We found algae-farming fish that domesticate tiny shrimp to help run their farms

Longfin damselfish (left) have domesticated mysid shrimps (right).
Rohan Brooker, Author provided

William Feeney, Griffith University and Rohan Brooker, Deakin University

Humans are experts at domesticating other species and our world would be unrecognisable without it. There would be no cities, no supermarkets, and no pets. Domestication is a special kind of cooperative relationship, where one species provides prolonged support in exchange for a predictable resource.

While humans have domesticated various plants and animals, these relationships are surprisingly rare in other species. It’s true some insects (ants, beetles, and termites among them) domesticate fungi, but few other examples exist outside the insect world.

In our new study, we describe what appears to be first example of a non-human vertebrate domesticating another animal.

Reef in Belize
On the coral reefs off the coast of Belize, in Central America, longfin damselfish create, manage and feed from algae farms.
By Andy Blackledge – P4120130, CC BY 2.0, CC BY

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Farming fish domesticate shrimps

On the coral reefs off the coast of Belize, in Central America, longfin damselfish create, manage and feed from algae farms. We noticed they regularly have “swarms” of tiny crustaceans called mysid shrimps floating above their farms.

We found this unusual, as most farming damselfishes chase away anything that ventures near their farm. We were unsure why these species associated with one another, so we decided to try to find out what was going on.

First, to see whether mysid shrimps and farming damselfish are regularly found together, we ran a series of what’s known as “transects”. In other words, we conducted a series of 30 metre swims along the reef, and during each one we recorded each time we saw mysid shrimps, as well as whether they were near farming damselfish or other fish species.

We found these mysids were far more likely to be found near farming species, like the longfin damselfish, than other species.

The Smithsonian’s Carrie Bow Cay Marine Research Station off the coast of Belize.
Rohan Brooker

Next, we wanted to know if the mysids specifically seek out their damselfish partners.

So, we collected mysid shrimps from the field, brought them into the lab and exposed the mysids to water soaked with different things. For example, do they avoid the smell of a predator? Are they attracted to the smell of a farming damselfish?

We found the mysids shrimps were attracted to the longfin damselfish, repulsed by a predator and indifferent towards a non-farming fish — and to the farm itself.

I help you, you help me

Many fish eat mysid shrimps, so we ran an experiment to see if longfin damselfish provided protection to the mysids when they are in the fish’s farm.

To do this, we placed mysid shrimps in a clear plastic bag and placed the bag either inside or outside a farm.

We found that when placed outside a farm, other fish tried to eat the mysid shrimps. When inside the farms, any fish that tried to come close to the bag was chased off by the longfin damselfish. This suggested the mysids seek out longfin damselfish, as they provide mysids with protection from predators.

Slippery Dick Wrasse is a common predator of shrimps.
Slippery Dick Wrasse is a common predator of mysid shrimps.
Brian Gratwicke/Flickr, CC BY

One question remained: do the mysid shrimps provide a benefit to the longfin damselfish?

Given the damselfish eat the algae they farm, we thought maybe by hovering above the farm, the mysid shrimps waste might act as fertiliser.

To test this, we examined the quality of the algae within farms that did, or did not have mysid shrimps. We also examined the body condition of fish that did, or did not, have mysid shrimps within their farms.

We found farms with shrimps had higher quality algae, and fish from farms with mysid shrimps were in better condition.

Insight into how domestication happens

These different analyses together suggest longfin damselfish have domesticated mysid shrimps. The longfin damselfish provide a safe refuge, and in exchange the mysid shrimps provide the damselfish with fertiliser for its farm.

This relationship is important, because while fantastic research has provided insight into the history of domestication in our ancestors, these things happened in the distant past.

In the longfin damselfish, we can watch the early stages of domestication occur as it’s happening.

This is fascinating because it’s very similar to the proposed series of events that led to our domestication of species such as chickens, cats, dogs and pigs.

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The Conversation

William Feeney, Postdoctoral Research Fellow in Evolutionary Ecology, Griffith University and Rohan Brooker, Casual Research Fellow, Deakin University

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

How is oxygen ‘sucked out’ of our waterways?

File 20190114 43544 kywlhi.jpg?ixlib=rb 1.1
Dead fish are a source of food for bacteria, which then extract oxygen from the river.

Stuart Khan, UNSW

A million fish have died in the Murray Darling basin, as oxygen levels plummet due to major algal blooms. Experts have warned we could see more mass deaths this week.

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Fingers have been pointed at poor water management after a long period of drought. However, mass fish deaths can also be caused by floods, and even raw sewage.

So what’s going on when oxygen gets “sucked out of the water”?

