Amazing adaptation: Spiny Brittle Star can Amputate Own Arm!

Spiny Brittle Stars, Ophioptrix angulata, have an incredible adaptation: they can detach their own arms to elude predators!

Brittle stars look a little like worms lurking within the folds of the sponge, but when you look closely, you can see the central disk within the five arms.  They often move quickly deeper into the sponge as they associate movement and light with danger.  

Recently washed up Redbeard Sponge
Red beard sponge as it begins to fade

Every time I find a clump of red-beard sponge on the beach, I like to look and see what else may have washed up with it.  In the ocean, the sponges are bright red, but after they wash ashore, they begin to turn brown.  If the sponge is still red, there’s a good chance of finding tiny creatures living in there, like a spiny brittle star, a tricolor anemone, or a sea squirt, or hermit crabs.  There might even be a sea whip! In this clump, we find an invasive porcelain crab, a thin striped hermit, some tiny snails, a stone crab, some rubbery bryozoan, and more.  Be careful, though, the sponge can cause contact dermatitis in some people. 

Spiny Brittle Star

I always like to look for brittle stars in these sponges; they feed on the sponge as well as using it for habitat.  

These echinoderms have 5 incredibly flexible arms which look like bristly worms, radiating out from a central disc.  These arms with spines and tube feet can capture phytoplankton and microscopic debris for feeding.  Incredibly,  there are also sensors in the arms that can detect light, which enable the sea star to dive back into the sponge to escape the light.

Spiny Brittle Star

Sutonomy: Detaching a limb on purpose

Sometimes we put the bright sponges in our nature center touch tank to study what lives there, and that’s what I was attempting to do with the one on my hand in the top video.  I was stunned when the brittle star simply pulled away from its arm, leaving it writhing on my hand.

spiny brittle star detaching arm
spiny brittle star

This is an adaptation called autonomy, which allows an organism to self-amputate a limb.  The advantage of this is mostly to confuse predators, giving them a chance to escape.  We’ve talked about other organisms who can do this: some species of lizards and skinks and salamanders, spiders and wasps, stone crabs, and even some octopi.  In almost all cases, the amputated part will regenerate.  If part of the central disc is amputated in some sort of injury or accident, an entire new brittle star can regenerate.  

Be Careful When Observing

In this case, I learned to be much more careful when handling the red beard sponge.  I was surprised to learn it can cause contact dermatitis due to the production of crinotoxins. I also don’t want the poor bristle stars to be dropping limbs: even though they will regenerate, the star will be 20% less effective at locomotion and feeding than it would otherwise be. We’ll check these out near with water’s edge without handling them so the interior organisms stay safe.

Further reading

I have a couple of go-to books for stuff I find on the beach, like Living Beaches of Georgia and the Carolinas, Sex, Drugs, and Sea Slime, and the South Carolina Beachcombers Guide (DNR).  

And I have always been interested in regeneration after reading Madeleine L’Engle’s Arm of the Starfish, first published in 1965.