Crustacea FAQs | Natural History Museum of Los Angeles

Crustacea FAQs

What is a crustacean?

A crustacean is an arthropod, meaning it is an animal that has an external skeletal support system and jointed legs and other appendages. Crustaceans differ from other arthropods in having two pairs of sensory appendages called antennae (insects have only one pair, and chelicerates, such as spiders and scorpions, have none). Unlike insects, which are nearly all recognizable in having three distinct body regions (head, thorax, and abdomen) and six legs, crustaceans come in all shapes and sizes, and can have many different legs and other appendages. Crustaceans are  primarily marine (living in or near the sea), but many also inhabit freshwater and terrestrial habitats. Their habitats range from the deep-sea to the highest mountain lakes, and from shallow warm waters of the tropics to the deepest and coldest ocean basins. More than 70,000 species of crabs, shrimps, lobsters, and their close relatives have been described — more than twice the number of all amphibians, reptiles, birds, and mammals combined. Although the insects still rule in terms of numbers, the crustaceans are the most diverse in terms of form. Crabs, shrimps, and lobsters are well-known crustaceans. However, barnacles, pill bugs, amphipods, copepods, krill, crayfishes, sea fleas, clam shrimps, fairy shrimps, and many others also belong to the Crustacea, an ancient group that arose in the early Cambrian nearly 600 million years ago.

What is an arthropod?

Arthropods are "invertebrate" (have no internal backbone) animals that have an external skeletal support system (sometimes called a cuticle, carapace, or shell) and jointed appendages. Arthropods are the largest major group (phylum) of animals on Earth. The phylum Arthropoda includes the insects, crustaceans, chelicerates (spiders, scorpions, ticks, mites, horseshoe crabs, harvestmen, and others), centipedes, millipedes, and others.

What are some common crustaceans?

Common crustaceans include: decapods (crabs, shrimps, lobsters, hermit crabs, crayfishes, and related forms), isopods (including the terrestrial pill bugs or "roly polys" and a lot of other forms), amphipods (a group that includes beach hoppers), copepods, barnacles, krill, fairy shrimps, and many more.

Where do most crustaceans live?

Crustacans are known from just about every major habitat on the earth, from montane cloud forests to deep-sea hydrothermal vents, and from tropical coral reefs to the frigid waters of Antarctica. Many live in freshwater, such as the familiar crayfish and a huge number of freshwater crabs, and some live only in ephemeral (temporary) pools that are dry for much of the year (fairy shrimp, tadpole shrimp, clam shrimp and their relatives). However, by far the majority of crustaceans are "marine," living in or very close to the sea.

Can you eat them?

Certainly you can eat many of them, and in fact the many edible species of shrimps, crabs, and lobsters make up a huge seafood industry worth billions of dollars each year to the world's economy. Other groups of crustaceans, such as barnacles, mantis shrimps, and "squat" lobsers (galatheids), are eaten in some parts of the world. A few crab species in the Indo-Pacific region are known to be poisonous if eaten. It is not clear if the poison is created by the crab or is caused by something in the crab's diet; you should always use caution and eat only "approved" species that are known to be safe. 

How big do crustaceans get?

The largest of the crustaceans include the the giant Japanese spider crab (Macrocheira kaempferi) with its four-meter (13 foot) legspan (measured across the outstretched legs), the Alaskan king crab (Paralithodes camtschatica), which can weigh more than 10 kilograms (22 pounds), and the giant Tasmanian crab (Pseudocarcinus gigas), which has been recorded at an impressive 14 kilograms (31 pounds). On the other end of the spectrum, some crustaceans never grow larger than 0.25 millimeters (0.01 inch), even as adults.

Is a garden “roly poly” (sow bug or pill bug) a crustacean?

Yes indeed, it is a member of a group of crustaceans called the "Isopoda," most species of which are marine, but there are many terrestiral ispods that are commonly called pill bugs, sow bugs, wood lice, or roly polys.

I found a bright purple roly poly in my garden. What caused it to be that color?

There is a type of virus called an "iridovirus" that attacks terrestrial isopods (roly polys or pill bugs) and forms crystals beneath their external skeleton (shell or cuticle). The accumulation of these crystals causes their appearance to change from the normal gray or brown to a bright blue, violet or purple. The virus is not dangerous to humans and cannot be "caught" from the roly poly.

Is a spider a crustacean?

No, a spider is a "chelicerate," a group of arthropods that do not have antennae (like crustaceans and insects do) and that have a pair of feeding structures called "chelicerae." Chelicerates include spiders, scorpions, ticks, mites, horseshoe crabs, harvestmen, and many others.

With an external skeleton, how does a crustacean grow?

Crustaceans, like all other arthropods, must grow by "molting" (ecdysis), which involves the shedding of their external skeleton.