Monday, December 7, 2009

The Echinoderm Christmas Tree?? Antarctic Cidaroid Sea Urchins!!!

Today, another SPECIAL Holiday-Themed Echinoblog!!! Allow me to explain!

I was just thinking about this:
Are there any kinds of echinoderms that are kinda like Christmas trees? They originate in cold places and you hang all sorts of weird crap on their branches???


Antarctic Cidaroid SEA URCHINS!!

Most people by now are probably saying "Okay, I'm not sure I understand a LOT of those words you just said"

So here's the Breakdown: There's sea urchins that live in Antarctica (and elsewhere-mostly in the deep-sea) that belong to a group known as the Cidaridae.

Cidaroid sea urchins are one of the more prominent groups found in the cold waters of the Antarctic and do some odd things. But what makes them of interest here is the LACK of skin on their spines!

"What? I didn't know sea urchins had skin on their spines????"

YES. Here is close up pic of a sea urchins spine. Notice the blue line around it?? ALL echinoderms, including sea urchins are actually COVERED by a thin, ciliated epidermis.

This epidermis is the primary reason most echinoderms you see don't get covered in encrusting organisms like algae or small grit and how stuff seems to flow over their body surface.
ALL groups of sea urchins have this epidermis EXCEPT for one group! The Cidaroids. Strangely enough, cidaroid sea urchins LACK a layer of epidermal covering on their spines!!!

As a result, some cidaroid sea urchin spines have become the substrate for encrusting animals that settle as larvae and cover over the spines!!

Thus, the sea urchin becomes a kind of host for a huge diversity of organisms! (shown here-and below is Austrocidaris)
What kinds of animals hang off these sea urchin spines the same way that lights and holly balls hang off a Christmas tree branch??
Here we got serpulid worm tubes of various sizes
....AND we got SPONGES that form around the spines, in and around the worm tubes!!

In addition to those-other encrusting animals recorded living on these spines include bryozoans, hydroids, small crustaceans, sea cucumbers (!), bivalves and foraminifera!

Multiple kinds of cidaroid sea urchins are known to host these many animal "decorations"! The one shown here is Austrocidaris, but others include (but are not limited to) Rhynchocidaris and Ctenocidaris.

But these "decorations" do MORE then just give the urchin that fine finished look!!! They can actually AFFECT the diversity of the area around it!!!

One study by Heterier et al. which focused on cidaroid sea urchins in the Weddel Sea found that the presence of these sea urchins actually PROMOTED higher species richness of these encrusting animals and increased their overall abundance!!

How does THAT work??
It turns out that the sea urchin spine surface is a ATTRACTIVE surface for encrusting animals to live!!

Most of these encrusting animals are filter feeders (i.e., they pick food out of the water) and living on sea urchin spines lifts them into the water water column and off the gritty, dirty bottom! Its all about prime real estate baby!

So, it is hypothesized that these different species of encrusting critters are actually MORE EFFECTIVE at selectively locating sea urchin spines to settle upon then on general rocky bottoms! These species become more "specialized" in finding a place to live.
BUT, the more "generalized" encrusting animals are left to settle on rocks in the surrounding area. So, the presence of urchins AND rocks apparently separates different SPECIES of these animals!

However, it does not alter the actual composition of overall diversity (i.e., many different kinds of foraminiferan species may settle but not different phyla of organisms).

So, the "decorations" on some of these cidaroid sea urchins can change (however subtly) the composition and richness of its surroundings !!!


Can a "normal" Christmas tree do that?
You decide.

Happy Holidays from the Echinoblog!

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