The nights of horseshoe crabs: arthropods are shedding their exoskeletons

The nights of horseshoe crabs: arthropods are shedding their exoskeletons

It is molting season for horseshoe crabs at the Primorsky Aquarium now: the animals are shedding their old tight exoskeletons (hard external shells) to grow larger. The arthropods gain weight between molts but the change in size only occurs after the animal has cast off its outer covering: an “upgraded” horseshoe crab is sometimes a third bigger than its shed shell.

Some horseshoe crabs housed at the Aquarium are undergoing their first postlarval molt, the others are quite large and have discarded their “clothing” more than once before. These living fossils shed 16-17 times during their life.

“To begin molting, the horseshoe crab must have proper nutrition and sufficient calcium for the formation of a new exoskeleton,” explained Marat Khaidarov, Head of the Department of Exotic Aquatic Species. “If the arthropod is short of necessary minerals, it will almost stop growing and thus will cease molting. We add special supplements to the feed for our horseshoe crabs so that the animals can get the same amount of useful nutrients as they would in the wild.”

Interestingly, the Aquarium’s horseshoe crabs tend to molt at night. The process starts with the carapace — a continuous shield that covers the cephalothorax — splitting open. It usually takes the horseshoe crab several hours to wiggle its way out of the shell: the older an individual is, the longer it lasts. To cast off its old exoskeleton, the horseshoe crab needs to swell up by taking in water. The expansion of tissues creates excessive pressure that allows the arthropod to exit the old shell.

The animal sheds its entire shell, including the coverings of its legs and tail. The new exoskeleton is initially soft but hardens in a few hours. “Molting is accompanied by regeneration. For instance, if an individual has an injury to its carapace, its severity will be reduced with each new exoskeleton. Lost limbs can be regenerated too,” said Marat Khaidarov. “When molting, the horseshoe crab also removes troublesome fouling organisms attached to its exoskeleton that can perforate it.”

The shell shedding has helped one of the Aquarium’s horseshoe crabs get rid of not only the old carapace but of its nickname as well. When the individual arrived at the institution, its shell featured an unusual pattern resembling the symbol of the buccaneer flag, due to which the animal was named Pirate. Most probably the marking looking like the Jolly Roger symbol has appeared on its carapace because of environmental influences, and the molt it went through at the Aquarium has deprived Pirate of the unique pattern.

General Information

The cast-off exoskeleton is called an exuvia. Its thickness depends on the age of the animal: exuviae of the first several molts are slightly thicker than wax paper. While shedding its shell, the horseshoe crab breaks one single seam running along the shell margin, and therefore the discarded exuvia remains intact.