A new species of the genus Sinonovacula
HUANG Rui,ZHANG Yun-fei(Haiyang Technology College of Xiamen,Xiamen 361012,China)
In 2002,in the research on artificial propagation of Sinonovacula rivularis along the Fujian coast,the authors found that some characters of this species were obviously diffrent from Sinonovacula constricta(Lamarck,1818).Experimental study indicates that S.rivularis is a separate species and named Sinonovacula rivularis sp.nov..This new species belongs to Bivalvia,Pharellidae,Sinonovacula.S.rivularis is laterally compressed and possesses a shell composed of two frail and thin valves that enclose the body like a column.When the two similar valves closed,the anterior and back ends are open.The shell is covered by periostracum,the colour of which depends on the age of shell and the amount of organic compounds in the habitation.When the age is young and the amount of organic compounds are low,the shell is yellowish green,on the contrary it is taupe.Because of bottom dwelling and friction caused by vertical motion,the periostracum of shell from umbo to peripheral falls off and white calcareous prisms layer is exposed.The shell length is 3.4 longer than the shell height,and is 5.2 longer than the shell width.The umbo is located at 1/3 of the forepart of the body, and there is a brown ligament after the umbo.In the posterior ventral edge of the shell there is an oblique and shallow groove.Beneath the umbo there is a hinge,and there are three cardinal teeth——anterior tooth,posterior tooth and medial tooth——located on the hinge line of the left valve.The medial tooth is bigger and bears several cusps;there are two cardinal teeth on the hinge line of the right valve.When the valves are closed,the cardinal teeth on the hinge line of the both valves are locked each other,in the other word,the anterior cardinal teeth of the right valve is between the anterior cardinal teeth and anterior cusp of medial teeth of the left valve,and the posterior cardinal tooth of the right tooth is between the posterior tooth and posterior cusp of medial tooth of the left valve.S.rivularis inhabitats in the medial tide and low tide of the intertidal zone of the estuary.They burrow in the soft mud or mud-sand substrata where the density of the seawater is low.When the density drops to 1.003,they can still live normally.They can even live in freshwater for more than 4 days in an experiment.For this reason,while flood lasts so long time that S.constricta living in the estuary all die,the S.rivularis can still survive.However,you can hardly find S.rivularis in the seawaters where the density is above 1.015;and when the density rises to 1.020,the two valves of S.rivularis are closed,and all of them die after 20h.Using the scanning electronic microscopy,the ultrastructural features of the sperm of S.rivularis are investigated.The spermatozoa of S.rivularis is primitive type,consists of a head,a midpiece and a tail.The head includs acrosome and cellular nuclear.Acrosome is composed of acrosomal head and acrosomal stem.The acrosomal head shows converse V with a length of 0.97 ～1.0 μm,and acrosmal stem is 1.6 μm long with a diameter of 0.12 μm;the exterior part of sperm nuclear is 0.8 thick,with a diameter of 2.0～2.2 μm,and is composed of 8～9 lamellas,and there are five spheral mitochondrias in the middle section(occasionally four).The diameter of mitochondria is 0.4～0.6 μm,surrounding the centrioles.The tail is about 54 μm long and with a diameter of 0.15 μm.The comparison of characters between S.rivularis and S.constricta from the same genera show in Table 1. Tab.1 Comparison of Sinonovacula rivularis sp.nov.with Sinonovacula constricta(Lamarck)ltems Sinonovacula rivularis sp.nov.Sinonovacula constrictashell length/shell height 3.41,the ratio of new species is 9% greater than that of S.constricta(picture 2) 3.12shell length/ligament length in D larval 1∶0.75 1∶0.6acrosome of the sperm The length of acrosome of S.rivularis is about 1/3 of that of S.constricta(picture 3e);the length of acrosome 2.4 μm;the head is 1.0 μm long,the length of acrosomal stem is about 1.6 μm;the maximum width of acrosome is 0.2 μm The length of acrosome is 7.0～7.5 μm;the head is about 1.1 μm long;the length of acrosomal stem is about 5.9 ～6.4 μmnuclear of the sperm The exterior part of sperm nuclear is 0.8 μm thick,with a diameter of 2.0～2.2 μm,and is composed of 8～9 lamellas The nuclear is near circular,with a diameter of 1.8 ～2.0 μm and a height of 0.83 μmdensity of thesurrounding seawater They can live normally in the sea,where the density of seawater is 1.003;they are rare when the density is 1.015,and die when the density is 1.020 They can live normally when the density is 1.020,but can not exist when the density is 1.005growth Rapid growth,the shell of two years old can reach above 80mm(picture 2) The shell of two years old cannot exceed 70mm S.rivularis is conchologically similar to Sinonovacula constricta,furthermore it possesses groove that is an important character of S.constricta. Consequently,S.rivularis is considered as S.constricta for long.However,the ratio of shell length to shell height of S.rivularis is greater than that of S.constricta,besides,the density range of surrounding seawater where S.rivularis lives is lower than that where S.constricta lives. More importantly,there is a remarkable difference in the ultrastructure of the sperm.The form and structure of sperm could be one of the taxonomic standards in the class of Mollusca,because the structure of sperm is characteristic in different species.Thus,Sinonovacula rivularis sp.nov.is inferred as a separate species.
【CateGory Index】： Q959