sea slug
Image taken at Aquarium of the Bay in San Francisco

The world of sea slugs is an amazing one.  First off, they are absolutely, stunningly gorgeous.  The one pictured above is comparatively subtle, with its pretty frosty spots, but sea slugs come in every color of the rainbow with fantastic and startling features.  Check it out, right now.  Go on.  Google image nudibranch and see what fascinating creatures pop up.  I’ll wait…

Finished?  Told you so.  Alright, so these animals are beautiful.  But they are even beautiful as eggs.  That’s right, they lay their eggs in gorgeous ribbons that look to me like underwater flowers.

egg ribbon
Sea slug egg ribbon.  Image taken at Aquarium of the Bay.

But beauty is not the reason why sea slugs are my favorite animals in the ocean.  No, that would be because of the multitude of ways these animals have to defend themselves.  So imagine for a moment that you decided that a porcupine would make a tasty treat.  You caught a porcupine and ate it.  And then all of a sudden you grow quills out of your back.  It’s absolutely absurd I know, but sea slugs do something similar.  Some species of sea slug eat sea anemones, which are animals that are related to jellyfish and like jellyfish, they have stinging cells called nematocysts.  (See a video of nematocysts firing here.) But the sea slugs don’t get stung by their prey.  No, instead as they make a meal of the anemone, the stinging cells pass through their system undischarged and then pop out of their back, so that the nudibranch has now essentially stolen its prey’s defenses to protect itself.

Other species of sea slug eat sponges instead.  Sponges are animals that are often poisonous, but the sea slug is not at all deterred by the toxins in their prey.  Once again, they will steal their prey’s defenses and instead of being poisoned, they will store their prey’s toxins and become poisonous.  Sponges are also often brightly colored, a warning to other organisms that they should be left alone.  When a sea slug eats its sponge prey, it will not only take their toxins, but it will steal their pigments as well, so when they are on their host sponge, they will be perfectly camouflaged and when they move away, they will exhibit wonderful, bright warning colorations.

Last, but not least, there are at least 2 species of sea slug that are pelagic, which means they float in the water column instead of crawl on the bottom of the sea floor.  One of these species actually eats the portuguese-man- of-war, a highly venomous animal (or rather, 4 types of animals working together) that is also related to jellyfish.  Pretty impressive, huh?

And I didn’t even get into the regenerating sea slugs or the sea slug that can photosynthesize!!  That’s right, the first animal ever found that can make food using only sunlight, like a plant.  (Note that it is not simply storing algae in its body and stealing the food the algae is making.  This is true photosynthesis.)

If you’re interested in learning more about sea slugs, the Australian Museum has this wonderful forum to start you off.