The ‘disco’ clam Ctenoides ales (Finlay, 1927) is an Indo-Pacific bivalve with an unusual reflective flashing display. The function of the flashing is unknown. Ctenoides ales is a suspension-feeder, so experiments were conducted to test the hypothesis that the flashing acts as a lure for planktonic prey items that are phototactic. First, the reaction of C. ales to a plankton stimulus was analyzed. Exposure to a concentrated plankton mix was followed by a significant increase in flash rate (Hz) for 5s, after which the rate returned to pre-stimulus levels. Next, two experiments were conducted to determine if potential prey (mixed phototactic plankton species <500µm, which were collected from the bivalves' habitat) were attracted to a simulation of C. ales flashing. The first experiment tested whether there was higher phototactic plankton density inside artificial clams with flashing lights than inside artificial clams that had constant lights, or lights that were turned off, but there were no significant differences between the three treatments. In the second experiment, a program played two illuminated lines on an electronic tablet screen that was exposed to experimental plankton. This second plankton experiment tested whether there was a higher phototactic plankton density collected over the line that flashed, mimicking the light spectra and frequency of C. ales, compared to the line that was constant. There was no significant difference in recorded densities between the two treatments. Plankton density was then measured inside the mantle cavity of C. ales in situ using SCUBA in Pemuteran, Indonesia (lat 8°17′ S, long 114°65′ E). Samples were taken twice daily to determine if phototactic plankton density was higher inside the clam when the flashing was visible (day) than when the flashing was not visible (dusk). Again there was no significant difference in the recorded plankton densities. Overall, the results from this study did not support the hypothesis that the flashing display of C. ales induced positive phototaxis in their planktonic prey.
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