Environmental parameters controlling gametogenesis in the echinoid Psammechinus miliaris

Abstract

The role of photoperiod and temperature as stimuli for the onset of gametogenesis in the echinoid Psammechinus miliaris was examined, and the resulting effect on energy allocation between gonadal and somatic growth was evaluated. The hypothesis tested was that P. miliaris is a ‘lengthening day’ species and that photoperiod and not temperature is the primary stimulus for gametogenesis. Six-month-old, hatchery reared P. miliaris were maintained under different photoperiod and temperature regimes from December 1988 to June 1999. The photoperiod treatments were either ambient, i.e. days lengthening, or fixed at 7 h of light, equivalent to the shortest day, to remove the stimulus of increasing spring day length. The temperature treatments were either ambient (6–14 °C) or heated so that the seawater temperature never fell below 9 °C to remove the cue of increasing spring water temperatures. Gonad indices, alimentary indices, test diameter and test organic content were measured monthly. Once gonad development commenced, samples were examined using phase-contrast microscopy on wet-squash preparations and also preserved for histological examination of the reproductive state. There was no significant difference in the gonad index (GI) between urchins from the various treatments either during or at the end of the trial. By the end of the trial, the urchins in the fixed photoperiod regime were of significantly smaller test diameter than those in the temperature treatments. In early spring (May), the urchins in all treatments were of a similar reproductive status with developing gametes. However, subsequent samples (June) showed a significant proportion of the urchins of both sexes in the fixed photoperiod regime, and female urchins in the temperature-controlled treatment had not completed gametogenesis, remaining premature. Failure to complete gametogenesis did not result in a measurable increase in energy allocation to somatic growth. The data suggest that lengthening days are an important cue for the completion of gametogenesis in P. miliaris in both males and females and that experiencing low temperature is also an important cue for the completion of vitellogenesis.