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Ecology

Damselfish farmers domesticated mysid shrimp

Farming and animal domestication are trademarks of establishing stable human civilizations. But we are not the only species to develop these methods. Reef-dwelling damselfishes known for farming their own algal gardens have recently been discovered tending to domesticated mysid shrimps. Read more about how and why this domestication developed.

Paper: Brooker, R. M., Casey, J. M., Cowan, Z. L., Sih, T. L., Dixson, D. L., Manica, A., & Feeney, W. E. (2020). Domestication via the commensal pathway in a fish-invertebrate mutualism. Nature Communications, 11(6253), 1–9. https://doi.org/10.1038/s41467-020-19958-5

 

 

Longfin damselfish (Stegastes diencaeus) image from Brooker et al., 2020, made available by creative common license.

We tend to think of domestication as a process by which humans tame or otherwise change an animal’s behavior to be better suited to our own needs. Certainly our own lives are interconnected with those of the plants and animals our ancestors domesticated – from the food we eat to the pets we keep, we rely on the mutually beneficial relationships our ancestors forged with other species.  But humans are not the only ones capable of domesticating other species. Ants, beetles, and termites are all known to cultivate their own domesticated fungal gardens. And now, a team of researchers led by Dr. Rohan Brooker has uncovered how damselfish domesticated mysid shrimps.

 

Many of these small, reef-dwelling damselfishes farm algae for food and display tenacious territorial behaviors, chasing much larger fishes (and even curious scuba divers) away from their algal gardens. Scientists were surprised to see swarms of mysid shrimps inhabiting some of the heavily-guarded algal gardens. A closer inspection of the reef area (located in Carrie Bow Cay in Belize) revealed that while some mysid shrimps were only found in some algal gardens, mysid shrimp appeared to rely on algal gardens and could not be found in surrounding waters. . The scientists decided to perform a set of experiments to determine why mysid shrimp appeared so reliant upon damselfish algal gardens.

 

Mysid shrimp depend on damselfish gardens

Close up of a mysid shrimp. Image from wikipedia.org

First, the team of scientists investigated what might attract the mysids to the algal gardens by exposing individual mysid shrimp to different types of scents found around the area: regular seawater, algae from the gardens, a species of farming damselfish (longfin damselfish), a non-farming damselfish (bicolor damselfish), and a known mysid predator (slippery dick wrasse).  Mysids showed no overall reaction to the scents of seawater, algal farms, or non-farming damselfishes, but actively avoided the scent of the known predator and were attracted to the scent of the farming damselfish species.

After determining that mysid shrimp were drawn to the scent of farming damselfishes, the scientists developed a set of experiments to test why mysids would seek out damselfish gardens. The scientists placed a swarm of mysid shrimp within a plastic bag and mounted bags at different locations within damselfish gardens and outside of gardens. Damselfish gardens apparently provided mysid shrimp some form of protection; the bags mounted outside of gardens were visited and attacked by far more predators than those within damselfish gardens. And when the scientists removed damselfish from some of the gardens, predators quickly swooped in to take advantage of the defenseless mysids.

 

Algal farming and mysid ranching

An algae garden (top) and associated mysid shrimp swarm (bottom). Images from Brooker et al., 2020, made available by creative common license.

But providing sanctuary to mysid shrimp comes at a cost to damselfish hosts. Damselfish harboring mysid shrimps spent far more time defending their algal farms and less time eating compared to damselfish lacking mysids in their farms. And yet, the mysids appear to be welcome guests within the algal gardens – damselfish hosts rarely attacked swarms of mysids within their gardens and those individuals hosting mysids even appeared healthier than those individuals that did not host mysids.

To understand why damselfish would exert energy defending mysids and how mysids were apparently improving the health of their damselfish hosts, scientists compared the algal gardens inhabited by mysids to those free of shrimp swarms. The mysid-hosting farms had a higher abundance of fleshy, brown macroalgae that serves as an optimal foundation for algal-dwelling invertebrates and more nutritious and delicious algal species damselfishes prefer to eat. Upon closer examination, the scientists found that mysid shrimp waste was fertilizing the damselfish algal farms, sustaining a more complex microhabitat that ultimately benefited the damselfish farmers.

The unintentional domestication of mysid shrimp

The scientists suggest that the damselfish-mysid relationship evolved much like the relationships humans have with many domesticated crops and animals. What was once a relationship of convenience has evolved into a greater reliance on one another. Now, some damselfishes reap the reward of hosting the fertilizer-producing mysid shrimps and it appears that the domesticated mysid shrimp cannot survive outside of the protective realm of the damselfish garden. Studying and understanding how damselfish unintentionally domesticated mysid shrimps could provide insights into other species interactions and elucidate our own history of domesticating species.

 

 

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