Gammaridae
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Crustacea - Woodlice, Crayfish etc.
Crustacea are a large group of Arthopod invertebrates. Many of them are aquatic, living in marine or freshwater environments, but the familiar terrestrial Woodlice are Crustacea in the Order Isopoda, along with freshwater Waterlice or hoglice, plus a few marine species. Crayfish are in the Order Decapoda - literally meaning 'ten footed' - which also include marine lobsters and crabs. Shrimps are now placed in the Order Amphipoda, which includes marine and freshwater species.
Crustacea include many thousands of tiny freshwater species that are planktonic (living in the water) or benthic (found in sediments). These include the Water-fleas (Diplostraca or Cladocera), Copepods (e.g. the Orders Cyclopoida, Harpacticoida and Calanoida) and Ostrocods. Their position at the lower levels of the aquatic food-chain means they are of great ecological importance.
The 'Practical Guide to Identifying Freshwater Crustacean Zooplankton' by L M Whitty (2004) has helpful guidance on how to identify these smaller species (despite being based on North American species). The Freshwater Biological Association has published keys for several groups of aquatic Crustacea.
Some Crustacea are parasitic, like the Fish lice in the family Argulidae.
