The influence of crowding upon the total number of vertebrae developing in Fundulus majalis (Walbaum)

Abstract

With epiboly one-third to one-half complete, embryos were divided into six unequal lots representing six levels of crowding (44, 66, 88, 110, 132, 154) and reared in screen-windowed containers in a fluctuating temperature regime (22·0°–26·0°C). Water space available to an individual embryo varied from a minimum of 16 ml at the start of the experiment in the most crowded container to a maximum of 72 ml near the end of the experiment, after mortalities, in the least crowded container. Solutes did not accumulate in the containers because water moved freely through the screened window from the reservoir of the incubation chamber. Embryos at the most crowded level developed the lowest mean vertebral count that was significantly different from the three highest mean counts (66-, 88-, and 132-embryo levels). Apparently crowding causes an acceleration in metabolic rate because the more crowded embryos hatched first and these also achieved 50% hatching in a considerably shorter time. Further, at the most crowded level, embryos developed only 33 vertebrae in almost twice the percentage of individuals, an indication of accelerated developmental rate. It is suggested that crowding induces an interaction between acceleration of metabolism and stress that has a real influence upon the number of vertebrae developing in F. majalis embryos.