Suckerfish are diving headfirst into unsuspecting manta rays' rear-ends

Suckerfish, it turns come out, ar non e'er the to the highest degree considerate travel companions. 

The little fish, also known as a remoras, are hitchhikers who traverse the oceans by latching onto larger marine animals like sharks, whales and manta rays. In return for the free ride, they gobble up parasites on their hosts. 

It may seem like a win-win situation, but new research suggests that, for manta rays, there may be a downside to having a remora along for the trip.

"These fish are heading up right into some manta ray rear-ends," Emily Yeager, a marine researcher at the University of Miami, told As It Happens host Nil Kӧksal. "It could be very uncomfortable for them."

Yeager, a PhD candidate, is the lead author of a new study about the phenomenon, published this month in the journal Ecology and Evolution.

It's called "cloacal diving," because the fish launches itself into its host's cloaca, a one-stop-shop orifice for reproduction and excretion.

Cloacal diving has been observed before between remoras and whale sharks, the latter of which are much bigger than rays. But this is believed to be the first documented evidence of this type of interaction between rays and remoras. 

Researchers noted seven instances, across different parts of the ocean and over a span of 15 years. But they suspect it happens far more often.

"We think this is an under-reported phenomena because, oftentimes, you just see the very tip of the tail poking out from the backside of the manta ray," Yeager said. "They're really wedging themselves into that area."

She suspects it's a fear response for the remora. In one video, a diver comes up behind a male Atlantic manta ray off the coast of Florida and seems to "startle" a nearby remora, she says.

"That remora jumps straight into the manta ray’s cloaca opening," she said. "That manta ray then shudders pretty violently before continuing on its way."

Watch this fish go 'cloacal diving' into a manta ray

Brooke Flammang, a professor of biological sciences at the New Jersey Institute of Technology, who has studied remora, says the little suckerfish may simply be looking for a tasty snack.

"The behaviour is likely coprophagy," Flammang, who was not involved in the study, said in an email. "Latin for eating poop."

Asked about the video of the seemingly spooked suckerfish diving for cover, she said remoras usually aren't skittish around divers. In fact, she says they'll sometimes even latch a human like any other host.

Instead, she suspects the fish was moving fast to get the food before another remora could beat it to the punch.

"They can be territorial about sharing space on the same host," she said.

Whatever the motivation, Flammang says that when it comes to remoras, nothing shocks her.

"As weird as it sounds, I honestly have to say I am not totally surprised by this," she said. "Remoras are just that weird."

It's impossible to know how the manta ray feels about this violation of its personal space, Yeager says, but she suspects it's probably not great.

She says there's been documentation of remoras causing skin damage to other host species with their suction cups.

"If they do that in the cloaca opening, which is likely much more sensitive than other parts of the manta ray's body, it could cause really severe damage and influence reproduction and also excretion of waste over time," she said.

If cloacal diving does, in fact, harm the manta ray, this complicates the way scientists categorize the relationship between the two species, Yeager said.

It's usually seen as either symbiotic, meaning everybody benefits, or commensal, meaning one species benefits while the other is unharmed. 

"Oftentimes when we think of nature, we like to put these relationships into pretty discrete boxes so we can define them easier," Yeager said "But what this has taught me is that these relationships probably exist on a spectrum, just like any relationship in your life."

Interview with Emily Yeager produced by Livia Dyring

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