Corals Recruit Gobies as Bodyguards

The Disney movie, Finding Nemo, popularized the understanding that coral reefs provide essential habitats for a large amount of marine biodiversity including hundreds of species of and fish, turtles, invertebrates, and many others.  Unfortunately, over the past thirty years, half of the Great Barrier Reef has been lost and nearly 80% of the live coral in the Caribbean has died.  With such a drastic decrease in the area that serves as feeding, spawning, and nursing grounds for such a wide array of marine organisms, one can only expect increasing competition for territory and a decrease in marine biodiversity.  In a new study, published in the November 9th edition of Science, researchers found that some of the reef inhabitants are helping to fight back against further coral reef loss.

Currently, major stressors including ocean acidification and heating, pollution, disease, and the overfishing of herbivores, enable some seaweeds to increasingly outcompeting the corals for territory.  Corals in the genus Acropora are essential for reef biodiversity as they provide the topography that other species rely on for colonization.  This study found that two reef dwelling gobies, Gobiodon histrio and Paragobiodon echinocephalatus, would help to protect the coral Acrospora nasuta from advances by the toxin containing seaweed Chlorodesmis fatigata.

The authors found that colonies of C. fastigata that wereplaced on corals inhabited by either or both species of gobies were quickly eaten or removed by the fish.  Abundance of the seaweed was decreased by 30% over the course of three days and the damage to A. nasuta was reduced by ~80% in comparison to that in corals lacking the gobies.  Additionally, the seaweed was found in the gut of the goby G. histrio, while it was not found in the gut of P. echinocephalatus.  It is also noted that G. histrio produces a toxic skin secretion, while P. echinocephalatus does not, though further investigation was unable to link the metabolites from the seaweed to the toxins in the goby.  These gobies are providing the coral with a fighting chance as these toxic algae continue to expand their range.

A goby on its way to remove C. fastigata from the coral Acrospora. Photo: Danielle Dixson

Recognition of the mutualistic relationship between the coral and the gobies was not enough, and the authors also set out to see what triggered the goby’s response to the encroaching seaweeds.  Using a cleverly designed experiment the authors found that chemicals released during the interaction between C. fastigata and A. nasuta along with cues from stressed coral after C. fastigata was removed caused the gobies to move toward the site of contact.  However, the seaweed on its own would not alert the gobies.  Further study using faux seaweeds controls and faux seaweeds infused with extracts from C. fastigata provided evidence that when compounds from C. fastigata come in contact with the coral, A. nasuta releases chemical cues, alerting the goby, which then arrive to remove the seaweed from the area.  When experiments were replicated using different coral species in contact with C. fastigata, no response from the gobies was detected.Though similar to other mutualistic relationships that have been well studied, the authors note that this appears to be the first time studies have shown one species chemically triggering a response in another species to help remove a competitor.

Additional research has shown that C. fastigata is particularly damaging to a range of Acrospora species and its continued presence will slow the recovery of corals that have been damaged (Bender et al., 2012).  A greater understanding of chemical cues may lead to other mechanisms that can be employed, slowing the loss of the earths remaining coral reefs.  Until that time comes, hopefully these little gobies can continue to combat C. fastigata, so corals like Acrospora can continue serving as the foundation that allows reefs to flourish and be large centers of biodiversity for generations to come.

References:

Bender, D., Diaz-Pulido, G., and S. Dove. 2012. Effects of macroalgae in corals recovering from disturbance. Journal of Experimental Marion Biology and Ecology. 429: 15-19.

Dixon, D.L., and M.E. Hay. 2012 Corals chemically cue mutualistic fishes to remove competing seaweeds. Science. 338: 804-807.

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