Crayfish (Malacastroca: Decapoda): Only one species of crayfish is reported from Saskatchewan. Their lobster like appearance is difficult to confuse with any other type of organism. It inhabits lakes, rivers and streams throughout mmuch of Saskatchewan. It is usually seen scuttling between rocks or swimming backwards through the water by violently flapping its tail. Often the bleached white legs and body are seen on rocks by the water's edge where some bird or other animal has made a meal of one.
Scuds (Amphipoda): There are at least three species of amphipods reported from Saskatchewan, Gammarus lacustris Sars, Hyalella azteca (Saussure) and Diporeia hoyi (Bousfield). All look superficially similar. The former two are abundant in lakes, ponds, and slow areas of running waters. D. hoyi only inhabits deep cold lakes.
Fairy shrimp (Anostraca): Fairy shrimp are usually associated with temporary ponds. They can become very numerous if conditions are right in mid-spring. They are usually less than 2 cm long. In the image below the female is above with an egg sac and a male below, distinguished by the large clasper-like antennae on the head.
Tadpole shrimp (Notostraca): These are relatively uncommon in Saskatchewan waters. They can reach 2 to 3 cm long in some habitats. At least three species are now known from Saskatchewan Lepidurus cousesii, L. lynchi and Triops longicaudatus. (Mann, H and M.V.S. Raju. 2005. A New Tadpole Shrimp, Triops longicaudatus, in Saskatchewan. Blue Jay. 63: 94-97.)
Clam shrimp (Conchostraca): Clam shrimps are common in shallow ponds and lakes. They first appear to be like small swimming clams. Length is about 1.5 cm or less.
Seed shrimps (Ostracoda): Ostracods are common in ponds and lakes. They initially resemble swimming seeds a couple of mm long.
Waterfleas (Cladocera): Cladocera can become so abundant in ponds and shallow water that they form reddish clouds in the water. They swim with a jerky motion using their antennae. They rarely get larger than a couple of mm.
Clifford, H.F. 1991. Aquatic Invertebrates of Alberta. University of Alberta Press.