Dr Henrique Bravo recording the presence of coral-associates using a transect tape during an underwater survey. May 2022, Little Cayman, Caribbean. Photo by Oriol Torres. Courtesy of Henrique Bravo.
Life within the Coral: A World of Tiny Hidden Creatures
Beneath the surface of a coral reef, life unfolds not only around corals but also inside them. It is easy to notice the many fish swimming among coral colonies, a familiar image of reef life. But when scientists look more closely, they discover that corals themselves are far from solitary species. Within the branches and skeletons of coral colonies live crabs, shrimps, worms, and many other tiny invertebrates that make corals their home, drawn to the shelter and food provided by the coral’s complex structures.
For Dr Henrique Bravo, coral reef ecologist and Research Fellow at the Earth Observatory of Singapore, Nanyang Technological University, these hidden inhabitants are key to understanding how coral reefs function. His research explores the many invertebrates that live in close association with corals and how these relationships shape the ecosystem.
‘’Corals are not just animals forming reefs’’, Henrique explains. ‘’They are ecosystems themselves, providing habitat and food for entire communities of organisms that live on, within, or around them’’.
Most of these small organisms are easy to miss. Some hide between coral branches, while others burrow in the coral skeleton itself, creating small pits, chambers or tunnels. To study them, Henrique combines underwater surveys and photography (used for 3D modelling) with laboratory techniques such as DNA analysis, sable isotopes, and micro-CT scanning, which can reveal what is happening both on the outside and inside of coral fragments. These methods allow scientists to detect animals that are often hidden from view and to better understand how they live in close association with corals.
Scientists refer to the many different roles played by these organisms as functional diversity: the variety of ways species contribute to the functioning of an ecosystem. On coral reefs, this diversity can influence how reefs grow, how they cycle nutrients, and how they respond to environmental change. Hermit crabs may inhabit empty holes in a coral for shelter, some shrimps take refuge among the coral’s structures, and certain tube-building worms sometimes alter the coral’s shape as they grow jointly. Some organisms also use corals as feeding grounds, catching food particles trapped by the coral or carried through the water.
Some coral-dwelling animals depend entirely on their hosts. For example, small gall crabs live in small chambers inside coral colonies, where they rely on the coral for shelter and feed mainly on its mucus. Other species, such as coral guard crabs, can also help protect their hosts by pinching and driving away predators such as crown-of-thorns starfish, which digest living coral tissue and leave behind bare white skeletons. In the case at Teluk Bakau in Bintan, it is the Diadema antillarum, or long-spined sea urchin that causes damage to the corals. All of this highlights the close and sometimes complex relationships between corals and the animals living within them.
Because many of these animals are so closely tied to their coral hosts, their presence (or absence) can reveal important clues about the health of a reef. A coral reef rich in coral-dwelling species often indicates a structurally complex and thriving ecosystem, with conditions that support many different coral hosts and the organisms associated with them.
For Henrique, studying these relationships helps shift how we think about coral reefs. Rather than focusing only on coral cover, scientists are increasingly paying attention to the intricate web of interactions that corals sustain. Many of these organisms are highly host-specific, meaning that the disappearance of particular corals may also mean the disappearance of the species that depend on them.
‘’Being able to understand how these communities are coping with current challenges and changing environments might tell us what they will look like as these conditions keep on worsening,’’ Henrique explains.
In fact, given how little is known about these coral-associated species, and how difficult it is to find them; some species may have already been lost, without us realizing. Understanding these hidden communities, and the roles they play, is becoming increasingly important as coral reefs face growing pressures from climate change, ocean warmin, pollution, and habitat loss. By studying the small creatures living inside corals, scientists can gain new insight into the complexity and vulnerability of these extraordinary ecosystems, before it is too late.
