One of the simplest and most efficient methods for qualitative analysis involves the use of cross-polarized light (Johnson 1995). Bivalve larvae are one of the few objects found in plankton samples that are able to refract light in two slightly different directions to form two rays (birefringent). Larvae are birefringent due to the crystalline calcite structure of their shells and glow as bright spots under polarized light.
Photographs courtesy of Dr. Ladd Johnson (University of California, Santa Barbara) and Renata Claudi (Ontario Hydro, Canada). The photograph on the left shows the zebra mussel veliger without the assistance of cross-polarized light. The photograph on the right shows it with the assistance of cross-polarized light.
Because of the arrangement of the calcite crystals, portions of the shell in line with the axes of the cross polarizing filters do not reflect the light and thus the veligers appear with small glowing "Maltese" crosses. Cross-polarized light will distinguish between items found in plankton samples except Corbicula larvae, larvae of other dreissenids, and ostracods. These organisms must be distinguished on the basis of morphology, behavior, size, shape, or other features. For more information, see the Larval Identification section.
The use of cross -polarized light requires a substage light on the microscope. One polarizing filter is attached to the microscope objective lens and another is placed between the substage light and the sample. To view, rotate the upper polarizing filter until the background is dark.