The Secret Lives of Coral Reef Fish

Anjani Ganase, marine biologist, looks into the world of coral reefs at some of the residents in these undersea cities. Down there, she says, it’s noisier than you think!

When we dive along coral reefs, we glimpse moments in the lives of the fish and marine creatures bustling about on their daily routines in underwater coral cities. At first glance, movements may appear arbitrary, but as you observe for a minute or two we start to recognise the activity the fish is carrying out, whether it is foraging or simply hiding out. However, for more rare activities or seasonal movements, such as at dawn or dusk or during mating season, this would require longer, more consistent times spent looking into the secret lives of fish. Here are a few observations of some reef fish behaviours:

Hunting buddies, the grouper and the moray eel

Grouper (Plectropomus pessuliferus) and Giant Moray Eel (Gymnothorax javanicus) make an excellent partnership for hunting on the coral reefs. How this partnership came about is not clear but it appears that the different strategies of the grouper and the moray eel compliment each other and therefore make for highly successful cooperative hunting of prey. Groupers tend to cruise over the coral reefs during the day, hunting fish that have strayed a bit too far off the reef. Moray eels are able to weave in between the reef framework, bombarding fish and backing them into tight corners. Morays also prefer to hunt at night, where low visibility gives them an advantage.

The grouper often initiates the hunting partnership, which is more likely to occur after an unsuccessful hunt when the grouper is hungry. The grouper will swim to a known resting spot of a moray eel, near to where it may have lost its prey. The grouper will then gesture to the eel in the form of a rapid shaking of the head in front of the eel’s face. The eel responds by swimming out on to the reef together with the grouper. If the uncaught prey is nearby, the grouper will then attempt to guide the eel to the location, usually a hole, where the prey previously escaped. The eel then uses its method of hunting. For the prey to escape the moray it may head to open water, where the grouper will be waiting. Alternatively, if the eel corners the prey, then the eel will have it for itself. Researchers have found that the grouper and moray eel could spend over half an hour hunting together, and both the grouper and the eel still benefitted overall from these arrangements, even though they did not share the meals (Bshary et al. 2006). On the occasions where the eel fed on the prey, the grouper did not appear to act aggressively towards the eel. Further research may be needed to determine whether the grouper will approach the same eel in the future. Groupers have also been observed gesturing for the aid of other predators. In an area of the reef where the prey is hiding, the grouper will remain near the opening and perform a headstand and shake its head to signal to others. Predators, such as napoleon wrasses, groupers and even moray eels may come over to assist in the hunt (Bshary et al. 2006).

Close up of a giant moray eel on Opal Reef, The Great Barrier Reef. Photo by Richard Vevers, The Ocean Agency, XL Catlin Seaview Survey.

Tool use by the wrasses

Only recently, certain species of wrasses, such as the black spot Tuskfish (Choerodon schoenleinii), yellowhead wrasse and sixbar wrasse, were recorded using corals as anvils to the crack the shells of cockles (Jones and Gardner 2011; Bernardi 2011). Tool use in the wild is associated with complex problem solving that extends the use of something other than a body part. The wrasses would uncover a bivalve, such as a cockle, by wafting the sand away with its fins. The wrasse would then pick the bivalve up using its mouth, swim over to a coral boulder and then pitch the bivalve towards the coral through sideways movement of its head. This action would be repeated multiple times until the shell of the bivalve is cracked and the fish could eat the insides (Bernardi 2011)


A school of Bumphead parrotfish hovers over a coral reef bommie. Photo by Richard Vevers, The Ocean Agency, XL Catlin Seaview Survey
Head butting bumphead parrotfish

On almost every dive along the northern Great Barrier Reef we would encounter schools of bumphead parrotfish, cruising along the shallows and foraging on corals. These fish may look intimidating with their bulbous head and chicklet smile but generally are not aggressive fish. Despite the frequent encounters, I’ve never experienced bumphead head-butting each other to win the favour of a female. This was the experience of scientist Roldan Muñoz and others, while diving in a remote marine reserve in the western Pacific: male bumpheads were observed repeatedly head-butting each other in an epic battle for the right to mate. The sounds that are produced from these repeated head bumps were quite loud and metallic. Encountering this aggressive behaviour also suggested that the marine reserve was also a spawning ground for bumphead parrotfish (Muñoz et al. 2012).

Perfect flatmates – the pistol shrimp and the goby

Living at the sandy edges of coral reefs, the goby sits beside a hole keeping a watch for predators. Meanwhile, his roommate, the pistol shrimp, busily maintains the burrow that they both share, by relocating sand and setting up coral and shell reinforcements along the sides and the opening of the burrow. The pistol shrimp is blind and therefore relies on movements of the Goby’s tail nearby to let it know when danger is around so that it can retreat back into the hole. The pistol shrimp has an enlarged claw that acts as another line of defence. When the claw clamps down, it sounds like a pistol blast, making it the noisiest animal in the ocean (relative to its size), beating even the sounds of whales and other large marine animals. The goby feeds on organisms, such as crustaceans and amphipods that live among the sand particulates stirred up by the shrimp. The shrimp, on the other hand, feeds on mostly detritus, some of it from cleaning the goby. After sunset, both shrimp and goby retreat back into the burrow, which becomes closed in by the end of the night either intentionally, or just from lack of maintenance overnight. In the morning, the goby pushes through the sediment and both goby and shrimp each begin their job of look out and excavator. The survival of the Goby and the shrimp is dependent on due diligence by the two creatures.

References:
Bshary R, Hohner A, Ait-el-Djoudi K, Fricke H (2006) Interspecific Communicative and Coordinated Hunting between Groupers and Giant Moray Eels in the Red Sea. PLoS Biol4(12): e431. https://doi.org/10.1371/journal.pbio.0040431

Jones, A.M., Brown, C. & Gardner, S. Coral Reefs (2011) 30: 865. https://doi.org/10.1007/s00338-011-0790-y

Karplus, I. (1987), The Association between Gobiid fishes and burrowing alpheid shrimps. Oceanogr. Mar. Biol. Ann. Rev., 25, 507 562

Muñoz RC, Zgliczynski BJ, Laughlin JL, Teer BZ (2012) Extraordinary Aggressive Behavior from the Giant Coral Reef Fish, Bolbometopon muricatum, in a
Remote Marine Reserve. PLoS ONE 7(6): e38120. doi:10.1371/journal.pone.0038120



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