Dugong and sea turtle poo sheds new light on the Great Barrier Reef’s seagrass meadows


Samantha J Tol, James Cook University; Alana Grech, James Cook University; Paul York, James Cook University, and Rob Coles, James Cook University

Just like birds and mammals carrying seeds through a rainforest, green sea turtles and dugong spread the seeds of seagrass plants as they feed. Our team at James Cook University’s TropWATER Centre has uncovered a unique relationship in the seagrass meadows of the Great Barrier Reef.

We followed feeding sea turtle and dugong, collecting samples of their floating faecal matter. Samantha then had the unenviable job of sifting through hundreds of smelly samples to find any seagrass seeds. These seeds range in size from a few centimetres to a few millimetres, and therefore can require the assistance of a microscope to be found. Once any seeds were found, they were stained with a chemical dye (Tetrazolium) to see if they were still viable (capable of growing).

PhD candidate Samantha Tol holding dugong poo collected from Cleveland Bay in Townsville.
TropWATER, JCU

Why is this important for turtles and dugong?

Green sea turtles and dugong are iconic animals on the reef, and seagrass is their food. Dugong can eat as much as 35 kilograms of wet seagrass a day, while sea turtles can eat up to 2.5% of their body weight per day. Without productive seagrass meadows, they would not survive.

This relationship was highlighted in 2010-11 when heavy flooding and the impact of tropical cyclone Yasi led to drastic seagrass declines in north Queensland. In the year following this seagrass decline there was a spike in the number of starving and stranded sea turtles and dugong along the entire Queensland coast.

The seagrass team at James Cook University has been mapping, monitoring and researching the health of the Great Barrier Reef seagrasses for more than 30 years. While coral reefs are more attractive for tourists, the Great Barrier Reef World Heritage Area actually contains a greater area of seagrass than coral, encompassing around 20% of the world’s seagrass species. Seagrass ecosystems also maintain vibrant marine life, with many fish, crustaceans, sea stars, sea cucumbers, urchins and many more marine animals calling these meadows their home.

These underwater flowering plants are a vital component of the reef ecosystem. Seagrasses stabilise the sediment, sequester large amounts of carbon from the atmosphere and filter the water before it reaches the coral reefs. Further, the seagrass meadows in the Great Barrier Reef support one of the largest populations of sea turtles and dugong in the world.

Seagrass meadows are more connected than we thought

Samantha’s research was worth the effort. There were seeds of at least three seagrass species in the poo of both sea turtles and dugong. And lots of them – as many as two seeds per gram of poo. About one in ten were viable, meaning they could grow into new plants.

Based on estimates of the number of animals in the coastal waters, the time it takes for food to pass through their gut, and movement data collected from animals fitted with satellite tags, there are potentially as many as 500,000 viable seeds on the move each day in the Great Barrier Reef. These seeds can be transported distances of up to 650km in total.

Green Island seagrass meadow exposed at low tide.
TropWATER, JCU

This means turtles and dugong are connecting distant seagrass meadows by transporting seeds. Those seeds improve the genetic diversity of the meadows and may help meadows recover when they are damaged or lost after cyclones. These animals help to protect and nurture their own food supply, and in doing so make the reef ecosystem around them more resilient.

Understanding recovery after climate events

Seagrass meadows have been under stress in recent years. A series of floods and cyclones has left meadows in poor condition, and recovery has been patchy and site-dependent.

This research shows that these ecosystems have pathways for recovery. Provided we take care with the environment, seagrasses may yet recover without direct human intervention.

The ConversationThis work emphasises how much we still have to learn about how the reef systems interconnect and work together – and how much we need to protect every part of our marvellous and amazing reef environment.

Samantha J Tol, PhD Candidate, James Cook University; Alana Grech, Assistant Director, ARC Centre of Excellence for Coral Reef Studies, James Cook University; Paul York, Senior Research Scientist in Marine Biology, James Cook University, and Rob Coles, Team leader, Seagrass Habitats, TropWATER, James Cook University

This article was originally published on The Conversation. Read the original article.

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How can we halt the feminisation of sea turtles in the northern Great Barrier Reef?


Ana Rita Patricio, University of Exeter

In the northern part of Australia’s Great Barrier Reef, the future for green sea turtles appears to be turning female.

A recent study has revealed that climate change is rapidly leading to the feminisation of green turtles in one of the world’s largest populations. Only about 1% of these juvenile turtles are hatching male.


Read more: What does climate change mean for sea turtles?


