Ocean corals could use a vacation.
As global temperature increases, the temperature of ocean water has followed suit. Corals rely on symbiotic bacteria that live inside the coral cell to photosynthesize the coral’s food. These bacteria, however, are fleeing the uncomfortably warm water for more hospitable environs, leaving the corals a bit peckish.
Along with increasing water temperature, ocean water pH also is changing (think a tad more like orange juice than milk). The impressive coral complex in which the corals live is crumbling in the increasingly corrosive ocean water. In their weakened state, corals are now subject to attacks by coral predators. But, in every great drama, a hero rises to the occasion. The champion of the coral? A genus of crab called Trapezia.
Trapezia tigrina – Red Spotted Guard Crab Credit: Steve Childs
The usual coral predators include two species of seastar (Acanthaster planci and Culcita novaeguinaeae) and one species of gastropod (Drupella cornus). These predators actively feed at night and leave visible scars on the exposed coral tissue. But, could a tiny crab stop these spiky predators?
Crown of Thorns (Acanthaster planci), Credit: Jon Hanson
While scientists around the globe have been actively studying coral reef systems to ensure these impressive ecosystems are not lost from the planet, for many years they discounted the importance of many species of Trapezia, due to their diminutive size. But a recent study in Mo’orea, French Polynesia proves that these tiny critters are up to the fight.
Despite their low numbers and small size, researchers found that the presence of the crabs not only reduced the occurrence of predation, but also reduced the amount of coral species consumed during an attack. Thirteen of the 21 species of Trapezia live at the French Polynesian site. These species provide mutualistic symbiosis in a diverse coral ecosystem. A coral reef is like a vast apartment complex and within each unit lives a coral polyp. Smaller Trapezia species are like dogs keeping kids from skateboarding outside the complex. Larger species of the crabs are like security guards – they keep like-sized jerks at bay. Or, if you prefer, the Trapezia species of various sizes confer improved survival benefits to the host organisms (the coral) by fending off a variety of antagonists (sea stars and gastropods).
And so, on behalf of ocean corals everywhere, thank you little Trapezia.
Article Citation: C. Seabird McKeon, Jenna M. Moore (2014) “Species and size diversity in protective services offered by coral guard-crabs”, PeerJ
Featured image courtesy of The Moorea Coral Reef Long Term Ecological Research Site