Differential recruitment of annelids onto tidal elevations in an estuarinemud flat

The recruitment of an annelid community was examined on 2 tidal elevations in 2 kinds of sediments along an inclined intertidal mud flat in an estuary in northern Taiwan. Recruitment onto defaunated and ambient intact sedimen ts (control) in both low and high tidal zones was followed weekly from 3 Ap ril to 1 May 1996. The overall density of recruits in the defaunated sedime nts reached the level of the control sediments within 3 wk in the low tidal zone, whereas in the high tidal zone the density of recruits did not reach the control level during the study period. Two polychaete species and 1 ol igochaete species were the major recruits, and each exhibited a different r ecruitment mode and rate. The spionid Prionospio japonica and the oligochae te Doliodrilus tener were among the most rapid recruits. P. japonica simult aneously exhibited 2 recruitment modes, larval settlement and postlarval tr ansportation, and its recruit density in the defaunated sediments in the lo w tidal zone reached the level of the control within 1 wk. D, tener recruit ed predominately by means of juvenile migration. In general, recruitment by settling larvae occurred at discrete intervals. whereas that by postlarvae or juveniles appeared to be continuous. The densities of P, japonica and o f D. tener exhibited a zonation pattern wherein density increased along a d own-shore gradient. The sedimentary characteristics also exhibited a simila r zonation pattern, wherein the sediment particles became smaller and silt/ clay content increased in the low tidal zone. This coupled phenomenon sugge sts that tidal action has a strong influence on the recruitment of P. japon ica and D. tener. These recruits are passively entrapped in the low tidal z one due to the depositional conditions created by slow flows. However, the tidal zonation of the capitellid Capitella sp, showed an opposite trend. wi th greater recruitment in the high tidal zone. indicating that the recruitm ent of Capitella sp. possessed an active component. D. tener recruited most ly into the low tidal zone, but a small, significant proportion of the recr uitment occurred in the high tidal zone. Capitella sp. and D, tener are sub surface burrowers, and thus, to some extent. their recruitment is less cont rolled by tidal action. Overall, the recruitment of this annelid community is influenced by both physical and biological controls.