Crustaceans

Later on in the same module, I studied Phylum Arthropoda. As my book puts it, "Phylum Arthropoda is a group of organisms that have literally taken over our planet." There are more species in this phylum than in all the other phyla in the animal kingdom combined. Arthropods can be found in all types of environments in creation. Some of the more common marine arthropods are shrimp, lobsters, barnacles, and crabs. All arthropods are bilaterally symmetric. Their bodies are covered with a "jointed suit of armor" encompassing their abdomens, mouth parts, and appendages. This armor is called an exoskeleton. It is made of chitin combined with minerals. This makes the exoskeleton very hard, and gives the creatures with it protection from predators. Arthropods must shed their exoskeleton in order to grow, and they must develop a new, larger one. This is called molting. When it starts molting, it sheds its exoskeleton; however, this leaves it very vulnerable to predators, so it usually hides while its exoskeleton re-grows.

Most of the marine arthropods are in class Crustacea. These creatures have gills for obtaining oxygen, special appendages for swimming, and an exoskeleton. They also have two pairs of antennae to sense their surroundings. Arthropods have two body segments: head, thorax, and abdomen. The head and thorax of krill and some other larger crustaceans are fused together, forming a cephalothorax. It is covered with a carapace, which protects the anterior part of its body. Shrimp, lobsters, and crabs belong to the order Decapoda, and are the largest of marine arthropods. They have five pairs of legs for walking. Shrimp and lobsters have a long abdomen, and many live on the ocean bottoms where they scavenge for leftover bits of food. Some shrimp feed on the parasites of fish; they set up a "cleaning station" where a fish swims up and the shrimp eats the parasites off of him, giving the shrimp food and the fish protection from parasites.

Bottom-dwelling crustaceans, like the shrimp and crabs, have appendages near their mouths called maxillipeds; they maneuver food into the mouth. Most crustaceans have teeth made of chitin, but they are inside their stomach. They also have an open circulatory system. They also have compound eyes. There are many types of planktonic larvae, depending upon the type of crustacean. Class Pycgnogonida contains sea spiders, class Merostomata contains horseshoe crabs, which are not really true crabs.

A blue crab:

Shrimp krill:

Lobster:

Sea spider: