Caridean shrimp as mesopredators on seaweed epifauna in northern New Zealand

Abstract

Marine foodwebs typically involve multiple predators, and the roles of small invertebrate predators in these systems are often overlooked in favour of larger, predatory fish. This study investigated the role of two caridean shrimp, Palaemon affinis and Hippolyte bifidirostris, as mesopredators on temperate rocky reefs around northern New Zealand. Their direct and non-consumptive effects on epifaunal populations associated with the brown macroalgae species Carpophyllum plumosum and Carpophyllum flexuosum were investigated experimentally under controlled conditions in the lab. The relationship between H. bifidirostris densities and epifaunal densities on C. flexuosum in shallow, subtidal rocky reefs along the coast of northern New Zealand was evaluated with samples taken from the field sites. In the lab both shrimp species reduced epifaunal densities across all prey sizes investigated. (0.25 mm to >2.8 mm). In the field, there was a negative relationship between epifaunal density and H. bifidirostris density, which was much stronger for smaller than larger epifaunal individuals. This relationship also correlated with environmental conditions. Epifaunal density increased, and H. bifidirostris density decreased, at sites with increasing wave exposure. In experiments on the non-consumptive effects of shrimp on epifaunal behaviour, gammarid activity did not change between day and night. Gammarid activity increased in the presence of H. bifidirostris, but there were no behavioural responses to the presence of P. affinis. The differences in the size range of epifauna affected by shrimp between the lab experiments and the field survey may be due to the absence of larger predators in the lab experiments. Predatory fish typically target epifauna above 1.0 mm (Edgar and Shaw, 1995), limiting prey sizes available in nature. Similarly, the epifauna’s behavioural response to shrimp as the sole predator in lab experiments may be different from their response in the presence of a combination of predators. Larger predators often limit the effectiveness of mesopredators and alter the influence mesopredators have on prey. Hippolyte bifidirostris and P. affinis have the capacity to affect epifaunal populations directly and indirectly. However, the extent of these effects in nature is likely modified by several factors including environmental conditions, seaweed characteristics, and the presence of other predators.