Estimating cladoceran birth rates: The importance of egg mortality and the egg age distribution1

Abstract

Three models of egg hatching in zooplankton populations gave significant differences depending on whether egg mortality was included. The differences were due to the effect of adult mortality on reducing the probability that eggs survive from the time of laying until they are released as live young. Adult mortality results in a population of eggs disproportionately dominated by younger stages, with resulting recruitment rates being lower than expected from development times alone. A technique based on the relative duration of various developmental stages was used to determine the age of eggs from two natural populations during periods of decline. The observed tendency for the age distribution of eggs to be dominated by younger eggs was as expected from models of egg hatching which included mortality but was not as expected from models of declining populations which did not incorporate egg mortality.

Predator‐induced phenotypic plasticity in Daphnia pulex: Life history and morphological responses to Notonecta and Chaoborus

“…The time to first reproduction was determined by recording the time and date each of the experimental broods was in midstage three (Threlkeld 1979). At l-h intervals, I also recorded the egg stage of each of the experimental females' first brood to determine when the embryos were midstage three (Threlkeld 1979).…”

“…time (h) elapsed from a stagethree embryo (Threlkeld 1979) to an adult whose first brood has developed to stage three. Age-specific survivorship:…”

“…Brood mates from a single D. p&.x dividual: body length, tail-spine length, egg female were randomly divided into four groups stage (Threlkeld 1979), and number of off-spring. This information was gathered for each animal through the sixth clutch.…”

No evidence for thermal transgenerational plasticity in metabolism when minimizing the potential for confounding effects

“…Eggs were defined as late in the development process if it was possible to observe the eyes inside the brood chamber (i.e. development stage 4-5 in [31]), and early if not. Individuals were excluded from the dataset (n ¼ 56) if we could not determine the contents of the brood chamber or where the individual had an ephippium present.…”

An experimental test of the egg‐ratio method with instantaneous birth rate as an independent variable

“…Birth rates can be calculated from models with field measurements of egg : female ratios (E) and egg development times (Edmondson 1960;Gabriel et al 1987) or egg age structure (e.g. Threlkeld 1979). Most "egg-ratio" models assume a stable age structure and exponential growth (Eq.…”