Morphological studies on the early development of Rapana venosa

Embryos and larvae of the prosobranch gastropod mollusk Rapana venosa were artificially incubated and hatched from egg masses under laboratory conditions,and morphological changes during the early development were observed. Fertilized eggs developed into veligers in about 16 days at temperature 25℃. Embryonic development included six stages: cleavage,blastula,gastrulae,trochophore in egg membrane,intra-membrane veliger,and veliger. The planktonic larval development was divided into five stages,including one-spiral whorl stage,tw o-spiral whorls stage(early,mid and late),early three-spiral whorls stage,mid and late three-spiral whorls stage,four-spiral whorls stage(early,mid and late). The highest growth rate of larvae was observed during the late tw o-spiral whorls stage with an increase of 44.45 μm in shell height per day,while the lowest was in early three-spiral whorls stage with an increase of 19.51 μm in shell height per day. The settlement and metamorphosis of veligers occurred in the mid-to-late four-spiral whorls stage with the shell height of 1250-1500 μm. Morphological changes during the settlement and metamorphosis of veligers could be summed up as: incrassation of edge of shell aperture, exstrophy of shell aperture,disappearance in the middle edge of shell aperture,degeneration of velum of veligers,and the broadened feet which could flex freely.