Octopus meat might be packed with protein, but for dolphins living off the coast of southwest Australia, snacking on the ocean's "eight-legged freaks" can be a dangerous affair.

When an Indo-Pacific bottlenose dolphin known as "Gilligan" washed up dead on Stratham Beach near the city of Bunbury back in 2015, the culprit was painfully obvious.

Image: Murdoch University/Marine Mammal Science

Researchers from Murdoch University's College of Veterinary Medicine conducted a necropsy on Gilligan, and a closer look at the otherwise healthy cetacean confirmed the team's initial suspicions: seven octopus tentacles were stuck to the back of the dolphin's throat.

"Rangers on Penguin Island and staff of [a] nearby ecotourism company ... have anecdotally reported similar octopus-related deaths in the past in both dolphins and Australian sea lions," write the researchers, who recently published their findings in the journal Marine Mammal Science. "Some have even recounted stories of having seen, in some instances when dolphins were observed holding an octopus in their mouths, the apparently live octopus reaching up towards the dolphin's blowhole."

Toothed whales can actually dislocate the larynx (or airway) to help them swallow large, awkward prey – but that's a risky manoeuvre because it breaks the seal that separates the lungs from the oesophagus. In this position, prey can move freely into the nasal passage. We saw that rare mishap last year when a pilot whale was asphyxiated by its flatfish dinner.

In Gilligan's case, however, death by suffocation occurred a bit differently: the octopus's powerful tentacles had indeed moved the larynx out of place, but they also flattened it, much like what would happen if you kinked a garden hose. This pinch would have prevented any air from reaching the lungs – even with the octopus sitting safely in the food pipe.

After removing both the airway and oesophagus from Gilligan's body, the researchers also discovered "innumerable circular lesions, many of which were discoloured red-purple." The suckers were holding so firmly that they gave Gilligan "throat hickeys" – impressive given that the octopus's brain was completely detached from the appendages.

Image: Murdoch University/Marine Mammal Science
Image: Murdoch University/Marine Mammal Science

"The real problem is that these arms stay active even after an octopus has been mortally wounded," explain Kate Sprogis, and David Hocking, who study Western Australia's cephalopod-slurping dolphins, but were not involved in the necropsy. "So even while a dead octopus is being processed, the suckers may still be able to find something to stick onto." In fact, lab tests have found that an amputated octopus arm may stay active for up to an hour after the animal dies.

Dolphins are certainly better breath-holders than humans are, but not by much. When swimming slowly at the surface, a typical adult dolphin will breathe between two and five times per minute – and exertion during hunting (or fending off a counter-attacking octopus) ups their need for air. In fact, experts have never observed a dolphin holding its breath for more than eight minutes.

"Despite this considerable ability, the dolphin would have been no match for the octopus's tenacity, and it is unknown how long this individual might have struggled to free its larynx from choking before succumbing," says the Murdoch team.

It's also possible that Gilligan died from cardiac arrest before he ran out of oxygen. The fight-or-flight response is known to release excessive amounts of the protein catecholamine in dolphins and their mammalian kin, including humans. This can have a direct toxic effect on the myocardium, the muscular tissue of the heart.

Clearly, octopus is a risky item to have on the menu, but new research does hint that dolphins have developed a few tricks to make it safer to eat.

The animals' simple, blunt teeth aren't meant for shearing, so individuals in some Australian pods opt for a "shake and toss" strategy when hunting octopuses. The manoeuvre is certainly effective at stunning and killing the dangerous prey, but local scientists are beginning to suspect it also makes those unwieldy suckered arms safe to swallow by "tenderising" them into submission. 

"Assuming an octopus carcass is sufficiently processed to render its arms into small enough fragments such that they and their suckers can be effectively and safely swallowed, their consumption must generally be a risk worth taking," the researchers note. "Although it did not play out well in this individual's case."



ht: New Scientist 

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