by Gary Ray, PhD
Typically, we think of great migrations of wildebeests in East Africa, caribou in Alaska, or whales in the southern oceans but crabs don’t often come to mind. According to our ‘guest post’ arriving today from Gary Ray, a researcher in the US Virgin Islands, we should widen our point-of-view. Steve Simonsen, the videographer a fine documentarian, captured video a very rare event---the migration of hermit crabs marching back nto the island’s forests after their ocean breeding season. Simonsen commented about his luck:
“This morning, I received an urgent telephone call from my good friend. She was terribly excited about an event just happening right before her eyes---a mass migration of soldier crabs, also known as hermit crabs. There were millions and millions of them she likened it to the migrations of Serengeti. I didn't need to hear anymore, I loaded my car with my video camera gear and was out the door. My friend told me that the migration began at sunrise at Nanny Point on St. Johns in the US Virgin Islands. I have heard about similar migrations and knew they occurred in August. The video shows the dramatic march of the crabs.”
The Caribbean version of the terrestrial hermit crab (Coenobita clypeatus) include females returning from (or on their way to) their egg dispersal in near-shore waters. The must redistribute themselves over large areas of the island to scavenge meals of downed fruit, excrement, and a wide spectrum of partially decomposed organic delights. Each year in late August to early September they begin their descent from the hills to particular shorelines, increasing in density as they approach the sea. As the aggregations grow, males partially “disrobe” from their shell, exposing sperm packets that are transferred to egg-bearing females. Fertilized eggs begin embryonic growth prior to dispersal into the sea, nourished by egg contents. Apparently guided by lunar cues, females take a dip en masse to release their eggs, which burst following seawater contact.
Adult soldier crabs (local Virgin Islands moniker) will select a variety of marine and intertidal zone mollusk shells as it grows to maturity, but overwhelmingly prefers the West Indian top shell (Cittarium pica), an intertidal snail. The snails, locally termed “whelks” (not true whelks), are harvested intensively by islanders. As a consequence, the larger shells no longer appear, a limiting factor for the soldier crabs, which desperately search for heavier, large substitutes for the snail shells. Most cannot find suitable replacements; begging the development of more sophisticated top shell harvest management. Soldier crabs have been observed carrying away baby food jars, old film canisters, and the like when shell availability wanes.
The video produced by Steve Simonsen captures a portion of this natural process well.
March of the Soldier Crabs (credit: Steve Simonsen)