WHAT is going ON HERE? A galloping herd of urchin! CRAZY Maretia planulata heart urchin aggregation!

Every once in awhile, I think that I might finally have run out of stuff to share with people and THEN something magic happens. Some fantastic new video or pic pops up and WOO! The diversity of echinoderms and the infinite resourcefulness of the Internet pop out of nowhere with some magic NEW biology to share!!

Case in point is this video, which was shot by "Dive Yos" showing various cool inverts spotted during a dive in Bali. The video was posted 2 months ago...

It starts with a few typical things, a sea anemone, sea horses, etc. a lot of typical fare...but then we see one, two, four heart urchins, six... and then at 0:27 into the video?  BOOOM!!! You got this HUGE heart urchin stampede!!!

Best as I can tell, these are the heart urchin, Maretia planulata, described by Lamarck in 1816. This species of urchin lives throughout the tropical Indo-Pacific, from Tanazania to the Marshall islands...

They live in relatively shallow water on the surface of sandy bottoms.

Here's what one individually looks like.

Heart urchins are "irregular urchins" which are closely related to sand dollars. I wrote up a post on a related animal called Echinocardium, which you can find here. 





But WOW! What is going on with this HUGE aggregation??

Here's a Flickr shot from 2008

As I've written before, other "regular" urchins sometimes form "urchin barrens" when the ecological balance of a particular region is out of whack and you have TOO many of them eating EVERYTHING in sight...

I have no idea if this huge "herd" of urchins is "natural" or not..

But the thing is, Maretia is a heart urchin (aka a spatangoid). They are sediment feeders, so they don't really eat kelp.  I suppose the absence of some predator might be the cause. And the huge numbers could STILL deplete food in a given region, but this high abundance seems to be a regular thing.

There are other websites which have also observed that they occur in high numbers like this... Such as this one displaying this aggregation.

Photo by Geoffrey Bertrand, on his website

The most I could find on the literature about this 'herd' of urchins was from 1986. Thomas Suchanek and Patrick Colin in the Bulletin of Marine Science , vol. 38(1): 25-34, noted that this species was abundant reaching 100-200 per square meter!!!  and that they processed "massive amounts of sediment"

I would love to know more about whatever is going on here. Or maybe this will be the beginning of someone's Phd thesis? Don't know. But the thought of 100s (thousands?) of these things galloping along the bottom of the Indo-Pacific sandy bottoms is just... farking AMAZING!