Cold-shock-induced polyspermy and low-temperature tolerance in Japanese sea urchin eggs

Abstract Low temperature of seawater is critical for success of fertilization and reproduction in some species of sea urchin. Cortical reaction was prevented and polyspermy was induced in sea urchin eggs subjected to the cold-shock in selective species. By the observation with two fluorescent probes connected phalloidin and lectin, it was revealed that the cortical granules moved to deeper layer from surface in the eggs after the cold-shock treatment, and the signal of actin filaments at the surface were detected more sharply than those in the control eggs. The degree of effects of cold-shock varied among the species of sea urchin. High sensitivities to cold-shock were observed in some sea urchins with reproductive seasons in summer. In contrast, several sea urchins from the northern part of Japan or those species with reproductive seasons in winter were insensitive to the cold-shock. A stronger tolerance to low temperature was observed in the eggs of the family Strongylocentrotidae by the phylogenetic analysis of Japanese sea urchins. This tolerance to low temperature may be influenced on evolutional processes. Recently, it has been reported that a part of this family Hemicentrotus pulcherrimus are distributed in the more northern areas in the Sea of Japan. Tolerance to low temperature may confer advantages to certain species of sea urchins in terms of distribution and reproductive season.