COLLECTION OF FOOD BY ORAL SURFACE PODIA IN THE SAND DOLLAR, ECHINARACHNIUS PARMA (LAMARCK)

¹ Department of Zoology, University of Toronto, Toronto, Ontario, Canada M5S 1A1

In Echinarachnius parma all spine types have cilia arranged in two bands along the shaft. Ciliary currents flow perpendicular to these bands and reversals were not observed. On the aboral surface the bands of cilia were oriented perpendicular to lines radiating from the apex. Flow visualization using dyes and particles showed that aboral currents flow radially towards the ambitus. In contrast, on the oral surface, currents flow from anterior to posterior and the bands of cilia are arranged at right angles to this axis. Oral surface ciliary currents do not carry particles to the mouth nor to the food grooves. At least 80% of the particles carried over the aboral surface are lost at the ambitus. Only particles < 20 µm, if any, pass around the ambitus and these were not seen to enter the food grooves.

Light microscope observations showed that oral surface and ambital podia continuously probe the substrate and draw particles towards the test. Particles are passed from podium to podium, to the food grooves, and thence to the mouth. The rate of particle collection and passage along the food grooves is much higher when concentrated diatoms are offered than with normal sand substrate. This oral surface activity has not been reported before in any sand dollar.

Echinarachnius parma discriminates against substrate particles larger than 230 µm in diameter, but does not especially select those less than 100 µm. Statistical analysis showed significantly fewer particles > 230 µm in the gut, when compared to natural sediment (P < 0.05). However, there was no significant difference in proportions of smaller particles. Thus there is no evidence that these sand dollars used an aboral sieving mechanism which would have concentrated particles < 100 µm from the substrate.

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