Small horseshoe crab shells have started littering the wrack line in Middle Bay, Maine. Every high tide brings more, sometimes half a dozen shells for every meter or so of wrack.

These are not dead crabs (the May full moon mating frenzy left plenty of those), but the discarded exoskeletons of molting juveniles. Young horseshoe crabs spend the first few years of their lives in the muddy bottom of the bay, just below the lowest reach of the tide. In August they molt, crawling head-first out of their old shells, then the animals swell up and harden their new armor. The cast-off shells wash ashore. Mature horseshoe crabs don’t molt, so these shed skeletons are all from growing youngsters. I’d guess the one in the photo is about two or three years old. This year’s hatchlings are still less than an inch across.

These are good things to find. Although the species is protected in Maine, horseshoe crab populations elsewhere have been hammered over the last decades by harvesting for bait and for bleeding (to yield chemicals used in human blood tests). Seeing new generations coming along is good news for the horseshoe crabs (who’ve been around largely unchanged for >400 million years) and for the migrant birds that depend on their springtime eggs for food.