Primary Study On Ontogenesis,Maturation Of Germ Cells,and Effect Of Environmental Factors On Early Development And Metamorphosis In Urechis Uniconctus

Urechis uniconctus is an unique specie of Xenopneusta along the seaboard in China, which is classified to Echiuroidea, Echiurida, Xenopneusta, Urechidae, Urechis. Because of its highly economic value and important function to the littoral ecology, it becomes a kind of marine invertebrate needing to be further exploited. In this work, the methods of microscope and electron microscope were used to study the development, maturation of germ cells, and effect of environmental factors on the early development and metamorphosis in Urechis uniconctus, to provide some basic knowledge of its reproduction, ecology and artificial breeding.There were two reproductive seasons one year: one was comparatively well-developed in May (lasting 50 days), and the other was in October (lasting30 days). The annual development of germ cells can be divided into 5 stages: early developmental stage(from July to the first ten days of August; from November to February of the next year), developmental stage(from March to April; the fist twenty days of August), mature stage(from the first twenty days of April to May; from September to the first of October), decline stage(the first twenty days of June; the last twenty days of October) and resting stage(the last ten days of June; the end of October). The ontogenesis of germ cells carried through in the coelomic fluid. When approaching the maturation, the sperm and egg moved into the nephridium. The ontogeny of bisexual gametes began in the form of cell mass in coelom fluid. The male germ cells deviated from the cell mass after maturation. When the diameter of oocyte reached 10μm, the oocytes began to grow in the coelomic fluid independently.Spermatogenesis included spermatogonia. spermatocyte, spermatid and spermatozoa. The spermatid phase could be divided into 3 stages: early, middle and late, according to the different statement of nucleus, mitochondria and acrosome. During the spermiogenesis, the cell size became smaller, and the nucleus changed from loose-condition to tight-condition. In the cytoplasm, the dispersed mitochondria assembled into one big mitochondrium gradually, which surrounded the centrosome. The cristae and matrix of the big mitochondrium got more and more developed. The well-developed Golgi complex located at the posterior part of nucleus. After they assembled into the pre-acrosome and moved to the anterior part of nucleus, the nipple-like acrosome was formed. The axial filament was formed by the distal centriole at the beginning of spermatid the early stage of spermatid, and the cross-section of axial filament showed typical "9+2" structure. According to the yolk statement, the oogenesis could be separated into 4 stages: pre-vitellogenesis, early-vitellogenesis, vitellogenesis and maturation. The nueleolus could be seen in the 4u.m egg. The yolk granule appeared at the egg diameter of 13urn. The yolk was formed by organelle such as mitochondria and Golgi complex, micropinocytosis and microvilli absorption. The different developmental stages have various formative modes. The double layer egg membranes may be formed by the electron dense material, microvilli and some tubule- secreted material. The inner was mainly formed by the electron dense material, and outer layer was formed by both two actions. The diameter of mature egg was 150±10.5 urn, and the germinal vesicle of some eggs broke.In this study, both the salinity and pH of seawater had the evident effect on the sperm vitality, fertilization rate, cleavage rate and larval hatching rate. At the comparative low salinity and weak alkaline seawater(S: 22—30, pH 7~9), the sperm vitality and embryonic development got the best result, and the optimal salinity was 26, pH 8. When the larvae hatching, the adaptive range of the salinity and pH became narrower, and the reason may be that the larva, which lost the egg membrane, contacted with the water directly. The temperature also had the effect on the sperm movement and fertilization rate. The low temperature can prolong the sperm life (For24 hours at room-temperature and 15 days at low-temperature, the sperm can maintain the vitality). The sperm movement and fertilization rate decreased along with preserving time.The effect of sediment on larval settlement and metamorphosis was also researched. The results show that, the mud and the mixture( consisted of mud and sdnd) are the good sediment for larval settlement and metamorphosis, especially the pure mud. After the larva turns benthic, it firstly gropes to choose the sediment, and then digs the U-shaped hole in the sediment.