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Shade & Light

Some reef fish require large table corals to conceal them.

James Cook University researchers James Kerry and Professor David Bellwood constructed artificial habitat to test the attraction of concealment for large reef fish.

By James Kerry

Climate change is reducing the complexity of coral reefs, with implications for the reef fish that require large table corals to conceal them from predators, prey, and even ultraviolet light.

Healthy coral reefs are architecturally complex environments sporting a diversity of structures that include corals, overhangs, grooves, tunnels and spurs. This structural complexity is one of the elements that makes coral reefs so visually appealing, but hidden within the reef complex there might be certain shapes that are particularly important for the survival of reef fish.

It is well-known that intricate branching corals are the preferred habitat for many species of small reef fishes, but there is only anecdotal evidence for the structures preferred by larger coral reef fish. We therefore set out to investigate whether or not larger reef fish show a preference for certain shapes of corals, and studied three of the most common growth forms: table corals, branching corals and dome-shaped corals.

We used underwater video cameras to record the behaviour of large reef fish interacting with these corals at a number of sites around Lizard Island in the northern Great Barrier Reef. Careful analysis of the video footage showed a very clear pattern for the majority of the large reef fishes – they vastly preferred table corals over branching or dome shapes.

There were consistently greater numbers of large reef fish gathering around table corals. These individuals tended to be significantly larger than the fish associating with the other shaped corals studied, and also tended to stay for longer periods. But why was this?

Clear structural differences exist between the different coral shapes. The branching corals that were studied had sufficient internal spaces to accommodate large reef fish but were often avoided. We questioned whether this is because the manoeuvrability of a fish would be severely restricted in the maze of a branching coral, reducing the possibility of a rapid escape in the event of an approaching threat.

Dome corals are relatively simple in shape compared with branching or table corals, and at first glance appear to offer few obvious structural benefits for large reef fish, which explains their apparent avoidance in our study. The larger dome corals may, however, prove more structurally useful than they first appear.

Professor Garry Russ of James Cook University has observed schools of large piscivorous fishes exploiting current flows around large dome corals when hunting. The older dome corals are frequently undercut by erosion so that they effectively create overhangs on the reef, producing a similar structural effect to the table corals that proved so popular in our study.

Table corals have at least one quality that immediately sets them apart from both branching and dome corals – the table top provides potential concealment and shade for reef fish. To examine this observation further we built artificial table corals from plastic storage containers, using the lid to mimic the table’s canopy. One type had the canopy painted entirely black while the other was left translucent. A number of artificial corals of both types were placed in the lagoon at Lizard Island, adjacent to existing coral reef.

In biology experiments of this kind it is common practice to allow what is referred to as a “soak time” for the introduced material. In this case we allowed the reef fish more than a week to acclimatise to the new habitat, and then conducted surveys of each artificial coral, noting the species and size of fish residing beneath them.

Once again a clear preference was evident. The large reef fish were highly selective, avoiding the translucent artificial corals and rapidly adopting the black artificial corals even after their relatively brief soak times. The black artificial corals were so attractive that our analysis of fish use of real table corals and fake black corals could not be distinguished statistically. It appears that there is something fundamentally important about the obstruction of vision and light that canopy structures on a reef provide, rather than the canopy shape itself.

In a number of cases the fish moved underneath the black artificial corals as we approached them. This suggests that large reef fish concerned about larger predators such as sharks consider themselves to be safer when they are wholly or partially concealed. A fish in shade is not only harder to detect but can also more clearly observe approaching threats.

Conversely, predatory fish that use an ambush mechanism to catch their prey will sit in shade waiting for an unwitting victim to pass by. In this case the ambush predator is both better able to see oncoming prey and at the same time is less easy to detect.

Interestingly, smaller reef fish such as damselfish tended to be more abundant near the translucent artificial corals than the black artificial corals. Although this might have been due to the displacement of smaller reef fish by larger reef fish, it is possible that smaller reef fish, which are more likely to be subjects of ambush predation, benefit from a clear field of vision in every direction.

Another potential attraction of shade provided by the canopy is the exclusion of harmful ultraviolet (UV) rays from the sun. There is evidence of tropical reef fish producing substantial quantities of UVA and UVB-absorbing compounds in their skin. In shallow reef conditions, like the ones studied here, it is possible that the production of these compounds represents a significant energy cost for fish that are frequently stationary. Table corals may reduce this cost by acting as a kind of sunscreen.

It is therefore concerning that table corals are especially threatened by climate change. Among the many shapes of corals, table corals are particularly susceptible to storms, coral bleaching and ocean acidification, all of which are predicted to increase in intensity in the coming decades.

One potential reprieve is that table corals are not the only structures on reefs that create canopies. Dome corals can erode to form overhangs on reefs, and other long-term processes can create ledges and tunnels that have similar physical properties as far as the fish are concerned. Nevertheless, the loss of table corals from reefs would reduce the quantity of shelter available for many large reef fishes, and may restrict this shelter to certain positions or depths on a reef.

It is still unclear just how critical this structural complexity is for large reef fish. The handful of studies that have correlated structural decline of reefs with communities of reef fish have not been able to give definitive patterns for large reef fish. Our study suggests that many species of large reef fish are especially drawn to table corals over other shapes, and this is probably because they provide important ecological benefits.

In the coming years our team will examine further questions about the importance of the structure of coral reefs in an attempt to better predict the indirect impacts of climate change on large coral reef fish, which are themselves important for the health of a reef.

James Kerry is a PhD student in marine biology at James Cook University. The research was funded by the ARC Centre of Excellence and is published in the scientific journal Coral Reefs.