Angel or Dragon?

The Blue Glaucus, Glaucus atlanticus, is a shell-less gastropod. Huh? A sea slug! These ethereal creatures live out to sea their entire lives; they are pelagic. Sometimes currents carry them onto shore, as in the picture. They grow to about an inch long and live in the Atlantic Ocean. They have no skeleton, so they are a bit floppy if you take them out of the water. They fit nicely on the tip of your finger.

Glaucus atlanticus

This gorgeous critter has a number of names: Blue glaucus, blue angel, blue dragon, blue sea slug.

All that beautiful blue color you see in the image is on their belly. Yepper, they float upside down. They’re shown here against a sandy bottom, but far out to sea, they are just as blue as the ocean, and impossible to spot. That’s the point. Hunting birds can’t see them.

Their backs are a boring silver. Why’s that? predators below see the shiny surface of the ocean and the Blue Glaucus is invisible, silver against silver. Us biologists call it countershading, because it’s more than just camouflage, it’s double camouflage!

They are predators. Their favorite meal is Portuguese Man O’ War. Are you kidding? Have you seen a Man O’ War?

Portuguese Man O’ War

This is a colonial organism. In other words, it’s a collection of smaller animals. The long, blue tentacles have “stingers” called nematocysts. These cool little weapons fire a barb and deliver toxin. There are hundreds of thousands on a tentacle. Man O’ Wars can have tentacles anywhere from 30 – 100 ft long!


How do they eat the Man O’ War? One bite at a time. Seriously! They have a good set of teeth and eat the tentacles. Yes, the 100 ft long ones with hundreds of thousands of nematocysts. Our little Blue Angel is quite the Blue Dragon! They don’t, of course, eat the entire tentacle, just what it takes to fill them up.

Here’s the really cool thing: they swallow the nematocysts and use them to hunt and protect themselves! The nematocysts are stored in the tips of their “fingers”, or cerata, usually 84 of them. Concentrated nematocysts. Even though I said they fit nicely on your fingertip, DO NOT pick them up.

Now that you’ve had a minute to think about this, you’re probably wondering why the bigger, deadlier Man O’ War doesn’t eat the little blue sea slug. These guys have protective armor, another reason to call them a dragon. They have a layer of mucus on their skin and hard disks under it so the nematocysts can’t hurt them.

Right about now I should be wrapping up this article, but wait, there’s more! Aren’t you curious how they float? They swallow some air! They keep a bubble in their stomach, which keeps them afloat. I assume they belch it out when they want to sink.

The very last thing about these lovelies is that they are hermaphroditic (reproductive organs from both sexes). They can mate if they come across others, but hermaphroditism is a good solution to spending your life alone in the ocean.

The Encyclopedia of Life was a great help in writing this article.

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