Scientists may have finally solved the 200-year-old “Darwin’s paradox”, according to a new study published on 23 May in Science (1). The findings suggest that tiny short-lived fish allow coral reef communities to thrive in parts of the typically “low-productivity” oceans.
The term “Darwin’s paradox” is used to describe how coral reefs manage to thrive and support swathes of diversity among the vast and relatively barren oceans – the conundrum has puzzled scientists for centuries. Apart from the microscopic plankton that can support some marine life, ocean ecosystems typically contain few nutrients.
Coral reefs are chock full of diverse fish and marine creatures. So, how do coral island hotspots exist among the oceanic desert? And where do vibrant reef communities obtain the food they need to flourish?
Plankton may contribute. In fact, a previous study published in Nature Communications showed a much higher percentage of phytoplankton biomass near coral reef islands — 86 per cent more— compared to oceanic conditions (2). But as it turns out, reef environments are also teeming with tiny fish — often less than a centimetre long — that may instead be the cornerstone of reef communities.
Around 3000 of these teeny tiny fish species are already known, and an estimated 1000 more may be present. The shy and mysterious fish hide in all the nooks and crannies of the reef. To learn more about them, the international team of researchers, led by Dr Simon Bandjl, a marine ecologist at Simon Fraser University in Canada, placed nets throughout coral reefs in Belize, French Polynesia, and Australia — 58 in total — and lured the fish inside in order to count them.
The researchers counted over 100 of the so-called cryptobenthic fish per square meter. The scientists also learned that most of these fish do not stray far from their birthplace. Then, assuming all of the fish die on the reef become food for other fish, they estimated that cryptobenthic fish account for around 60 per cent of the fish consumed by other marine life on the reef.
However, the most interesting finding is the astonishing number of larvae that cryptobenthic fish can produce to maintain the food web. In fact, two-thirds of all larvae come from just the 17 types of cryptobenthic fish. And the larvae seem to have a higher survival rate than the larvae of larger reef fish.
The findings reveal just how important even the tiniest creature can be to ecosystems. But cryptobenthic fish are also threatened by warming waters as more corals continue to die off due to climate change. As the fish slowly lose their habitat, it could mean the vibrant reefs will gradually become more like the barren oceans. And as Brandl and colleagues point out, 500 million people also rely on reefs for fishing and livelihoods.
(1) Brandl, S.J. et al. Demographic dynamics of the smallest marine vertebrates fuel coral-reef ecosystem functioning. Science (2019). DOI: 10.1126/science.aav3384
(2) Gove, J.M. et al. Near-island biological hotspots in barren ocean basins. Nature Communications (2016). DOI: 10.1038/ncomms10581