Character variation and evidence for spine length selection in the invertebrate predator Bythotrephes (Crustacea: Cladocera) from Lakes Michigan, Huron, and Erie

Abstract

Bythotrephes cederstroemi from Lakes Erie, Huron, and Michigan differ in size and mass. Animals are typically smallest in Erie and largest in Michigan. Nitrogen contents per unit mass are similar among lakes, suggesting that the differences do not reflect variations in protein content. The animals from different lakes are not merely miniaturized or enlarged versions of a common shape, however. There are striking differences among the lakes in the allometry of spine length to body lenght, and in the ratio of spine to body length with instar. The smallest individuals, from Lake Erie, exhibit proportionately the longest spines as adults. The most conservative morphological feature of the populations seems to be the length of the spine borne by adult females, which is roughly 8 mm in the three lakes, even though adult female body lengths, and the total weights, differ by up to 50 %. Thus, the smallest animals from Lake Erie invest proportionally more structural tissue in 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 spine lengths, than are exhibited by first instar animals. Because the spine is not lost at molting, the pattern strongly indicates differential survival of animals with longer spines at birth, and thus that the spine has adaptive value. These differences in allocation of energy and biomass suggest that neonate size and adult spine length may be targets 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 strong linear function of clutch size, demonstrating that accrual of reproductive tissue and offspring mass far exceeds somatic tissue growth by these animals.