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.)