Invasion of a reef-builder polychaete: Direct and indirect impacts on the native benthic community structure

In this work, we evaluate the effect of the introduced reef-building polychaete Ficopomatus enigmaticus on the benthic community and on sediment characteristics of a southwestern Atlantic coastal lagoon. When reefs were experimentally added, density of the native crab Cyrtograpsus angulatus dramatically increased in a short time period. After reefs and crabs were experimentally transplanted as a unit, they decreased the density of the free-living soft-bottom polychaetes Heteromastus similis and Laeonereis acuta. Exclusion/inclusion caging experiments showed that Cyrtograpsus negatively affect the density of soft-bottom polychaetes (H. similis, L. acuta, Nephtys fluviatilis) and ostracodes. Our results showed that this effect is much higher in areas populated by reefs because of the increased density of crabs that find shelter under the reefs. Thus, reefs have a cascading effect on the native benthic community within the areas colonized by them. Analysis of crab stomach contents indicated that crabs feed on a wide variety of prey, including infaunal organisms, small gastropods and also algae. When reefs and crabs were experimentally added, the amount of bivalve shells on superficial sediments increased. Our results suggest that this bivalve shell accumulation and sediment composition are due to the reworking activity of Cyrtograpsus in the sediment where they dig burrows. The invasive habits of Ficopomatus may be favoring crabs to have a major effect on the integrity of the native community in the lagoon. Ficopomatus should be considered a bioengineer organism by creating and regulating refuge for other species, altering the interactions between preexistent species and also by changing the physical factors of the invaded environment.