By David Wartinbee, for the Redoubt Reporter
The group Arthropoda includes those animals with jointed legs and an exoskeleton made of chitin. It’s a large, diverse family, to be sure.
Insects, the most abundant animal group in the world, are Arthropods. Insects are well-known for having six legs and they are primarily found in terrestrial or freshwater habitats. Crustaceans are another large group of Arthropods that are mostly marine creatures with a few freshwater members. In Alaska we are quite familiar with crustaceans like the king crab, tanner crab and Dungeness crab. Crustaceans range from microscopic copepods to the giant, 8-foot-wide Japanese spider crab.
Crustaceans are perhaps best-known from marine habitats, but they are also found in freshwater streams, lakes and ponds. Many years ago while working on a jungle stream in Venezuela, I was startled to find a number of small but aggressive black crabs in my collection net. A well-known freshwater crustacean from the Central and Southern states is the crayfish.
In Alaska, we have neither freshwater crayfish nor crabs, but there are a number of much smaller crustaceans in our lakes and streams. Perhaps the most commonly encountered crustaceans are the amphipods. These small, shrimplike creatures have a couple long antennae and a whole bunch of legs. Scientifically, they are called Gnathopods, Pareopods, Pleopods, Uropods or Maxilopeds, but I’ll just call them legs. The name “amphipod” comes from the fact that some legs are specialized for crawling while others are specialized for swimming. The name might also imply they are able to move in and out of the water but they rarely leave their aquatic home.
Just about every lake, pond or stream in our area has an abundant population of the amphipod genus Gammarus. In some special areas, like springs, the number found per square meter can reach 10,000 individuals. Most Gammarus are fairly small and will be between 2 and 12 millimeters in length. Interestingly, huge Lake Baikol in Siberia has more than 300 species of amphipods, and some of them reach 60 millimeters in length. Our freshwater amphipods are found crawling or swimming along the bottom of the water body, hiding in organic debris. It is this organic debris that they use for food, as well as protection from predators. They are particularly important organic material processors in many freshwater habitats because they can consume more than their body weight in organic material each day.
The predators on amphipods include large aquatic insects, like beetle larvae, stonefly nymphs and dragonfly nymphs. Additionally, they can be prey to various birds and a variety of fish. Since they are active and abundant year-round, they are a favorite prey species for trout.
Ask any ice fisherman to look inside the stomach of the trout they caught and I’m betting you’ll find a bunch of Gammarus. Since the amphipods are closely related to shrimp, it’s no wonder shrimp are a favorite bait when trout fishing. In some states, like Minnesota, fishing bait dealers sell live Gammarus for catching panfish.
Their laterally compressed body form enables them to crawl through thick plant and algal masses. They are adept swimmers when disturbed and they quickly seek shelter by burrowing into organic debris. Interestingly, because of their compressed body, when they swim, they do it on their sides. They have sometimes been called side swimmers because of this trait.
The amphipods in our streams, lakes and ponds are rarely seen unless we go specifically looking for them. However, they play very important roles in their communities, as they process extensive amounts of organic material and are a major food source for a variety of freshwater predators.
David Wartinbee, Ph.D, J.D., is a biology professor at Kenai Peninsula College’s Kenai River Campus. He is writing a series of columns on the ecology of the Kenai River and Cook Inlet watershed.