EFFECTS OF TEMPERATURE ON THE SIZE OF AQUATIC ECTOTHERMS - EXCEPTIONS TO THE GENERAL RULE

1. Of 61 studies of aquatic ectotherms, increased rearing temperature (apparently not stressful for growth and development) caused a reduction in organism size at a given developmental stage in 55 cases (90.2%) and an increase in only six (9.8%). 2. The six exceptions to the size-reduction rule included one diatom (Phaeodactylum tricornutum), one copepod (Salmincola salmoneus) and four species of mayfly (Ephemeroptera). The extent to which these exceptions could be explained by each of four mechanisms was investigated by comparing their life cycles and niches with those of other closely-related species. 3. No satisfactory explanation could be found for the response of P. tricornutum, but mechanisms consistent with the response of S. salmoneus were: reduced risk of oxygen shortage, low risk of ectotherm predation and seasonal constraints on the life cycle. The latter may also help explain the four mayfly exceptions.