East Coast - Everything About A Sea Star Is Bizarre
It took way more effort than I initially expected, but at long last I found one of the most iconic creatures of the Atlantic coast: a sea star, or starfish. They’re a common, prolific species, but they - among their relatives the echinoderms - are anything but ordinary. Today we’re going to see exactly how strange sea stars are by focusing in on just one little bit of them: their incredible tube feet.
EPISODE NOTES
No joke about how hard it was to find a sea star. I think I had amalgamated every tide pool I visited as a kid on the east coast into one urpool which was filled to the brim with urchins, anemones, sculpins, eel pouts, hermit crabs, regular crabs, snails, whelks, limpets, chitons and, yes, sea stars. And while I did come across some pretty magical tide pools in my time there, none of them quite hit the fantasy version I had in my head. I’m very grateful to the one little sea star in this video. It really feels like it got plopped down by a deity who had started feeling really bad for me.
TRANSCRIPT
Across the sandy beaches and rocky shores of Nova Scotia, I was determined to find one of the Atlantic Coast’s most iconic critters.
From my memories of growing up here I’d convinced myself that they were hiding under every rock, plastering the sides of every tide pool. In reality, it took days of dedicated searching across multiple beaches until finally, in one unassuming tiny shallow sandy little puddle of a pool, I managed to find exactly one… starfish.
Or as scientists would prefer we call them: sea stars. That’s to make absolutely clear that this is not a fish. In fact it’s not much like any other animal on earth.
Sea stars belong to the class Asteroidea or, wait for it: ‘star-like’ - in the phylum Echinodermata, which in turn means ‘spiny skin’. Same group of animals as sea urchins, sand dollars and sea cucumbers. They might not look like they have much in common but they all exhibit pentaradial symmetry - a five-sided, or, star-like body plan that is unique to echinoderms.
And all of them are… well, they’re weird. They’re very weird. Even the common everyday Atlantic Sea Star.
I mean, for a start we could talk about how they can just ditch one of their five limbs as a defensive measure and then regrow them or, even better, grow an entire body back from a detached limb.
We could talk about how they eat: by ejecting their stomachs out to envelop their prey, digesting them, then sucking their stomachs back in.
We could talk about how they don’t have brains. Or how they use filtered seawater in place of blood.
But to give you a real idea of how fascinating every single aspect of the sea star is, why don’t we zoom way in on just… their feet.
See, one thing that sets sea stars apart from other echinoderms is their skeleton. Sea urchins and sand dollars have fused bony plates that make them less mobile. Sea cucumbers’ skeletons have been reduced to microscopic ‘ossicles’ of calcium carbonate.
But sea stars have bony plates that function like flexible joints, so they get protection and mobility - thanks to hundreds and hundreds of these.
Each of these little tube feet is a little miracle, doing multiple jobs for the sea star in some unexpected ways.
For one thing, they don’t directly work on muscle power. They run on hydrostatic pressure. The sea star forces water into the tube to expand and extend it, and relaxes to retract it. Those simple actions, the feet on the leading edge expanding as the feet on the trailing edge retract, performed in waves over hundreds of feet, propel the sea star over the ocean floor at a blistering clip of about 15 centimetres a minute.
And those feet don’t just provide locomotion, but adhesion. Sea stars can climb up rocks just as easy as moving across sand. But the ‘grab and release’ action they use there is unusual as well.
Lots of animals who can climb walls - maybe most famously geckos - do so using physical structures. Gecko foot pads have millions of tiny hairs to give them their grip. It’s the same deal with insects.
But sea stars, naturally, just have to do things the weird way. Every single time every single one of those feet reaches out, it’s doing a rapid chemical gluing, and ungluing. The foot secretes an adhesive substance to stick it to the surface. And when it’s time to let go, it secretes a second substance to break the adhesive bonds and free the foot.
And those feet ain’t just for walkin’ - or climbin’. Turns out they’re for eatin’ too! Sea stars love feeding on bivalves, like mussels, clams and scallops. And it’s the same strength of that chemical adhesive on their feet that lets them split open the shells to get at the meat inside.
And if that’s not enough for ya, they even respire through their feet, with the thin membrane at the tips allowing for gas exchange.
Just like every other part of the sea star, a miraculously weird set of adaptations.
There are more than 2,000 species of sea star worldwide, from the intertidal to 6,000m down in the abyss, from polar regions to the tropics. A new species was just discovered in Japan.
And in the Atlantic Coast’s littoral environment, they’re what’s known as a ‘keystone species’ - so crucial to the proper functioning of the food web that their presence or absence alone is a great indicator of the health of the whole ecosystem.
So, weird or not - whatever it is they’re doing - boy is it ever working.