Seasonal changes of Bosmina (Crustacea, Cladocera) in Frains Lake, Michigan: laboratory observations of phenotypic changes induced by inorganic factors

Summary In Frains Lake, individuals of the species Bosmina longirostris decrease relative mucro and antennule lengths during the spring and summer, and increase them again during the fall. The cyclomorphosis involves both embryonic and post‐embryonic events, since the relative sizes of parts in neonates and the relative growth rates of parts in later instars change seasonally. Antennules grow during winter, but may even regress during summer. Mucrones show an opposite pattern, growing maximally during late summer and fall. The kinds of cyclomorphotic changes in Bosmina suggest evolutionary adjustments to two principle types of predators: visual vertebrates and blindly grasping invertebrates. While laboratory experiments implicate temperature and turbulence as proximate factors in the cyclomorphotic changes, some unknown factor (or factors) also appears necessary to stimulate the magnitude of responses observed in nature.