缢蛏属一新种

A new species of the genus Sinonovacula

笔者对福建省闽江口、长乐市梅花镇海区滩涂的一种蛏进行研究,发现该种的壳长与壳高比、精子细胞形态、遗传基因(另做报道)等方面有别于缢蛏.新种的贝壳壳长与壳高比值较缢蛏大,生活水域的比重较缢蛏低.精子细胞核外缘呈瓣状,具8-9瓣,顶体长度约为缢蛏的1/3,为福建河口区缢蛏属一新种(软体动物门,双壳纲,灯塔蛤科),定名为近江蛏.

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, Sino novacula. 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. rivuiaris 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 uhrastructural features of the sperm of S. rivulads are investigated. The spermatozoa of S. rivulads 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 O. 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 mitochon- dria 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, rivulads and S. constricta from the same genera show in Table 1. S. rivularis is conchologically similar to Sinonovacula constricta, furthermore it possesses groove that is an important character orS. constricta. Consequently, S. rivularis is eonsidered as S. constricta for long. However, the ratio of shell length to shell height of S. rivularis is greater than that of S. corutricta, besides, the density range of surrounding seawater where S. rivuiaris lives is lower than that where S. corutricta lives. More importantly, there is a remarkable difference in the ultrastrueture 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.