Crustaceans, Subphylum Crustacea

Subphylum Crustacea, Crustaceans

Crustaceans belong to the phylum Arthropoda, along with insects, arachnids, millipedes, centipedes, and fossil trilobites. However, crustaceans occupy their own subphylum, Crustacea. The term crustacean derives from the Latin crusta, meaning crust or hard shell. In some references, the crustaceans are classified at the class level, but I choose to follow the classification outlined in Borror and DeLong’s Introduction to the Study of Insects, 7th edition.

The subphylum Crustacea is subdivided into 10 classes:

• Class Cephalocarida – horseshow shrimps
• Class Branchiopoda – tadpole, fairy, and brine shrimps
• Class Ostracoda – ostracods, seed shrimp
• Class Copepoda – copepods, fish lice
• Class Mystacocarida
• Class Remipedia – cave-dwelling blind shrimps
• Class Tantulocarida
• Class Branchiura
• Class Cirripedia - barnacles
• Class Malacostraca – lobsters, crayfish, crabs, shrimps, amphipods, isopods (including pillbugs and sowbugs), ad mantis shrimps

Description

Most of the 44,000 species of crustaceans live in saltwater or freshwater. A small number of crustaceans live on land. Whether marine or terrestrial, crustaceans share certain traits that determine their inclusion in the subphylum Crustacea. As with any large group of organisms, exceptions to these rules will occasionally apply.

Typically, crustaceans have functional mouthparts and two pairs of antennae, although one pair may be greatly reduced and difficult to discern. The body may be divided into three regions (head, thorax, and abdomen), but is often limited to two (cephalothorax and abdomen). In either case, the abdomen will be clearly segmented, usually with a non-segmented area or extension at the hind end (called a terminal telson). In some crustaceans, a shield-like carapace protects the cephalothorax. Crustaceans have biramous appendages, meaning they divide into two branches. All crustaceans breathe via gills.

Diet

We usually think of crustaceans as food, rather than as feeders. The smaller crustaceans – tiny shrimp and amphipods, for example – play an important role as food for larger marine organisms. Most crustaceans are themselves either scavengers or parasites. Terrestrial crustaceans often live on the ground, hidden under rocks or debris in moist, humid environments, where they can feed on decaying vegetation.

Life Cycle

 Because the subphylum Crustacea is such a large and diverse group, their development and natural history varies greatly. Like other arthropods, crustaceans must molt and shed their hardened cuticles (exoskeletons) in order to grow. The crustacean life cycle begins with the egg, from which the immature crustacean emerges. Crustaceans may undergo either anamorphic or epimorphic development, depending on the taxon. In epimorphic development, the individual that hatches from the egg is essentially a tiny version of an adult, with all the same appendages and segments. In these crustaceans, there is no larval stage.

In anamorphic development, the individual crustacean emerges without all the segments and appendages of the mature adult. As it molts and grows, the immature larva gains segments and acquires additional appendages, until it reaches adulthood.

In very general terms, anamorphic crustaceans will develop through three larval stages:

• naupli - In the naupli stage, the larva is basically a floating head, with a single eye, and three pairs of appendages which it uses for swimming. Some anamorphic crustaceans skip this larval stage and emerge from the egg at a more advanced level of development.
• zoae - In the zoae stage, the larva has both a cephalon (head) and thorax. By the end of this stage, it will add abdominal segments as well. Zoae swim using biramous, thoracic appendages, and may also have a pair of compound eyes.
• megalopae – By the megalopae stage, the crustacean has added the segments of all three body regions (cephalon, thorax, and abdomen), as well as its appendages, including at least one pair of swimmerets. It looks like a smaller version of an adult​ but is sexually immature.

Sources

Borror and DeLong’s Introduction to the Study of Insects, 7th edition, by Charles A. Triplehorn and Norman F. Johnson.

Natural History Collections: Crustacea, University of Edinburgh. Accessed May 28, 2013.

Subphylum Crustacea, Florida International University. Accessed May 28, 2013.

Crustacea, H-B Woodlawn Biology and AP Biology pages. Accessed May 28, 2013.

Subphylum Crustacea Tree of Life, Virtual Fossil Museum. Accessed May 28, 2013.

Crustaceamorpha, University of California Museum of Paleontology. Accessed May 28, 2013.