Bioassay on horseshoe crab embryo survivability in cultures contaminated by Bacillus thuringiensis

Horseshoe crab embryos are highly sensitive to temperature and microbial stress. In this bioassay, embryos were tested under ambient, constant temperature (32 °C), dark conditions and another set with Bacillus thuringiensis. Six developmental stages were recorded for ambient and constant temperature settings. Survivability (100%) was maximum only in ambient temperature, it declined for 32 °C to 76 % and it was 70.8 % together with darkness. Constant 32 °C accelerated hatching to 37 days in comparison to 40–12 days for ambient group. Exposure to B. thuringiensis arrested the embryogenesis in first embryonic moult stage and survivability was only 32.8%. There was abnormal coloration from degraded eggshell and these embryos were reduced in size and weight. Later, the advanced embryo had impaired limb buds. Infection rates were lowest at ambient temperature with darkness (20%) and highest under 32 °C with darkness (67.2%). Basically, thermal manipulation was stressful to horseshoe crab embryos because the hatching success rate and immunity was lowered. The presence of B. thuringiensis affirmed that stress increased vulnerability to pathogens. Understanding the presence of combined stressors is critical for biodiversity conservation and aquaculture management of C. rotundicauda.