What Is an Eyestalk?

Eyestalks are common in mollusks and crustaceans such as crabs, lobsters and snails. The eyestalk is a long, slender appendage that extends from the head of the animal and ends with an eye. It can be easily moved around so that the animal can look in any direction, and it can be extended or retracted at will.

When an animal with an eyestalk feels threatened, such as when the stalk is touched, it will retract its eyestalks, thus bringing its eyes close to its head in order to protect them. It does this by pumping blood in or out of the eyestalk. When it is full of blood, it is extended, and when the blood is removed it is retracted.

In addition to providing an extended field of view, the eyestalk contains various hormones that affect different aspects of the creature’s life. Experiments have been done where the eyestalks have been removed from different animals, and the removal has impacted far more than just visual ability. The specific hormones and their functions vary somewhat by species and by sex, but in general the hormones relate to the growth and reproductive activities of the animals.

Blue crabs, for example, depend on the hormones in their eyestalks to help them to regulate their growth. Because crabs have an exoskeleton, they must molt in order to grow. The eyestalk of the blue crab affects how much and how quickly these crabs grow by releasing hormones that regulate how often they lose their external shells.

In spider crabs, the eyestalk has a significant impact on the ability to breed. When scientists removed the eyestalks of some of these crabs, the females produced significantly more eggs than those that had their eyestalks left intact. This has led scientists to conclude that this part of the spider crab produces a hormone that regulates reproductive activities and the production of eggs.

In other creatures the eyestalks perform a different function. The males of certain insect species, such as the stalk-eyed fly, use eyestalks to attract a mate, and the bigger the male’s eyestalks, the more likely he is to be able to breed. It can also provide a hosting spot for parasites. The amber snail is sometimes seen with a large striped eyestalk. This is the result of its having been infected with a parasitic worm that takes up residence there, causing the eyestalk to swell and making it impossible for the snail to retract.

An eyestalk is a protruding extension that supports an eye, commonly found in invertebrates, especially crustaceans like crabs, lobsters, and shrimp. These structures enhance the animal's field of vision by elevating the eyes above the main body, allowing for a better view of their surroundings and potential threats or prey.

Eyestalks provide several advantages, including increased visual range without the need to move the whole body, which can be crucial for spotting predators or prey. They also allow for more discrete movement, as the animal can survey its environment with minimal exposure. Additionally, eyestalks can retract or rotate, offering protection and varied perspectives.

Eyestalks work by elevating the eyes away from the body, which can significantly expand an animal's field of view. The eyes on the stalks are often highly mobile and can move independently, providing a panoramic view. This setup helps the animal to better detect movement and judge distances, crucial for survival in their habitats.

Many crustaceans with eyestalks have the remarkable ability to regenerate lost or damaged appendages, including their eyestalks. This process can take several molting cycles to complete, during which the animal will grow a new eyestalk to replace the one that was lost, ensuring they maintain their crucial visual capabilities.

Beyond vision, eyestalks in some crustaceans also play a role in hormone regulation. For instance, the eyestalks of certain species contain the X-organ-sinus gland complex, which is involved in the secretion of hormones that regulate growth, reproduction, and color changes, according to studies in crustacean endocrinology.

Yes, certain species have unique adaptations of eyestalks. For example, the mantis shrimp possesses eyestalks with complex eyes that can see polarized light and have trinocular vision in each eye, allowing them to perceive depth and distance with just one eye, as reported in research on their visual systems. This adaptation is particularly beneficial for their predatory lifestyle.