Sinonovacula rivularis sp. nov., found in Minjiang Estuary Waters of Fujian Province,was identified as a new species after study,and it belonged to Sinonovacula,Pharellidae,Bivalvia.The outer shape for the shell of S. rivularis sp. nov. was similar to that of S.constricta (Lamarck,1818), and it was likely that S. rivularis sp. nov. and S. constricta had a relatively closer affinity.Because the morphological structure of spermatozoon had the specificity of species,the differences between the species could be shown by studying and comparing the ultrastructure of spermatozoa with relatively closer affinity and could be used as the auxiliary evidence for related classification.This article observed the ultrastructure of mature sperms of S.rivularis sp. nov. and S.constricta respectively by scanning electron microscope and transmission electron microscopy aiming at providing a certain basic data and related figures for the reproductive biology research of S.rivularis sp.nov. and also providing strong evidences for differentiating S.rivularis sp.nov. and S.constricta. The samples of S.rivularis sp.nov. and S.constricta were collected in mating season(in November).Samples of S.rivularis sp. nov. were collected from coastal waters of Meihua town,Changle city,Fujian,while samples of S.constricta were collected from Xiamen Seafood Market.The sample preparation method of scanning electron microscope:dissecting the mature brood stock,extracting the sperms by suction tube and fixating by glutaric dialdehyde of 2.5%(V/V);taking 1-5 mL of the fixated and concentrated sample,and filtering it into the nuclear porous filter membrane at the size of 1.8 μm;then washing them three times with sterilized sea water and distilled water respectively,each time 10 minutes,dehydrate it once using 0%,50%,70%,90% and 95% graded ethanol respectively; and then dehydrate it twice using absolute ethyl alcohol,10 minutes for each grade.The sample is to be dried in CO₂ Critical Point Dryer; the sample after being dried is to be sprayed with metal for 25 s;and using LEO1530 field emission scanning electron microscope to observe and take photographs.The sample preparation method of transmission electron microscopy:selecting male S.rivularis sp. nov. with full and well-developed gonad,absorbing spermary by the suction tube,and quickly placing it into the fixative solution(2% paraformaldehyde 2.5% glutaraldehyde 0.05% CaCl₂,pH 7.6,0.1 MPBS),and then fixating it in an environment of 4 ℃,after dehydration and embedding,and then cutting it into slices by NOVA ultramicrotome; dyeing it with 3% uranyl acetate-lead citrate; using JEM-2100HC High contrast TEM to observe and take photographs.The ultrastructures of spermatozoa of Sinonovacula rivularis sp. nov. and Sinonovacula constricta were examined by transmission electron microscopy(TEM)and scanning electron microscopy(SEM).The results showed that there were a great deal of differences between S.rivularis sp. nov. and S.constricta. The mature sperm of S.rivularis sp. nov. was a typical primitive type,consisting of head,middle piece and tail. S.rivularis sp. nov. and S.constricta both have ellipse-shaped acrosome,but the length of acrosome for S.rivularis sp. nov. was shorter than S.constricta. The nucleus of S.rivularis sp. nov. was oblate with 8-9 arc-tubes in outer margin,whereas the nucleus of S.constricta was spherical.The middle piece of S.rivularis sp. nov. contained five mitochondria,and typically a proximal and a distal centriole.Few mature sperm of S.rivularis sp. nov. contained 4 or 6 mitochondria.There were 5 or 6 mitochondria of S.constricta based on published information.The flagellum exhibited the typical 9 2 microtubule structure in both species.