Why are fish carrying sea anemones? A hidden partnership revealed

The symbiosis between clownfish and sea anemones, made famous by the movie Finding Nemo, is just one chapter in a much bigger story.

Researchers have found that open-ocean fish youngsters – from filefish and driftfish to pomfrets and even a juvenile jack – appear to pick up larval anemones and button polyps and carry them around like living shields.

The behavior, rarely seen until now, has finally come to light. “Blackwater” photographers dive at night over deep water, shining their lights on the drifting, miniature life of the open ocean.

“Some species of vulnerable larval or juvenile fish use invertebrate species apparently for defensive purposes,” said diver and contributor Rich Collins of the Florida Museum of Natural History. “They’ll find something that’s noxious or stingy, and they just carry it around.”

Cameras unlock the open nursery

Lead author Gabriel Afonso, a Ph.D. student at William & Mary’s Batten School of Coastal & Marine Sciences at VIMS, credits blackwater diving with making these encounters visible.

At night, many larvae and juveniles ride daily vertical migrations into shallower water. Suspended over hundreds or thousands of feet of open blue, divers can capture crisp, close-up images of tiny animals that would otherwise go unseen.

The photos in the study are striking. They show delicate larval tube anemones and button polyps clenched gently in small fish mouths. Filaments and tentacles trail behind them like warning flags.

It’s an image that upends familiar reef-shelf dynamics. It shows the open ocean – far from coral heads and sea anemone colonies – as a place of inventive survival strategies.

Anemone armor for baby fish

Why would a fish tote around an anemone? The leading idea is defense. Even as larvae, anemones carry stinging cells that can make any would-be predator’s mouth tingle in all the wrong ways.

“The sting from a larval anemone might not be enough to kill a predator, but it would be unpalatable,” Afonso said. In other words, one distasteful bite may be all it takes to persuade a hunter to spit and move on.

The partnership could be two-way. Anemones are slow, especially at the larval stage. Being ferried by a quick juvenile fish could help them ride currents and find new places to settle.

“As far as I know, this is the first relationship of an open water fish interacting physically with an anemone that looks to be carrying the invertebrate,” Afonso said.

If confirmed, that would mark a new form of mutualism in the pelagic zone. The fish gains protection, and the anemone gets a free ride.

Fish drifters with defensive partners

Adult reef fish sometimes hunker down among corals or anemones to rest or hide. What’s different here is the life stage and the setting.

These are larval and juvenile fish adrift in the open ocean – the planet’s largest nursery – improvising with whatever defenses they can find.

The images show filefish, driftfish, pomfrets, and a young jack employing the same trick: grasping a stinging invertebrate and wearing it like a portable hazard sign.

Collins says blackwater nights are full of such surprises. He’s seen tiny filefish ferrying box jellyfish – despite their fearsome sting – using the same carry tactic.

The new photos extend that behavior to anemones and suggest the strategy may be more widespread than anyone realized.

Mysteries behind the stings

The study raises as many questions as it answers. Do specific fish species prefer certain anemones? How long do they carry them?

Do the anemones survive the ride and ultimately settle, or are they swapped out like disposable armor? And what cues – chemical, visual, tactile – trigger a juvenile fish to grab an anemone in the first place?

Answering those questions won’t be easy; these are fleeting interactions in a vast, dark habitat. But blackwater photography is rapidly expanding what scientists can observe.

Each image becomes both documentation and hypothesis: proof the behavior occurs, and a prompt to investigate how common it is and what it means for open-ocean food webs.

A new lens on ocean life

Afonso hopes the work draws more eyes – and cameras – to the pelagic night shift. Blackwater diving has already revealed larval forms, behaviors, and partnerships that never make it to reefs or shorelines.

This latest glimpse suggests the open ocean is full of creative alliances, improvised defenses, and relationships that don’t fit the reef-centric stories we know.

“Blackwater photography is showing us a previously unseen world. These interactions are happening in plain sight – just at night, in the middle of the ocean, at a scale we’re only now starting to capture,” Alfonso concluded.

The study is published in the Journal of Fish Biology.

Image credit: Linda Ianniello

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