Crustaceans: Beyond the Cranky Crayfish
Thursday, March 1st, 2007
by Andy McAllister, WPCAMR Watershed Coordinator
Crustaceans make up a significant portion of animal communities in all aquatic habitats, from the most extreme ephemeral desert pools to the oceans and everywhere in between. Crustaceans are probably the most well known arthropods because of their contributions to aquatic and terrestrial food webs — not to mention their impact on the economics of the world.
Though arguably the most numerous animals in the marine environment, crustaceans haven’t been quite as successful over the millennia in freshwater and terrestrial ecosystems where they share their habitat with more successful arthropods (principally insects). Still, the diversity of crustacean life in freshwater is astounding.
Crayfish
courtesy Andrews University
As crustaceans feed, they grow, and as they grow they must shed their hard outer covering, called an exoskeleton, and produce a larger one; this process is called molting. Crustaceans molt as they grow throughout their lives, but they molt most frequently during a process called metamorphosis, as they change from larvae to adults.
Seed Shrimp
courtesy Iowa State University
In addition to the most commonly recognized freshwater crustaceans in our streams, crawfish and scuds, aquatic ecosystems are home to some more unusual crustaceans. Ostracods, commonly called seed shrimp, are among the smallest crustaceans. At about the size of a grain of sand, they are easily missed in a stream sample. An ostracod looks like a small clam, but it is actually a type of crustacean that makes this clam-like shell. Although they spend most of their time in the bottom scavenging for tiny particles of dead and rotting material, they can and do swim readily, propelling themselves around like a jet-ski.
All Crustaceans brood their young: as the eggs are laid, they are collected by the female and incubated under or on the outside of the mother’s hard shell. Once the young crustaceans hatch, they leave the mother and are off on their own.
Crustaceans, like many other stream invertebrates, are adversely affected by pollutants, including acid mine drainage (AMD). The hard shell of most crustaceans consists largely of calcium compounds, which are very susceptible to being dissolved by the acids in AMD. According to many reports, crustacean diversity begins to be negatively affected at stream pH levels less than 6.0.