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.