THE RELATIVE IMPACT OF PHYSICAL DISTURBANCE AND PREDATION BY CRANGON-CRANGON ON POPULATION-DENSITY IN CAPITELLA-CAPITATA - AN EXPERIMENTAL-STUDY

In a laboratory experiment, the short-term (hours-days) responses of l aboratory-reared populations of Capitella capitata (Annelida: Polychae ta) to predation by Crangon crangon (Crustacea: Decapoda) or physical disturbance of the sediment, were tested in relation to stable environ mental conditions. Population density (measured as active tubes at the sediment surface) remained stable ill the control (140.8 +/- 12.8 ind /100 cm(2)),whereas rapid, significant changes were recorded in the pr edation treatment and the disturbance treatment (0-level reached withi n three days of predation, and immediately after sediment disturbance) . Recovery rates following both treatments were rapid (48 h after sedi ment disturbance and 11 d after predation by Crangon). Simultaneously, no predatory effects were recorded on meiofauna in the sediments. It is concluded that Crangon potentially is an important predator on Capi tella, but that this predation to a large extent is in the form of cro pping the tails of the polychaete (seen in gut contents of the shrimp) , which allows rapid recovery. Sediment disturbance destroys the tubes of the worms, and forces them deeper into the sediment, and after an event of disturbance the Capitella-population rapidly recovers in term s of activity at the sediment surface.