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
II. HABITAT AND DISTRIBUTION
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).
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
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
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).
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).
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).
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).
IV. PHYSICAL TOLERANCES
Thecacera pennigera is a temperate species with a broad thermal tolerance,
inhabiting waters as thermally dissimilar as Brazil and Massachusetts.
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.
V. COMMUNITY ECOLOGY
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 principle prey 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).
VI. INVASION INFORMATION
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
Potential to Compete With Natives:
The potential to compete with native species is unknown but is assumed to be
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 http://data.daff.gov.au/marinepests/index.cfm?fa=main.spDetailsDB&sp=6000016382.
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.
J. Masterson, Smithsonian Marine Station
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Page last updated: October 4, 2007