Monday, March 26, 2012

Shape Shifters of the Sea!

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.


  1. 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.

  2. This 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?

  3. Very interesting. I always forget that they are an actual organism and not just the shell we hope to find on the beach!

  4. During 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!

  5. To 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.

    To 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.