Sunday, July 13, 2008

Asymmetry.

For several years I shared lab space with echinoderm biologists. I don’t believe there is a specific “ology” for echinoderms, and although not being directly involved in whatever my lab-mates were doing was called, I managed to learn a fair amount about echinoderms – specifically sand dollars and other irregular urchins – for they were everywhere. Every shelf, every countertop, every 10-gallon pail that competed for leg space beneath tables. A lot were pickled in ethanol, many were dried, and slid around in shallow boxes and trays. There were echinoid fragments everywhere, for sand dollar tests (what the dried shells are called ) are nature’s finest, miniature version of interlocking paving stones. Drop them, they shatter.

I learned the names: Echinarachnius, Mellita, Leodia, Laganum, Clypeaster, Arachnoides, Dendraster. I liked Leodia. I thought that if I ever had a daughter (a crazy dream, parenthood, at that age), I might name her Leodia. Thankfully, I forgot all about that when it actually happened.

I enjoyed learning about and looking at sand dollars. They are remarkable things, more mineral than animal, and with a fascinating, alien symmetry. Pentameric -- you can chop them into five identical parts – almost. There is only one anus, and it ends up near an edge, which foils the perfection for even the most symmetrical. In others, there is clearly an aspect of bilateral (side to side) symmetry that overlays the fiveness; they have a long axis and a short one.

Heck I’m taking a long time to get to this one: Dendraster excentricus, the sand dollar that lives in my back yard. (Or would, were my back yard Boundary Bay.) I remember them well, scattered on the lab lunch table, picking them up, idly looking at the patterns of plates and grooves on their top and bottom surfaces, and being intrigued by the very different secondary bilateral symmetry in this species – not only side-to-side, but also back-to-front. Back then, in the centre of the continent (Toronto), I doubt I would have believed that one day I would be able to get up, take a short drive, walk out into the ocean and pick up a live one, accompanied by a daughter named Leodia.

But see how unpredictable life is. Here I am, and here I did, albeit with my son. Leodia was not thrilled with the plan: “It’s muddy!” so stayed with mom.



Dendraster excentricus, amid the eel grass.

In Boundary Bay, at low tide, you have to walk about a kilometre out on the mud/sandflat to find the eel grass beds that contain Dendraster excentricus. First you spy the dead, bleached tests, and then you start finding live ones.

About the symmetry, you see it best in dead tests:


Dendraster excentricus, test, aboral surface.

This is the top, the aboral surface. Not that the hump where the petalloids (which are the rows of pores where the respiratory tube feed emerge on the surface of the test) come together in not smack in the centre, but toward the bottom (in this orientation) of the test.

Dendraster excentricus, test, oral surface.

This is the bottom, the oral surface, with the mouth in the centre and the anus toward the edge. Notice that there is more test above the mouth than below it, and compare the arrangements of the branching grooves, which are where in life tube feet would be transporting, bucket-brigade-style, mucus and food particles (diatoms, etc.) toward the mouth. On the anus-side of the mouth (there must be a better way of putting that), the food grooves are well-defined. On the other side of the mouth, they sort of peter out.


Thanks to the staining, anoxic muck this test was buried in, you can see that the food grooves from the oral surface actually curl around the edge of the sand dollar onto the aboral surface. I’m not certain, but I think this is unique to this species.

There may be a functional relationship to this fore-and-aft disparity in food grooves related to a feeding behaviour in Dendraster. In certain conditions, groups of sand dollars (what is the collective term?) will partially burrow into the sand, so that one “end” (the end with the anus and the well-developed food grooves) is sticking up into the water column. The end with the poorly-defined food grooves is buried in the sand. The sand dollars may be reducing their surface area to prevent being blown away by the current, or they may be presenting a larger feeding surface to the water column. As the water flows over the standing sand dollars, eddies form and may carry more or different planktonic food to the surfaces, oral and aboral (there are, uniquely, food grooves on both).


Dendraster excentricus, live, somewhat nibbled.

This functional story returns to me as I hold the sand dollar tests -- as I held them at the lunch table when a student. A big difference is that now I can also hold the live ones, which is a thrill. A living sand dollar is made of velvet. They look and feel like it. I hand it to my son. “It feels like fur!” he says.

I’ll have to drag Leodia out to see them.


5 comments:

Anonymous said...

Very interesting piece on sand dollars. Boundary Bay is a great place. So much to see. But I can't say I've ever seen a sand dollar. So thanks for the blog.

Don

Wanderin' Weeta said...

I brought some home, dead, but still with the "fur" on. Trouble is, no matter how gingerly I handle them, by the time the dollar is completely dry, most of it has fallen off.

I found one in my collection (in a cookie jar) that still has a bit of fuzz. And I can see the food grooves on the oral side, and a few on the topside, on the short axis.

Thanks for the good explanation!

Hugh said...

Don, you have to go out quite far at a low tide to find them. Start at Centennial Beach, and walk toward Mt. Baker. They're in the deeper "troughs" about a Km out from the beach.

WW, Once they are dead and the epidermis that covers the spines decomposes and/or dries out, I don't think there's any way to keep the fur on. I remember there were spine-bearing ones in the lab, but they may have been fixed in formalin prior to drying.

A cookie jar is a very appropriate place to keep them.

matt said...

hey, have you seen the mucous filled sacs that are all over the place in boundary bay intertidal zone? they are about the size of a lime,... i have been trying to figure out what they are. i was thinking they were associated with lugworms, but now i think they are not.

Hugh said...

Matt, I had thought them associated with the lugworms too (a jelly sac where the larvae develop), but every description of lugworm life cycles I have found says that larvae emerge from the female's burrow and attach themselves to the surface in a "tube." No pictures. I wonder if instead they may be associated with the larvae of other polychaete worms, such as the ragworm, Nereis --although I'm not sure of the numbers and distribution of Nereis in Boundary Bay. Nereis would be my alternate suspect.