2026 Volume 21 Issue 2 Pages 111-120
The periodicity of vulvar opercular decalcification, mating, and oviposition was examined in two sentinel crabs, Macrophthalmus japonicus and M. abbreviatus. For both species, the percentage of decalcified females varied with the semi-monthly tidal cycle of peaks around spring tides. Most females with decalcified opercula oviposited eggs within two days of sampling, and their opercula were recalcified within one day of oviposition in M. japonicus and within half a day in M. abbreviatus. Both surface copulation and underground copulation have been observed in M. japonicus. However, only surface copulation has been observed in M. abbreviatus. Nevertheless, copulations were rarely observed in either species. The ovigerous females of both species fed actively on the surface and spawned the next brood shortly after hatching. These results suggest that opercular decalcification occurs after ovarian maturation and that, likely, the eggs are fertilized by stored sperm in many females. Although the influence of opercular calcification on mating systems remains unclear, short decalcification periods may enable females to avoid multiple matings. Fertilization using stored sperm allows successive oviposition and reduces the risk of predation.
