Embattled coral reefs in French Polynesia have found a colorful ally in crabs. In exchange for shelter and secreted nutrients, different species of coral guard-crabs fiercely defend their coral hosts against corallivores ranging in size (and voraciousness) from giant predaceous starfish to small sea snails. The work was published in PeerJ this week.
Crown-of-thorns sea stars (Acanthaster planci), which grow to the size of trashcan lids, can threaten entire reef communities. After inverting its stomach, the sea star drapes it over a coral and secretes digestive enzymes that dissolve the coral tissue, which is then absorbed by the stomach. But if the coral is housing guard-crabs from the genus Trapezia, they’ll emerge to pinch, shake, and nip off the sea star’s tube-like feet. “Basically, they annoy the sea star until it goes away,” Jenna Moore from the University of Florida tells Smithsonian Science. For these protective services, the coral offers shelter and produce fatty deposits in their tips for the crabs to snip off and eat.
To further examine the symbiotic relationship between these crabs and the corals they defend, Moore and C. Seabird McKeon from the Smithsonian Institute tested the effectiveness of several Trapezia species at repelling sea stars and other corallivores in a series of experiments near the island of Mo’orea in the Indo-Pacific.
“We found that diversity in both species and size of coral guard-crabs is needed to adequately fend off coral predators,” McKeon says in a news release. “Seemingly small differences among crabs guarding their coral homes can have big effects on coral survival.”
In one of their experiments, the duo removed the largest species of coral guard-crab, T. flavopunctata (pictured, right), from 45 Pocillopora coral colonies in the path of a crown-of-thorns sea star outbreak during 2008 and 2009, and then watched what happened. Undefended corals -- and even corals with other species of guard-crabs -- were eaten overnight in 64 percent of the colonies. For comparison, 18 percent of corals with all their symbionts intact were attacked. The sea star leaves behind visible white scars of exposed coral skeleton where tissue was removed (pictured below).
This crab-coral symbiosis is well documented, but what’s new here is the extent of functional diversity among Trapezia species. The level of protection provided vary by both size and species. Smaller guard-crabs called T. serenei and T. punctimanus defend their homes from coral-eating sea snails Drupella cornus, a threat that larger crabs ignore. Medium-sized crabs (including T. bidentata and slightly larger T. serenei) protect them from cushion stars, Culcita novaeguineae. Guard-crab diversity, the findings show, offers multiple lines of defense.
Different species of guard-crabs reside inside the same coral, but only one monogamous male-female pair of the same species can live there. And host corals aren’t the only ones to benefit from a diverse guard-crab ecosystem: Smaller species of corals were also shielded from the onslaught.
Images: David Liittschwager (coral with crab, top), Seabird McKeon (crab, middle), Jenna Moore (sea star on coral, bottom)