A Juvenile Crab in the Spotlight

What am I looking at?

This is a juvenile crab collected off the coast of Massachusetts, in the eastern United States. Under ultraviolet (UV) light, different parts of a crab shine in different colors due to the presence of naturally occurring fluorescent molecules in its body – a process is called autofluorescence. Its eyes, mouthparts, and front and back legs appear blue (1) because these body parts are low in chitin, the hard substance that makes up a crab’s shell. The parts of this crab that have a more developed shell and thus more chitin, such as its back and its middle legs, appear red/orange (2).

Click on the right arrow to see a crab at an earlier stage of development, also under UV light.

Biology in the background

When a crab first hatches from its egg, the stage shown in the additional image, it is known as a zoea. At this point in its development, it doesn’t have a hard shell and doesn’t resemble an adult crab. But like the juvenile crab in the main image, a zoea is free-floating, also known as planktonic. At the second stage of development, a crab is known as a megalopa; during this stage, it begins to resemble an adult crab. Next, it reaches the juvenile stage, as shown in the main image, when it looks like a very small crab but still has little to no hard shell.

Finally, it reaches the adult stage, during which its exoskeleton hardens, it begins to search for food on the sea floor or coastline, and it can mate. Crabs and other crustaceans incorporate calcium carbonate in their exoskeleton in a process called biomineralization, which gives their bodies mechanical rigidity and strength.

Juvenile crabs vary in size, but this one is about 10 millimeters across, or roughly half the width of a human thumbnail.

Technique

This image was created using macrophotography under UV light.