When coral dies, tiny invertebrates boom. This could dramatically change the food web on the Great Barrier Reef

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Kate Fraser, University of TasmaniaThis week, international ambassadors will take a snorkelling trip to the Great Barrier Reef as part of the Australian government’s efforts to stop the reef getting on the world heritage “in danger” list.

The World Heritage Centre of UNESCO is set to make its final decision on whether to officially brand the reef as “in danger” later this month.

To many coral reef researchers like myself, who have witnessed firsthand the increasing coral bleaching and cyclone-driven destruction of this global icon, an in-danger listing comes as no surprise.

But the implications of mass coral death are complex — just because coral is dying doesn’t mean marine life there will end. Instead, it will change.

In recent research, my colleagues and I discovered dead coral hosted 100 times more microscopic invertebrates than healthy coral. This means up to 100 times more fish food is available on reefs dominated by dead coral compared with live, healthy coral.

This is a near-invisible consequence of coral death, with dramatic implications for reef food webs.

When coral dies

Tiny, mobile invertebrates — between 0.125 and 4 millimetres in size — are ubiquitous inhabitants of the surfaces of all reef structures and are the main food source for approximately 70% of fish species on the Great Barrier Reef.

These invertebrates, most visible only under a microscope, are commonly known as “epifauna” and include species of crustaceans, molluscs, and polychaete worms.

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When corals die, their skeletons are quickly overgrown by fine, thread-like “turfing algae”. Turf-covered coral skeletons then break down into beds of rubble.

We wanted to find out how the tiny epifaunal invertebrates — upon which many fish depend – might respond to the widespread replacement of live healthy coral with dead, turf-covered coral.

A sample of epifauna under the microscope.
Kate Fraser

I took my SCUBA gear and a box of lab equipment, and dived into a series of reefs across eastern Australia, from the Solitary Islands in New South Wales to Lizard Island on the northern Great Barrier Reef.

Underwater, I carefully gathered into sandwich bags the tiny invertebrates living on various species of live coral and those living on dead, turf-covered coral.

But things really got interesting back in the laboratory under the microscope. I sorted each sandwich bag sample of epifauna into sizes, identified them as best I could (many, if not most, species remain unknown to science), and counted them.

I quickly noticed samples taken from live coral took just minutes to count, whereas samples from dead coral could take hours. There were exponentially more animals in the dead coral samples.

The Great Barrier Reef may soon be listed as ‘in danger’
Rick Stuart-Smith

Why do they prefer dead coral?

Counting individual invertebrates is only so useful when considering their contribution to the food web. So we instead used the much more useful metric of “productivity”, which looks at how much weight (biomass) of organisms is produced daily for a given area of reef.

We found epifaunal productivity was far greater on dead, turf-covered coral. The main contributors were the tiniest epifauna — thousands of harpacticoid copepods (a type of crustacean) an eighth of a millimetre in size.

In contrast, coral crabs and glass shrimp contributed the most productivity to epifaunal communities on live coral. At one millimetre and larger, these animals are relative giants in the epifaunal world, with fewer than ten individuals in most live coral samples.

Dead coral rubble overgrown with turfing algae.
Rick Stuart-Smith

These striking differences may be explained by two things.

First: shelter. Live coral may look complex to the naked eye, but if you zoom in you’ll find turfing algae has more structural complexity that tiny epifauna can hide in, protecting them from predators.

A coral head is actually a community of individual coral polyps, each with a tiny mouth and fine tentacles to trap prey. To smaller epifauna, such as harpacticoid copepods, the surface of live coral is a wall of mouths and a very undesirable habitat.

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Second: food. Many epifauna, regardless of size, are herbivores (plant-eaters) or detritivores (organic waste-eaters). Turfing algae is a brilliant trap for fine detritus and an excellent substrate for growing films of even smaller microscopic algae.

This means dead coral overgrown by turfing algae represents a smorgasbord of food options for the tiniest epifauna through to the largest.

Meanwhile, many larger epifauna like coral crabs have evolved to live exclusively on live coral, eating the mucus that covers the polyps or particles trapped by the polyps themselves.

Harpacticoid copepod are just an eighth of a millimetre in size.
Naukhan/Wikimedia, CC BY

What this means for life on the reef?

As corals reefs continue to decline, we can expect increased productivity at the base level of reef food webs, with a shift from larger crabs and shrimp to small harpacticoid copepods.

This will affect the flow of food and energy throughout reef food webs, markedly changing the structure of fish and other animal communities. The abundance of animals that eat invertebrates will likely boom with increased coral death.

We might expect higher numbers of fish such as wrasses, cardinalfish, triggerfish, and dragonets, with species preferring the smallest epifauna most likely to flourish.

The dragonet species, mandarinfish, feeds on the smallest harpacticoid copepod prey.
Rick Stuart-Smith

Invertebrate-eating animals are food for a diversity of carnivores on a coral reef, and most fish Australians want to eat are carnivores, such as coral trout, snapper, and Spanish mackerel.

While we didn’t investigate exactly which species are likely to increase following widespread coral death, it’s safe to say populations of fish targeted by recreational and commercial fisheries on Australia’s coral reefs are likely to change as live coral is lost, some for better and some for worse.

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The Great Barrier Reef is undoubtedly in danger, and it’s important that we make every effort to protect and conserve the remaining live, healthy coral. However, if corals continue to die, there will remain an abundance of life in their absence, albeit very different life from that to which we are accustomed.

As long as there is hard structure for algae to grow on, there will be epifauna. And where there is epifauna, there is food for fish, although perhaps not for all the fish we want to eat.

