Most youngsters look a lot like their parents. They have the same number of arms and legs in roughly the same arrangement – but not sand dollar larvae. While their parents are disc shaped bottom dwellers, the shuttlecock shaped larvae bob around freely in the ocean. ‘They are plankton that drift with the currents,’ explains Tansy Clay from the University of Washington. Over the course of their development the larvae grow additional arms. Starting off with four, they develop another two and finally a total of eight before eventually settling on the seabed. And sand dollars are not the only larvae to take this strange shape. The larvae of several unrelated species are also shaped like shuttlecocks. ‘So what is the purpose of this crazy shape and why would different organisms independently evolve it?’ puzzles Clay. Intrigued by the larvae's bizarre appearance, Clay and her supervisor, Daniel Grünbaum, decided to find out how the different larval stages swim. Knowing that their coastal water habitats are turbulent, the duo decided to focus on how the larvae fare in the flows in turbulent eddies (p. ).
According to Clay, previous attempts to understand how plankton move in turbulent eddies had represented them as simple spheres and ellipsoids. The calculations suggested that rounded plankton tilt in vertically circulating water so that they begin travelling horizontally toward downwelling water and are sucked down. Would the shuttlechock-shaped larvae suffer the same fate or would their strange morphology direct them into up-welling water?
The duo took a two-pronged attack: running computer simulations of the movements of all three larval life stages; and filming real larvae bobbing about in a tank. Basing the computer simulations on model larvae built from cylinders, Clay calculated how all three-life stages moved and found that they were directed into up-welling flows in mild turbulence found in a calm estuary. But as the turbulence increased, Clay's calculations suggested that the four- and eight-armed larvae tilted and swam horizontally until they became trapped in downward plumes of water, while the intermediate six-armed larvae were tilted and drawn toward up-welling flows.
Clay admits that she was surprised that the differences in the larvae's shapes had such a significant effect on their calculated movements. ‘I expected some differences but not that extreme,’ she says.
Turning to live larvae bobbing about in tanks, Clay generated vertical water flows in the middle of the tank by warming one side of the chamber and cooling the opposite side while she filmed the larvae's movements in the central portion. Tracking the fluid movements with algal cells, Clay could see that the four- and eight-armed larvae behaved as her simulations had predicted, travelling horizontally until they were engulfed in a plume of down-welling water. But when she looked at the six-armed larvae, they were doing something completely different: they didn't move horizontally like the simulations. ‘It seems that they were resisting horizontal movement towards down-welling water,’ says Clay.
So the larvae's strange shapes do change the ways in which they swim and could allow them to move selectively to different locations within the water column at different stages of development.
