CHARACTER VARIATION AND EVIDENCE FOR SPINE LENGTH SELECTION IN THE INVERTEBRATE PREDATOR BYTHOTREPHES (CRUSTACEA, CLADOCERA) FROM LAKES MICHIGAN, HURON, AND ERIE
Ca. Sullivan et Jt. Lehman, CHARACTER VARIATION AND EVIDENCE FOR SPINE LENGTH SELECTION IN THE INVERTEBRATE PREDATOR BYTHOTREPHES (CRUSTACEA, CLADOCERA) FROM LAKES MICHIGAN, HURON, AND ERIE, Archiv fur Hydrobiologie, 142(1), 1998, pp. 35-52
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.