Pill bugs, crabs, crayfish, and lobsters are some of our most familiar animals. All are members of the subphylum Crustacea, a vast group comprising more than sixty-seven thousand species. Yet despite their diversity and the fact that they inhabit much of our planet, most crustaceans go unnoticed.
In general, Crustaceans are marine creatures, although there are exceptions – including crabs that climb trees and woodlice that survive the extreme aridity of the Namib Desert. While insects have radiated onto the continents to become key components of almost every terrestrial habitat, crustaceans long ago evolved to fill most available niches in the oceans as well as in many streams, rivers, and lakes.
Crustaceans are an ancient group that arose early in the Cambrian period, nearly six hundred million years ago. They are extremely diverse in size and form from essentially sedentary barnacles to free-swimming oceanic krill. The largest crustaceans include the Japanese spider crab, which has a thirteen-foot leg span, and the Alaskan king crab, which can weigh more than twenty-two pounds. At the other end of the spectrum, there are crustaceans that never grow larger than a hundredth of an inch (a quarter millimeter) in length, even as adults.
With such diversity it is no surprise that many crustaceans face threats. More than five hundred crustacean species are on the IUCN Red List and, in the United States, twenty-two are protected under the Endangered Species Act. According to a study reported by The Nature Conservancy and NatureServe in 2000, crayfish are the second most endangered group of all plants and animals in North America – second only to freshwater mussels. The stories of many of these at-risk animals often involve very specialized habitats that are quite vulnerable to human activities.