2.1. Preparation of Water-Accommodated Fractions (WAF) of 380# Heavy Fuel Oil (HFO)

HFO 380# was purchased from Dalian Marine Fuel Co., Ltd., China. The natural seawater used for the WAF preparation and sea urchin culture was from Dalian Xinghai Park, with a salinity of 34 parts per thousand (ppt). HFO WAF was prepared according to the method of a previous study, with minor modifications [20]. Briefly, HFO WAF was prepared with the prefiltered natural seawater (salinity: 34 ppt) at an oil-water ratio of 1: 40 (w/v). The mixture was stirred for 18 h with low mixing energy using a magnetic stirrer and was then settled for 6 h. After settling, the WAF solution was separated from the oil phase and stored at 4 °C in the dark until subsequent treatment tests.