Population dynamics of a free-living nematode Terschellingia longicaudata in relation to sedimental environment on the Tanakagawa tidal flat, Mie Prefecture, Japan

The free-living marine nematode Terschellingia longicaudata is a cosmopolitan species inhabiting muddy tidal flats, which has tolerance to pollution and low oxygen concentration in sediments. Although T. longicaudata is known to be a useful indicator of tidal flat environments, we currently have little ecological information on this nematode. In this study, we examined the distribution and population dynamics of T. longicaudata on a small tidal flat at the mouth of a short river (the Tanakagawa River) along the western coast of Ise Bay, central Japan. Sampling was undertaken at 13 sites on the flat once in June 2013, and also every 3 months from July 2013 to April 2014. T. longicaudata was found to be highly abundant at two sites, St.1 and St.13, which were characterized by reductive sediment with a high organic matter content. The density of T. longicaudata was in the range of 1.7−2.3×103 ind./10 cm2 at St.1 and 0.5−1.7×103 ind./10 cm2 at St.13. From August 2013 to July 2014, sampling was also undertaken every month at these two sites dominated by T. longicaudata. Juveniles and adults, including gravid females, occurred throughout the year at both sites. On the basis of monthly data for environmental conditions and T. longicaudata density, the density showed negative correlations with the ORP and salinity of the interstitial water. In contrast, the rate of gravid females showed a positive correlation with sediment temperature. In this study, we observed that T. longicaudata reproduced throughout the year. Further, the density of this species in the high-density distribution regions suggested that it is more sensitive to variations in the salinity and redox potential of interstitial water than to seasonal environmental variations.