Environmental parameters controlling gametogenesis in the echinoid Psammechinus miliaris

The role of photoperiod and temperature as stimuli for the onset of gametog enesis in the echinoid Psammechinus miliaris was examined, and the resultin g effect on energy allocation between gonadal and somatic growth was evalua ted. The hypothesis tested was that P. miliaris is a 'lengthening day' spec ies and that photoperiod and not temperature is the primary stimulus for ga metogenesis. Six-month-old, hatchery reared P. miliaris were maintained under different photoperiod and temperature regimes from December 1988 to June 1999. The ph otoperiod treatments were either ambient, i.e. days lengthening, or fixed a t 7 h of light, equivalent to the shortest day, to remove the stimulus of i ncreasing spring day length. The temperature treatments were either ambient (6-14 degreesC) or heated so that the seawater temperature never fell belo w 9 degreesC to remove the cue of increasing spring water temperatures. Gon ad 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 preserv ed 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 t he end of the trial, the urchins in the fixed photoperiod regime were of si gnificantly smaller test diameter than those in the temperature treatments. In early spring (May), the urchins in all treatments were of a similar rep roductive 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-con trolled trea tment had not completed gametogenesis, remaining premature. Failure to comp lete gametogenesis did not result in a measurable increase in energy alloca tion to somatic growth. The data suggest that lengthening days are an impor tant cue for the completion of gametogenesis in P. miliaris in both males a nd females and that experiencing low temperature is also an important cue f or the completion of vitellogenesis. (C) 2001 Elsevier Science B.V. All rig hts reserved.