Temperature characteristics of the first cleavage cycle of Japanese sea urchins

First cleavage—cycle times (FCCTs) from fertilization to the first cleavage of Hemicentrous pulcherrinus and Peronella japonica were measured at different temperatures, (Fig. 1) and the temperature coefficients of FCCTs were calculated by the Arrhenius’ equation, according to Crozier (1924, J. Gen. Physiol. 7:189). Between 24 to 8°C , the former urchin cleaved normally, and the latter did between 18 to 28 °C . The temperature coefficients of the former species were µ ₁ =9,200 for 24–15 °C (an optimal temperature range) and µ ₂ =19,100 for 12–8 °C (a suboptimal temp, range), and the coefficients of the latter species were µ ₁ =8,400 for 28–24 °C and µ ₂ =19,300 for 24–18 °C . The ratio of µ2 / µ1 is 2.1 for the former urchin and 2.3 for the latter.