Neon Shuttlecock

What am I looking at?  

This is the skeleton of a sand dollar larva under cross-polarized illumination. The skeleton is made of a single crystal of calcite, which is optically active, meaning it can rotate a plane of polarized light to the right or left. This change in the plane of polarized light results in the colors visible in this image.

Biology in the background  

Sand dollars, also called sea biscuits, are classified as echinoderms, which means they are related to other marine invertebrates like sea stars (also called starfish) and sea urchins. The larval stage of these animals is free-swimming, and the long spikes (1) that you can see clearly in this image help with locomotion, sensing, and protection against predators.

As adults, sand dollars settle on the sea floor and grow a hard endoskeleton (inside their body) in the shape of a disk that protects their body. They can be found all around the world, mostly in shallow coastal waters – although some species live in deeper waters farther from the coastline. They eat by using small spines and tube-like “feet” on the bottom of their bodies to comb through the sand for food particles that they guide to their mouth at the center of their bodies.

A sand dollar larva can grow up to about 600 micrometers, or roughly eight times longer than the width of a human hair. Adult sand dollars can grow up to 30 centimeters, or roughly 11 inches long.

Technique  

 This image was created using polarized light microscopy.