Japanese spider crabs, also known by their scientific name Macrocheira kaempferi, reproduce by internal fertilization. Once a year, during the mating season between January and March, male crabs insert sperm into abdomens of female crabs with the foremost pair of their chelipeds.
Every mating season, female spider crabs lay up to 1.5 million eggs that are smaller than 1 millimeter in diameter. They carry the fertilized eggs for about 10 days, at which point they hatch into larvae and enter the first zoeal stage. Immediately after hatching, the spider crabs larvae are left to fend for themselves, which is why only a few of them survive.
Zoeal stages in crab larvae are characterized by development of spines on the carapace and end with shedding of the exoskeleton after 12 to 37 days. Three-quarters of the larvae survive the first zoeal stage and enter the second stage. After another 12 to 37 days, the megalopa, or post-larval, stage begins. The megalopa stage ends after 30 days with the metamorphosis of 33 percent of the larvae that survived into adult animals. A fully grown Japanese spider crab, with a body size of 15 inches, leg span up to 13 feet and a weight of over 40 pounds, is the largest arthropod and can live up to 100 years.