Thanks to the Monterey Bay Aquarium and  their research and technology partner MBARI   for partnering with us on  this episode of SciShow.   They are super excited for folks to learn about   the deep sea. [ intro ] In ocean waters around the world,  from the poles to the tropics, you can find tiny, nearly transparent sea slugs with a pair of appendages in the shape of wings.

As far as slugs go, these so-called  sea angels are pretty adorable. But they’re also pretty ferocious predators.

They’ve evolved that way because  for as long as they’ve been around, they’ve been in an evolutionary arms race  with their main prey: the sea butterfly. And by exploring just how these  angelic-looking slugs conquer their prey, we can uncover the fascinating adaptations  this species has developed to survive. Now, it can be a little hard to imagine these creatures as terrifying predators because, aside from the angel wings,  they’re not exactly huge.

Like, the biggest ones are  around 7 centimeters long, and many measure less than 2 centimeters. Plus, aside from the wings, they  basically look like regular land slugs. They’re technically pteropods, a group that falls under the same taxonomic  umbrella as land snails and slugs.

But out in the ocean, far from being the  slow, innocuous garden pests we know, s ea angels are ruthless hunters. They feed on other swimming snails,  including the sea butterfly, a close relative of theirs that lives in a shell. And thanks to some incredible adaptations, sea angels have become experts at  hunting down and devouring their prey.

One advantage the sea angels  have is that despite being slugs, there’s nothing snail-paced about them. They can zip through the water  at 10 centimeters per second, almost as fast as a goldfish! Which is  enough to easily outpace their prey.

Sea butterflies’ top speed  is less than half as fast. Sea angels get this speed from their  wings, which act like oars on a boat, propelling them straight through the  water with up to three strokes a second. Oddly enough, their wings have evolved  to be smaller than their relatives’,   but thanks to their swimming  technique, sea angels are still faster.

They’re not always zooming around, but they’re  capable of bursts of speed when they need it. But sea angels don’t just rely  on speed to capture their prey. Sometimes they do the opposite  of chase: They just wait.

For instance, one species of sea  angel has basically figured out how to be the world’s laziest predator.  It literally gets its prey to reel it in. The reason it works is because this sea  angel preys on a type of sea butterfly that feeds with the help of a mucus web. It’s roughly the same concept as a  spider web, except, instead of silk, it’s made of mucus that the sea  butterfly expels from its foot.

Which, yes, a sea butterfly has  feet! Or, well, foot, singular. They’re really just modified swimming appendages.

Anyway, normally, sea  butterflies use this web to catch plankton and other floating particles. Then, when it’s mealtime, they pull the web  back in and eat the food they’ve caught. Except… sometimes they end up reeling  in something that wants to eat them.

Because now and then, a hungry sea angel  attaches itself to one of these webs. Then, when the sea butterfly  pulls it in, the tables turn, and the sea angel eats the sea butterfly. Among other animal species, hunting is often an extremely expensive  part of a predator’s energy budget.

But this passive hunting technique  is a remarkable adaptation that lets this species of sea angel conserve  energy while keeping itself well fed. Whether a sea angel chases down its meal or has itself delivered straight to  its prey, it wastes no time digging in. And it essentially uses built-in silverware  stored in the top of its head to eat efficiently.

It can’t just devour the sea butterfly  whole because it’s in a shell. So the sea angel opens its mouth and extends  a set of terrifying-looking tentacles called buccal cones to  scoop the sea butterfly out. These tentacles are tricked out with hooks, barbs… Some even have suction cups  like octopus tentacles.

So, the sea angel reaches these  into the opening of the shell and uses the hooks on them to yank tissue  from the sea butterfly toward its mouth. With its mouth, it holds onto that tissue while its tentacles reach back into the  shell to grab more of the sea butterfly, until the whole thing is out  of the shell and ready to eat. This process is so fast that sometimes, the sea angel devours its  whole meal in just two minutes.

There’s no other creature that hunts  and eats quite like the sea angel. And one important reason  why these whimsical-looking   creatures developed such specific and ferocious hunting tactics is because  they eat an incredibly specific diet. Some species eat only sea butterflies.

So, as sea butterflies evolved  to outsmart sea angels, sea angels evolved further  to keep up with their prey. The two have basically been  playing evolutionary chess forever. And it shows us just how many  evolutionary moves a species is capable of when it meets its match.

MBA

Outro: Thanks for watching  this episode of SciShow, which was a collab with Monterey Bay Aquarium! They worked together on an exhibition, “Into The

Deep: Exploring Our Undiscovered Ocean,”  to give visitors to the Aquarium a rare look at some of the animals that thrive in the  least-explored area of the planet, the deep sea! So go check out our fronds at the Aquarium! Their YouTube channel has awesome  videos about the deep sea, and on the Aquarium’s website, you can make a donation to support  their exhibits, education programs, and ocean conservation work. [ outro ]