Kate Fraser, Marine Ecologist, University of Tasmania

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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Drugs in bugs: 69 pharmaceuticals found in invertebrates living in Melbourne’s streams

Thanks to their consumption of invertebrates, Melbourne platypus likely receive half the recommended human dose of anti-depressants every day.
Denise Illing

Erinn Richmond, Monash University and Mike Grace, Monash University

Pharmaceuticals from wastewater are making their way into aquatic invertebrates and spiders living in and next to Melbourne’s creeks, according to our study published today in Nature Communications.

We found pharmaceuticals in every bug we sampled – over 190 invertebrates – from six different streams. These included caddisfly larvae, midge larvae, snails and dragonfly larvae. We also found pharmaceuticals in spiders living in stream-side vegetation.

We found 69 different drugs in the bugs, including fluoxetine and mianserin (anti-depressants), fluconazole (an anti-fungal), and non-steroidal anti-inflamatories (NSAIDs), often used to treat arthritis.

While we don’t know how these drugs are affecting these invertebrates, we know from other studies pharmaceuticals do affect the lifecycles of other organisms.

We also calculated that animals that eat these aquatic invertebrates, such as platypus, would be receiving half the daily recommended dose of anti-depressants for humans.

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Drugs everywhere

We know wastewater is a contributing factor to pharmaceutical contamination in aquatic organisms, so we sampled from a range of streams with different wastewater inputs. These included a site just downstream of large-scale wastewater treatment facility, and areas with ageing septic systems.

Sassafrass Creek, one of the streams sampled in the study.
Erinn Richmond

We also included a stream within a national park to attempt to obtain samples we thought would be free of pharmaceuticals. We sampled aquatic invertebrates and stream-side spiders and tested them for 98 pharmaceutical compounds.

To our surprise, we found up to 69 different pharmaceuticals in aquatic invertebrates and up to 66 in riparian (streamside) spiders. Contamination was greatest downstream of the high capacity waste water treatment plant.

Moreover, every insect we sampled contained pharmaceuticals, including at the site in a national park, possibly due to septic systems in the drainage area of the stream that contribute small amounts of waste water.

The fact we detected drugs, admittedly in very low concentrations, in this seemingly pristine site suggests finding places “free” from pharmaceutical contamination may be difficult. Recent studies by other researchers detected pharmaceutical contamination in surface water in Antarctica and in national parks in the US.

We also found spiders living on the stream edge (the “riparian zone”), also contain a wide variety of pharmaceuticals in their tissues. These animals primarily consume adult insects and are an indication other animals that eat adult aquatic insects, such as birds, reptiles and bats, may also be exposed.

Spiders living in stream-side vegetation take up pharmaceuticals from the insects they eat.
Erinn Richmond

The dark side of our pharmaceutical use

We take and are prescribed pharmaceuticals to improve our quality of life. These medications are designed to be biologically active – they are meant to treat us; for example, we take paracetamol to alleviate a headache. For all the benefits drugs afford us, there is an often overlooked dark side to our extensive use of them.

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When we take a pharmaceutical, our bodies do not always use all of the drug and we excrete drug residues into our waste water and the drugs then move into our sewage system. Unfortunately, waste water treatment facilities are not always designed to, or are capable of, removing pharmaceuticals. So they’re often discharged into our streams, rivers and coastal waters.

Lead author, Erinn, sampling invertebrates in Brushby Creek.
Keralee Brown

We have known from many studies over almost two decades that the drugs we take are found in waterways around the world. There are thousands of drugs available, but very little is known about their occurrence and movement through aquatic food webs.

Our research team has previously studied the effects these pharmaceuticals have on organisms living in streams. For example, we found fluoxetine, a common anti-depressant, increased stream insect emergence (the important phase of an insects’ life where it metamorphoses from a stream dwelling larvae to an aerial adult).

We also found this antidepressant, and other drugs, alter the rates of photosynthesis in algae, the important base of stream food webs.

Happy platypus?

Platypus and trout live in or nearby the streams we studied. These animals feed almost exclusively on aquatic invertebrates. Although we did not directly sample trout or platypus, we were able to use previous studies on the feeding rates of these animals to estimate what proportion of a human daily dose of drugs they may be exposed just by eating the aquatic invertebrates we did measure in the streams we studied.

Based on these calculations, a platypus living in a creek receiving waste water could be exposed to over half of a human daily dose (per kg body weight) of antidepressants, just by eating aquatic invertebrates. Trout, too, would be exposed to these drugs, but would be exposed to a lower dose.

Studies have shown single drugs can alter the behaviour of fish, but just what consuming 69 different pharmaceutical compounds might do to a fish or platypus remains unknown and worthy of future research.

Global pharmaceutical use is increasing, with many benefits to humankind. However, our recent publication makes it clear pharmaceuticals are accumulating and moving through stream food webs and expose spiders, and likely birds, bats, fish, and platypus to a wide array of drugs. We are yet to fully understand the broader ecological consequences of this type of pharmaceutical contamination.

We know in humans, there are health risks associated with taking multiple drugs because of drug interactions. Is the same true for animals? Like so many studies, our research leaves us with many unanswered questions.

The one thing that is abundantly clear is the drugs we so frequently use are ending up in nature and are moving through food webs.

This article was co-authored by Emma Rosi, an aquatic ecologist at the Cary Institute of Ecosystem Studies.

Erinn Richmond, Research Fellow, School of Chemistry, Monash University and Mike Grace, Associate Professor, Monash University

This article is republished from The Conversation under a Creative Commons license. Read the original article.