Videos reveal the Russian doll parasitic world of the deep seas

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Videos reveal the Russian doll parasitic world of the deep seas

By Emily Benson

Deep below the ocean’s surface, parasitic crustaceans are latching on to fish that eke out a living. Video from the ocean depths has revealed their diversity and range for the first time.

Parasites may be an indicator of a healthy ecosystem, as they can regulate animal populations and keep them from growing out of control, says Andrea Quattrini at Harvey Mudd College in Claremont, California.

“And yet virtually nothing is known about their importance in the deep sea,” she says. “The first step is really just to document their diversity, and how many and what species are being infected.”

Quattrini and Amanda Demopoulos at the US Geological Survey studied films collected by a remotely operated underwater vehicle (ROV) surveying the sea floor off the coast of the north-eastern US.

 

Hitch-hiking in the deep sea

NOAA Okeanos Explorer Program

One third of the fish species spotted during 43 separate ROV dives – and up to half of fish of some species – had up to several dozen visible parasites: isopods and copepods. Some of these have hooks on the ends of their legs that they embed into the fish. Others bury their head into the flesh.

One parasitic copepod even had parasites of its own: eight leeches clung to it (see video, above).

The researchers also documented a new association: their cameras captured a clip of the first time an isopod from the Cymothoidae family has been found on a type of fish called a silver roughy.

Without physically collecting the animals, it’s not yet clear if any of them are new species, says Quattrini.

 

Close-up of a parasite

NOAA Okeanos Explorer Program

ROVs may be the best method for gathering information on how deep-sea parasites interact with fish, says Iain Barber at the University of Leicester, UK.

Dredging or trawling for fish and dragging them to the surface can destroy the fish and dislodge their parasites. “There’s no other way that you could actually collect these data,” he says.

The parasitic crustaceans were familiar to researchers from shallow water environments, but the new study showcases their diversity and abundance in a different setting, says biologist Geoff Boxshall at the Natural History Museum in London.

“Seeing them in their natural condition is a bit of an eye opener that in fact parasitism is probably much more common in the deep sea than perhaps we had previously imagined,” he says. “Parasitism is an extremely common mode of life, and we’ve only just begun to sample the diversity of it.”

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