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.  The green and brown morphs do not seem to be able to change color readily and may in fact be different races or subspecies. Males have generally been said to have a green or white stripe running down their back (photo), but we have observed both males and females with this coloration.  Said to be up to 4 cm long; but the male below is 5 cm long.

How to Distinguish from Similar Species:  According to Kozloff's key, no other local isopod has a concave posterior margin to its pleotelson. However, Rafi and Laubitz (1990) state that another species with a concave pleotelson, Idotea rufescens, can be found from southeast Alaska to the Strait of Juan de Fuca. The two species can be distinguished by the fact that I. rufescens has only 4 segments on the palp of its maxilliped and the corners of the posterior pleotelson are rounded, while P. resecata has 5 segments on the palp of its maxilliped (as an adult) and the corners of the posterior pleotelson are more sharply pointed.

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).  Those found on kelp are almost always brown, while those found on eelgrass are green. The brown coloration is due to the exoskeleton, and the green coloration is due to a combination of the exoskeleton and the green hemolymph. These animals appear to be temperature and salinity sensitive. A temperature of 15 degress Celsius is an optimum temperature for P. California.  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. Juveniles have a convex tip to the pleotelson when released from the mother, but develop the distinctive concave tip on later molts. The species blends in with the kelp or eelgrass that it is on, and can also swim well using its first three pleopods. It is a faster swimmer (about 0.2m/sec) than its congener Pentidotea wosnessenskii is (Anderson and Chen, 1990). The last two pleopods are used for respiration (breathing).

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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:

Alexander, David E., Jeffrey Blodig, and Shiu-Yie Hsieh, 1995. Relationship between function and mechanical properties of the pleopods of isopod crustaceans. Invertebrate biology 114:2 pp 169-179

Alexander, David E. and Tao Chen, 1990. Comparison of swimming speed and hydrodynamic drag in two species of Idotea (Isopoda). Journal of Crustacean Biology 10:3 pp. 406-412.

Anderson, Sarah J., Faith E. Hunnicutt, and David L. Cowles, 2017. Activity levels of Pentidotea resecata support conclusion that the animal is photosynthetic.  Poster presented at ASLO Aquatic Sciences Meeting, Honolulu, HI

Cowles, Joanna M., 2015. Does photosynthesis take place in the gut of Pentidotea resecata?  M.S. thesis, Walla Walla University

Cowles, Joanna M. and David L. Cowles, 2015. Photosynthesis and the green isopod Pentidotea resecata. Poster at SICB annual meeting, West Palm Beach, FL. 

Cowles, J.M., N. Sullivan, D.L. Cowles, and G. Stone, 2011. Is the isopod Pentidotea resecata photosynthetic? Poster for Murdock Scientific Conference, Seattle, WA.

Dann, Leah E., 2017. Antioxidant capacity in the hemolymph of the marine isopod Pentidotea resecata. M.S. thesis, Walla Walla University

Dann, Leah E., C. Judelle Johnson, and David L. Cowles, 2017.  Does Pentidotea resecata hemolymph protect the isopod from oxidative stress?  Poster presented at ASLO Aquatic Sciences Meeting, Honolulu, HI

Hunnicutt, Faith E., Sarah J. Anderson, and David L. Cowles, 2016.  Where do all the isopods go?  A population study of Pentidotea resecata.  Poster at Murdock Conference, Spokane, WA, November 2016

Johnson, C. Judelle, 2017. How does the green eelgrass isopod Pentidotea resecata protect its tissue against highly fluctuating oxygen conditions? M.S. thesis, Walla Walla University 

Johnson, C. Judelle, Leah E. Dann, and David L. Cowles, 2017.  How does the green eelgrass isopod protect its tissue against highly fluctuating oxygen conditions?  Poster presented at ASLO Aquatic Sciences Meeting, Honolulu, HI

Jones, L.G., 1971. Studies on selected small herbivorous invertebrates inhabiting Macrocystis canopies and holdfasts in southern California kelp beds. pp. 343-367 in W.J. North (ed.), The biology of giant kelp beds (Macrocystis) in California. Verlag von J. Cramer, Lehre, Germany.

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

McLarty, Shelley J., 2015. Gut content and pigment analysis in the marine isopod Pentidotea resecata.  M.S. thesis, Walla Walla University

McLarty, Shelley J. and David L. Cowles, 2015. Gut content and pigment analysis in the marine isopod Pentidotea resecata.  Poster at SICB annual meeting, West Palm Beach, FL

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

Rafi, F., and Diana R. Laubitz, 1989. The Idoteidae (Crustacea: Isopoda: Valvifera) of the shallow waters of the northeastern North Pacific ocean. Canadian Journal of Zoology 68: pp2649-2687

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-
CSS coding for page developed by Jonathan Cowles

Salish Sea Invertebrates web site provided courtesy of Walla Walla University