THE IMPORTANCE OF SHELTER, DISTURBANCE AND PREY INTERACTIONS FOR PREDATION RATES OF TUBE-BUILDING POLYCHAETES (PYGOSPIO ELEGANS (CLAPAREDE)) AND FREE-LIVING TUBIFICID OLIGOCHAETES

In laboratory experiments, the hypothesis that tubes provide shelter f or soft-bodied annelid prey from epibenthic predation was tested. The prey (free-living tubificid oligochaetes and the tube-building polycha ete, Pygospio elegans (Claparede)) were given different acclimation pe riods (1, 2 or 24 h) prior to predator (brown shrimp, Crangon crangon L.) introduction. Oligochaetes and Pygospio were tested both separatel y and combined at prey densities of 7500 ind.m(-2). Results showed tha t increasing acclimation period increased the survival of both prey ty pes when tested separately. Significant differences were found between 1 and 2 h, but not after 24 h of acclimation. The survival of Pygospi o was higher than that of oligochaetes. Survival of prey did not, howe ver, increase with increasing acclimation period when the species were combined. Instead, the survival of oligochaetes was significantly low er after 24 h than after a l-h acclimation period. Overall, the result s gave only weak support for the a priori hypothesis, but indicate tha t physical disturbance (simulated by short acclimation period) may inc rease the predation effect of Crangon on both oligochaetes and Pygospi o. When present together the interference (competitive interaction) be tween the prey species particularly decreases the survival of oligocha etes. (C) 1997 Elsevier Science B.V.