Meiofaunal Ecology In Various Marine Habitats And Phylogenetic Analysis Of The Order Enoplida (Phylum Nematoda)

Meiofauna is well-recognised as an abundant and ubiquitous component of metazoa and plays a pivotal role in sediment stability,nutrient cycling and energy flow.So it is an important indicator to monitor marine sedimentary environment.Purported to be the most abundant metazoan group,nematodes play an important role in meiofauna ecology.While nematodes are inherently small animals,identification of them often requires high-resolution light microscopy.This factor makes nematode identification with morphological characteristics is often difficult and laborious,and no doubt contributes to the limited taxonomy in this group.With the developing of molecular tools,some phylogenetic confusion of nematodes has been resolved.Yet the representation of nematodes'sequences is insufficient,so many phylogenetic relationships are still ambiguous.On the one hand,this study focused on investigating and analyzing meiofaunal community and environment in Laizhou Bay of Bohai sea,Muping offshore area of Yellow sea,Nanji Islands'intertidal zone of the East China sea and the Northwestern Pacific ocean.On the other hand,combining taxonomic characters and molecular sequences,we analysed phylogenic relationships of free-living marine nematodes,such as Enoplida,Monhysterida and Chromadorida,which were abundant in the ocean.The result about meiofaunal abundance in various marine habitats revealed,meiofauna in Laizhou Bay,Muping offshore area,Nanji Island and the Northwestern Pacific ocean is 1979 ind/10 cm~2,2741 ind/10 cm~2,556 ind/10 cm~2 and 147 ind/10cm~2,respectively.In Laizhou Bay and Muping offshore area,the content of chlorophyll-a had a significant positive correlation with meiofaunal abundance.According to reference values in different types of sediments,the nematode/copepod ratio(N/C)could be used to evaluate the condition of sediment organic pollution in the two sea areas.In Nanji Island,seasonal sampling revealed temperature and the content of chlorophyll-a had significant positive correlations with meiofauna abundance.There was higher abundance in HKA because of higher primary productivity,and seasonal fluctuation of meiofaunal abundance was much heavier than in DSA.Tardigrades were rich in high tide of HKA where had higher content of chlorophyll-a than in low tide,and the fluctuation of their abundance was opposite to other groups'during one year.Tourists visiting decreased meiofaunal abundance and diversity in sallow layer(0~8 cm)and the community would recover within three monthes during off season.Meiofaunal community in the Northwestern Pacific ocean was weakly related with environmental parameters in our study,and a solid conclusion was still absent in similar researches.Analyzing the meiofaunal structure and environmental parameters among these habitats,grain size negatively influenced the vertical distribution of meiofauna.Intertidal zones and sallow seas were easier to be disturbed by anthropogenic disturbance.The meiofaunal bundance,N/C,and vertical distribution of meiofauna were useful to monitor the influence of anthropogenic disturbance.In the chapter of nematoda phylogeny,we mainly sampled nematodes from Qingdao and Nanji Island,and captured their morphological characters using multifocal images before getting their sequences.Finally we got 63 SSU and 44 LSU gene sequences of 45 nematode species and analyzed them combining with some sequences from database.Both Maximum Likelihood and the Bayesian Inference method supported Dorylaimia as the earlist branch of Nematoda.As one of the richest nematode groups,Enoplida consisted of five clades in our research.Leptosomatidae belonged to Ironina in current phylogeny of Nematoda was placed in Enoplina with high support;Alaimina was poorly supported monophyletic clades;some families were paraphyletic,such as Oxystominidae,Oncholaimidae,Anoplostomatidae and Ironidae.In other groups,Monhysterida and Araeolaimida were recovered as paraphyletic clades in our analysis.And Microlaimoidea was placed outside the main group of Desmodorida and closer to Chromadorida,so we supported Microlaimoidea was separated from Desmodorida.Five families within Chromadorida recovered monophyletic with high support,but Chromadorida was paraphyletic with low support value.Similar ambiguities are still remaining and to be resolved.