What animal can eat itself?

That bizarre thing in the photo below is not an alien life form, but a regular old animal found in our oceans. That doesn’t begin to describe its weirdness, however. It is not a mollusc, or a sponge, or any other kind of distantly related invertebrate; it’s our cousin. Believe it or not, this creature is a chordate, belonging to the same grouping of animals as birds, frogs, dogs, cats, and you or I. It is known as a sea squirt.

Sea squirts and their relatives have a supportive structure running along their body called a notochord - essentially, a primitive backbone. The human spine is basically just a very complicated, advanced notochord. This, along with a host of other features, proves that these strange animals are the closest relatives of vertebrates in the world.

They don’t possess this notochord throughout their whole life cycle, however. It’s a trait present only in the earliest phase of sea squirt life, when they exist as larvae. The larvae are far too small to be seen with the naked eye, but almost resemble tadpoles when viewed under a microscope.

Within a day or two of being born, these free-swimming larvae find a nice spot on the seabed, settle down, and metamorphose into the bizarre adult form. In so doing, they lose their notochord, but also their primitive brain, which is formed of bundles of nerves. This brings us to the topic of the question. Believe it or not, the developing sea squirt digests its own brain, and uses it as nutrition to fuel its transformation!

This is a rare example of self-cannibalism in the animal world. After metamorphosing, the sea squirt takes on an immobile form, staying rooted to the rocky sea floor. It siphons the surrounding water through its body - as it passes, the tasty plankton it contains is trapped by the creature’s mucus-coated inner linings, and digested. Adult sea squirts can be solitary:

Live in clusters:

Or even form vast colonies which together function as a single super-organism:

These incredible colonies are one of nature’s great wonders. They are typically made up of countless genetically identical clones. In some species, the clones are almost melded together, such that nutrients, blood, waste and such can flow freely between individuals, in a massive sort of shared circulatory system. They reproduce in a wide variety of amazing ways.

Sea squirts as a whole are likewise varied. There exists extraordinary diversity within the group. Some are translucent:

Others are brilliantly colourful:

Some… look like this…

Or like this…

Or this…

Or whatever this is.

The last couple of examples are especially fascinating. They are colonial species which have taken the whole “super-organism” thing to another level entirely. In the photo above, each of those tiny white cups is actually its own individual sea squirt. They have fused together, formed groups, and grown larger shared tissues. Each individual in the group has its own inhalant siphon (think of it as a mouth), but they all share that big orange opening, the exhalant siphon (think of it as… the other end).

Creatures like these challenge our very idea of what an organism even is… and what it isn’t. Thank you for giving me an excuse to talk about sea squirts! Yes, they eat their own brains, but they are amazing for so, so much more. I could go on about these guys all day, but I’ll leave it at that for now. Thank you all very much for reading, I hope you found some of it interesting, and have a great day!