MUKILTEO — State biologists reassure community members that what appears to be dead crabs littering beaches near Mukilteo are mostly molted shells.

People have reported large numbers of Dungeness crab shells on the beaches of Mukilteo Lighthouse Park, Silver Cloud hotel and near the Boeing Pier since Dec. 30, Washington Fish and Wildlife biologist Don Velasquez said in an email.

“These Mukilteo beaches represent a location where Dungeness crab molt shells have accumulated many times in the past on the shoreline,” he wrote. “Crabs of certain size categories and sex tend to molt at about the same time of year, and the shells get noticed by the public.”

On Monday, Velasquez and Amy Pumputis, a lead science technician for Fish and Wildlife, collected data on washed up shells at reported beaches.

The scientists measured the top shells, called carapaces, and recorded when shells were attached to the body and legs, or broken off.

“All the complete shells amongst our sample were male molts and not dead crab,” Velasquez said.

Dungeness crabs part of the animal group called arthropods, making them distantly related to spiders. Arthopods have exoskeletons, hard shells that protect the animals from the enironment or predators.

But the exoskeletons don’t grow with the animals like human’s bones do. In order for crabs to grow, they must shed the outgrown shells.

Dungeness crabs molt roughly a dozen times before they reach two-years-old, and then molt about once a year for the rest of their lives, according to the state’s Department of Fish and Wildlife.

“Molted shells are light and they collect where the wind and currents naturally concentrate them,” Velasquez wrote. “These Mukilteo beaches represent a location where Dungeness crab molt shells have accumulated many times in the past on the shoreline.”

Eliza Aronson: 425-339-3434; eliza.aronson@heraldnet.com; X: @ElizaAronson.

Eliza’s stories are supported by the Herald’s Environmental and Climate Reporting Fund.