Welcome to Echinoderms, Part 2! Who's excited! I am because the two classes we're talking about today are awesome (just like the rest of them :P ). Today's topics are brittle stars and sea cucumbers! Just like what we talked about in
Part 1, these guys are part of the Echinoderm phylum. If you haven't seen Part 1 you should! I covered Starfish in there and they are some freaky creatures!
So let's just get started, shall we?!?!?
Brittle Stars / Basket Stars - Ophiuroidea
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The central disc of brittle stars are reminiscent of starfish.
Source |
Brittle stars and basket stars are very similar to starfish. They have the same general shape, just a bit more lengthy, and they have tube feet, just like starfish. But they are very different in their own ways.
To begin, brittle stars are just a different version of starfish. They only have 5 arms and have the same body shape. Only.... they don't. A central disc makes up their body and their arms are snake-like extensions. Unlike starfish, the internal organs of brittle stars are compressed into the central disc and have a one way digestive system, meaning they don't have an anus and that food passes in and out of the mouth (gross right?). Also, their madreporite, which allows them to suck in water for their circulatory system, is found on the underside of their body.
Their arms, instead of being solid and stiff, are segmented by plates and are extremely flexible. Small ventral plates make up their arms, connecting their internal organs (the same set up as starfish) and allowing their tube feet to exchange water to attach to surfaces and food. These arms can be voluntarily dropped at a moments notice just by disconnecting the plates at a specific point in order to avoid predators or get themselves out of a sticky situation. These plates can then be grown back through regeneration. On top of voluntarily dropping their arms, the segmented structure makes brittle stars the fastest echinoderms out there. Each plate has a spine on each side, allowing them to grip onto surfaces better than tube feet alone would. This gives them snake-like movement through the water, which fits their class, "Ophiuroidea", which means "snake-like".
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A basket star wrapping it's branched arms around a
coral colony. Source |
Now, for Basket Stars. They are similar to brittle stars only.... their 5 arms branch like crazy, which is where they get their name. Their exceedingly complicated network of branched arms creates as "basket" of sorts in order to catch prey. Instead of having exposed ventral plates like brittle stars, their body is covered in a thin leathery skin. This allows them to grip surfaces and catch prey better. This skin is extremely thin and is easy to rip off of them to expose their ventral plates beneath.
Basket stars only come out at night to catch prey and can look pretty creepy. When "sleeping", they curl up their tentacle like arms and can easily be mistaken for some sort of growth on rocks or coral (pictured over there << ). But when unfurled in the current, they look monstrous. If you came upon one of those on a night dive, you'd probably be in stunned aw at the size it could be come. They climb on top of corals or rocks or anything really that gets them into the gentle currents of the water and unroll their crazy tentacle arms, swaying in the water to catch plankton or small fish and crustaceans.
So... If you're ever go night diving... Be careful of these creepy cool creatures!
Sea Cucumbers - Holothuroidea
Sea cucumbers are what I consider to be the "Redheaded step child" of the Echinoderm phylum. Why is this? Well, they really don't look like a starfish... or a brittle star... or a sea urchin.... or anything really. And if you just look at it, it doesn't seem to have the same symmetry as the rest of the phylum, which it doesn't. Instead of being radially symmetrical, sea cucumbers are bilaterally symmetrical. This means that they can only be equally split in half lengthwise (like you would a lobster).
Sea cucumbers are cylindrical in shape and have tube feet around their bodies in rows, multiples of fives like the rest of the echinoderms. But! In addition to that, they have these weird tentacles around their mouth that are actually modified tube feet that are used to catch food and shove it into their mouths. Pretty ingenious for an animal that looks and feels like a slug without a shell.
In terms of defense, these guys have some weird ways of doing it. When moved, picked up, or taken out of the water, sea cucumbers do this weird thing with their molecular structure to make them feel like... well, goo. Their whole body goes limp and becomes this gelatinous mess. This action discourages any attack because there's really not much to attack. Plus it feels really really weird in your hands. Now, when they feel there is danger about, they spit out their insides. It's called self-evisceration. Poisonous threads that sit in their digestion tract are expelled to ward off would be attackers and allow the sea cucumber to slowly wiggle away! These threads can be regenerated over time so that they can do it all over again. So... just imagine fending off a bully by pooping threads out of your bum..... Weird right?
I hope you've enjoyed this weeks Marine Biology Facts! Come back next week for more! And if you have any questions or comments, please, leave them below!
Tata for now!
Information:
http://brittlestars.weebly.com/fun-facts.html
http://www.starfish.ch/reef/echinoderms.html
http://www.realmonstrosities.com/2012/04/basket-star.html
http://www.starfish.ch/reef/echinoderms.html
Awesome Sam! Love it!
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