The Sand Dollar
It is typical that a juvenile or baby will look like its
parents, at least to some degree. However, this isn't always the case, and it certainly is not the case with the sand dollar. Everybody knows what a sand dollar looks like. They are disc
shaped with the little star or flower shape in the middle. They are flat on the bottom
because they are bottom dwellers.
The sand dollar larvae however, are very different. They are
“shuttlecock” shaped (shuttlecock is like the shape of a 'birdie' from bad-mitten) and float around freely in the water. They are planktonic and freely drift with the water currents. As the larvae develop, they will
actually begin to grow some arms.
They begin with four and will end up with eight. By the time
that they have a total of eight, they have found their place on the seabed
where they being their life as bottom dwellers. This method of development,
although it may be strange, is not uncommon to find in many other organism. The
question then is, why? What is the purpose and reasoning behind this odd method
of development? To get a better understanding of this, studies were done on the
swimming patterns of the different stages of larval development of the sand
dollar.
It is known that they live in turbulent waters. They thought
that maybe the shape of the sand dollar larva help save them from the fate of
other types of larva in these types of waters. Commonly, larvae will get stuck
in vertically moving water and are forced horizontally and eventually are
sucked down. It was predicted that the sand dollar larva’s unique shape could
help them into up-welling water instead of being sucked down.
To test and study this method, computer simulations of the
movements of all three larval life stages were down. Then, real larvae bobbing
about in a tank were filmed and were compared to the simulations. Doing this, allowed
them to determine how all three life stages moved. It was found that the larvae
went into up-welling flows in mild turbulence like predicted. However,
as the turbulence of the water increased, it was discovered that the larvae
with four and eight arms were forced horizontally until they became trapped in down
flow of the water. It was the larva with six arms that was drawn toward
up-welling flows.
Even though differences were expected, these differences
were much more extreme. The drastic differences in the swimming behavior of the
different stages of larva indicated that their strange shapes do indeed change
the ways in which they swim. It can allow them to choose where they swim to in
the water column depending on their stage of development.
It is apparent that there is a significant reason as to why
this organism has such a different method of larval development. The point is
so that the larvae can choose where they travel to in the water column. Maybe different swimming behaviors are better at different stages to ensure an optimal spot to settle down on
the on the seafloor to live the remainder of their lives as bottom dwellers.
Clay, T. W. and Grünbaum, D. (2010). Morphology–flow
interactions lead to stage-selective vertical transport of larval sand dollars
in shear flow. J. Exp. Biol. 213, 1281-1292.
http://jeb.biologists.org/content/213/8/i.2.full.pdf+html?sid=4d95b461-22db-4c6b-a655-a4f584ab9ab6http://biologyblog.edublogs.org/2008/03/22/creatures-clone-selves-in-face-of-danger/
WOW! This is a very interesting study. To think that sand dollars can swim seems to be odd, yet they choose where they settle. I find that very unique for this animal because it is a bottom dweller. Also poses the idea of reproduction with these creatures as well. How many larvae are produced at one time? Do they all to swim off to different directions? This post has me really curious to learn more about these animals.
ReplyDeleteThis is so interesting! I have seen so many adult sand dollars on vacations, but I had never once thought about its larval stages and any developmental stage for that matter. It is very neat that they have the ability to swim at a stage and can choose where they would like to settle. Do you know how long it takes them to settle?
ReplyDeleteVery interesting. I always forget that they are an actual organism and not just the shell we hope to find on the beach!
ReplyDeleteDuring spring break last year in Tybee Island Georgia, there was a crazy amount of sand dollars on the beach. I have been to several different beaches in my lifetime and at different times of the year and I had never seen anywhere near the amount I saw. Did you come across anything about their abundance in certain areas at certain times of the year? And it is awesome to collect some and study their little leg like structures on the bottom side. When you let me sit out of the water for a little (but not to dry out) and touch their undersides, their leg-like structures harden. Its very cool!
ReplyDeleteTo Katelyn, I'm not too sure if they all swim off and go in the same direction. They might not since they usually start off as free swimming larva that float freely with the currents. Also, I'm also not sure of the number of eggs produced each time, but I did read somewhere that the female can produced around 350,000 eggs each year.
ReplyDeleteTo Kelly, I do not know and exact time, but I do know that they begin dropping and settling after their hard skeleton begins to form. Usually, they will begin as free swimming larvae and go through several stages before their skeleton forms.
To Angie, I'm sorry to say that I did not come across anything on the abundance of sand dollars in different areas.