Crabs are very special for a number of reasons, making scientists take special interest in the study of their organs and the way they function. The compound and complex eyes of crabs are certainly among the most studied organs of crustaceans. Here is what makes the crab’s eye so special.
Crabs prefer murky and muddy environments where visibility is low, so they need very sharp eyes to be able to capture their prey and to protect themselves against predators that can come from all directions. Like most arthropods, crabs have apposition eyes, meaning two large, primary eyes located on the sides of their head and moving separately on their individual stalks.
These primary eyes are what scientists call compound eyes, that is, organs that consist of thousands of individual units called ommatidia that work in a way very similar to lenses – some species have eyes made up from 8,000 tiny lenses . The units are placed on a convex surface, which also means that each of them points in a different direction, even if the distance between them is tiny. The apposition eyes of crabs collect the different images and transmit them to the animal’s brain, where the complete picture becomes assembled, allowing the crab to see in all directions at the same time.
The eyes located in different areas of the crab’s body are structured differently. The eyes that look ahead are usually made up of larger lenses because they need to cover only a relatively small area, but must provide high resolution images. The eyes that point upwards are there only to warn the crab if there is a bird flying in the sky, and therefore they do not need to provide very sharp images.
Crabs rely primarily on the compound eyes found around their head, but many crab species have primitive eyes distributed on their bodies to help them monitor their surroundings all around. One of the oldest crab species on Earth, the horseshoe crab has not only compound apposition eyes on its head and primitive eyes on its body, but also median eyes on the top of its carapace to sense visible light, an endoparietal eye for UV light, ventral eyes on its belly to signal predators coming from below, plus multiple light receptors along its tail to improve the animal’s navigating skills in the almost-darkness of the sea bed.
Some crab species are not only known to have a very acute eyesight, but they are also able to detect UV light. The discovery that crabs are sensitive to blue and ultraviolet light has puzzled scientists for a while – after all, why would anyone need the ability to see light in an environment that is almost completely dark? Marine biologists have recently come up with the idea that UV sensitivity is a very useful quality for crabs, as it allows them to distinguish between prey and harmful creatures such as anemones.