Coral Guard Crabs
The guard carbs – Trapezia – are a group of dwellers living among the branches of hard corals. The crabs form heterosexual pairs and are territorial to their coral, tolerating other crabs only from other species on the same coral colony. It is possible to observe up to five different pairs of Trapezia species on one coral colony. These crabs belong to the Family Trapeziidae of the Order Decapoda, and are found in shallow tropical reefs worldwide.
The young guard crabs settle out of the plankton looking for their coral host. They are thought to be able to smell appropriate coral hosts and it is likely that different corals emit different scents, allowing a young crab to find its preferred host. Young crabs will tend to occupy smaller coral colonies rather than extensive coral heads inhabited by aggressive adult crabs.
The Trapezia have a strong mutual symbiosis with their coral hosts; wherein the coral provides lodge and board and in exchange the crab provides protection from corallivorous (coral-eating) creatures and cleaning services. Some Trapezia species will protect the corals from a group of corallivorous snails from the genus Drupella. Bigger crabs will defend their coral host from the cushion starfish (Culcita novaeguineae) by nipping of the tube feet of the echinoderm. The biggest and most protective crustaceans defend corals from the ferocious corallivore, the Crown-of-Thorns Seastar (COTS – Acanthaster planci) by pinching off the COTS’s integumentary spines, otherwise able to consume the whole coral colony in a matter of hours. Studies have shown that coral colonies that contain crabs are less likely to be eaten by these corallivorous sea-stars.
The presence of crabs stimulate the coral to concentrate protein-filled fat bodies in the tips of their tentacles, that are supposedly in excess and not needed for the coral. These fat bodies are providing a feed source for the crabs, wherein they go between the tentacles and clip these for a feed. Equipped with a brush and comb on their specialized-feeding legs, the crabs scrape the tissue of their coral host, collecting on the way mucus, debris and bacteria for a yummy nutritional meal.
In addition to the protective services the crabs provide, their activity between the coral branches has other benefits to its calcareous host. Whilst moving around the coral’s branches, the crab helps clean its host of sediment and also helps the flow of fresh oxygenated seawater in the tight spaces in the coral colony.