Humans have dogs, damselfish have shrimp

Researchers have discovered that a species of coral fish uses shrimp to help fertilise its algae farms, which, they suggest, is the first evidence of a non-human vertebrate domesticating another species. Longfin damselfish (Stegastes diencaeus) are known to aggressively defend the farms they rely on for food – but not, it seems, against planktonic mysid shrimps (Mysidium integrum).

“We found that the damselfish keep swarms of mysid shrimps within their farms, providing them with a long-term safe haven from predators,” says Rohan Booker from Australia’s Deakin University, lead author of a paper in Nature Communications.

“The mysids, in return, swim over that farm all day and passively pump out waste material. All that extra waste acts as fertiliser, improving the farmed algae, and, in turn, the condition of the farmer, the damselfish.”

This is known as a “domesticator-domestica relationship”, a mutually beneficial arrangement where one species provides ongoing support to another in exchange for predictable benefits, such as cleaner fish picking parasites off other fish or insects pollinating flowers.

Humans have had such relationships with many different animals since domesticating dogs around 10,000 years ago, selectively breeding them for certain appealing characteristics such as tameness.

Other examples of non-human domestication are best known in insects that tame plants such as fungi-farming ants. This study shows non-human vertebrates also domesticate other animals and suggests it may be more commonly than previously known, says Booker.

In non-human species, he notes, the process of domestication likely occurs without conscious intent but rather through co-evolution over millennia. It’s suspected that human relationships with other animals originated in the same way.

Human domestication of grey wolves (Canis lupus), for instance, is thought to have started when the wolves were attracted to human camps; as the two species coexisted and became used to each other, humans started deriving benefits such as more successful hunting.

Similarly, chickens and cats may have been drawn to human encampments to feed on scraps, which then developed into a symbiotic, or mutually beneficial relationship, through the benefits they provided such as eggs, meat or hunting rodents.

“I think the really exciting thing we were able to do was use the fish-mysid relationships to experimentally test the commensal pathway which is one of the ways human relationships with animals are thought to have first emerged,” says Brooker.

“The territorial behaviour of the fish created a brand-new niche on the reef which the mysids could opportunistically take advantage of for shelter, similar to how chickens or cats may have taken advantage of human settlements for food, leading to better survival.

“These animals, in turn, provided a benefit to the niche creator and so positive feedback strengthened the relationship over time.”

These affiliations have ecologically important ramifications, the authors say, as mutual benefits evolve through the generations, shaping behaviour, adaptation and survival, and even landscapes and biodiversity.

Brooker first spotted the phenomenon with damselfish and mysid shrimp when swimming around coral reefs of Belize five years ago. He and colleagues from Deakin and Griffith universities in Australia, as well as from France, the US and the UK, then followed up with a series of field studies and behavioural experiments.

He is hopeful that future research can verify the phenomenon in other reefs around the world where mysid shrimp are reported to inhabit damselfish territory, such as Lizard Island on Australia’s Great Barrier Reef.

“Our ability to domesticate other organisms has contributed to the success of our species and transformation of the world’s ecosystems,” the team writes. They suggest new insights could be gleaned by further exploring how this happened in other animals.

The Royal Institution of Australia has an Education resource based on this article. You can access it here.