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.
