Spider Crabs

Spider crab

The Japanese spider crab is the largest crab in existence

Special to The Star
Published: Thursday, February 13, 2014 at 01:44 PM.

By Tom Baird


While bay waters are crystal clear right now, snorkeling is probably not on your “to do” list. However, warmer days – and warmer water – are coming. When scallop season arrives, you’ll be back in the water and see many things in the bay waters besides scallops. A regular sight for snorkelers is the spider crab. They are hard to miss as they slowly lumber over the seagrasses or sandy bottoms of the bay. The species of spider crab we have in St. Joseph Bay is the Portly Spider Crab (Libinia emarginata), and they are easily seen if you snorkel over the grass flats at Eagle Harbor.

Spider crabs are in many ways opposite to the familiar and tasty Blue Crabs (Callinectes sapidus). While blue crabs are beautiful with their stream-lined body, red-tipped claws and blue carapace; spider crabs are dull tan and rather ugly. Blue crabs can swim and will dart away when you get too close. Spider crabs can’t swim, are slow, and are easy to pick up. The meat of the blue crab is delicious, and they are the target of both commercial and recreational collectors. Spider crabs have little muscle but are edible. Nevertheless, few people bother. Blue crabs can deliver a painful pinch if not correctly handled. Their sharp claws can even puncture a finger nail. Spider crabs, on the other hand, have small, weak pinching claws and are easy to handle and inspect. Blue crabs scuttle sideways, as do most crabs. Spider crabs walk in a forward motion.

Then again, spider crabs have some very interesting adaptations. First, they are a scavenger, which means they can survive on a wide range of organic material. They will, however, also eat starfish and small shellfish, like mussels, and will prey on anemones. They are masters of camouflage. They will often tear apart sponges and place the pieces on their backs as camouflage. The back of their carapace is often covered by living bryozoans, hydroids, algae and sponges, enabling them to blend with their surroundings. They have sensitive tasting and sensing organs located on the tips of their walking legs which allow them to identify food in the water or on the substrate as they walk.

Despite the name, portly spider crabs do not have eight legs, but are a true Decapod with ten legs. They are somewhat spider-like in appearance however, and when the legs are outstretched can reach a spread of up to a foot. The central body reaches a maximum of four inches front to back. The result is a relatively small body with long legs.

The Portly Spider Crab’s range is from Nova Scotia to the western Gulf of Mexico, and to depths of 150 feet. They have few defenses other than their camouflage, however when attacked or threatened they will wave their pincers over their heads as a warning gesture. The males grow slightly larger than the females, and aggressively protect the females when they are carrying eggs.

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