Thecamoebian species and assemblages reported from various limnological environments in North America are not appreciably different across a climatic gradient ranging from semi-tropical to Arctic latitudes. However, certain morphological characteristics of individual species referred to as “strains” do appear to be very sensitive to changing environmental conditions. Thus, the identification of strains is a useful technique in the interpretation of paleoenvironmental conditions. Other features considered diagnostic of Arctic conditions in particular, are assemblages dominated by one or two species, and large and coarse tests.
Some thecamoebian specimens found in lakes on Richards Island, Northwest Territories, Canada, are two to three times larger than those reported from temperate locations. These “giant” specimens are perhaps related to very short periods of warm water temperatures which appear to be favorable for reproduction. Tests grow to a large size after extended feeding periods during the short Arctic summers. For the first time, continuous annual lake temperature data confirm the hypothesis that the presence of abundant large tests in an assemblage may be indicative of cold climates, due to a short reproductive season.