Zaops ostreum

Geographic Range

Pinnotheres ostreum is found along the Atlantic coast from North America to Brazil, and also in the Pacific Ocean where its hosts live.

• Biogeographic Regions
• atlantic ocean atlantic ocean
• pacific ocean pacific ocean

Habitat

Males are probably free-living, but females parasitize various marine bivalves. Pinnotheres ostreum females of stages three, four, and five have only been found on the gills in the mantle cavities of the oyster Crassostrea virginica along the Atlantic coast of North America, but in cases of heavy infestation young crabs may be found throughout the oyster's water conduction system. One oyster may harbor multiple young crabs, but infestations of more than one stage five female are very unusual.

• Habitat Regions
• saltwater or marine saltwater or marine

• Aquatic Biomes
• coastal coastal

• Other Habitat Features
• intertidal or littoral intertidal or littoral

Physical Description

In the first crab stage, the invasive stage, the carapace of P. ostreum is flattened and is around .61 mm long and .59 mm wide. Males tend to be slightly larger than females but the sexes are indistinguishable except for differences in genital openings and by microscopic examination of the pleopods. The carapace has no spines but seven to nine setae on each lateral edge. Plumose setae, feelers that act as swimming hairs, are found only on the last two segments of the third and fourth legs. There are two small, white spots on carapace and sternum, around which the carapace is hardened. Pinnotheres ostreum then passes through pre-hard stages in which the carapace is rounded and no swimming hairs are found. Next is the hard stage: the carapace is rounded and very hard, swimming hairs are once again found on the third and fourth legs, and there are large, white spots on the carapace and sternum. Males are usually larger than females and do not mature beyond this stage. Females continue through four more stages in which they lose swimming hairs and their abdomens grow wider and cover the coxae until they become a mature female. Females become ovigerous (egg bearing) in their first summer but do not reach their maximum size until their second or third summer. Fully grown ovigerous females can measure from 9.4 to 10.8 mm in width. Also, orange or reddish gonads may be seen through the thin carapace.

• Other Physical Features
• ectothermic ectothermic
• heterothermic heterothermic
• bilateral symmetry bilateral symmetry

• Sexual Dimorphism
• male larger

Development

The first crab stage is the invasive stage. Males tend to be slightly larger than females but the sexes are indistinguishable except for differences in genital openings and by microscopic examination of the pleopods. Pinnotheres ostreum then passes through pre-hard stages in which the carapace is rounded and no swimming hairs are found. Next is the hard stage: the carapace is rounded and very hard, swimming hairs are once again found on the third and fourth legs, and there are large, white spots on the carapace and sternum. Males are usually larger than females and do not mature beyond this stage. Females continue through four more stages in which they lose swimming hairs and their abdomens grow wider and cover the coxae until they become a mature female. Females become ovigerous (egg bearing) in their first summer but do not reach their maximum size until their second or third summer. Orange or reddish gonads may be seen through the thin carapace.

Reproduction

In both sexes it is the hard stage that is the copulatory stage. Males leave their hosts in the hard stage and enter other oysters in search for females. Males may copulate with one or two females in June or July, after which the males disappear. Females need to eat a lot of food for rapid growth and development before they deposit their eggs. Fully grown ovigerous females carry from 7957 to 9456 eggs for 3 to 5 weeks. One batch of eggs is produced the first year, while two batches may be produced during the second and third years. In their second and third years the female may become ovigerous earlier because they already have sperm in their spermathecae from the previous year.

• Key Reproductive Features
• gonochoric/gonochoristic/dioecious (sexes separate)
• sexual sexual
• fertilization fertilization
  • internal internal
• oviparous oviparous

• Parental Investment
• pre-fertilization
  • provisioning
  • protecting
    • female
• pre-hatching/birth
  • protecting
    • female

Behavior

The male is thought to be free swimming like the first stage female. First stage females invade oysters in late summer and autumn. The crab faces away from the gill with its carapace under the normal gills in order to efficiently take the food brought in for the oyster. In order to maintain the crabs' position its dactyli of the walking legs often pierce the gill tissue. Walking legs also are used to scrape the gill in search of food and small crabs cause the most damage by this mechanism. Considerable damage is done to the host when the oyster is upside down because the crab is forced to consistently maintain its dactyli in the gill tissue. Larger crabs cause more damage in this last case.

