The wharf roach Ligia exotica is a large (up to 4 cm), gray-brown isopod found living in high intertidal to semi-terrestrial habitats It's long paired antennae, long uropoda (posterior
appendages), large, insect-like paired eyes, and supralittoral ("splash zone") habit aid in
Potentially Misidentified Species
The similar looking congener Ligia oceanica, a possible European
introduction, may be encountered from Rhode Island to Maine on the
Atlantic coast of the United States. On the Pacific American coast
another congener, L. occidentalis, occurs from Central California south
to Central America. While yet another Ligia species, L. baudiniana
occurs from South Florida to Brazil, Bermuda, and the Caribbean. In the
IRL region of Florida, L. exotica is unlikely to be confused with any
other species (Van Name 1936, Rupert et al. 2004).
HABITAT AND DISTRIBUTION
On the Atlantic coast of the US, the nonindiginous species L.
exotica has been reported from New Jersey south through Florida.
The species also occurs in coastal Texas and, as of 1996, has been
recorded from Hawaii as well (Eldredge and Smith 2001).
Sizable numbers of L. exotica are commonly observed scurrying
across the surfaces of rocks and pilings above the water line in
harbors and on jetties throughout the IRL region.
LIFE HISTORY AND POPULATION BIOLOGY
Age, Size, Lifespan
The NEMESIS database
indicates that adults are typically in the 2-3 cm size range but
that adult males can grow to nearly 4 cm. NEMESIS indicates that although
the lifespan of this species is not definitively known, the typical
lifespan averaged among several congeners is approximately 2
L. exotica is commonly encountered in sizable numbers in
intertidal/supratidal habitats where they resiide. Annectdotal
information from local marine ecology and invertebrate zoology
instructors indicates that 50 or more individuals can easily be
collected at sampling sites they frequented.
Lopes et al. (2006) report the reproductive season of L exotica at
two localities in Brazil to extend from approximately September or
October to approximately May or June. These authors note that
over a breeding season clutches from Ligia populations in Brazil
contained between 25 and 142 eggs/embryos, with a mean
ranging from around 73 to 88 eggs/embryos.
Females can reproduce multiple times a year and they exhibit a
trait called ovigery, which means they carry their fertilized eggs
outside their body, holding onto them with specialized egg-bearing
appendages (Chesapeake Bay Introduced Species
Fox (2006) indicates that direct development within eggs is
typical, but that some members of the genus Ligia may also hatch
out as free-swimming larvae. Juveniles molt through a series of
instar stages until they become mature.
L. exotica is most abundant in warm-temperate to subtropical
regions, and considerably less prevalent in tropical areas
(Schultz 1974). New Jersey represents the northernmost extent of
its introduced eastern North American range (Kensley and Schotte
1989, Kensley et al. 1995).
Although L. exotica is considered a supralittoral coastal organism, this
represents only a partial reinvasion of the marine environment by
a species belonging to the same terrestrial isopod suborder
(Oniscoidea) as pillbugs and wood lice (Rupert et al. 2004).
Some authors (e.g., Schultz 1977)
describe L. exotica as an herbovore primarily grazing on algal
crusts and diatoms growing on hard substrates of the high
intertidal. Eldredge and Smith (2001) consider it to be a
scavenger, feeding mostly on detritus and plant debris.
Ligia exotica is a good example of a cryptogenic species, i.e., a
widely distributed species whose original native range is in dispute
or not entirely known.
Although some sources (e.g., Eldredge and Smith 2001) cite the
native range of L. exotica as encompassing the northeastern Atlantic
and Mediterranean, other sources cast doubt on this assertion. The
initial description of the species by Roux in 1828 was based on
organisms collected from Marseilles on the French Mediterranean
coast. The species name ('exotica') given by Roux, suggests it
was not then considered to be native to the region (Roman 1977,
Ligia exotica has been widely and unintentionally spread through
shipping probably since the days of the first wooden sailing
vessels. L. exotica could have survived transport via early wooden sailing
vessels both within damp cargo and dry ballast, and also in
moist cracks and crevices occurring above the waterline (Lippson and
Lippson 1984, Schultz 1977).
