Sunday, October 19, 2014

Underwater Basket Weaving

Hey everyone, this week we're diving into a bit of a mystery. There's a pretty good chance that by now this little gem has come across your social media feed!

The original of that video had 8 million views on facebook alone last I checked. I've gotta say it's really cool that people are so curious about ocean animals. But the question on everyone's mind seems to be, as my mom succinctly put it: "What the heck is it?" Despite its facehuggerly appearance this is a native of the earth, or should I say the sea? What you're looking is a basket star. I should also mention that I'm not the first to identify this guy/gal. Both the Echinoblog, and IFLScience have tackled this mystery.

First off basket stars aren't actually a true sea star. You may remember from the post on catch connective tissue that sea stars are members of the echinoderm phylum. More specifically the sea stars we're most familiar with make up the asteroidea class (a class is one grouping more specific than a phylum) So if you're feeling pedantic and mischievous you can tell people you found tons of asteroids on the beach and not be lying. However the basket star is not an asteroid! Basket stars are part of a class of animals called ophiuroids (pronounced "off-yer-roids), and are more commonly called brittle stars.

 Jazz Hands!
Courtesy  Paul Thompson via Flickr

Even though most brittle stars look quite a bit like traditional sea stars, being in a separate class means they are as different from a true sea star as a sea urchin is. One of the most notable differences between sea stars and brittle stars is in how they get around. Sea stars use their hundreds of suction cup tube feet to grip tightly to the bottom and cruise along. Their rays (also referred to as arms) act as more of a platform for those strong tube feet to operate from. Brittle stars don't use their tube feet to walk. Instead they pick themselves up on their rays and stroll or slither like something out of the Nightmare before Christmas. Their tube feet lack suction cups and are used to grab food and help move it towards their mouth.

Basket stars are a really cool specialized group of brittle stars. They are well adapted for collecting plankton out of the water with their arms. In the above video you can only catch the view for a second, but at one point the basket star opens all its arms, and you can see the central disk. The disk is pentagonal and one trunk-like arm grows out of each side. Each of those five arms then branches dozens of times to create a wide net. The arms of the basket star are covered in microscopic hooks, a nice coating of mucus, and are capable of coiling around themselves to form traps that hold onto their planktonic prey. Below you can watch as some euphasiid shrimp are added to a basket star's tank at the Seattle Aquarium.

That video is a little sped up, but you can see how those branches form a wide net and are waved back and forth to sweep for more food. Grabbing food out of the water like this is called suspension feeding. Sometimes you'll hear it called filter feeding, but that's a bit different. When there isn't an obvious load of plankton around them, basket stars usually cling to a hard surface or the branches of corals. They curl their rays up above their bodies into the current forming a basket shape. Hence the name.

What a basket ca...I'm not even gonna let myself finish that joke
By Peter Southwood (Own work) [CC-BY-SA-3.0 (], via Wikimedia Commons

Once a basket star has enough food trapped on one of their rays they'll slowly move it towards their star shaped mouth. Incidentally brittle stars don't have an anus, so they excrete their waste through the same hole they consume food. Anyway inside the mouth are five sets of comb-like teeth. The star slides its arms over the teeth and the prey are scraped off like frosting from a fork. Am I the only one who does that? I can't be the only one who does that.

Basket stars are found throughout the world from shallow water to the abyssal plane. The one from the original video is probably Euryale aspera which is a shallow living basket star found throughout the Indian ocean and tropical western Pacific. One of the things I think is coolest about basket stars is that they seem to have a strong association with a variety of coral species. Not only do coral branches make a good holding place for adult basket stars, they may even be an important nursery for juveniles. Young of the species most commonly found around N. America, Gorgonocephalus eucnemis, are usually found living just inside the polyps of the sea strawberry coral (Gersemia spp.). While this seems to be some type of symbiotic relationship, it isn't entirely clear if the little basket stars are stealing food from the polyp they're living on, or just using their mouth as a platform to feed from.


Stöhr, S., O’hara, T., & Thuy, B. (March 2nd 2012) “Global Diversity of Brittle Stars (Echinodermata: Ophiuroidea)” PLOS ONE DOI: 10.1371/journal.pone.0031940,
Accessed via

"What is that weird thing on facebook???" The Echinoblog

Gorgonocephalus eucnemis” Encyclopedia of Life,

"Gersemia” Encyclopedia of Life, 

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