Hide and Seek on a Coral Reef

Latest update July 2, 2019 Started on June 7, 2019
sea

Fish love to hide inside the complex architecture that corals build. But on many reefs, corals are being replaced by more tolerant organisms, including sponges. Can soft-and-wobbly sponges provide a safe habitat for fish on future reefs?

June 7, 2019
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Preparation

In the coming weeks, as we prepare for our fieldwork, we will take some time to introduce you to some of our favourite Caribbean coral, sponge and fish species.


Starting today, with Acropora palmata or elkhorn coral, named after the obvious resemblance of its branches to elk antlers. Once a dominant coral in the Caribbean, over the past decades this species has suffered from diseases, bleaching, coastal development and other man-made stressors, and so it is now much harder to find. ‘Critically endangered’ is its official status on IUCN’s Red List of Threatened Species.

On Curacao, you can still find quite a few elkhorn corals in the shallows. In between the horns, fish find the perfect hiding spots. So well-hidden are the fish that you can hardly even see them in these snapshots!

The south side of Curacao is sheltered, and the reefs here are often separated into three zones based on their depth: a shallow zone (from the shore to 8 metres depth), an intermediate zone (8-20 m depth), and a deeper zone (>20 m). The shallow and intermediate zones will be our focus when we start our fieldwork. The shallow zone is dominated by branching corals such as elkhorn coral, whereas in the intermediate zone you will often find boulder-shaped corals. Both these zones can be easily accessed by conventional scuba diving. So we are hoping our video analysis will allow a better look at the fish that choose elkhorn coral as their home.

But we are also very curious to see what the relationship between fish and corals/sponges is in the deeper zone, where plate-like corals dominate. As we cannot go that deep routinely, we’d love to have some technology that can go there instead. ‘Technology that takes you places’, or in other words a Trident Underwater drone, would be a great tool to explore the twilight reefs of Curacao. The Trident would be employed to literally deepen our research questions, and to allow us to describe how fish interact with corals and sponges in waters where we would otherwise not set a fin.

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Expedition Background

From water pollution to global ocean warming, reef-building corals are facing a suite of pressures that are transforming the face of tropical reefs worldwide. On changing Caribbean reefs, more tolerant organisms such as sponges are gaining ground, often replacing corals. But can sponges also replace the complex architecture that corals build to house thousands of species of reef-dwellers worldwide?


Previous research has shown that in deep waters, and in environments where sand would dominate if it wasn't for sponges, sponges do provide habitat to fishes. But we simply don't know enough about the relationship between sponges and fish on shallow, tropical coral reefs.

Sponges form soft structures that often lack the intricate nooks and crannies of the coral framework where reef fish hide. If reef fish potentially have less space to live, grow and reproduce, their already decreased populations might suffer further losses, with critical downstream effects for the food chain of reef ecosystems. Not to mention the effects for fisheries and food security of local communities and beyond.

So our mission is to shed light on the potential of sponges to provide refuge and shelter to Caribbean fishes. First we will describe the 'architectural complexity' of sponges and corals. That is, we will use underwater photography to create 3D models of different coral and sponge species, and based on these models we will evaluate how much hiding space each organism offers. Then we will film these organisms on the reef for extended periods of time to quantify the size, numbers and diversity of fishes associated with sponges, in comparison to corals.

We have chosen to do this study on the coral reefs of Curaçao in the southern Caribbean. Curaçao is a young island nation formerly part of the Netherlands Antilles. Compared to elsewhere in the region, many reefs here are relatively healthy, with a decent coral cover. And they are also rich in sponges!

A collaboration between the Caribbean Research and Management of Biodiversity Foundation (CARMABI) on the island, and the California Academy of Sciences (CAS) in San Francisco, our project will kick off later this year. For now, we will dive into field work planning and logistics, but we invite you to stay tuned as our project unfolds!

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