, Richard C. (1976) The opisthobranch Thecacera pennigera in New Zealand, with a discussion of the genus., Veliger 18(4): 347-352

Wirtz, Peter (1998) Twelve invertebrate and eight fish species new to the marine fauna of Madeira, and a discussion of the zoogeography of the area., Helgolander Meeresuntersuchungen 52: 197-207