Not All Reefs Are Equal — And Sharks Know It
- 2 days ago
- 4 min read
Our new study reveals that the living community of a reef — its fish, its coral, its food web — determines which sharks and rays show up, and why.
Imagine you're a reef shark. You're not going to hang around just any patch of underwater rock. You want prey worth hunting, cleaning stations to stay healthy, a community worth belonging to. Our latest paper, published in Scientific Reports, confirms what the animals seem to already understand: the biological character of a reef — who lives there, what grows there, who eats whom — is a powerful driver of which sharks and rays call it home. Just like in human communities.
Elasmobranchs (the group that includes sharks, rays, skates, and guitarfish) are among the ocean's most threatened animals. Across the Indian Ocean, populations have declined sharply due to overfishing and habitat loss. To protect them, we need to know not just where they go, but why they go there. That's the question we set out to answer.
Cameras, coral, and twelve months of data
With the help of our volunteers, we deployed Remote Underwater Video Stations (RUVS) on four reefs off the coast of southern Mozambique, an area known for its remarkable elasmobranch diversity. Over the course of a year, the cameras logged 140 hours of footage across 48 deployments, capturing which sharks and rays visited each reef. After every deployment, researchers also swam transects to measure coral cover, cataloguing the types and abundances of corals present. Together, this built a detailed biological portrait of each reef.
The four reefs sit close to one another geographically and share broadly similar physical conditions such as depth range, water temperature, and currents. If only physical factors mattered, you'd expect similar communities at each site. That's not what the data showed.
140
Hours of video recorded
4
Distinct reef communities
12
Months of deployments
∞
Conservation implications
Diversity drives diversity
The northern reef hosts the richest and most abundant community of bony fish, particularly piscivorous (fish-eating) species. The higher up the food chain, the more prey there is for apex predators. Sharks noticed and responded accordingly, dominating the elasmobranch community here. A pyramid isn't built from the top down - without a solid base in the food chain, there is no support for higher predators.
The southern reef's fish community is characterised by herbivores and, critically, cleaner fish. Cleaner fish and shrimp set up stations where other animals come to have parasites removed, a beautiful example of a mutualistic relationship that acts like a magnet for rays and other elasmobranchs. Mobula rays and guitar fish were the primary visitors here.
The shallowest reef shows reduced coral cover and has the lowest overall abundance of both bony fish and elasmobranchs. Depth shapes coral communities, and when the food web is weakened at the bottom, and those effect ripple all the way to the top predators.
Why the living reef matters
Previous elasmobranch research has largely focused on physical factors: depth, water temperature, currents, lunar phase. These clearly matter. But this study adds an important layer: the biological structure of a reef, i.e. how its community is organised and what functional groups are present, shapes elasmobranch use just as powerfully. Think about it, nobody wants to live in a beautiful but empty ghost town.
Reefs where piscivores dominate attract predators higher up the chain. Reefs with active cleaning stations attract species that rely on those services. A shallow reef with poor coral cover and a sparse fish community simply offers less for everyone. The reef's ecology is, in effect, an advertisement to passing sharks and rays. "Come over here, we just got a shipment of fresh tuna!"
This matters enormously for conservation. Elasmobranchs occupy the upper rungs of the reef food web, and many species are listed as Vulnerable, Endangered, or Critically Endangered by the IUCN. Protecting them requires protecting not just the water they swim through, but the living systems that draw them there. A no-take zone on a biologically impoverished reef may do little. A protected area that preserves ecological diversity (think healthy coral, diverse fish communities, functional relationships like cleaning stations) could make all the difference.
"Including reefs with different biological characteristics within local conservation strategies may promote rare and vulnerable regional elasmobranch species — from stingrays and guitarfishes to reef sharks and pelagic rays"
A Conservation Blueprint for Mozambique and Beyond
Southern Mozambique sits within one of the Indian Ocean's most biodiverse marine regions. The coastline around Inhambane has long attracted researchers and divers for its remarkable concentration of threatened marine megafauna — whale sharks, manta rays, hammerheads. This study deepens our understanding of why certain reefs are so important, and points toward a more nuanced approach to marine protected area design.
Rather than protecting large swathes of water with similar characteristics, managers should consider building MPA networks that capture the full range of biological reef types. A reef dominated by piscivores, a reef with active cleaning stations, a reef with high coral structural complexity — each serves different members of the elasmobranch community. Together, they could provide a safety net for the full suite of species.
The implications extend well beyond Mozambique. Across the tropics, reef degradation from bleaching, overfishing, and coastal development is homogenising what were once ecologically distinct habitats. If reefs come to resemble one another biologically, we may lose not just coral, but the specific ecological conditions that keep sharks and rays anchored to them.
This study is a reminder that conservation is not just about stopping the bad. It's about understanding and actively maintaining the intricate biological fabric that makes a reef worth living on.
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