One of the simplest and most efficient methods for distinguishing between items found in planktonic samples involves the use of cross-polarized light. Bivalve larvae are one of the few birefringent objects found in plankton samples. Larvae are birefringent due to the crystalline calcite structure of the larval shell and they glow as bright spots under 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. Corbicula larvae, larvae of other dreissenids, and ostracods must be distinguished on the basis of morphology, behavior, size, shape, or other features. However, given the quantity of extraneous material that may be present in a plankton sample, cross-polarized light provides a simple way to narrow the range of possibilities.
Example of sample showing use of cross-polarized light
Zebra mussel veligers viewed with the assistance of cross-polarized light. 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 the larvae shell, and glow as bright spots under 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 all items found in the plankton sample except Corbicula larvae, larvae of other dreissenids, and ostracods.
Close-up of sample showing use of cross-polarized light
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).
In addition, this picture shows the relationship between the size of immature zebra mussels and other commonly collected organisms.
Size comparison of zebra mussel larvae with other common planktonic organisms. Sizes are approximate.
Notice the size relationship between the dreissenid larvae and other commonly collected planktonic organisms.