The secret lives of stubbies

Isopods: the name means "same legs". Because their 7 pairs are all more or less the same, as compared to the hermits and crabs, whose "legs" can be anything from huge claws to rudimentary grabbers and tiny brooms.

Stubbies, Gnorimosphaeroma oregonensis, on the underside of a rock.

The stubby isopods hang out in large groups, hidden away out of the glare of sunlight. When I turned over this stone, several dozen scrambled for cover. A handful rolled up into balls and rolled away. These few opted for pretending to be part of the rock. The largest are about 1 cm long.

Isopod females carry their young internally until they are ready to join the crowd. They do not have a swimming larval stage, like crabs do. The tiniest ones in this group of stubbies are youngsters, mostly female. After they reach adulthood and raise a family, they become males, large enough to compete for smaller females.

Wosnesenski's isopod. 4 times the size of a large stubby.

Crustaceans molt. We're used to seeing the remains of a crab molt on the beach; the upper carapace, sometimes with legs and lungs still attached. Hermit crabs back out of their hardened upper body shells, dragging the eyes and legs after themselves. I find their empty exoskeletons floating near the bottom of my tank, look around, and find a new-looking, clean, and slightly larger hermit, often in a new, larger shell.

The isopods are the only crustaceans that molt in two stages; first, they squirm out of the rear section, then later, back out of the front, eyes, antennae and all.

As adults, isopods differ from other crustaceans in that moulting occurs in two stages known as "biphasic moulting". First they shed the exoskeleton from the posterior part of their body and later shed the anterior part. (Wikipedia)

Crabs and hermits and other hard-covered invertebrates mate in the brief interim between the molt and the hardening of the new exo-skeleton. Isopods mate in between the two half molts.

(It always makes sense to re-read Wikipedia articles: I often notice things I had missed on an earlier read. This time, I noticed the line about molting, then searched Google on isopod molting and found out about the mating strategy.)

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.