Reproductive biology of the intertidal and infralittoral mysids Archaeomysis kokuboi and A. japonica on a sandy beach in NE Japan

Abstract

Seasonal reproductive biology was investigated in 2 species of Archaeomysis, each of which occurs in distinctive habitats, intertidal and infralittoral areas of a sandy beach of NE Japan. The seasonal reproductive patterns of the 2 species are similar, with 3 breeding peaks from spring to autumn. The populations are initially dominated by larger sized overwintering and then smaller sized spring/summer generations. However, intertidal A. kokuboi exhibits a more extended breeding period than infralittoral A. japonica. The brood size (number of Stage I larvae female-1) increased with increasing size of females and averaged 12 to 72 for A. kokuboi and 10 to 55 for A. japonica, showing a higher brood size in the overwintering individuals of A. kokuboi. The larval size and development time and the interval between broods decreased with increasing temperature; and development took much longer in A. japonica at the lower temperature range. In overwintering generations, the larval (Stage I) production rate of A. kokuboi (0.9 to 3.4 larvae female -1 d-1) was higher than in A. japonica (0.6 to 2.1), while after July the rate of both species fluctuated in almost the same range (0.4 to 1.9). The specific larval production rate of both species was significantly related to water temperature and was 17 to 38 % higher in A. kokuboi than in A. japonica throughout the year. This study found that A. kokuboi retains a higher reproductive effort to settle into the intertidal zone of sandy beaches, but does not show any specialization to the basic reproductive features, such as an altered generation pattern or a change in larval size. The retention of a high reproductive potential in A. kokuboi is an adaptation to the intertidal zone of sandy beaches which is characterized by a high mortality rate due to unpredictable events such as storms, while efficient reproduction during the high water temperature season in A. japonica is an adaptation to the infralittoral habitat where more predictable events, such as seasonal variation of temperature, prevail as environmental variables.