Gonad development and recruitment pattern of the oysters Crassostrea ariakensis and Crassostrea sikamea in a subtropical bay in China

Abstract

The extent of oyster stocks and their habitat loss experienced on the Chinese coasts suggests that oyster restoration will be necessary to rebuild direct fisheries and recover lost ecosystem services. Limited understanding of oyster biology and ecology is among the potential barriers to successful restoration along the Chinese coasts. At a natural oyster bed in Jiantiao Bay, Sanmen County, China, the study compared gonad development and spawn duration of the Sumonie oyster Crassostrea ariakensis and the Kumamoto oyster Crassostrea sikamea through histological sections from May 2020 to March 2021. Their temporal and vertical recruitment patterns were simultaneously observed along an increasing tidal elevation gradient (0.1 m, 0.6 m, 1.1 m, 1.6 m, 2.1 m). The gonad index did not differ between the oysters (P>0.05), with the maximum values in June–August, but C. sikamea had a longer spawning duration (May–December) than C. ariakensis (May–October). Crassostrea ariakensis recruited to only the lower 3 tidal zones (0.1 m, 0.6 m, 1.1 m), and its recruits varied linearly and significantly as an inverse function of tidal elevation. Significant 2nd degree polynomial fits best described the relationships for C. sikamea recruits and tidal elevation. The greatest recruits were commonly observed at the mid intertidal zones (0.6 m, 1.1 m and 1.6 m). An evident vertical settlement pattern emerged for C. sikamea at both 1-day and 3-day experiments; the greatest settlement rates occurred in the 1.6 m tidal zone, intermediate rates occurred in the 1.1 m and 0.6 m, the lowest rates occurred in the 0.1 m and 2.1 m. The study concluded that C. sikamea sustained longer spawning and recruitment durations than C. ariakensis, and a strong zonation pattern in recruitment emerged; C. sikamea achieved its maximum recruits in the mid-tidal zone, while the recruits of C. ariakensis linearly decreased with increasing tidal elevation.