Crustaceamorpha: More on Morphology

Molt of a sergestid shrimp (Decapoda) sinking through the water of the open ocean. This sergestid species lives at around 400 m (1200 feet), thousands of meters above the sea floor and spends most of its time swimming. Image from video taken by the remotely operated vehicle Ventana, and unmanned submersible operated by the Monterey Bay Aquarium Research Institute.


Crustaceans, like other Arthropods, have an exoskeleton made of chitin and proteins that protects them from drying out and provides a hard framework to which their muscles attach. Because of their hard exoskeleton, to increase in size they must molt or shed their exoskeleton and produce a new one (see box at left). Between the time that they shed the old exoskeleton and produce a new one is a very vulnerable time for crustaceans because they are easy prey for many predators.

Crustaceans have an open circulatory system meaning that all their blood is not contained within vessels, instead, blood is drawn in to the heart through holes called ostia, then pumped out again to circulate through the tissues and return again to the heart.

Line drawing of a stomatopod labeled with general crustacean morphology. Illustration used with permission from Roy Caldwell, UC Berkeley.

Crustaceans are usually segmented with one pair of appendages per segment. Segments are arranged into distinct tagmata (regions that are speciallized for specific purposes) such as the cephalon (head, cephalic = adjective), thorax (thoracic = adjective), and abdomen. In some taxa the thorax and abdomen are not distinguished from each other and are then called the trunk. Within a region, fusion of segments is common, particularly in the cephalon where it is seldom possible to differentiate each segment. The fusion of the cephalic segments is most obvious when a cephalic shield or carapace is present. A cephalic shield is a fusion of the dorsal (normally the uppermost part, in humans it is the back) portion of the cephalic segments. A carapace, or cephalothorax, is similar to a cephalic shield but is fused further around to the ventral (underside) portion. The table below summarizes the general morphology of the different crustacean groups.

Group or taxon Body regions (# segments) Carapace Appendages
Remipedia head (5), trunk (<32) cephalic shield All trunk appendages similar
Cephalocarida head (5), thorax (8), abdomen (11) cephalic shield phyllopodus (leaf-like) on thorax, abdominal absent
Branchiopoda head (5), trunk (unidentifiable to 32) none or carapace phyllopodus on thorax, abdominal lacking or similar to thoracic
Maxillopoda head (5), thorax (6), abdomen (4) cephalic shield or carapace thoracic often reduced, abdominal absent
Malacostraca head (5), thorax (8), abdomen (7 in Phyllocarida, 6 in Eumalacostraca) carapace, or may be lost Eumalacostracan with thoracic and abdominal, not phyllopodus; Phyllocarida with phyllopodus thoracic and reduced abdominal

The specialized crustaceans living today are quite different from what ancestoral crustaceans are thought to have been like. They were thought to have consisted of many similar segments, each with a pair of biramous appendages that were also similar to each other. All the variations in morphology seen today in the crustacean groups arose from ancestors with this basic body shape. The Remipedia most closely resemble this ancestor, though they do have several differences and specializations.