Among sea turtles, incubation temperatures above 29ºC produce more female offspring. When incubation temperatures approach 33ºC, 100% of the offspring are female. Cooler temperatures yield more males, up to 100% near a lower thermal limit of 23ºC. And if eggs incubate at temperatures outside the range of 23-33ºC the risk of embryo malformation and mortality becomes very high.

As current climate change models foresee increases in average global temperature of 2 to 3ºC by 2100, the future for these turtles is in danger. Worryingly, warmer temperatures will also lead to ocean expansion and sea-level rise, increasing the risk of flooding of nesting habitats.

How scientists are tackling the problem

Green sea turtles’ sensitivity to incubation temperatures is such that even a few degrees can dramatically change the sex ratio of hatchlings.

Sea turtles are particularly vulnerable because they have temperature-dependent sex determination, or TSD, meaning that the sex of the offspring depends on the incubation temperature of the eggs. This is the same mechanism that determines the sex of several other reptile species, such as the crocodilians, many lizards and freshwater turtles.

Scientists and conservationists are well aware of how future temperatures may threaten these species. For the past two decades they have been investigating the incubation conditions and resulting sex ratios at several sea turtle nesting beaches worldwide.

This is mostly done using temperature recording devices (roughly the size of an egg). These are placed inside nest chambers among the clutch of eggs, or buried in the sand at the same depth as the eggs. When a clutch hatches (after 50 to 60 days) the device is recovered and the temperatures recorded are analysed.

Research has revealed that most nesting beaches studied to date have sand temperatures that favour female hatchling production. But this female bias is not immediately a bad thing, because male sea turtles can mate with several females (polygyny). So having more females actually enhances the reproductive potential of a population (i.e. more females equals more eggs).

But given that climate change will likely soon increase this female bias, important questions arise. How much of a female bias is OK? Will there be enough males? What is the minimum proportion of males to keep a sustainable population?

These questions are being investigated. But, in the meantime, alarming reports of populations with more than 99% of hatchlings being female stress the urgency of science-based management strategies. These strategies must be designed to promote (or maintain) cooler incubation temperatures at key nesting beaches to prevent population decline or even extinction.

The challenge of reversing feminisation

There are two general approaches to the problem:

  1. mitigate impacts at the most endangered nesting beaches
  2. identify and protect sites that naturally produce higher proportions of males.

Several studies emphasise that the natural shading native vegetation provides is essential to maintain cooler incubation temperatures. Thus, a key conservation action is to protect beach vegetation, or reforest nesting beaches.

Coastal vegetation also protects the nesting beach against wave erosion during storms, which will worsen under climate change. This strategy further requires coastal development to allow for buffer zones. Construction setback regulations should be enforced or implemented.

When natural shading is not an option, clutches of eggs can be moved either to more suitable beaches, or to hatcheries with artificial shading. Researchers have tested the use of synthetic shade cloth and found it is effective in reducing sand and nest temperatures.

Other potential strategies involve adding light-coloured sand on top of nests. This can help by absorbing less solar radiation (heat) compared to darker sand. Beach sprinklers have also been tested to simulate the cooling effect of rainfall.

The effectiveness of these actions has yet to be fully tested, but there is concern about some potential negative side effects. For example, excess water from sprinklers may cause fungal infections on eggs.

Finally, as much as mitigation measures are important, these are always short-term solutions. In the long run, prevention is always the best strategy, i.e. protecting the nesting beaches that currently produce more males from deforestation, development and habitat degradation.

Our recent research on the largest green turtle population in Africa reports unusually high male hatchling production. We found almost balanced hatchling sex ratios (1 female to 1.2 males). We attributed this mostly to the cooling effect of the native forest.

This, and similar nesting beaches, should be designated as priority conservation sites, as they will be key to ensuring the future of sea turtles under projected global warming scenarios.

Sea turtles face an uncertain future

Sea turtles are resilient creatures. They have been around for over 200 million years, surviving the mass extinction that included the dinosaurs, and enduring dramatic climatic changes in the past.

There is potential for these creatures to adapt, as they did before. This could be through, for example, shifting the timing of nesting to cooler periods, changing their distribution to more suitable habitats, or evolution of critical incubation temperatures that produce males.


Read more: Turtle hatchlings lend each other a flipper to save energy


But the climate today is changing at an unprecedented rate. Along with the feminisation of these turtles in the northern Great Barrier Reef, sea turtles globally face many threats from humans. These include problems associated with by-catch, poaching, habitat degradation and coastal development, plus a history of intense human exploitation.

The ConversationIn 2018, the prevalence of these species depends now more than ever on the effectiveness of conservation measures.

Ana Rita Patricio, Postdoctoral research fellow, University of Exeter

This article was originally published on The Conversation. Read the original article.