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.

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 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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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.

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How shrimp farming wreaked havoc on Sri Lanka’s coasts

Mark Huxham, Edinburgh Napier University

We are not far from the ocean here. The air smells of salt and sulphur, of marine life. But the square of black, cracked mud in front of us, bounded by its four crumbling walls of sand, is no place for living things. It was previously a pond for cultivating tiger prawns, the lucrative species that was the reason for cutting the lush mangrove forest that once covered this area. The recent history of this abandoned place is sadly representative of the story of thousands of hectares in this region in the west of Sri Lanka.

A swelling appetite for shrimps and prawns in America, Europe and Japan has fuelled industrial farming of shellfish in the past few decades. The industry now has a farm-gate value of $10bn (£6.4bn) per year globally and the prawn in your sandwich is much more likely to have come from a pond than from the sea. While the industry is dominated by the likes of China, Vietnam and Thailand, a large number of other countries have invested heavily in cultivation too.

One is Sri Lanka, which saw the industry as a passport to strong economic growth and widespread employment. Just outside the world’s top ten producers, it accounts for approximately 50% of the total export earnings from Sri Lankan fisheries. More than 90% of the harvested cultured prawns are exported, going mostly to Japan.

Yet the picture is decidedly mixed on a closer inspection. The country saw an explosion of unregulated aquaculture on the island in the 1980s and 1990s, bringing riches to a few and the hope of riches or at least an income to many more. But poor coastal management also brought white spot syndrome virus, a virulent disease that spreads in water and on the feet of birds, and can kill all the prawns in a pond in under a week.

Abandoned shrimp farm in western Sri Lanka
Estibaliz Diaz Tejedor

Crowding shrimp together in warm little pools full of nutrients creates the perfect conditions for an outbreak. It contributes to the fact that here and elsewhere in the tropics, most intensively farmed ponds remain productive for only five to ten years (the other main reason is the build-up of an organic ooze, rich in uneaten food and prawn faeces). Such ponds are then abandoned in favour of new areas of wetland to convert for another brief harvest. The disease kills off prawns in the wild in large numbers too.

A bird’s-eye view

To get a sense of how bad the problem has been in Sri Lanka, I was one of a group of researchers who studied the Puttalam area on the west coast, one of the first in the country where large-scale aquaculture was introduced.

Google Maps

We looked at satellite imagery from 1992 to 2012, which showed an explosion in prawn farms from less than 40ha in our study area to over 1,100ha (a rise of over 2,700%). This combined with a decline in natural habitats – mangroves lost some 36% of their area over the period. Yet most of these historic ponds are now unproductive or abandoned.

The evidence from the satellite images combined with interviews with local people suggest that a staggering 90% of ponds are lying idle. The story is unlikely to be quite as bad across the country as a whole, since Puttalam was one of the early areas to be cultivated. Detailed figures are thin on the ground, but certainly overall shrimp exports in 2012 were 65% below their 1999 peak.

Prawn aquaculture has been likened to slash-and-burn cultivation – find a pristine spot, remove the vegetation and farm it for a few years before moving on. But the analogy is misleadingly benign. Slash-and-burn systems on a small scale can be sustainable, since the cut plots can recover afterwards.

Don’t mess: mangroves
Wikimedia, CC BY-SA

In the case of prawn farming, a better phrase would be “slash and sink”. Mangroves are among the most carbon-dense of all ecosystems, often storing more than 2,000 tonnes of carbon per hectare in sediments beneath the forest floor, according to research that our group has yet to publish. Cut them down and this carbon is oxidised and emitted into the atmosphere as CO₂.

We estimate that nearly 192,000 additional tonnes of carbon have been added to climate change as a result of these land-use changes in Puttalam, Sri Lanka alone. And that of course excludes any emissions during farm operations and the potential for the lost mangroves to capture carbon in future.

An additional issue is the sinking shoreline. In the face of global rising sea levels of more than 3mm a year, healthy mangrove forests are among the best protection since they bind together sediments and even elevate their soils to match the rising tide. Lose them and the chances of coastal subsidence, erosion and storm damage goes up.

In fact, mangroves are such useful ecosystems that destroying them almost never makes sense, even from a narrow economic perspective. A recent analysis in southern Kenya showed that conserving and restoring the forests was worth at least $20m more in present value than allowing current cutting to continue.

So what about Sri Lanka? A positive recent development was that the government announced that it would protect all of its remaining mangroves, totalling some 8,800 hecatares. It also promised to replace a further 3,900 – a task that will require careful restoration of the right tidal conditions and planting trees where necessary. Another positive sign is that there are now local movements that are coordinating production among zones and farms to avoid disease and achieve better sustainability. This is on the back of a commitment by the government in 2010 to expand the industry.

The country should also look to return some of its abandoned ponds to production, provided producers are supported to adopt best practice and work together to avoid disease outbreaks and pollution in future. As for us in the West who import these shellfish in vast quantities each year, we need to think harder about the real costs of that cheap prawn sandwich. Without knowing where it has come from and what farming practices have been used, we would do well to steer clear.

Mark Huxham, Professor of Teaching and Research in Environmental Biology, Edinburgh Napier University

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