Friday, April 29, 2011

Hidden harems

Each visit to the beach turns up a new mix of critters and plants to investigate. This Wednesday, we arrived at the White Rock beach just after high tide, a low high; there was plenty of barnacle-covered stony beach to keep us busy.

Stubby isopods like to hang out where the salinity is low. On this beach, that means around the openings of the culverts bringing runoff from the steep south slope of the town. Usually there are a few there, crawling around the tiny barnacles and mussels on the bottom side of the rocks.

This time, they were everywhere, in great numbers. Every stone I turned over had a busy colony.

Stubby isopods, aka Oregon pillbug, Gnorimosphaeroma oregonense. Look closely; the eyes are visible, mostly on the pale brown ones.

These isopods are very similar to the woodbugs (aka pillbugs, woodlice, roly-polies, etc.) that feed in our gardens. They grow to about 1/2 inch long at maturity, although most of the ones we saw were the smaller harem females. Like the land-based pillbugs, they roll up in a tight little ball if they are disturbed. Just brushing them off a stone will do it; they land rolling.

Pillbug "pill" on the barnacles, lower right.

It's spring; party time, time to find mates and start the next generation. For the stubby isopods, that means sex changes and harems. The baby isopod hatches as a female. She becomes a member of a harem "owned" by one male, produces one brood, then begins a series of molts which transforms her into a male, ready to find his own harem of immature, smaller females. If the harem's male is removed, or dies, the dominant female in the group becomes a male to take his place.

(Most other crustaceans that switch sexes have a reverse pattern; the immatures are male, and they become female at maturity.)

The spring population explosion will not last; crabs and hermit crabs love a snack of unrolled isopod. Like the tides, life on the beach ebbs and flows.

More info: UAS.Alaska, Protogynous Sex Change in the Intertidal Isopod Gnorimosphaeroma oregonense, WSU Beach Watchers.

1 comment:

  1. Yay! Let's hear it for sex-changing marine organisms!

    You missed one name for their terrestrial relatives, though: we used to call 'em "potato bugs."


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