Different Temperature Regimes Affect Gonadal Development and Energy Budget in Apostichopus japonicus

Apostichopus japonicus is a temperate marine echinoderm whose growth and reproduction are strongly influenced by temperature. We compared adult sea cucumbers exposed to stepwise warming (10–12, 12–14, and 14–16 °C across three successive 20-day stages) with animals maintained at 15 °C. Growth indices, gonadosomatic index (GSI), gonadal histology, and energy-budget components were evaluated in females and males. The 60 d experiment used three replicate tanks for each treatment-by-sex combination. Compared with the constant 15 °C treatment, the stepwise-warming regime showed a higher female GSI at stage III; representative gonadal sections documented stage-related structural changes in both treatments. No significant treatment effects were detected for final wet weight, specific growth rate, daily growth rate, or feed conversion ratio. Excretory energy decreased during stages II and III, while the proportion allocated to gonadal growth increased. Stage-specific within-sex comparisons showed higher gonadal growth energy under stepwise warming in both sexes during stage II, while the clearest GSI difference occurred in females during stage III; treatment × sex interactions were not significant. The clearest differences between the two temperature regimes were observed during stages II and III (12–16 °C). The findings provide an integrative physiological basis for temperature management during broodstock conditioning of A. japonicus.