FLANNERY, J.A.*; MARTIN, K.L.M.; Pepperdine Univ.; Pepperdine Univ.: Temperature Sensitivity and Stages of Grunion Egg Development

The California grunion Leuresthes tenuis is a marine fish that deposits its eggs in beach sand by emerging from the ocean to spawn at high tides. The eggs remain out of water in this moist terrestrial environment throughout incubation. We measured field temperatures in the incubation environment at the appropriate depths during the spawning season. Additionally we examined eggs at 18�C over the entire time of incubation in order to generate a table of morphological developmental landmarks for grunion embryos. Then we set up an experiment to show temperature effects on the rate of development. One set of eggs was incubated at 18�C, and another set was incubated at 20�C. A third set of eggs was incubated in a fluctuating air temperature that was 18�C for 18 hours of the day, increased to 26�C for 6 hours in the afternoon. This reflects the range of temperatures seen in the prime spawning months of the grunion, and it is equivalent to a mean daily temperature of 20�C. Each day, several eggs were taken from each set and assessed microscopically for morphological stage of development. The eggs in the variable treatment, with a mean daily temperature of 20�C, developed at the same rate as the eggs held at a constant 20�C. These two sets of eggs developed significantly faster than eggs incubated at 18�C. A small 2�C difference in incubation temperature makes a significant difference in the rate of development of grunion embryos, yet eggs develop at the same rate for the same mean temperature, whether experienced continuously or as a mean of daily temperature fluctuations. This work was supported by the National Science Foundation, DBI-99-87543.