CHARACTER VARIATION AND EVIDENCE FOR SPINE LENGTH SELECTION IN THE INVERTEBRATE PREDATOR BYTHOTREPHES (CRUSTACEA, CLADOCERA) FROM LAKES MICHIGAN, HURON, AND ERIE

Bythotrephes cederstroemi from Lakes Erie, Huron, and Michigan differ in size and mass. Animals are typically smallest in Erie and largest i n Michigan. Nitrogen contents per unit mass are similar among lakes, s uggesting that the differences do not reflect variations in protein co ntent. The animals from different lakes are not merely miniaturized or enlarged versions of a common shape, however. There are striking diff erences among the lakes in the allometry of spine length to body lengt h, and in the ratio of spine to body length with instar. The smallest individuals, from Lake Erie, exhibit proportionately the longest spine s as adults. The most conservative morphological feature of the popula tions seems to be the length of the spine borne by adult females, whic h is roughly 8 mm in the three lakes, even though adult female body le ngths, and the total weights, differ by up to 50 %. Thus, the smallest animals from Lake Erie invest proportionally more structural tissue i n spine growth than do the largest animals from Lake Michigan. Animals which survive to the second instar often exhibit longer distal spine lengths on average, and lower coefficients of variation among distal s pine lengths, than are exhibited by first instar animals. Because the spine is not lost at molting, the pattern strongly indicates different ial survival of animals with longer spines at birth, and thus that the spine has adaptive value. These differences in allocation of energy a nd biomass suggest that neonate size and adult spine length may be tar gets of additional selection pressure among the lakes. As a structural investment, the spine represents a mass equal to about one-third of a neonate. The relative mass gained from birth to primaparity is a stro ng linear function of clutch size, demonstrating that accrual of repro ductive tissue and offspring mass far exceeds somatic tissue growth by these animals.