Giant Spider!

Tuesday, April 15, 2025

Giant Spider!

Today’s special animal friend is the Japanese giant spider crab, Macrocheira kaempferi. It lives in the ocean around Japan, and also near Taiwan, and it is really big, with a leg-span up to twelve feet. This is the largest leg-span of any known arthropod. Their carapace can be 16” across, and they can weigh up to 42 lbs. Japanese spider crabs are decapods, which means they have ten legs, not eight like an arachnid. However, they use only eight of the legs for walking. They other two have claws adapted for tearing and grasping. They are called “chelipeds.” Males’ chelipeds are much longer than their walking legs, while females’ chelipeds are shorter.

They have orange coloring over a taupe base color and sometimes have white spots on the legs. Their exoskeleton is tough and bumpy, protecting them from predators such as octopi. The color and texture also help camouflage them against the ocean floor, where they are found at depths between 600 and 2,000 feet, often near vents and holes in the substrate.

They are omnivorous, with a diet emphasizing ocean floor debris such as dead and decaying fish, invertebrates, and plants. Sometimes they pry open shells and eat living bivalves, catch small marine invertebrates like worms, or tear up living kelp to eat. Mating is accomplished when male and female press their abdomens together, and the male uses his chelipeds to put a spermatophore into her cloaca. A female can lay up to 1.5 million eggs per breeding season. She carries the fertilized eggs on her abdomen until they hatch into planktonic larvae. Only a small percentage reach adulthood.

They spawn in shallow water, and the juveniles remain nearer the surface, a couple of hundred feet, and move into deeper, cooler water after reaching adulthood. As they grow, the young crabs, who are more vulnerable to predators than adults are, camouflage themselves by picking up tube worms, sponges, and other invertebrates from the sea floor and sticking these animals onto their shells. The crab secretes gunk that causes the animal to adhere to its shell, and the passengers, as it were, resume growing in their new location.

The giant spider crab’s legs keep growing throughout its life, which may be 50 to 100 years. Adults have no predators other than large octopi, which may tear off a leg to snack on. The crab will regenerate the leg the next time it sheds its exoskeleton.

I assume that, like Atlantic lobsters, they die when their growth rate slows so much that the exoskeleton physically deteriorates before they generate a replacement. IUCN has not evaluated the population of giant Japanese spider crabs. They are commercially fished in Japan, but, because most of them live in deep water, they are not caught all that often. Because of a decline in the number caught in recent decades, they are now protected from fishing during their mating season.