Posted by cat hopson on March 25, 2006

Q: Why are the crayfish eyes located on stalks?

A: 1.Why are the crayfish eyes located on stalks?

"Why" questions are difficult, if not impossible, to answer in science. One can certainly ask what is the adaptive value of something but "why" an organism has this or that feature implies a purpose. Organisms evolve to survive so what we see when we look at any organism is what has helped it survive over time. We can say that eyes on stalks help a crayfish see and monitor its surroundings and probably help it survive.

2. What are the advantages of the armorlike exoskeleton? A big advantage is that it is difficult for predators to overcome the hard, outer covering and make a meal of the crayfish. For a fish to eat a crayfish, it usually can't break through the shell so it needs to consume the animal whole and must therefore be rather large in comparison to the crayfish. Smaller fish may pick at a given crayfish, but the shell protects the animal and gives it a chance to escape.

3.Are there any possible disadvantages? Absolutely. Along with any anatomical advantages in the natural world, there are always some disadvantages. There is no perfect physical arrangement. The one that comes to mind first is that in order for a crayfish to grow, it must shed its exoskeleton periodically. When it does shed its shell, (fisherman call these softcraws) it is totally soft for a time, and almost unable to move about and protect itself. At this time it is especially vulnerable and can easily be eaten by predators.

4. What possible advantage does the exoskeleton of the crayfish have over that of the clam? Of course, these two animals are very, very different. Each has evolved in a different way and live very different kinds of lives. But, the crayfish's jointed exoskeleton allows it to move about freely. The clam's shell - it's not an exoskeleton and grows by adding layers over time - is not jointed and serves to protect this sedentary (doesn't move around much) animal.

5.What is the advantage of well-developed sensory organs (eyes, statocytes, antennae, and antennules) to the way of life for a crayfish? The better an animal is able to interprete its environment, the better it is able to survive and reproduce. The bottom line for biological "success" is an ability to pass on one's genes. So if sensory organs help crayfish survive, this is an important and valuable adaptation. Hope this helps.

John Wiessinger ( March 27, 2006)