Population and reproductive biology of an intertidal sandstone-boring isopod, Sphaeroma wadai Nunomura, 1994

Abstract

Population structure, growth and reproductive activities were investigated in a field population of an intertidal isopod, Sphaeroma wadai, burrowing in sandstone for 1 year. The sex ratio was biased toward females throughout the year (male/ female: 31.9–79.7%). Heterosexual pairs living in the same burrow occurred from November to July, and their sizes were positively correlated to each other only in the main pairing season. Ovigerous females were observed from April to September, with the brood size ranging from 1 to 37 (18.8, SD 9.35). The number of broods per female is estimated to be one per year. The earliest stage of juveniles occurred from June to September, with most of them cohabiting with mature females in the same burrow. With successive developmental stages of juveniles, the proportion of juveniles cohabiting with mature females decreased. The maximum body length was greater in females (10.70 mm) than in males (10.20 mm). The life span was estimated at almost 2 years or about 15 months. We conclude that the distinct life history characters of S. wadai (sexual dimorphism, prolonged mate-guarding, cohabiting of females and offspring) are a consequence of its boring habit.