<P> Females are larger than males; C. rotundicauda is the size of a human hand, while L. polyphemus can be up to 60 cm (24 in) long (including tail). The juveniles grow about 33% larger with every molt until reaching adult size . </P> <P> During the breeding season, horseshoe crabs migrate to shallow coastal waters . A male selects a female and clings to her back . Often, several males surround the female and all fertilize together, which makes it easy to spot and count females as they are the large center carapace surrounded by 3 - 5 smaller ones . The female digs a hole in the sand and lays her eggs while the male (s) fertilize them . The female can lay between 60,000 and 120,000 eggs in batches of a few thousand at a time . The eggs take about two weeks to hatch; shore birds eat many of them before they hatch . The larvae molt six times during the first year . Raising horseshoe crabs in captivity has proven to be difficult . Some evidence indicates that mating takes place only in the presence of the sand or mud in which the horseshoe crab's eggs were hatched . It is not known with certainty what is in the sand that the crab can sense, nor how they sense it . </P> <P> Horseshoe crabs use hemocyanin to carry oxygen through their blood . Because of the copper present in hemocyanin, their blood is blue . Their blood contains amebocytes, which play a similar role to the white blood cells of vertebrates in defending the organism against pathogens . Amebocytes from the blood of L. polyphemus are used to make Limulus amebocyte lysate, which is used for the detection of bacterial endotoxins in medical applications . This means there is a high demand for harvesting the blood, which involves collecting and bleeding the animals, and then releasing them back into the sea . Most of the animals survive the process; mortality is correlated with both the amount of blood extracted from an individual animal, and the stress experienced during handling and transportation . Estimates of mortality rates following blood harvesting vary from 3 - 15% to 10 - 30% . Approximately 500,000 crabs are harvested annually . </P> <P> The mortality rate of bled horseshoe crabs could be as high as 29% . Bleeding may also prevent female horseshoe crabs from being able to spawn or decrease the number of eggs they are able to lay . Up to 30% of an individual's blood is removed, according to the biomedical industry, and the horseshoe crabs spend between one and three days away from the ocean before being returned . Some scientists are skeptical that certain companies return their horseshoe crabs to the ocean at all, instead suspecting them of selling the horseshoe crabs as fishing bait . </P>

What is the color of horseshoe crab blood