Throughout human history, the ocean has been a dangerous place for ships.

Storms, icebergs, and reefs have been the bane of sailors since we first put to sea. But they’re not the only things that can sink a vessel.

Living things can be rough on a ship, too. And we’re not talking about creatures of legend, like sea monsters or the Kraken. There’s a much humbler, more mundane animal that actually exists that’s more than capable of destroying boats, by eating them.

It’s the boring clam confusingly known as the shipworm. [♪♪ INTRO ♪♪] Now, when I say it’s a boring clam, I don’t mean that it’s not interesting. I mean that it literally bores into wood, tunneling through it and leaving it riddled with holes, the way a termite does. It scrapes these burrows using its shell, the most obviously clam-like part of its body.

The rest is, distinctly worm-y, which is where the “worm” part of “shipworm” comes from. There are around 70 species of shipworm, ranging from a few centimeters to more than 1.5 meters in length. The biggest ones are actually the longest extant bivalve the group that includes scallops, clams, and oysters.

And the full-size adults don’t even live in ships or wooden structures at all. They’re found buried in ocean mud in the Philippines. And – this is super-weird – the symbiotic microbes that live in their gills subsist on stinky hydrogen sulfide as their energy source.

They use this energy to make carbon molecules, which their shipworm hosts consume. Does that even count as eating? I’m not sure.

Generally, shipworms can be found in oceans and seas around the world, where they’re capable of wreaking havoc on any wood that comes their way, be it a boat, driftwood, or a dock or pier. And, with the exception of the giant shipworms in the Philippines and another species there that appears to ingest limestone, shipworms actually eat the wood they excavate to create the tubes they inhabit. The bits of wood they scrape off with their shells are carried to their mouths by cilia, which are tiny hair-like structures on the shipworm’s body.

From there, the wood shavings travel to a specialized part of the clam’s gut for digestion. But their gut doesn’t use acid to break down its food, like ours does. Instead, like the giant shipworms, most other shipworms also have symbiotic bacteria that live in their gills that contribute to this process.

But, unlike the giant shipworms, their bacteria make enzymes to break down the cellulose in wood. These enzymes meet up with other enzymes produced by the shipworm itself in their gut, where they digest the wood. The whole system feels kinda like if the bacteria in our noses made our stomach acid.

And digestion isn’t the only physiological process that they do in an unexpected way. Shipworm sex can be pretty weird, too. All shipworm larvae start off their lives as what we might call males – but they don’t necessarily stay that way, otherwise, where would more shipworms come from?

According to one researcher, adult shipworms “can be anything at any time” – they can have eggs, sperm, both, or switch back and forth between having eggs and sperm. And shipworms don’t just reproduce in a single way. Some do what’s called ‘broadcast spawning,’ where they just release eggs and sperm into the water, in the hope that they’ll meet up.

This is the way most bivalves reproduce. But other shipworms fertilize their eggs internally by collecting free-floating sperm from the water, then brooding their offspring in a special pouch before releasing them into the world. And at least four species do what’s called “pseudocopulation.” To explain how that works, first, you need to know some basic shipworm anatomy.

The head-end of the shipworm is where the shell and mouth are – this part of the shipworm hangs out inside its burrow. And the two bits that poke out of the burrow are called siphons. One is basically for taking in water and particles suspended in it, and the other is for expelling waste and other things, like sperm.

What happens during pseudocopulation is that one shipworm takes its expelling siphon, sticks it into the intake siphon of another shipworm, and deposits its sperm there. And because shipworms can have both eggs and sperm at the same time, the shipworm that’s receiving the sperm can also be using its other siphon to deposit sperm into another shipworm’s siphon! It’s a much more “hands-on” mating approach than the broadcast spawning of most other bivalves.

But when you make your home in things like free-floating bits of driftwood that you’re also actively eating, you kinda have to take all the mating opportunities you can get before you finish eating your house. Especially now, when humans have come up with a lot of other building materials for ships and docks that they can’t eat. Don’t feel too bad for the shipworms, though, they used to have it tougher.

For most of the 1900s, the delicious wooden wharves and pilings of major ports like those in New York and Los Angeles used to be inaccessible to them because the waters there were too polluted. But after the passage of the United States’ Clean Water Act in 1972, they were back on the menu. And today, the warmer oceans brought on by global climate change may be giving shipworms another boost, since they tend to breed and start their wood boring when temperatures are higher.

These changes in climate might also help shipworms expand their range into new waters and stay active longer, which potentially means being able to cause more damage. We do have at least one other defense against them, though. They are just oddly-shaped clams after all, and lots of different cultures around the world have come up with plenty of ways to deal with clams… Maybe the time has come to turn the tables on the shipworms.

Sure, they can eat our boats…but we can eat them. The Bizarre Beasts pin club subscription window is open again, from now through the end of August 7th. When you sign up, you’ll get this incredible shipworm-meets-sea monster pin in the middle of the month, and the pins after that around the time each new video goes live.

For more fun facts about shipworms and other bizarre beasts, you can like and subscribe here, and follow us on Twitter @BizarreBeasts, and on Instagram and Facebook @BizarreBeastsShow. And, as always, profits from the pin club and all of our merch go to support our community’s efforts to decrease maternal mortality in Sierra Leone. [♪♪ OUTRO ♪♪]