Crabs more susceptible in larvae stage

The mass deaths of hundreds of tiny crabs discovered at Po‘ipu Beach Park and areas around the park may have an explanation.

Upon learning of the discovery by water safety officers Tuesday, Jean Souza, the Kaua‘i program coordinator of the National Oceanic and Atmospheric Administration’s Hawaiian Islands Humpback Whale National Marine Sanctuary, went to visit the site and sent out inquiries to several agencies for possible explanations into the deaths of the juvenile crabs.

In an e-mail reply, Loren Scott Godwin, a graduate student at the University of Hawai‘i Institute of Marine Biology said the crabs are not adult crabs but planktonic crab larvae that are at a stage called megalopae.

“At this stage, they are exclusively in the water column as part of the plankton and are active swimmers,” Godwin, considered to be an expert in Hawai‘i marine invertebrates, said. “As active swimmers they tend to be attracted to the light at the ocean surface and one of the things that tends to cause mass mortality in a group of megalopae is air bubbles forming under their shells due to being tossed around at the ocean surface.”

Godwin said the megalopae are generally unable to rid themselves of these bubbles easily and results in them being too buoyant. The megalopae are then at the mercy of the ocean swells and can be pushed to shore by a combination of wind and waves.

“Crab megalopae are one of the best sources of planktonic food for small fish and sea birds,” Godwin said. “They are a great source of lipids.”

Godwin said once they wash on the beach, the ghost crabs will make short work of them so their demise is a benefit to others.

A check with Po‘ipu Beach Park lifeguards revealed that although more crabs were washing ashore, the quantities were substantially less than the Tuesday amounts, and on Thursday, most of the crab larvae was gone with the exception of an isolated few mixed in with the sand.