The phenomenon is very well known to water quality engineers; we call it “biochemical oxygen demand”. To understand it, we need to talk about a little bit of biology and a little bit of chemistry.

When oxygen meets water

Oxygen molecules are soluble in water in the same way that sugar is soluble in water. Once its dissolved, you can’t see it (and, unlike sugar, oxygen is tasteless).

The maximum amount of oxygen that you can dissolve in water depends on a number of factors, including the water temperature, ambient air pressure, and salinity. But roughly speaking, the maximum amount of dissolvable oxygen, known as the “saturation concentration” is typically around 7-10 milligrams of oxygen per litre of water (7-10 mg/L).

This dissolved oxygen is what fish use to breathe. Fish take water in through their mouths and force it through their gill passages. Gills, like our lungs, are full of blood vessels. As water passes over the thin walls of the gills, dissolved oxygen is transferred into the blood and then transported to the fish’s cells. The higher the oxygen concentration in the water, the easier it is for this transfer to occur.

Once in the cells, the oxygen molecules play a key role in the process of “aerobic respiration”. The oxygen reacts with energy-rich organic substances, such as sugars, carbohydrates and fats to break them down and release energy for the cells. The main waste product from this process is carbon dioxide (CO₂). This is why we all need to breathe in oxygen and we breathe out carbon dioxide. Fish do that too. A simple way to express this is:

Organic substances + Oxygen Carbon dioxide + Water + Energy

The Hunter River in NSW suffered a ‘blackwater’ event in 2016 when floodwaters washed organic matter into the river.
Andrew S/Flickr, CC BY-SA

What is the biochemical oxygen demand?

Just like fish and people, many bacteria gain energy from processes of aerobic respiration, according to the simplified chemical reaction shown above. Therefore, if there are organic substances in a waterway, the bacteria that live in that waterway can consume them. This is an important process of “biodegradation” and is the reason our planet is not littered by the carcasses of animals that have died over many thousands of years. But this form of biodegradation also consumes oxygen, which comes from dissolved oxygen in the waterway.

Rivers can replenish their oxygen from contact with the air. However this is a relatively slow process, especially if the water is stagnant (flowing creates turbulence and mixes in more oxygen). So if there is a lot of organic matter present and bacteria are feasting on it, oxygen concentrations in the river can suddenly drop.

Obviously, “organic substances” can include many different things, such as sugars, fats and proteins. Some molecules contain more energy than others, and some are easier for the bacteria to biodegrade. So the amount of aerobic respiration that will occur depends on the exact chemical nature of the organic substances, as well as their concentration.

Therefore, instead of referring to the concentration of “organic substances”, we more commonly refer to the thing that really matters: how much aerobic respiration the organic substances can trigger and how much oxygen this will cause to be consumed. This is what we call the biochemical oxygen demand (BOD) and we usually express it as a concentration in terms of milligrams of oxygen per litre of water (mg/L).

Like us, bacteria don’t consume all of the food which is available to them instantly – they graze on it over time. Biodegradation therefore can take days, or longer. So when we measure the BOD of a contaminated water sample, we need to assess how much oxygen is consumed (per litre of water) over a specified period of time. The standard period of time is usually five days and we refer to this value as the BOD5 (mg/L).

Murray cod pull oxygenated water through their gills, transferring it to their bloodstream. Without oxygen in the water, they die.
Guo Chai Lim/Flickr, CC BY-NC-SA

As I mentioned earlier, clean water might only have a concentration of dissolved oxygen of up to around 7-10 mg/L. So if we add organic material in a concentration which has a higher BOD5 than this, we can expect it to deplete the ambient dissolved oxygen concentration during the next five days.

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This phenomena is the main reason for which biological sewage treatment was invented. Raw (untreated) municipal sewage can have a BOD5 of 300-500 mg/L. If this were discharged to a clean waterway, the typical base-level of 7-10 mg/L of oxygen would be consumed, leaving none available for fish or other aquatic organisms.

So the purpose of biological sewage treatment is to grow lots of bacteria in large tanks of sewage and provide them with plentiful oxygen for aerobic respiration. To do this, air can be bubbled through the sewage, or sometimes surface aerators are used to churn up the sewage.

By supplying lots of oxygen, we ensure the BOD5 is effectively consumed while the sewage is still in the tanks, before it’s released to the environment. Well treated sewage can have a BOD5 as low as 5 mg/L, which can then be further diluted as it’s discharged to the environment.

