Analysis of different patterns in marine communities with Arbacia lixula and Paracentrotus lividus : annual and day-night changes in algal cover and response to predation

Abstract

Sea urchins are considered as one of the main herbivores of marine ecosystems, to such an extent that they are able to modify the landscape given their great activity of grazing on seaweeds. Therefore, there are very different situations present in littoral habitats regarding the degradation due to the activity of sea urchins, in a very large area. This activity can also be modified by various factors such as the circadian rhythm, the season of year or the presence of signals related to their predators. This reaction to a risk signal is known as landscape of fear, and it can structure the landscapes significantly. In order to evaluate all these factors, we conducted a photographic monitoring of 16 plots with mixed populations of the sea urchins Arbacia lixula (Linaeus, 1758) and Paracentrotus lividus (Lamarck, 1816). The selected points have different levels of barrens and hiding (crevices). With this monitoring, it was possible to record the high capacity of modeling the landscape that sea urchins have, since in the same plot the algal cover changes significantly throughout the year. There was high correlation between the percentage of barren areas and the density of the A.lixula specimens, this correlation was not found for P.lividus. In contrast, in day-night comparisons, no significant differences were obtained for the behavour of the two species. On the other hand, we carried out experiments in aquariums with different specimens from both species, exposing them to the presence of dead sea urchins. Their movement rate was analyzed for two days and two nights (48 hours) and significant differences were found in the case of A. lixula, which reduces its activity with a risk situation. In addition, its activity increases at night, which could be associated with fear to daytime predators. Furthermore, in the tests made with P. lividus the results were not conclusive