EFFECTS OF TIDAL LEVELS ON THE RECRUITMENT OF AN ANNELID COMMUNITY IN AN ESTUARINE INTERTIDAL MUD FLAT
Hsieh, H.-L.1; Chen, C.-P.1,2* & Shu, C.-F.2
1 Institute of Zoology, Academia Sinica, Taipei, Nankang, Taiwan, 115, ROC
2 Institute of Fisheries Science, National Taiwan University, Taiwan, Republic of China.
The effects of tidal elevation and of established infauna on the recruitment of an annelid community were examined in the inclined intertidal mud flat of an estuary located in northern Taiwan. Recruitment onto defaunated and control sediments at both the low and high tidal zone was followed weekly from April to May in 1995. The overall recruit density in defaunated sediments reached the level found in ambient control areas within one month. Prionospio japonica and the oligochaete Doliodrilus tener were among the most rapid colonists. The recruit density of the former in defaunated sediments reached the level in ambient control within 2 weeks and this rate was faster than that in D. tener. In addition, P. japonica exhibited two recruitment modes in which both larval settlement and postlarval migration were operating simultaneously, whereas D. tener recruited predominately by means of juvenile migration. Recruitment by larval arrival occurred at discrete but regular intervals, whereas that by postlarval or juvenile movement appeared at a continuous but irregular schedule. Tidal elevation profoundly affected the distribution of the recruits; by contrast, established infauna had no influence on recruit distribution. Zonal distribution of the recruit populations was formed at the recruiting phase. The densities of the whole recruit community, of both P. japonica and of D. tener, exhibited a zonation pattern where density increased along a down-shore gradient, with greater density in the low tidal zone than in the high tidal zone. This density gradient was parallel to the grain size gradient, where the sediment particles became smaller and silt/clay contents became greater in the lower tidal zone. This coupled phenomenon implies a strong influence of tide actions in the recruitment in which the colonists are passively entrapped in the low tide zone because the flow regime there creates deposition conditions. However, the zonal distribution of the capitellid, Capitella sp., showed an opposite trend, with greater recruitment found in the high tidal zone. Both the capitellids and the oligochaetes are sub-surface burrowers, and thus, to some extent, their recruitment is less controlled by tidal flows. Therefore, the recruitment in these two cases exhibits an active component and is not completely under the physical control of tidal flow. Moreover, the recruits are quite insensitive to the presence of established infauna despite high adult population densities, suggesting that recruit-adult interactions are unimportant. Alternatively, the intensity of recruitment may not be great enough to evoke such biological interactions.