FACTORS INFLUENCING EGG SIZE IN THE GAMMARID AMPHIPOD GAMMARUS-INSENSIBILIS

Developmental and seasonal changes in egg volume were examined in a po pulation of the amphipod Gammarus insensibilis Stock occurring on the south coast of England, towards its northern limit of distribution. Re sults showed a marked increase in egg: volume during development (2.9 times by Egg Stage V), resulting from water uptake and from the conver sion of yolk reserves into structural elements. The maximum rate of in crease coincides with the period of organ and limb development. At hat ching, after initial rupture of the egg membrane by urosome spines, eg g volume increases rapidly over a short period (15 to 20 min) by a fur ther 30% (uptake rate 3.6 x 10(-5) mm(3) s(-1)), followed by a post-ha tching decrease in juvenile volume. Increase in size at hatching is th e result of drinking by embryos, although changes in body-wall permeab ility may contribute. Females carrying eggs in an advanced stage of de velopment exhibit egg-collecting behaviour. This is seen as an adaptat ion to an increased likelihood of egg loss with increase in volume of the brood as hatching approaches. Seasonal changes in Stage I (early) egg size are marked in this species, with winter eggs as much as 60% g reater in volume than summer eggs. Egg size is inversely related to th e temperature during oocyte development. A simple model has been deriv ed to account for the observed seasonal pattern in egg size. The conse quences of seasonal variation in egg and juvenile size are considered.