A landlubber's guide to deep sea dining

Last Updated 22 January 2018, 18:25 IST

You'll never go to dinner in the deep sea. It's dark, vast and weird down there. If the pressure alone didn't destroy your land-bound body, some hungry sea creature would probably try to eat you. Fortunately for you, something else has spent a lot of time down there, helping to prepare this guide to deep sea dining.

For nearly three decades, robots with cameras deployed by the Monterey Bay Aquarium Research Institute (MBARI) have glided through the ocean off the coast of central California at depths as deep as 2.5 miles below. Cameras on these remotely operated vehicles captured the feeding habits of anything that didn't flee them. They revealed 242 unique feeding relationships comprising 84 different predators and 82 different prey items. Building on prior research using other methods, these videos enhance understanding of the deep sea food web, particularly the jelly dishes and diners.

Foraging for food

It was once thought that these wobbly mounds of water were not worth being eaten. But thanks to the cameras mounted on the researchers' underwater probes - and elsewhere on penguins, monk seals and sea turtles - we now realise that gelatinous animals aren't just ravenous predators invading the ocean, but major food items in a complex web of interactions.

You're probably more familiar with that web as a chain, ending in the tuna on your dinner plate. That beautiful hunk of red meat was once a top predator. But if it weren't for the food web deep under the ocean - a whole collection of crustaceans, worms, fish, jellies and squids feasting on one another miles below the fishing boat that caught your tuna - there'd be no food to forage and no tuna to catch.

"It's really exciting and really important," said Anela Choy, a marine biologist at MBARI, who led the study that was recently published in the journal Proceedings of the Royal Society B. "It's taking a bigger view and allowing you to see a lot more of the connectivity of the ocean ecosystem." So let's go eat.

Find something to eat and grab it: No one knows exactly what species one creature is, but Anela calls it a galaxy siphonophore. It waits in the water for whatever swims into its orange curtain of tentacles. The deep sea can be a tough place to find food, and the creatures that live down here have adapted to its fickle abundance. They don't just use tentacles to grab unwitting prey. Consider detritivores, including crustaceans and even some jellies that eat them: they munch on decaying organic matter called 'marine snow' that sinks down to the bottom from sloppy feeders or phytoplankton near the surface. And the black swallower fish: it uses its big jaw to swallow prey bigger than itself whole, like a snake. These different species show there are diverse ways to fill your belly in an unforgiving environment.

Learn to tolerate gory table manners: One of the most common interactions that Anela and her colleagues observed were cephalopods like the gonatid squid preying on fish. They are abundant in midwaters and play the role of both predator and prey in the food web. Endowed with an insane metabolism, the voracious cephalopods are constantly eating. They dine on deep sea fish including lantern fish, owl fish and dragon fish.

The species ranges in size from six inches to one foot long, but it can consume fish bigger than its own body. To do so, the squid grasps onto its prey with tentacles lined with hooks and suction cups. Then it pierces the fish's brain with its beak, which is creepily located right between the squid's eyes. It bites off pieces of fish flesh, which it chews and swallows through an oesophagus in the centre of its brain.

Sometimes you have to eat your own kind: Sometimes, a gonatid squid eats other gonatid squid. This kind of cannibalism is common in the deep sea. And for the squid it can be beneficial. By eating competitors from within its species, a gonatus may free up more food and find more opportunities to mate. But they don't just eat one another. Other species of squid, swordfish, bottle-nosed whales, sperm whales, hooded seal and other marine animals eat gonatus too.

Eat just about anything: In the deep sea, jellyfish from the Narcomedusae order are quite abundant. The institute's recordings revealed that they are major predators, consuming nearly two dozen different sea creatures including other gelatinous animals, especially ctenophores or comb jellies, worms and krill. One example they recorded dining on a ctenophore was solmissus, which is also called a dinner plate jelly.

Hopefully you're not allergic to shellfish: Crustaceans, hard-bodied creatures like krill and shrimp, are like dinner rolls of the deep sea. They're always around, and practically everyone eats them. Physonect siphonophores, gelatinous animals that live in long chains, have a boundless appetite for these tiny creatures. Some eat all kinds of crustaceans, the researchers found, but Nanomia, siphonophores that are quite abundant off the central California coast, feed almost exclusively on krill.

(Published 22 January 2018, 10:24 IST)

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