In the case of the Darling river, the high BOD load was created by algae, which died when temperatures dropped. This provided a feast for bacteria, lowering oxygen, which in turn killed hundreds of thousands of fish. Now, unless we clean the river, those rotting fish could become fodder for another round of bacteria, triggering a second de-oxygenation event.The Conversation

Stuart Khan, Professor of Civil & Environmental Engineering, UNSW

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

Explainer: what causes algal blooms, and how we can stop them

Michele Burford, Griffith University

Outbreaks of algae have killed up to a million fish in the Murray Darling Basin over the last two weeks. The phenomena of “algae blooms”, when the population of algae in a river rapidly grows and dies, can be devastating to local wildlife, ecosystems and people. But what are algae blooms? What causes them, and can we prevent them?

Microscopic algae are fundamental to life on earth. These tiny plants provide the fuel that drives marine and freshwater foodwebs, and via photosynthesis, they gobble up carbon dioxide to help counteract emissions, and provide us with oxygen to breathe. Besides rivers, streams, lakes, estuaries and the coast, they can also be found in diverse environments such as snow, soil, and in corals.

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But when humans channel agricultural run-off, sewerage and stormwater discharge into waterways, we dramatically increase the amount of nutrients such as nitrogen and phosphorus. This creates an imbalance, because some microscopic algae are supremely effective at mopping up nutrients and can grow very quickly, dividing up to once a day and quickly overtaking other species. The result is an algal bloom.

Cyanobacteria (blue-green algae) under a microscope.
Author provided

So why don’t we have algal blooms all the time? This is because algae don’t just require nutrients to grow. Like any plant, factors such as temperature and light availability are also important in determining how quickly algae grow and whether they form blooms. Blooms also need slow moving or still water to become established.

In Australia, our algal blooms are typically in freshwaters. The main group of algae responsible for this are known as blue-green algae, or more accurately, cyanobacteria. They regularly bloom in warmer weather in our reservoirs, lakes and slow flowing rivers. In 2016, for example, 1,700km of the Murray River was affected by an algal bloom.

There are many ways they impact the environment and economy. Some algal blooms are toxic, requiring expensive water treatment and – in extreme cases – shutdown of water supplies. This isn’t just a problem in Australia. In 2014, some 500,000 people in the US were left without drinking water due to a toxic algal bloom in Lake Erie.

The toxins can also affect domestic animals, such as dogs, when they drink contaminated water, and limit use of lakes and rivers for swimming, boating and fishing. Even when algal blooms are not toxic, they unbalance the food web, reducing the number of species of animals and plants.

They can also reduce oxygen levels at night, as they switch from photosynthesis (producing oxygen) during the day, to a process called respiration at night where they use oxygen. Low oxygen can stress and even kill fish and other animals if they cannot escape this.

At some point, algal blooms crash when conditions become unsuitable. The resulting dead algae break down, providing an ideal food source for bacteria. This is when waters can become smelly, often with a rotten egg smell. As the bacteria multiply, they suck the oxygen out of the water. At this point, oxygen levels become low both day and night.

If the area of low oxygen is extensive, such as a whole lake or many kilometres of a river system, fish and other animals may not be able to escape to more suitable oxygen levels, and major fish deaths typically occur.

In other areas of the world, algal blooms have caused such severe oxygen conditions that thousands of square kilometres of ocean around the world are now known as dead zones, where no animals can live. These vast dead zones are not something we ever want to see in Australia.

So what can be done about blooms?

There are a wide range of treatments that can be used to control blooms, for example, aerating the water, and adding clays and chemicals, but the catch is they are very expensive on a large scale.

Ideally, the problem should be tackled at the source. This means reducing nutrient loads to our waterways. There has already been progress on this in our cities where sewage treatment plants have been upgraded to reduce nutrient loads to waterways. But tackling nutrients coming from agriculture – erosion, fertilisers, animal waste – is much more challenging and expensive because of the vast areas involved. So this remains work in progress.

It’s also very difficult to predict when blooms will occur; despite being simple plants, algae have an amazing range of strategies to grow and survive. But as we learn more about their complexity our ability to model and predict blooms will improve. This is crucial to managing risks to water supplies and preventing major environmental effects, such as fish deaths.

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Ultimately there are no quick fixes to algal blooms. Given the pressure we put on our waterways, they are here to stay. In fact they are likely to increase due to increasing temperatures and more extreme conditions, such as droughts. We know what we need to do to reduce the scale and likelihood of blooms: the challenge is devoting the resources to achieve it.The Conversation

Michele Burford, Professor – Australian Rivers Institute, and Dean – Research Infrastructure, Griffith University

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

China: The red Rocks of Mount Gongga

The link below is to an interesting article on the algal growth on rocks over the slopes of Mount Gongga in China.

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