The Damselfish Farmer: Keeping Shrimp as Algae Fertiliser

Every diver knows that the most aggressive fish in the ocean is a damselfish. Through its highly territorial behaviour, it chases away everything and everyone that comes just a bit too close to its home. But one species of damselfish is known for something else. The Longfin damselfish (Stegastes diencaeus) grow algae farms and protect tiny shrimp (Mysidium integrum) like bodyguards.

This is new! Because although it is completely normal for us humans to domesticate plants and animals, it rarely occurs in the animal world. Until now, this was only known from insects that cultivate fungi. Let us now meet the fish in the ocean that operates its own algae farm while domesticating small crustaceans as „fertiliser suppliers.“

The Underwater Farmer and His Fertiliser Suppliers

The Longfin damselfish lives in Caribbean reefs. These fish have long been known for establishing algae farms. They aggressively defend specific areas of the reef against other fish, allowing algae to grow there, which they then eat. What stood out was a peculiarity of these farms.

Dense clouds of tiny, transparent crustaceans – so-called mysid shrimp – often hovered above the algae patches. This was unusual, since damselfish normally chase away every intruder.

Longfin Damselfish

Image from Brooker et al. (2020),
Nat Commun 11. Licensed under CC BY 4.0.

Shrimps on Farm

Image from Brooker et al. (2020),
Nat Commun 11. Licensed under CC BY 4.0.

The mysid shrimp is a tiny crustacean (plankton*) measuring only millimetres, which normally drifts through open water as a free-swimming swarm. What is extraordinary is that these tiny shrimp return to the same algae patch every night.

A behaviour one would not normally expect from plankton. They leave their safe spot to feed and swim up at night, but return to the damselfish farm at dusk. This intergenerational site fidelity is the first sign of something extraordinary.

The Beginning of a Special Partnership

Scientists wanted to find out why these two species lived together so closely and conducted several experiments:

Who Seeks Whom?

Through observations, they found that the shrimp were indeed significantly more above damselfish farms than at other locations or with other fish species. Next, researchers tested in the lab if shrimp actively approached damselfish. They found that they were attracted by the scent of the Longfin damselfish, repelled by the scent of their predators, and indifferent to the algae farm itself. The tiny shrimp thus actively seek the company of the damselfish.

What Do the Shrimp Get Out of It?

Mysid shrimp are zooplankton, a classic component of plankton that many fish feed on. The researchers suspected that the damselfish provides the shrimp with protection. They conducted some tests by placing shrimp in plastic bags either inside or outside the algae farm.

• Outside the farm: Predatory fish immediately attempted to attack the shrimp.

• Inside the farm: The damselfish chased away every fish that approached the bag.

grown algae

Image from Brooker et al. (2020),
Nat Commun 11. Licensed under CC BY 4.0.

The shrimp benefit by receiving protection from predators within the damselfish’s territory.

What Does the Damselfish Get?

The shrimp hover above the algae farm and constantly excrete waste there. This waste acts as fertiliser and helps the algae grow. When they examined the algae and damselfish, this was confirmed:

• Farms with shrimp had better algae quality (more nutrients)

• The damselfish from these farms were in better physical condition (they were healthier and better nourished)

The fish benefit: the shrimp „fertilise“ their food source and thus provide better yields.

Domestication Without Intent

This is the first documented example of domestication by a non-human vertebrate. As fascinating and clever as this coexistence of the two species sounds, the process follows no bigger plan. The damselfish did not deliberately „domesticate“ the shrimp – it was simply territorial and aggressive. The shrimp were likely attracted by its behaviour and saw a survival opportunity in it.

What began as commensalism* (only one species benefits without harming the other) has evolved over time into a true mutually beneficial relationship (mutualism*). It is important to note that there are damselfish with algae farms that they manage themselves – and the mysid shrimp can survive globally without the damselfish.

How a Fish Teaches Us Ancient History

This discovery paints a picture of classic domestication – only underwater. The Longfin damselfish provides the mysid shrimp with protection, which in return supplies its algae farm with fertiliser. The process mirrors what scientists believe to be the origins of our own domestication: cats around grain stores, dogs around campfires – opportunistic niches that became partnerships.

This relationship demonstrates the complexity of coral reef ecosystems. Where we humans would expect a simple predator-prey relationship, nature instead reveals highly developed mutual partnerships that evolved over time.

Your`s Lea

Glossary of Terms

Plankton: Tiny, suspended organisms in water. There is, for example, zooplankton (animal plankton) and phytoplankton (plant plankton). Without plankton, there would be no functioning food chain in the ocean.

Commensalism: Describes a relationship between two species in which only one species benefits, while the other neither benefits nor is harmed.

Mutualism: Describes a mutually beneficial relationship in which both species benefit.

Source

Brooker, R.M., Casey, J.M., Cowan, ZL. et al. Domestication via the commensal pathway in a fish-invertebrate mutualism. Nat Commun 11, 6253 (2020). https://doi.org/10.1038/s41467-020-19958-5

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