The Marine Life Information Network

Information on the biology of species and the ecology of habitats found around the coasts and seas of the British Isles

A sea slater (Idotea pelagica)

Distribution data supplied by the Ocean Biodiversity Information System (OBIS). To interrogate UK data visit the NBN Atlas.Map Help



A dorso-ventrally flattened crustacean with an elongated rounded oblong shaped body. Body mostly dark purple to brown with white diamond-shaped patches or stripes down the midline and white markings along the edges of the body. Females tend to be darker than males. Males range in length from 4-11 mm and females from 7-10 mm. The distinctive head bears two dorso-lateral eyes, a pair of short antennules, and a pair of antennae. Most of body is taken up by a thorax composed on seven sections (somites). The body ends in a short abdomen (two somites) and a distinct tail-piece (the pleotelson). The antennule extend to the third segment of the antenna. The antenna is robust with a short flagellum that is densley covered with hairs in males. The pleotelson in adults is characteristic, with straight or slightly convex sides, a rounded end and only an indistinct middle tooth or process.

Recorded distribution in Britain and Ireland

Probably distributed all around the coasts of Britain and Ireland but poorly represented in surveys.

Global distribution

Recorded form Norway to the French coast but not entering low salinity waters of the inner Baltic.


Found on wave exposed rocky shores amongst barnacles, mussels and stunted fucoids.

Depth range


Identifying features

  • Dorso-ventrally flattened and oblong, oval body.
  • Abdomen (pleon) consists of two complete somites and one partial suture.
  • Antennule just or equal in length to the third section of the antenna.
  • Antennal flagellum shorter than its peduncle, less than one sixth of the body length, and densely covered in hairs in males.
  • Pleotelson sides straight or slightly convex, rounded with only an indistinct, blunt tooth.
  • Tops of the legs bear broad coxal plates that widen posteriorly.
  • Legs are very robust and bear a relatively larger claw than other Idotea.

Additional information

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Listed by

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Biology review


AuthorityLeach, 1816
Recent Synonyms


Typical abundance
Male size range4-11mm
Male size at maturity
Female size rangeSmall(1-2cm)
Female size at maturity
Growth form
Growth rate
Body flexibilityLow (10-45 degrees)
Characteristic feeding methodGrazer (fronds/blades)
Diet/food source
Typically feeds onFucoids, associated epiphytes.
Environmental positionEpifaunal
DependencyNo text entered.
SupportsNo information
Is the species harmful?No

Biology information


Habitat preferences

Physiographic preferences
Biological zone preferences
Substratum / habitat preferences
Tidal strength preferences
Wave exposure preferences
Salinity preferences
Depth range
Other preferencesNo text entered
Migration Pattern

Habitat Information

There are a total of 8 members of Idotea in British waters which can be distinguished using the keys provided in Naylor (1972) and Hayward & Ryland (1995b). The various species are not typically sympatric, but are ecologically segregated. For example, Idotea pelagica is found in the lower intertidal and shallow subtidal on exposed rocky reefs and is replaced in less exposed areas by I. granulosa which in turn is replaced by I. chelipes on sheltered, estuarine shores (Naylor, 1955).

Life history

Adult characteristics

Reproductive typeGonochoristic (dioecious)
Reproductive frequency See additional information
Fecundity (number of eggs)11-100
Generation time1-2 years
Age at maturity
Life span1-2 years

Larval characteristics

Larval/propagule type-
Larval/juvenile development Brooding
Duration of larval stage-
Larval dispersal potential -
Larval settlement periodSee additional information

Life history information

  • Reproductive season
  • The reproductive season of Idotea pelagica is closely linked to temperature (Leifsson, 1999). Reproductive effort is at its greatest during periods when the sea water temperature is between 5 and 12&176; C (Sheader, 1977; Healy & ONeill, 1984; Leifsson, 1999). While ovigerous females are found all year round in southern Irish coasts, the highest proportion of ovigerous females are found between December and August (Healy & ONeill, 1984), while on the Northeast coast of England ovigerous females were found only between April and August (Sheader, 1977). Icelandic populations have their reproductive period further reduced to May to July (Leifsson, 1999). In the more southern extent of its range, the reproductive season of Idotea pelagica would be expected to shift further towards winter.
  • Fecundity
  • Females brood up to 80 eggs for 6-8 weeks (Leifsson, 1999), as with most isopods there is no larval stage and the juveniles appear as the adults, but with 6 pairs of pereopods not 7. Once the eggs hatch, females may then moult and produce a second brood (Healy & O'Neill, 1984).

    Sensitivity reviewHow is sensitivity assessed?

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    Chemical pressures

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    Heavy metal contamination
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    Hydrocarbon contamination
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    Radionuclide contamination
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    Changes in nutrient levels
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    Biological pressures

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    Additional information

    Importance review


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    Importance information



    1. Bruce, J.R., Colman, J.S. & Jones, N.S., 1963. Marine fauna of the Isle of Man. Liverpool: Liverpool University Press.

    2. Hayward, P., Nelson-Smith, T. & Shields, C. 1996. Collins pocket guide. Sea shore of Britain and northern Europe. London: HarperCollins.

    3. Hayward, P.J. & Ryland, J.S. (ed.) 1995b. Handbook of the marine fauna of North-West Europe. Oxford: Oxford University Press.

    4. Healy, B. & O'Neill, M. 1984. The life cycle and population dynamics of Idotea pelagica and I. granulosa (Isopoda: Valvifera) in South-East Ireland. Journal of the Marine Biological Association of the United Kingdom, 64(1), 21-33

    5. Kroer, N. 1986. Distribution and habitat segregation of four species of Idotea (Isopoda) in a Danish fjord. Ophelia, 25(3), 199-207

    6. Leifsson, B.R. 1999. The life cycle and population dynamics of Idotea pelagica and I. granulosa (Isopoda: Valvifera) in South-East Ireland. Sarsia, 83(1), 1-13

    7. MBA (Marine Biological Association), 1957. Plymouth Marine Fauna. Plymouth: Marine Biological Association of the United Kingdom.

    8. Naylor, E. 1955. The ecological distribution of British species of Idotea Journal of Animal Ecology

    9. Naylor, E., 1972. British marine isopods. London: Academic Press. [Synopses of the British Fauna, no. 3.]

    10. Picton, B.E. & Costello, M.J., 1998. BioMar biotope viewer: a guide to marine habitats, fauna and flora of Britain and Ireland. [CD-ROM] Environmental Sciences Unit, Trinity College, Dublin.

    11. Sheader, M. 1977. The breeding biology of Idotea pelagica (Isopoda : Valvifera) with notes on the occurrence and biology of its parasite Clypeoniscus hanseni (Isopoda : Epicaridea) Journal of the Marine Biological Association of the United Kingdom, 57(3), 659-674.


    1. Environmental Records Information Centre North East, 2018. ERIC NE Combined dataset to 2017. Occurrence dataset: accessed via on 2018-09-38

    2. NBN (National Biodiversity Network) Atlas. Available from:

    3. OBIS (Ocean Biodiversity Information System),  2023. Global map of species distribution using gridded data. Available from: Ocean Biogeographic Information System. Accessed: 2023-03-31


    This review can be cited as:

    Tyler-Walters, H. 2005. Idotea pelagica A sea slater. In Tyler-Walters H. and Hiscock K. Marine Life Information Network: Biology and Sensitivity Key Information Reviews, [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 31-03-2023]. Available from:

     Download PDF version

    Last Updated: 23/05/2005