Light intensity impacts on growth, molting and oxidative stress of juvenile mud crab Scylla paramamosain

An 8 weeks regression study was performed to test the effects of increasing light intensities from darkness to 30 W m- 2 on growth performance, molting, antioxidant capacity, and gene expression of molting and apoptosisrelated genes in Scylla paramamosain. No significant differences were found in survival rates between treatments (ranging from 71.7 to 87.3% at the end of the experiment). However, weight gain and specific growth rate over the study period displayed a curvilinear response to light treatments with peak values in crabs exposed to 10 W m- 2. Linear (BLM), quadratic (BQM), and 4-parameter saturation kinetic (4-SKM) models show the optimal light intensities for SGR were 12.98, 18.27, and 11.36 W m- 2, respectively. The light intensity did not appear to impact molting. However, crabs reared in darkness showed significantly reduced molt frequency (3.51 +/- 0.16) and extended intermolt intervals compared to other treatments. Melatonin levels in the eyestalks were significantly higher in crabs exposed to darkness (502.52 +/- 56.24 pg mL-1) than light intensities of 10-30 W m- 2 (413.50 +/- 32.38 and 384.99 +/- 15.56 pg mL-1). Cortisol levels were significantly lower in the 0 and 5 W m- 2 groups. Light intensity significantly impacted the activity of antioxidant enzymes, with crabs showing a significant increase in total antioxidant capacity (T-AOC) under 10 W m- 2, catalase (CAT), and superoxide dismutase (SOD) under 15 W m- 2 and lower malondialdehyde (MDA). Gene expression of the molt-inhibiting hormone (MIH) was downregulated in eyestalks from crabs exposed to 10 W m- 2 compared to darkness and 20-30 W m- 2. Expression of apoptosis-related genes did not show clear light intensity trends. Taken together, these results suggest light intensity can impact S. paramamosain growth, molting, stress levels, and antioxidant capacity. As such, light regimes used in crab farming should be carefully considered to optimize productivity and welfare.