The invasive and suspected non-native green porcelain crab Petrolisthes armatus is a small, flat crab that
is usually orange-brown to dark brown as an adult, with a speckled and
somewhat lighter appearance as juveniles. Individuals may also be olive green
to dark green in color, giving rise to the common name. Mature specimens of
both sexes have portions of their mouthparts pigmented brilliant blue. The
chelae (pincers) are long and each possesses a distinct orange spot that
is visible when they are open.
As is characteristic for all porcelain crabs, the antennae are inserted external to
the eyes, not between them as is observed in most other crab families (Knott
and King undated, IRL Field Guide).
Potentially Misidentified Species
The body plan and the position of the antennae relative to the eyes (see above)
can be used to differentiate porcellanids from other crab species. Petrolisthes armatus can be distinguished from co-occurring porcellanids by means of its
characteristic blue mouthparts and orange spot on the chelae.
HABITAT AND DISTRIBUTION
Petrolisthes armatus is believed native to Brazil and occurs in shallow subtidal and low
intertidal habitats such as oyster reefs and rock rubble (Hartman and Stancyk
2001, Knott and King undated). Other investigators differ in their opinion, suggesting P.
armatus is native to the Gulf of Mexico and Florida's Gulf Coast.
Modern distribution of P. armatus is broad. In the Atlantic, it includes
tropical western Africa, Ascension Island, Bermuda, the Bahamas, West Indies
and Caribbean, the Gulf of Mexico and South America to southern Brazil. In the
eastern Pacific, this crab occurs from the Gulf of California south to Peru.
The current range of P. armatus in the southeastern U.S. includes the
South Atlantic Bight from South Carolina down to the southern tip of Florida,
and the species has also been reported from Mississippi (Knott et al. 1999,
Hartman and Stancyk 2001).
IRL distribution records are poor, but the species is presumed to occur throughout the entire length of the estuary.
LIFE HISTORY AND POPULATION BIOLOGY
Age, Size, Lifespan
Petrolisthes armatus is a diminutive crab. Boudreaux et al. (2006) report a size
range of 6-8 mm body width and a weight range of 0.3-0.6 g for the specimens
they collected in the Mosquito Lagoon. Knott and King (undated) indicate P.
armatus from South Carolina can attain a somewhat larger maximum body width
of 12 to 14 mm. The authors report collecting female specimens that are
sexually mature at just 3-4 mm body width.
Petrolisthes armatus can be found in is some abundance within parts of its
introduced range. A Florida field study of the Crassostrea virginica
oyster reef macrofauna within the Mosquito Lagoon portion of Indian River
Lagoon system revealed the green porcelain crab to be among the most abundant
mobile species present (Boudreaux et al. 2006). Monthly lift net sampling for
11 months in 2004-2005 at six IRL oyster reef sites collected more than 500 P.
armatus specimens. Sampling in North Inlet -Winyah Bay, SC, from
July-November 2000 yielded average P. armatus densities of
47.2 individuals per square meter in subtidal samples and 98.5 individuals per
square meter in intertidal samples (Hartman and Stancyk 2001).
Substantially higher densities have been reported as well. Hollebone (2006)
noted that mean densities of P. armatus on South Atlantic Bight oyster
reefs up to several thousand individuals per square meter. Lohrer (2001)
claims mean densities have been as high as 10,000 individuals per square meter
in South Carolina salt marshes, while localized dense aggregations have exhibited maximum
densities more than three times as high.
P. armatus populations exhibit a remarkable ability to rapidly increase
from tens or hundreds of crabs per square meter in the winter to thousands and
tens of thousands of crabs per square meter in the summer (Hollebone 2006).
Details of reproduction in Petrolisthes armatus are similar to those of other
crabs. After mating, females can store sperm-containing spermatophores until
eggs are ready for release and fertilization. Fertilized eggs are then held in
a mass between the abdominal flap and body and the eggs are aerated by
continuous beating of the pleopods until they hatch.
P. armatus sex ratios observed in their putative native range in Brazil
remain close to 1:1 throughout the year (Oliveira and Masunari 1995).
Investigations in Georgia have also revealed ratios that are essentially 1:1
(Hollebone 2006), whereas ratios reported from South Carolina were male-biased
at a ratio of 1.5:1 (Hartman and Stancyk 2001).
When Petrolisthes armatus eggs hatch, they metamorphose through two planktonic
zoeal stages and then one planktonic megalopal stage before finally becoming
settlement-ready juveniles (Knott and King undated). There is some evidence
suggesting gregarious settlement in which the presence of P. armatus
individuals at a site cues the settlement of conspecifics (Hollebone 2006).
The temperature range within the putative native range of P. armatus is
16-29ºC (Oliveira and Masunari 1995). Northward expansion of the introduced
range of this species may therefore be limited by low temperatures. Knott and
King (undated) note that the time frame during which the crab successfully
expanded north through Georgia and into South Carolina coincided with a period
in which winter-time surface temperatures (measured in Charleston Harbor) were
as much as 3-4°C above the multidecadal average and this may have facilitated
the range expansion.
