The crustaceans belong to the phylum Arthropoda, which includes the familiar insects, spiders, and shrimps and crabs. This HUGE group may have over 50 million species. There are more arthropod species in 1 square mile of tropical rain forest than there are species of vertebrate animals in the whole world. 

Despite this variety, they all share a hard exoskeleton (a tough outer coating), have jointed limbs, grow by molting off their old skin, and usually are of one sex per individual. Within the crustacean there is wide range of forms. Copepods, amphipods, isopods, krill, brine shrimp and many other small crustaceans make up the base of many food chains. The three conspicuous major groups that are found in a salt water aquarium are: the barnacles, the shrimp, and the crabs. 

Exactly what is a shrimp, and what is a crab, is the subject of much taxonomic debate. Forgetting what I know about taxonomy, simply put, a shrimp has a free abdomen, and a crab has a reduced abdomen and it is tucked under the cephalothorax. Maybe a bit of anatomy will clarify this. Shrimps and crabs are grouped together in an order called Decapoda. This literally means that they have ten legs. The head and trunk (from which are attached the ten legs) are fused into a structure called the cephalothorax. The tail (the tasty part of shrimps and lobsters) is called the abdomen. There will be more on anatomy later, here are a few pointers for all crustaceans. 

New crustaceans should be acclimated slowly, as they are quite sensitive to fluctuations in salinity. Add about 1/4 of the bags volume of water from the aquarium every 10 minutes for about an hour. The tank should have plenty of hiding spaces since the crustaceans will use these when they need to molt. 

Molting is the process where by the animal casts off its old skin and grows a new slightly larger one. After molting, the skin is soft and the animal is vulnerable and needs some shelter. Leave the old molt in the aquarium, as the animal may eat it to gain back valuable minerals lost in the molting process. 

All crustaceans can carry out autonomy. They can cast off a leg if necessary (say if a predator has a hold on it), and can regenerate a new one in a few molts! 

The pH of the tank should be at least 8.1, and calcium supplements should be used to insure that the shell is in good condition. 

As a rule, crustaceans are not too finicky about their diet, but there will be more on that in the later sections. I want to wedge in a note on barnacles here since these are occasionally seen in the hobby, but do not require a whole paper of their own. These bizarre crustaceans actually settle to the ground, and cement their head to it. After settling a barnacle creates a shelter around itself and use its legs to filter food from the water. These animals require a fairly frequent feedings of phytoplankton to survive for a long period. As a rule they like areas of high flow and reduced lighting. They are harmless, and actually may fall prey to large fish and crustaceans.