Most people are familiar with one type of amphipod, the sand flea, a term that actually refers to several species. However, these little crustaceans are rather versatile and have found their way into all sorts of habitats. The nearly 2000 known species inhabit a range of terrestrial, marine and freshwater environments. Some are specialized to live in restricted conditions, for example caves. The one defining condition is that the habitat must contain some moisture. Unlike insects, amphipods don’t have a completely waterproof exoskeleton and eventually will die in arid conditions.
Those sand fleas hopping along the tideline are not the only marine amphipods. Amphipods are found from the beach to the deep sea floor. Some species live on the underside of Arctic sea ice and some drift through the oceans with the plankton. Overall the group has shown itself to be remarkably adaptable, and it didn’t stop at the oceans; some moved into freshwater and terrestrial habitats.
Amphipods have colonized a range of freshwater habitats, mainly cooler ones, including lakes, over- and underground rivers, ponds and marshes. They also are commonplace in brackish water, which is salty but not as much as the ocean; a difficult environment for organisms especially when the salinity changes with the tides. Because of their permeable skins, amphipods are sensitive to pollution, making them, on occasion, what the US Fish and Wildlife Service (FWS) describe as “the canary in the coalmine” -- an indicator species that is one of the first to decline when a habitat becomes contaminated.
Land amphipods are associated with the leaf mold of woodland floors, probably because this creates a nice damp environment. They sometimes move to other areas after heavy rainfall, which is the time you are most likely to see them as they venture over paved areas and sometimes into buildings. Unless the rain is continuous, they are unlikely to live in their new homes for long. Once the environment dries out, the amphipods dehydrate and die.
Because they are small, not especially colorful and not enormously interesting to look at unless you have a microscopic, it is unlikely you have bought amphipods for your fresh or saltwater tank. Chances are high, however, that at least one species has arrived along with other animals, plants or accessories. In the wild, they form an important part of the ecosystem, often acting as scavengers and providing food for all sorts of larger creatures. In your tank, they’ll probably perform the same functions and are unlikely to be a pest. They also are unlikely to be able to multiply to great numbers -- your fish and invertebrates will eat most of them.
- sandy beach image by Adkok from Fotolia.com