The complete original native range is not known with certainty, but
the species is likely to be native to the Pacific from Japan and
China south to Madagascar and South Africa in the Indian Ocean. L.
exotica is also found on the West African coast and in the Azores,
likely a consequence of introduction (Roman 1977, NEMESIS).
In the United States collected specimens date back to 1880 from
North Carolina, and to the same decade from Florida (Key West, Cedar
Key), Brazil, and Uruguay (Van Name 1936). The widespread
occurrence of this non-native invertebrate suggests early and, most
likely, repeated accidental introduction. In subsequent decades
throughout the first half of the 20th century, L. exotica was
reported from Texas and Mississippi along the Gulf of Mexico,
throughout much of coastal Florida, and up the east coast of the
United States to New Jersey.
Potential to Compete With Natives
Ligia exotica is often a numerically dominant high
intertidal/supratidal invertebrate, but competitive impacts on
co-occurring natives remain largely unstudied. Where L. exotica occurs in
abundance some effects of grazing on communal dietary resources may
be assumed. There is no indication as to whether or not this
leads to dietary resource limitation for other community members,
Possible Economic Consequences of Invasion
Despite widespread introduction of Ligia exotica, no detailed
economic studies have been conducted. This may be due to the
difficulty in assessing a meaningful baseline condition in areas
where the species has occurred for several decades. L. exotica is
considered to be naturalized throughout much of its introduced
range. NEMISIS suggests that, aesthetics aside, the ecological and
economic impacts of non-native L. exotica are unlikely to be
significant. Likewise, the ecological/economic importance of
this non-native organism as either a resource consumer (i.e., through
grazing) or as a prey item (e.g., for shorebirds) remain unreported.
Eldredge L.G. and C.M. Smith 2001. A guidebook of introduced marine species in Hawaii. Bishop Museum Tech Rep. 21. 70 p.
Fox R. 2006. Lander University Invertebrate Anatomy OnLine.
Kensley B. and M. Schotte. 1989. Guide to the marine isopod crustaceans of the Caribbean. Smithsonian Institution Press, Washington, D.C. 336 p.
Kensley B., Nelson W.G., and M. Schotte. 1995. Marine isopod biodiversity of the Indian River Lagoon, Florida, Bulletin of Marine Science
Lippson A.J. and R.L. Lippson.1984. Life in the Chesapeake Bay, Johns Hopkins University Press, Baltimore. 293 p.
Lopes E.R., Blasina C.A, Dumont J.R., and C.F. Luiz. 2006. Reproductive
biology of Ligia exotica (Crustacea, Isopoda, Ligiidae) in Rio Grande, Rio
Grande do Sul State, Brazil. Iheringia, Ser. Zool. 96:5-12. (Available online here).
Roman M.L. 1977. Les oniscoides halophiles de Madagascar (Isopoda,
Oniscoidea), Beaufortia 26: 107-151
Ruppert E.E., Fox R.S., and Barnes R.B. 2004. Invertebrate Zoology, A
Functional Evolutionary Approach. 7th ed. Brooks Cole Thomson, Belmont CA. 963
Schultz G.A. 1974. Terrestrial isopod crustaceans (Oniscoidea) mainly from the
West Indies and adjacent regions. I. Tylos and Ligia., Studies on the Fauna of
Curaao and Other Caribbean Islands 45: 162-173.
Schultz G.A. 1977. Terrestrial isopod crustaceans (Oniscoidea) from St.
Catherines Island, Georgia, Georgia Journal of Science 35: 151-158
Van Name W.G. 1936. The American land and fresh-water isopod Crustacea. Bull.
American Mus. Nat. Hist. 71:1-535.