The morphological time of fixation of the total number of vertebrae in Fundulus majalis (Walbaum)

Abstract

Three lots of embryos with epiboly about one-third complete were reared continuously in different fluctuating temperature regimes (16° – 20°, 22°–26° and 28°–32°C). At certain somite-number stages (3–6, 9–12, 15–23, 24–28 and 29–32) paired lots of embryos were transferred from the intermediate temperatures to the warmer and cooler regimes simultaneously. Mortalities were low (7.0%) to moderate (28.6%) in the 13 treatments. Embryos reared continuously in the three temperature regimes developed mean vertebral numbers showing a linear inverse relationship between temperature and mean vertebral count; on the average decreasing temperature induced an increased number of vertebrae. Among the transferred embryos only the 3–6 and 9–12 somite lots transferred to the cold regime responded to temperature changes. The response of presumptive vertebral tissue to cold temperature begins to decrease during the formation of the fifth and sixth somites and continues gradually until fixation occurs during formation of the tenth through twelfth somites. Cold temperatures influenced vertebral number more than warm and it is suggested that vertebral number in fishes spawning on rising water temperatures may be influenced more by cold water than by warm; conversely, fishes spawning on falling water temperatures may be influenced more by warmer than by colder water.