Pentidotea resecata (Stimpson, 1857)

Common name:  Concave isopod, eelgrass isopod, cut-tailed isopod, seaweed isopod, kelp isopod, transparent isopod
Synonyms:  Idotea resecata, Idothea resecata
Phylum Arthropoda
 Subphylum Crustacea 
  Class Malacostraca
   Subclass Eumalacostraca
    Superorder Peracarida
     Order Isopoda
      Suborder Valvifera
       Family Idoteidae
Pentidotea resecata about 5 cm long found by Heidee Leno at Shannon Point. The brown stringy growth seen at several places on the body is chains of diatoms attached to the cuticle.
Photo taken by Heidee Leno, July 2002
Description:  As in all valviferan isopods, the uropods of this species form platelike folds over the ventral side of the abdomen (pleon) (photo).  Pentidotea resecata has a flattened body with 7 free pereonites (thoracic segments).  The lateral margins of the cephalon do not have a deep incision.  The eyes are lateral.  The pleon has two free pleonites plus the pleotelson, which has a pair of lateral incisions showing where a third pleonite is partially fused to the pleotelson.  The palp of the maxilliped has 5 articles (photo).  The posterior margin of the pleotelson is concave.  A green pigment is deposited in the endocuticle while a red pigment appears in the exocuticle, often resulting in a brownish overall body coloration, especially when living on Macrocystis. Those living on eelgrass (Zostera) are more green.  Males generally have a green stripe running down their back (photo).  Said to be up to 4 cm long; but the male below is 5 cm long.

How to Distinguish from Similar Species:  No other local isopod has a concave posterior margin to its pleotelson.

Habitat and Geographical Range:  Pentidotea resecata generally appear from Alaska to the tip of Baja California and Mazatlan (Mexico).  They prefer to be among eelgrass (Zostera) or kelps (Macrocystis and Pelagophycus).

Depth Range:  Intertidal to 18 m

Biology/Natural History:  When placed on a substratum, the isopod will reorient itself along the long axis of its substratum and if placed otherwise, it immediately reorients itself. P. resecata prefers to feed on algae such as Macrocystis, Pelagophycus porra, Eisenia arborea, Pterygophora californica, and Egregia laevigata, or on eelgrass (Zostera).  These animals appear to be temperature and salinity sensitive. A temperature of 15 degress Celsius is an optimum temperature for P. resecata.  If temperatures reach between 29 and 31 degrees Celsius, the animal will die.  If the isopod is placed in fresh water it survives a little over an hour in comparison with 33 hours in 125% seawater.  Development of these isopods is a bit unclear, but studies show that the general body form changes little during growth.  Features such as segment number and setae number vary directly with size, each molt providing further change.  The species blends in with the kelp or eelgrass that it is on, and can also swim well using its pleopods.

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Dichotomous Keys:
  Flora and Fairbanks, 1966 (as Idothea (Pentidotea) resecata)
  Kozloff, 1987, 1996
  Smith and Carlton, 1975

General References:
  Johnson and Snook, 1955 (as Pentidotea resecata)
  Kozloff, 1993
  Lamb and Hanby, 2005
  Morris et al., 1985
  O'Clair and O'Clair, 1998
  Ricketts et al., 1985

Scientific Articles:
Lee, W. L. and B. M. Gilchrist, 1972.  Pigmentation, color change, and the ecology of the marine isopod Idotea resecata (Stimpson, 1857).  Journal of Experimental Mararine Biology and Ecology 10: 1-27

McCluskey, Richard Laverne, 1966.  Dietary regulation of growth and mortality in the isopod, Idotea resecata (Stimpson).  Master's thesis, Walla Walla College.  23 pp.

Menzies, R.J. and R.J. Waidzunas, 1948.  Post-embryonic growth changes in the isopod Pentidotea resecata (Stimpson) with remarks on their taxonomic significance.  Biological Bulletin 95: 107-113

General Notes and Observations:  Locations, abundances, unusual behaviors, etc.:

This male, 5 cm long not counting antennae, was found clinging to eelgrass in Padilla Bay.  Note that this individual, from eelgrass, is greener than is usually seen in individuals which are feeding on brown algae.  Males often have a prominent greenish stripe (or greenish-white in this indvidual) down the dorsal side.  Photo by Dave Cowles, July 2008

This underside view of the same male as the above photo shows the flaplike uropods which enclose the pleopods on the ventral side of the pleon (to the left).  Photo by Dave Cowles, July 2008

In this photograph of the mouthparts, the scalpel is holding out the maxilliped.  The main, subchelate branch of the maxilliped can be seen just below and partly overlapping the scalpel blade.  The maxilliped  palp, which has 5 articles (difficult to count in this photo), crosses the scalpel blade in front of the brown blotch.  The darkened, sclerotized mandibles can be seen just above and to the left of the scalpel blade.  Photo by Dave Cowles, July 2008

Baby from pouch
This baby, 3 mm long, was taken from the marsupium of a mother who died.  There were many more babies in the marsupium.
At this early stage the pleotelson is still convex.  Photo by Dave Cowles, August 3, 2016

Authors and Editors of Page:
Created original page: Heidee Leno
Edited by: Hans Helmstetler 11-2002; Dave Cowles 2008, 2012