With the summer tourist season unofficially kicking off, it isn't just shoobies and snowbirds converging on the Jersey Shore. The annual invasion of horseshoe crabs, those ancient mariners who each spring make their way to beaches along the Delaware Bay to spawn, is in full swing.

Worldwide, there are four species of horseshoe crabs, though in this area we're most acquainted with Limulus polyphemus, those familiar yet funky-looking creatures sometimes called living fossils.

Horseshoe crabs are among those animals that almost defy explanation - not even a crustacean, they're closer cousins to scorpions and fleas than a lobster or blue crab, and can live more than 20 years.

Aesthetically, they're not about to win any beauty pageants, with nine eyes, a helmet-shaped carapace, and that scary swordlike tail. But they're a vital part of the life of the red knot, an at-risk shorebird species that makes an annual migratory pit stop along the Delaware Bay to feast on horseshoe crab eggs.

They're important in medicine, as well, as their blood contains a clotting agent that provides a rapid, reliable test to detect infectious bacteria in drugs and some medical prosthetic devices.

The horseshoe crab is a protected species. It's illegal in New Jersey to harvest or even possess a live one without a proven scientific purpose. Three men were recently issued summonses in Ocean City after police found them with a pickup truck loaded with 132 horseshoe crabs. It's unclear why the men wanted the crabs, as the local species is only considered useful as bait for eels and whelk.

So if you're looking for a souvenir to take home from your Memorial Day at the Shore, stick with saltwater taffy or those little hermit crabs for sale at boardwalk shops (most of which come from the waters off Haiti), and leave the horseshoe crabs alone.