Winter low temperatures in Georgia and South Carolina may well exceed the lower
thermal limit of this crab which may, in part, drive the large winter decrease in
densities that have been reported (Hollebone 2006).
Petrolisthes armatus exhibits a degree of tolerance to salinity variation.
Salinities experienced by the crab within its putative native range
typically fall between 6.7 ppt and 31.5 ppt (Oliveira and Masunari 1995).
Roesijadi et al. (1974) demonstrated that if P. armatus with blood
chloride levels isotonic to a 15 ppt external environment were transferred to
salinities between 7 and 35 ppt, they would attain new steady-state chloride
levels within 12 hours.
Like other porcelain crabs, Petrolisthes armatus is primarily a filter feeder that
extends large, feathery maxillipeds (feeding mouthparts) and waves them back
and forth to strain plankton from the water. Internal mouthparts then transfer
food from the sieve-like hairs to the mouth for ingestion.
P. armatus and other porcelain crabs are also capable of scavenging
larger bits of food using their claws.
Green porcelain crabs preferentially occupy intertidal and subtidal oyster reef and rock rubble habitats and therefore co-occur and potentially interact with members of these communities.
Knott et al. (1999) report that Petrolisthes armatus was collected from south
Florida waters in Biscayne Bay and Miami Beach as early as the 1930s.
Northward spread of the species appears to have been slow; by 1977, P.
armatus was only rarely encountered in the Indian River Lagoon. Since
then, however, the green porcelain crab has become well established in the IRL.
The first records of P. armatus occurring north of Cape Canaveral date
to 1994. By the fall of that year, the crab had spread north to Georgia, and by
the spring of 1995, low numbers could be found off South Carolina as well.
By 1999, P. armatus had become well established as far north as Winyah
Bay, SC (Knott et al. 1999).
The introduction pathway for this species into the southeastern U.S. remains a
matter of speculation, but the most likely human-facilitated pathways involve
transport (probably of larvae) in ship ballast water or as accidental
introductions as part of commercial mollusc shipments (Knott and King undated)
Potential to Compete With Natives
The extraordinary densities achieved by Petrolisthes armatus in the South Atlantic
Bight strongly suggests that the populations could significantly affect oyster
reefs and the ecological communities they support. Because the arrival of the
species to much of the region is relatively recent, however, long-term
ecological effects on oyster reef communities have yet to be fully understood
Hollebone (2006) reported that any initial biotic resistance to community
alteration due to P. armatus invasion is ultimately overwhelmed by the
sheer numbers of new crabs recruiting to the system.
Possible Economic Consequences of Invasion
The potential economic impacts of invasive P. armatus have yet to be
estimated, but they could be significant if oyster reefs become severely
impacted. Among other potential impacts, filter-feeding green porcelain crabs
may measurably affect juvenile oyster recruitment by ingesting oyster larvae.
Boudreaux M.L., Stiner J.L., and L.J. Walters. 2006. Biodiversity of sessile
and motile macrofauna on intertidal oyster reefs in Mosquito Lagoon, Florida.
Journal of Shellfisheries Research 25:1079-1089.
Hartman M.J. and S.E. Stancyk. 2001. Distribution of an invasive anomuran
decapod, Petrolisthes armatus, in the North Inlet-Winyah Bay national
Estuarine Research Reserve (NERR) on the South Carolina coast. Abstract. Second
International Conference on Marine Bioinvasions. April 9-11, 2001. New Orleans,
Hartman M.J. 2003. Population dynamics and trophic interactions of
Petrolisthes armatus, an invasive decapod crustacean. Unpublished
Doctoral Dissertation. University of South Carolina. 128 p.
Hollebone A.L. 2006. An Invasive Crab in the South Atlantic Bight: Friend or
Foe? Unpublished Doctoral Dissertation. Georgia Institute of Technology. 133 p.
Knott D.M., Boyko C., and A. Harvey. 1999. Introduction of the green porcelain
crab, Petrolisthes armatus (Gibbes, 1850) into the South Atlantic Bight.
In: J. Pederson (Ed.). Marine Bioinvasions: the Proceedings of the First
National Conference. Massachusetts Institute of Technology, Cambridge,
Massachusetts. pp. 404.
Knott D.M and R.A. King, undated. Petrolisthes armatus - an introduced
species in the South Atlantic Bight? Southeastern Regional Taxonomic Center. 4
Lohrer A.M. 2001. The threat of invasion in South Carolina estuaries: A focus
of exotic decapod crabs. Paper presented at the 16th Biennial Conference of the
Estuarine Research Federation, November 4-8, 2001. St. Petersburg Beach, FL.
Oliveira E. and S. Masunari. 1995. Estrutura populacional de Petrolisthes
armatus (Gibbes) (Decopoda, Anomura, Porcellanidae) da Ilha do Farol,
Matinhos. Parana, Brasil. Revista Brasilia Zoologica 12:355-371.
Roesijadi G., Petrocelli S.R., Anderson J.W., Presley B.J., and R. Sims. 1974.
Survival and chloride ion regulation of the porcelain crab Petrolisthes
armatus exposed to mercury. Marine Biology 27:213-217.