So, when last we left the Echinoblog......We looked at Sand Dollars (highly specialized sea urchins in the order Clypeasteroida) with lunules (i.e., the holes or notches in their body) and how those lunules were used as mechanisms to prevent sand dollars from being picked up and washed/"blown" away by the water currents around them.
But if you don't have lunules, then how do you cope?
Answer: Get a WEIGHT BELT!
This part of the story begins in 1973 (I would have only been 3 years old at the time!) with a paper in Science by famous echinoderm biologist Fu-Shiang Chia which is most simply summarized by their abstract:Juvenile sand dollars (Dendraster excentricus) selectively ingest heavy sand grains from the substrate and store them in an intestinal diverticulum which may function as a weight belt, assisting the young animal to remain in the shifting sandy environment. The sand disappears from the diverticulum when the animal reaches the length of 30 millimeters.That's pretty much as simply amazing as it sounds (Plus, how often do sand dollar papers make it into Science?).
Several of these "un-lunuled" sand dollars apparently have a very specialized intestine which has a bunch of tubes and pouches in the peripheral part of the animal's body.
In, at least some species, this part of the intestine is filled with selectively heavier sand grains!
(Dendraster excentricus from SFSU biblio page)
(Image from Geology.com)
However, Chia's study focused on juveniles and it wasn't clear what the ramifications were for OTHER sand dollars or for that matter for adult Dendraster!
Weight Belts & the Evolution of Sand Dollars!
In 1996, a neat paper by Rich Mooi and Chang-Po Chen was published in the Bulletin of Marine Science which surveyed weight belts across the many different types of sand dollars.How many sand dollar taxa have a "weight belt"?
Was having a weight belt more of an incidental evolutionary response?
Or were these actually a broad phylogenetic character event? or Something that evolved across a whole GROUP of sand dollars?
It turns out that a WHOLE bunch of sand dollars have weight belts! The black in the figure above shows where sand-filled weight belts were present. Incidentally, it turns out that SOME sand dollars with lunules DO have weight belts (although they seem more weakly developed to me...)
Also, as it turns out, weight belts are a unique feature (i.e., a synapomorphy) of a major clade (i.e., evolutionary group) of sand dollars called the Scutellina, which includes not only 'un-lunuled" but also the "lunuled" sand dollars.
As it turns out, the weight belt is strongly expressed in juveniles of both types of lunuled and "non-lunuled" taxa..So it seems that weight belts start out as a mechanism for keeping baby sand dollars from getting 'washed away'...
Unfortunately, there was is not yet data for a clear cut conclusion of how much weight belts compensate for resisting hydrodynamic forces in "non-lunuled" sand dollars vs. "lunuled" forms
but expression of weight belts IS an important factor in different species.
For example, in Dendraster , the shallow-water species, D. excentricus, which lives in high-energy environments retains its weight belt at GREATER sizes then does D. laevis, a deeper-water form that would presumably not be as vulnerable to high-energy currents.
So, two things help keep sand dollars stabilized where they live
- lunules that essentially break up the hydrodynamic flow and
- weight belts that keep the body forms stabilized and "weighted" down on the sandy bottom.
Maybe I should stuff my bank accounts with sand...
ReplyDeleteThank you for yet another enlightening post about my favourite animal!
ReplyDeleteWe shore nature guides have a corny joke about sand dollars: "Where's the best place to keep your sand dollars safe?"
"In the sand bank!" we would announce proudly, to groans.
Now it would seem, the bank is inside the dollar. WOW!
Why would lunules be restricted to tropical species? Surely tropical tidal flow isn't that different from northern waters? There must be some secondary advantage, maybe a greater opportunity for tropical sand dollars to collect food particles from the water, not so much needed in nutrient rich northern waters. Just my two cents, anyway.
ReplyDeletePossibly..there could also just be phylogenetic/biogeographic constraints. Or it could be constraints on calcification of the larvae that led to how they form?
ReplyDeleteSand dollars are neat!
Interesting topic - our Bio 355 class (Dr Rebecca Johnson) at SFSU was able to listen to a presentation by Rich Mooi today at CalAcademy - he mentioned the function of lunules but didnt mention weight belts. Is there a disadvantage to sand dollars being overweighted, resulting in them being anchored to the bottom?
ReplyDeleteErik
Have you seen the Pansy Shells found on South African beaches. If you break one open they have tiny shell birds inside them
ReplyDeleteI wonder if the tropical ones have lunules because the water is warmer so it flows differently. Since cold water is heavier and sinks, maybe the cold-water sand dollar would just ensure it was heavier. That cold water isn't going to try to rise anytime soon, not until it starts to warm anyway.
ReplyDelete