These Fish Keep Shrimps As Pets To Help Them Farm Algae

'Will provide protection in exchange for poop' - Longfin damselfish, 2020. Stubblefield Photography/Shutterstock.com

Domestication has seen humans achieve great things, from agriculture and aquaculture to turning the wolf into a pug. Such domestication has been seen in non-human invertebrates such as ants who farm aphids and spiders that have pet frogs for keeping parasites at bay. Now, new research published in the journal Nature Communications details an example of domestication in a non-human vertebrate, specifically the longfin damselfish (Stegastes diencaeus). These fish employ planktonic mysid shrimps (Mysidium integrum) to help them fertilize algae farms on which they feed.

Longfin damselfish will spend their lives defending a patch of algae from other sea creatures who might try to eat it. They run patrols and if there’s no free real estate on which they can farm, they will suck on polyps in the coral until they die and the algae take its place. They are considered a menace for dying reefs for this reason, and the better they are at farming the larger their garden will grow, killing off more of the coral in the process. Parrotfish are another species that feed on the algae and will target longfin farms, but despite the enormous size difference between the two, the longfin is so aggressive it can scare parrotfish away.

This new research wanted to establish why longfins, who usually fiercely defend their algae from all living things, will tolerate swarms of mysid shrimp floating over their yards. They first needed to establish if this behavior was widespread and so swum a series of 30-meter (98-foot) laps of a reef in Belize, making notes of when they saw mysid shrimps and which species they were near. Their results showed that mysids were found near fish farmers such as the longfins far more than any other species.

Longfin damselfish must protect their algae farms from hungry species like the parrotfish. Ronsmith/Shutterstock.com

Then, to find out if this association was accidental or a choice made by the shrimps, they collected some mysids and exposed them to water contaminated by different “smells”. The mysids revealed that they are attracted to the smell of farming damselfish but are indifferent to non-farming fish.

To ascertain why mysids would seek out damselfish, they put some of the shrimps in small, clear plastic bags and placed them inside and outside of longfin farms to see how they reacted. They found that the mysids inside the farms gained a benefit from the longfins as they would defend the shrimp from other fish who wanted to eat the mysids. Those shrimps outside of the farms were not protected and were on the receiving end of multiple predation attempts by other fish.

The final piece of the puzzle was to ascertain if the longfins were benefiting from keeping these swarms of mysids above their farms. They suspected that since the shrimps were above the algae, their waste might act as a fertilizer, meaning the longfins would enjoy a bumper crop. Sure enough, an analysis of longfin farms with and without revealed that farms with mysids were more bountiful than those without.

Longfin damselfish with shrimps over their farms experience a bumper crop of algae compared to those without. MYP Studio/Shutterstock.com

“These different analyses together suggest longfin damselfish have domesticated mysid shrimps,” wrote the study authors of the research in a piece on The Conversation. “The longfin damselfish provide a safe refuge, and in exchange the mysid shrimps provide the damselfish with fertilizer 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.”

[H/T: The Conversation]

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