Description: As with other valviferan
isopods, the uropods
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).
has 2 free pleonites
plus the pleotelson
1 is narrower laterally than in the middle (photo).
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)
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.
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.
Pentidotea wosnesenskii (Brandt, 1851)
Common name(s): Rockweed isopod, Olive green isopod, Vosnesensky's
|Synonyms: Idotea wosnesenskii,
|Pentidotea wosnesenskii from Padilla Bay.
|(Photo by: Dave Cowles, July 2006)
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
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
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
and Fairbanks, 1966
and Carlton, 1975
and McConnaughey, 1985
et al., 1980
and O'Clair, 1998
et al., 1985
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. doi.org/10.2307/1548330
General Notes and Observations: Locations, abundances,
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).
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