Pentidotea wosnesenskii (Brandt, 1851)

Common name(s): Rockweed isopod, Olive green isopod, Vosnesensky's isopod

Synonyms:  Idotea wosnesenskii, Idothea wosnesenskii
Phylum Arthropoda
 Subphylum Crustacea 
  Class Malacostraca
   Subclass Eumalacostraca
    Superorder Peracarida
     Order Isopoda
      Suborder Valvifera
       Family Idoteidae
Pentidotea wosnesenskii from Padilla Bay.
(Photo by: Dave Cowles, July 2006)
Description:  As with other valviferan isopods, the uropods are ventral to the pleotelson, forming a valvelike covering over the pleopods (photo).  Pentidotea wosnesenskii is one of the largest intertidal isopods in this area.  It has a dorsoventraly flattened body with 7 free pereonites and all pereopods similar (with sharp claws) (photo).  The lateral margin of the cephalon does not have a deep incision.  The eyes are lateral (photo).  The pleon has 2 free pleonites plus the pleotelson (photo).  Pleonite 1 is narrower laterally than in the middle (photo).  The pleotelson has lateral incisions indicating the end of another partly free pleonite (photo).  The palp of the maxilliped has 5 articles (photo).  The posterior margin of the pleotelson is convex and usually ends with a small apical tooth.  The posterolateral margin of the coxal plate of the 7th pereonite is pointed (acute) (photo).  Pereonite 1 is wider by at least 1/10 than the cephalon (photo), and the pereonites near the middle of the body are wider than the anterior or posterior ones.  The pleotelson is also wider than the cephalon.  The eyes are kidney shaped when viewed from the side.  Usually dark colored, dark olive green to brown but sometimes red (when among coralline algae) or nearly black.  May have a few white spots on the body, especially at the posterior end.  Up to 3.5 cm long.

How to Distinguish from Similar Species:  Of the species with a convex posterior margin to the pleotelson, several species look similar but pleonite 1 is not narrower laterally than in the center.  Several others have a more slender body with all the pereonites about the same width.  Idotea aculeata, in the southern part of the range, looks similar but has round eyes instead of kidney-shaped.

Geographical Range: Alaska to central CA; Sea of Okhotsk, USSR.

Depth Range:  Mid intertidal to 16 m

Habitat:  Often abundant in kelp, in mussel beds, and under intertidal rocks.

Biology/Natural History:  Eats algae.  May also eat the egg capsules of Nucella emarginata.  Predators include fish such as the spotted kelpfish Gibbonsia elegans and dwarf perch Micrometrus minimus.  This species can swim well, using its pleopods (it opens the flaps of the uropods to expose the pleopods).  Males are usually larger and paler than females and have thicker legs.  Ovigerous females have been found in July, and a female was seen brooding 3-4 mm young in November in SE Alaska.

The 5 pairs of pleopods on the abdomen are enclosed within a branchial chamber formed by the uropods, which serve as operculae. The first three pairs are designated as swimming pleopods and the last 3 as ventilatory, but all 5 are used in ventilation using a stroke similar to that used while swimming. Ventilatory strokes are usually slower than swimming strokes and are often separated by resting phases. Sometimes the operculae are opened farther to allow a larger-amplitude ventilatory stroke. It takes about 3 ventilatory strokes to pump water all the way through the branchial chamber  (Alexander, 1991). This species has a higher drag coefficient and is a slower swimmer (about 0.1 m/second) than is Pentidotea resecata (Alexander and Chen, 1990).

The species is named for the Russian zoologist Ilya Gavrilovich Vosnesensky, who collected and studied species from Siberia, Alaska, and California from 1839-1848

Return to:
Main Page Alphabetic Index Systematic Index Glossary


Dichotomous Keys:
  Flora and Fairbanks, 1966
  Kozloff 1987, 1996
  Smith and Carlton, 1975

General References:
  Harbo, 1999
  Kozloff, 1993
  McConnaughey and McConnaughey, 1985
  Morris et al., 1980
  Niesen, 1997
  O'Clair and O'Clair, 1998
  Ricketts et al., 1985
  Sept, 1999

Scientific Articles:
 Alexander, David E., 1991. Mechanics of branchial ventilation in the valviferan isopod Idotea wosnesenskii (Crustacea). Journal of Zoology 224:4 pp 607-616

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.

Web sites:

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

In this closeup of the head (cephalon) one can see the lateral eyes (kidney-shaped when viewed from the side) and the fact that the first pereonite is much wider than the cephalon.

In this view of the underside of the head one can see the palp along the side of the third maxilliped, which is covering the mouth (the dark mandibles are visible between the third maxillipeds).
The palp has 5 segments.

This view shows the last two pereonites (at top), then the two free pleonites and the front of the pleotelson.
The lateral plates seen on the pereonites are actually coxal plates.  Note that the posterolateral margin of the coxal plate from the last (7th) pereonite is pointed (acute)
The first pleonite is narrow or acute at the edge, much narrower than it is mid-dorsally.
Note the notch at the sides of the front of the pleotelson showing the margin of a third partly free pleonite.

In this ventral view one can see the 7 similar  pereopods on the pereon, and the flaplike ventral uropods which are characteristic of suborder Valvifera and cover the pleopods on the pleotelson.

Authors and Editors of Page:
Dave Cowles (2006):  Created original page