Zaops ostreumoyster pea crab

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Geographic Range

Zaops ostreum is found along the Atlantic coast from North America to Brazil, and also in the Pacific Ocean where its hosts live. (Bower, et al., 1994; Cheng, 1973; Kaplan, 1988)

Habitat

Males are probably free-living, but females parasitize various marine bivalves. Zaops 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. (Cheng, 1973; Stauber, 1945)

Physical Description

In the first crab stage, the invasive stage, the carapace of Z. 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. Zaops 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. (Cheng, 1973; Sandoz and Hopkins, 1947)

  • 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. Zaops 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. (Cheng, 1973; Sandoz and Hopkins, 1947)

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. (Cheng, 1973; Christenson and McDermott, 1958)

  • 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. (Stauber, 1945)

Communication and Perception

Pea crabs chemically detect hosts in the water. (Dando and Burchett, 1996)

Food Habits

The larval stages of Z. 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. Zaops ostreum also eats small crustaceans that are brought into the oyster that are not part of the oysters' diet. (Cheng, 1973; Stauber, 1945)

  • Other Foods
  • microbes

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.

Species Used as Host

Economic Importance for Humans: Positive

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

  • Positive Impacts
  • food

Economic Importance for Humans: Negative

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

Conservation Status

Other Comments

Approximately 80% of oysters harboring Z. ostreum are males. Since female oysters can be induced to change sex by simple starvation, it is assumed that Z. ostreum interferes with food intake of the oyster and therefore cause the oyster to produce sperm instead of the more energy-consuming eggs. (Christenson and McDermott, 1958)

Contributors

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

Glossary

Atlantic Ocean

the body of water between Africa, Europe, the southern ocean (above 60 degrees south latitude), and the western hemisphere. It is the second largest ocean in the world after the Pacific Ocean.

World Map

Pacific Ocean

body of water between the southern ocean (above 60 degrees south latitude), Australia, Asia, and the western hemisphere. This is the world's largest ocean, covering about 28% of the world's surface.

World Map

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

causes or carries domestic animal disease

either directly causes, or indirectly transmits, a disease to a domestic animal

chemical

uses smells or other chemicals to communicate

coastal

the nearshore aquatic habitats near a coast, or shoreline.

detritivore

an animal that mainly eats decomposed plants and/or animals

ectothermic

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature

fertilization

union of egg and spermatozoan

filter-feeding

a method of feeding where small food particles are filtered from the surrounding water by various mechanisms. Used mainly by aquatic invertebrates, especially plankton, but also by baleen whales.

food

A substance that provides both nutrients and energy to a living thing.

herbivore

An animal that eats mainly plants or parts of plants.

heterothermic

having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.

internal fertilization

fertilization takes place within the female's body

intertidal or littoral

the area of shoreline influenced mainly by the tides, between the highest and lowest reaches of the tide. An aquatic habitat.

motile

having the capacity to move from one place to another.

oviparous

reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.

parasite

an organism that obtains nutrients from other organisms in a harmful way that doesn't cause immediate death

phytoplankton

photosynthetic or plant constituent of plankton; mainly unicellular algae. (Compare to zooplankton.)

planktivore

an animal that mainly eats plankton

saltwater or marine

mainly lives in oceans, seas, or other bodies of salt water.

sedentary

remains in the same area

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

tactile

uses touch to communicate

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.