Curious Kids: how do fish sleep?



The Ringtail Unicornfish, which occurs in tropical marine waters of the Indo-Pacific. All fish sleep, even the weird-looking ones.
Bernard Spragg/Flickr

Culum Brown, Macquarie University


How do fish sleep? Do they keep swimming or do they sleep somewhere? – Anna, age 5, Thornleigh, NSW, Australia.



Nearly all animals sleep. Sleep is very important for refreshing the mind and body. When people sleep we close our eyes and lie motionless for a long time. We may be less aware of what is going on around us and our breathing slows down. Some people are very heavy sleepers and it takes a LOT to wake them up!

Fish don’t have eyelids — they don’t need them underwater because dust can’t get in their eyes. But fish still sleep. Some sleep during the day and only wake up at night, while others sleep at night and are awake through the day (just like you and I).

A happy puffer fish.
Flickr

How do fish know when it’s bedtime?

It’s pretty easy to tell when fish are sleeping: they lie motionless, often at the bottom or near the surface of the water. They are slow to respond to things going on around them, or may not respond at all (see some sleeping catfish here). If you watch their gills, you’ll notice they’re breathing very slowly.




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People with fish tanks at home will know that when the lights go off at night, the fish become far less active. If you turn a light on in the middle of the night you’ll see how still they are.

Like people, fish have an internal clock that tells them when to do things like sleep and eat. So even if you accidentally leave the lights on at night, the fish may settle down and go to sleep anyway.

A video showing sleeping catfish.

Some scientists have studied sleep in fish that live in caves where it is always dark. Even in some of these species there are times of low activity that look just like sleep. Of course there is no sunrise or sunset in caves so their rhythm is often different to fish that live at the surface in bright sunshine.

Some fish, like tuna and some sharks, have to swim all the time so that they can breathe. Its likely that these fish sleep with half their brain at a time, just like dolphins.

Parrot fish make a mucus cocoon around themselves at night — a gross, sticky sleeping bag which might protect them from parasites attacking them while they sleep.

Fish don’t need eyelids because dust can’t get in their eyes – but they still sleep.
Gavin Leung/Flickr

Fish may dream like people do!

One wonders if fish dream while they are sleeping. So far we don’t have the answer to that question but recent video footage of a sleeping octopus showed it changing colours, which suggests it may have been dreaming about hiding from a predator or sneaking up on its own prey (which is why octopuses change colour when they’re awake).




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Believe it or not, fish sleep is being studied to help us better understand sleep in people. Most of these studies use zebrafish and try to understand things like the effects of sleep deprivation (lack of sleep), insomnia (trouble getting to sleep) and circadian rhythm (sleep cycles).

Here is a cool video about sleep in animals, including fish.


Hello, curious kids! Have you got a question you’d like an expert to answer? Ask an adult to send your question to curiouskids@theconversation.edu.au — —The Conversation

Culum Brown, Professor, Macquarie University

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

Australia’s only active volcanoes and a very expensive fish: the secrets of the Kerguelen Plateau


Evening light on a Heard Island icescape. The island is part of the Kerguelen Plateau, which is being jointly studied by France and Australia.
Matt Curnock

James Dell, University of Tasmania

Stretching towards Antarctica lies a hidden natural oasis – a massive underwater plateau created when continents split more than 100 million years ago.

Straddling the Indian and Southern Oceans, the Kerguelen Plateau is three times the size of Japan. It’s farthest depths are four kilometres below the surface; its islands form one of the most isolated archipelagos on Earth. These include Heard Island and McDonald islands, Australia’s only active surface volcanoes.




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Australia and France share a territorial border across the Kerguelen Plateau and work together to study it. The most recent findings, The Kerguelen Plateau: Marine Ecosystems and Fisheries, have been published by the Australian Antarctic Division.

The collaboration has fostered new knowledge of the Kerguelen Plateau as a unique living laboratory – and as the home to one of the world’s most expensive fish.

Bird activity behind a research vessel near the Kerguelen Plateau.
Paul Tixier

Tracking the Patagonian toothfish

Volcanic activity pumps vast amounts of minerals such as iron into the water, making the Kerguelen Plateau a biological hotspot.

The plateau hosts populations of Patagonian toothfish, or Dissostichus eleginoides, a predatory fish that lives and feeds near the bottom of the Southern Ocean. The brownish-grey fish grow up to 2 metres long, live for 60 years and can weigh 200kg. The species is often marketed as Chilean seabass.

Australia and France have worked together since the early 2000s to eliminate illegal, unregulated and unreported fishing, to understand the toothfish’s population dynamics and surrounding ecology. As a long-lived top predator with a broad diet, they have a key role in the structure of communities inhabiting the seafloor.

A location map of the Heard and Macquarie islands.
AAD

The toothfish is also economically important. Its snow-white flesh is prized as rich, good at carrying flavour and rich in omega-3 fatty acids. Catches command high market prices: prepared fillets have sold for more than A$100 per kg in recent years.

Approved commercial fishing vessels catch Patagonian toothfish around the plateau. Over the past few decades, scientific observers on fishing boats have tagged and released more than 50,000 toothfish at the Australian islands. This, along with annual surveys, biological sampling and data collection, has shed light on the species’ biology and population ecology.

This informs management measures such as total allowable catches and “move on” rules, where vessels must cease fishing in an area once a predetermined weight of non-target fish has been caught.




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The nations continue to manage toothfish populations, as well as fish, seabirds and marine mammals that interact with fishing activity.

The shallow banks of the plateau support a spectacular diversity of long-lived sponges, brittle stars, anemones, soft and hard corals and crustaceans. These fragile and slow-growing communities are vulnerable to disturbance. Fishing gear fitted with automated video cameras helps locate and protect sensitive areas, and Australia and France have established marine reserves and managed areas across the plateau.

Patagonian toothfish are prized in the restaurant industry for their rich flesh.

A unique underwater oasis

The plateau’s islands are incredibly isolated and provide the only breeding and land-based refuge for birds and seals in this part of the Southern Ocean.

Submarine volcanoes, some of them active, surround the islands and are particularly abundant around the younger McDonald Islands.

The plateau cuts across the strong current systems that sweep around the South Pole. This thrusts deep, cold water, enriched with volcanic minerals, to the surface then back to the seafloor. In turn, this powers a food chain stretching from small zooplankton to fish and predators such as Patagonian toothfish, penguins and albatross, and diving marine mammals such as elephant seals and sperm whales.




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Carbon and nutrients returned to the seafloor support diverse communities of invertebrate and fish species that could not inhabit this location if not for the plateau.

The orientation and location of the Kerguelen Plateau make it a canary in the coalmine for understanding the southward shift in marine ecology due to climate change. As sea temperatures rise and ocean currents shift, plant and animal species will move south in search of cooler waters.

Recent modelling suggests those species most at risk from climate change in this region are those sedentary or slow-moving invertebrates, such as sea urchins.

King penguins at Corinthian Bay, Heard Island.
Matt Curnock

Policy backed by science

Work continues to build comprehensive maps of the seafloor, deploy a network of ocean robots to collect physical and biological information, and use French and Australian fishing fleets for research.

The plateau’s waters are in the region overseen by the Commission for the Conservation of Antarctic Marine Living Resources, an international treaty body. French-Australian research is presented to the commission at meetings in Hobart each year to guide management decisions.

The cross-country partnership is a model for international scientific cooperation and fisheries management. In the context of a changing climate, these efforts will provide insight into future impacts on natural systems throughout the Southern Ocean.The Conversation

James Dell, Post Doctoral Fellow, University of Tasmania

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