Technically, these are called 'joint-legged animals,' and are in the Phylum Arthropoda, which is one of 36 phyla in the Kingdom Animalia (there are 5 kingdoms containing flora and fauna). In fact, representatives in 28 of the 36 are found in marine environments with some not visible to the human eye. Nevertheless, for marine hobbyists those of interest, e.g., shrimp-like creatures are discussed here, with others in this Phylum, such as barnacles, crabs, lobsters, and sea spiders discussed elsewhere in the Aquarium Library in separate sections.
As to shrimp-like creatures, they fall into the Subphylum Crustacea, which has over 32,000 described species and many yet undescribed species. Before I go further, past thinking had the Order Decapoda (shrimp) placed in the 'Phylum' Crustacea, which was then divided into two suborders, 'Nantantia' containing the more interesting shrimp, and 'Reptantia' containing lobsters, crabs, and hermit crabs. The latest thinking is that 'Crustacea' should now contain six classes and be a 'subphylum' of the 'Phylum Arthropoda.'
Only a few of these classes are of interest to hobbyists. The Class Maxillopoda containing barnacles is discussed elsewhere in this library. The remaining two classes of major interest are Branchiopoda and Malacostraca, which contain by far the majority of animals we think of as crustaceans. Those in the Class Branchiopoda are mostly freshwater or brackish species, such as brine shrimp, usually incorrectly called Artemia salina, as this species is thought to be extinct. Yet one of the known species from the San Francisco area, Artemia franciscana, is regularly cultured for the aquarium trade. Another, the very common Daphnia pulex, would be familiar to freshwater aquarists, however, its also used as a marine fish food.
Nevertheless, those in the Class Malacostraca are of far more interest as it contains crabs, lobsters, prawns, shrimp, and crayfish. It's further divided into three subclasses, Phyllocarida, Hoplocarida and Eumalacostraca. As for Phyllocarida, its mostly ancient/primitive creatures, with most extinct. In the Subclass Hoplocarida, its Order Stomatopoda contains those dreaded mantis shrimps that are often undesirable in our aquariums.
In the Subclass Eumalacostraca, there are several superorders, two of which are of interest to marine aquarists. Its Superorder Peracarida contains small bottom-dwelling isopods and amphipods, along with small opossum shrimps we know as mysid shrimps.
The Superorder Eucarida contains two orders of significance. Order Euphausiacea contains the Pacific krill (Euphausia pacifica), which is an excellent foodstuff, either frozen or freeze-dried for our fishes. And probably the most interesting is the Order Decapoda (meaning 10 feet), which includes shrimps, prawns, lobsters, hermit crabs, and true crabs, and is composed of about 68 families containing almost 10,000 species.
This order is further divided into two suborders, Dendrobranchiata and Pleocyemata where those within it are classified by the structure of the gills and legs, and the way larvae develop. Those in the Suborder Dendrobranchiata release their eggs into the water and hatch as nauplii. And if shrimp are a people food in your household, a whole industry has been built around the culture of 'prawns' in this suborder. As for those in the Suborder Pleocyemata, its members cement their eggs to their pleopods (abdominal appendages) until they hatch, and these are of more interest to aquarists.
And once we move into the Suborder Pleocyemata, there are numerous 'infraorders' dedicated to various types of creatures, e.g., shrimp, lobsters, crabs, etc., and two have special interest if 'ornamental' type shrimp are of interest. In the Infraorder Stenopodidea, Family Stenopodidea, there's a very familiar species called the Banded Coral/Boxer shrimp Stenopus hispidus. However, it's not really considered a shrimp! It's considered more a lobster-like creature. And the Infraorder Caridea is where the more familiar 'true shrimp' genera begin to show up.
Lets stay here with shrimp-like creatures, and if interested in other type joint-legged animals, such as barnacles, crabs, lobsters, and sea spiders, checkout their sections elsewhere in this library.
Please click on the Shrimp group you are interested in viewing.