• Key Behaviors
• parasite parasite
• motile motile
• sedentary sedentary

Communication and Perception

Pea crabs chemically detect hosts in the water.

• Communication Channels
• tactile tactile
• chemical chemical

• Perception Channels
• tactile tactile
• chemical chemical

Food Habits

The larval stages of P. ostreum feed on plankton while the adults are parasitic on oysters. The adults eat the oyster's food. The adults' food consists mainly of the minute organisms, mostly diatoms and dinoflaggelates, found in the food-laden mucous strings formed by the host. This food is part of the oyster diet. Pinnotheres ostreum also eats small crustaceans that are brought into the oyster that are not part of the oysters' diet.

• Primary Diet
• herbivore herbivore
  • algivore
• planktivore planktivore
• detritivore detritivore

• Plant Foods
• algae
• phytoplankton phytoplankton

• Other Foods
• microbes

• Foraging Behavior
• filter-feeding filter-feeding

Predation

Other animals may feed on this species. These crabs are considered a delicacy by humans.

Ecosystem Roles

This species parasitizes various bivalves, including mussels, clams, cockles, oysters, and scallops.

• Ecosystem Impact
• parasite parasite

Economic Importance for Humans: Positive

Pinnotheres ostreum , the oyster crab, is considered a delicacy and costs twice as much as oyster meat.

• Positive Impacts
• food food

Economic Importance for Humans: Negative

Pinnotheres ostreum damages the gills of oysters, resulting in less flesh for consumption by humans. Economic loss highly variable, depending on P. ostreum population size.

• Negative Impacts
• causes or carries domestic animal disease causes or carries domestic animal disease

Conservation Status

Other Comments

Approximately 80% of oysters harboring P. ostreum are males. Since female oysters can be induced to change sex by simple starvation, it is assumed that P. ostreum interferes with food intake of the oyster and therefore cause the oyster to produce sperm instead of the more energy-consuming eggs.

Additional Links

Encyclopedia of Life

Contributors

Ann Walker (author), University of Michigan-Ann Arbor, Teresa Friedrich (editor), University of Michigan-Ann Arbor.

References

Abele, L., B. Felginhauer. 1982. Crustacea: Decapoda. in Parker, S.B. (ed.) Synopsis and Classification of Living Organisms. vol. 2 . New York: McGraw-Hill.

Bower, S., S. McGladdery, I. Price. 1994. "Pea Crabs in Oysters" (On-line). Synopsis of infectious diseases and parasites of commercially exploited shellfish. Accessed November 05, 2004 at www-sci.pac.dfo-mpo.gc.ca/shelldis/pages/pcboy_e.htm .

Cheng, T. 1973. General Parasitology . New York and London: Academic Press.

Christenson, A., J. McDermott. 1958. Life history and biology of the oyster crab, Pinnotheres ostreum Say. Biological Bulletin , 114: 146-179.

Dando, M., M. Burchett. 1996. SeaLife, a complete guide to the marine environment . Washington, D. C.: Smithsonian Institution Press.

Kaplan, E. 1988. A Feld Guide to Southeastern and Caribbean Seashores . Boston: Houghton Mifflin Co..

Sandoz, M., S. Hopkins. 1947. Early life history of the oyster pea crab, Pinnotheres ostreum. Biological Bulletin , 93(3): 250-258.

Stauber, L. 1945. Pinnotheres ostreum, parasitic on the American oyster, Ostrea (Gryphaea) Virginica. Biological Bulletin , 88: 269-291.