Maybe there is too much alum. Sea urchins will melt after a while. Many manufacturers put some alum in order to keep the shape of sea urchins, but some manufacturers may put too much, which will make sea urchins bitter. The shell of horse manure sea urchin is hard and hemispherical, with a diameter of 30 ~ 40mm and a maximum of 60 mm. The feeding rate of sea urchin from horse manure to Undaria pinnatifida and kelp is significantly higher than that of Ulva pertusa, but its feeding absorption rate to Ulva pertusa is higher than that of kelp and Undaria pinnatifida.