This is the one everybody knows about and the feeding mode with which most people are most familiar. Many sea urchins prefer kelp and various other seaweeds and marine "plants." There are several species found in cold-temperate water habitats.. California, New Zealand, Chile.. to name a few, and all these places have sea urchin species in abundance.
Here is the famous purple sea urchin, Strongylocentrotus purpuratus engaged in some kelp feeding!
|Image by Todd Jackowski
Here is a nice Shape of Life video that shows feeding by the Purple sea urchin Strongylocentrotus purpuratus.
Echinoderms: Urchin Time-lapse: Eating Kelp from Shape of Life on Vimeo.
Most people (even several biologists) don't realize that sea urchins can be pretty flexible in their diet. If kelp isn't available, they will obtain whatever nutrition happens to be available.
Some large examples of food here, but feeding also includes microalgae (such as diatoms), encrusting algae, moss animals (i.e. bryozoans) and etc.
Here we have what Strongylocentrotus spp. in the Arctic or sub Arctic feeding on what looks like a wayward or moribund jellyfish..
|Image by Alexander Semenov
Most people don't think of sea urchins as aggressively chasing down and pursuing ACTIVE prey...
Probably one of the most dramatic deep-sea/paleontology events in the last few years was the discovery that, not only could stalked crinoids (echinoderms with a ring of feeding tentacles and a stalk) crawl BUT they did so with some urgency!
It turns out that they were running from Cidaroid SEA URCHINS. I've written up these papers here.
|Calocidaris -a cidaroid urchin. Image by D. Pawson
WARNING! Those who do not wish to watch crinoids being brutalized and devoured should avert their eyes!
Further evidence on deep-sea crinoid skeletal pieces is here. That notch is a scar left over from where an Aristotle's Lantern has gotten to this animal...
|Urchin bite on a crinoid stem by T. Baumiller
2. Deposit Feeding/Sediment Feeding
Whew! That was quite a violent section for sea urchins, wasn't it??? Let's go to something a bit more majestic!
One of the NEATEST stories in sea urchin evolution is how a major sub group, the "irregularia" aka the sand dollars, sea biscuits, and spatangoid ('heart urchins') sea urchins all evolved from a more open lifestyle with the feeding modes shown above (herbivory, predation, omnivory) to a specialized series of body forms that involve plowing through sediment/mud/sand in order to obtain food. Deposit or sediment feeding. A more involved post of this story can be found in this post.
Feeding in these animals is intrinsically connected with their life mode and body shape. Here are some videos that show some of these animals plowing through sand...sometimes just to get around but maybe also to feed?
Japanese sand dollar plowing through sand...but check out the food going to the mouth at 1:05
Note all the spines moving through the sediment...
Again.. check out the spines!
1. Filter/Suspension Feeding
So, now that I just got done telling you that sand dollars and their relatives are deposit feeders.. I will immediately point out the exception! Perhaps the most UNUSUAL feeding mode in sea urchins is filter feeding, i.e., obtaining food from water currents using some kind of sieve or screen.
Urchin morphology tends to be...counterproductive where this sort of feeding is concerned.. Except in TWO unusual examples.....
Dendraster excentricus-is the so-called "Eccentric Sand Dollar"which lives along the west coast of North America. So named for the very erratic pattern of its feeding grooves on the oral surface.
|Image by fiveinchpixie
|Image by Peter_r
Dendraster uses its tube feet, pedicellariae and spines to pass along food caught from the water currents to the mouth.. More info on this species to be found here.. This is a pretty commonly encountered animal, but really when you look at them in this fashion, they are freaky deaky!
Dermechinus horridus is a strange deep-sea sea urchin which has a body, literally shaped like a cactus, with sharp, needle-like spines to boot! It is among the oddest of the sea urchins known.
|Image by NIWA
Here is some of the FIRST available video of this species in its natural habitat occurring next to brisingids, which are suspension feeding asteroids...
And although I don't wish to steal his thunder, let's just say that info from a recent International Echinoderm Conference presents some intriguing possibilities!
|Image by NIWA