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Species Name:    Pinnotheres maculatus
Common Name:              Squatter Pea Crab



Kingdom Phylum/Division: Class: Order: Family: Genus:
Animalia Arthropoda Malacostraca Decapoda Pinnotheridae Pinnotheres

Species Name: 
Pinnotheres maculatus Say, 1818

Common Name(s):
Squatter Pea Crab

The integrated taxonomic information lists Pinnotheres maculatus as an invalid name for Tumidotheres maculates Say, 1818.

Common Name(s):
Squatter Pea Crab

Species Description:
Pinnotheres maculatus is a member of the family Pinnotheridae, a group of decapods that live symbiotically within other marine invertebrates (Bierbaum and Ferson 1986). Females are soft shelled living their entire adult life in one host. Males move from host to host.


Regional Occurrence:
Occurs from Massachusetts to Argentina (Kruczynski 1992).

IRL Distribution:
Pinnotheres maculatus occurs in Crassostrea virginica and other bivalves in the Indian River Lagoon.


Age, Size, Lifespan:
Females are larger with a carapace measuring 8 - 12 mm. The males are "dwarfs", the carapace grows to approximately 6mm. Females produce eggs after they reach a minimum carapace size of 6 mm, at an age of at least one year (Bierbaum and Ferson 1986).

There is considerable geographic variation in the presence of Pinnotheres maculatus in hosts. In Bogue Sound in North Carolina, the mean density of P. maculatus in Mytilus edulis was reported to be approximately 3 per meter squared, low compared to other studies (Kruczynski 1973). In Quicks Hole, Massachusetts, 97.6% of the blue mussel M. edulis hosted P. maculatus (Kane and Farley 2006). In Woods Hole, Massachusetts, P. maculatus were found in half of the blue mussels collected at depths of 3 m or greater but were almost absent in the mussels living shallower than 3 m (Kruczynski 1992).

Reproduction and embryology in Pinnotheres maculatus is complex, involving both the pea crab and its host. Reproduction occurs when females and males reach an anomalous juvenile instar stage at which time they leave their hosts and engage in copulatory swarming in the water column. During this time P. maculatus juveniles have a well-calcified hard carapace measuring approximately 3.3 cm. After the swarm, the female crabs will soon re-enter their host while the males remained in the water column longer. Ovigerous females that survive the winter will carry the eggs under their abdomen until they begin to hatch in August (Pearce 1964).

Pinnotheres maculatus larvae have five zoeal stages and one megalopa planktonic stage (Costlow and Bookhout 1966). The females have 7 development stages and live their entire lives in the host leaving only to participate in a copulatory swarm (Pearce 1964, Kane and Farley 2006). The megalopa molts into the first true crab in mid-September and then leaves the plankton in search of a host (Pearce 1964).


Larval activity of Pinnotheres maculatus decreases with decreasing water temperature (Welsh 1932).

Adult pea crabs do not survive well in salinities less than 20 ppt (Kruczynski 1973).


Trophic Mode:
Pinnotheres maculatus larvae are planktotrophic and can be reared in the laboratory on Artemia nauplii and Arbacia eggs (Costlow and Bookhout 1966). Adult females use their chelae to remove food from the gills of the host (Bierbaum and Ferson 1986) while adult males feed independently of the host (Kane and Farley 2006).

Associated Species:
Pinnotheres maculatus is a small commensal crab commonly found in the scallop Aequipecten irradians, the blue mussel Mytilis edulis, and other bivalves (Sastry and Menzel 1962, Costlow and Bookhout 1966, Kruczynski 1992).

P. maculatus is a generalist and does not show host specificity (Bierbaum and Ferson 1986). They will live symbiotically with one of 21 species of bivalves and polychaetes including Mytilus edulis, Crassostrea virginica, and Argopecten irradians concentricus. The symbiosis is either commensal or parasitic (Derby and Atema 1980, Kane and Farley 2006). Females crabs steal food strands from the gill of their hosts and therefore have the largest impact on the growth and survival of the host (Bierbaum and Ferson 1986).

Pre-adult T. maculates settle in individual Argopecten irradians concentricus in the fall and mature by the spring. Females stay in their original host while the males move from scallop to scallop (Kruczynski 1972). The female P. maculatus removes phytoplankton from the gills of the host resulting in slower host growth than those scallops occupied by male squatter pea crabs. The females are trapped in these smaller homes. Females appear to prefer larger hosts and will grow to a larger size if there is room in the host (Kane and Farley 2006).

There is evidence that suggests that P. maculatus locate their hosts using chemical cues (Sastry and Menzel 1962, Derby and Atema 1980).


Special Status:

Economic/Ecological Importance:
As a commensal associate with a variety of species, the ecological importance of thi sspecies may be greater than might be expected.


Bierbaum RM and S Ferson. 1986. Do symbiotic pea crabs decrease growth rate in mussels? Biological Bulletin 170:51-61.

Derby CD and J Atema. 1980. Induced host odor attraction in the pea crab Pinnotheres maculates. Biological Bulletin 158:26-33.

ITIS. Integrated Taxonomic Information System. Available online.

Kane K and GS Farley. 2006. Body size of the endosymbiotic pea crab Tumidotheres maculates: larger hosts hold larger crabs. Gulf and Caribbean Research 18:27-33.

Kruczynski WL. 1992. Relationship between depth and occurrence of pea crabs, Pinnotheres maculates, in blue mussels, Mytilis edulis, in the vicinity of Woods Hole, Massachusetts. Estuaries and Coasts 15:167-169.

Kruczynski WL. 1973. Distribution and abundance of Pinnotheres maculatus Say in Bogue Sound, North Carolina. Biological Bulletin 145:482-491.

Kruczynski WL. 1972. The effect of the pea crab, Pinnotheres maculatus Say, on growth of the bay scallop, Argopecten irradians concentricus (Say). Estuaries and Coasts 13:218-220.

Pearce JB. 1964. On reproduction in Pinnotheres maculates. Biological Bulletin 127:384. (Abstract)

Sastry AN and RW Menzel. 1962. Influence of hosts on the behavior of the commensal crab Pinnotheres maculatus Say. Biological Bulletin (Woods Hole) 123:388-395.

Welsh JH. 1932. Temperature and light as factors influencing the rate of swimming of larvae of the mussel crab, Pinnotheres maculatus Say. Biological Bulletin 63:310-326.

Report by: Melany P. Puglisi, Smithsonian Marine Station
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Page last updated: October 1, 2008