The association of a non-native amphipod with an invasive reef-building polychaete as an enhancer of biological invasion processes

Abstract

Several population dynamics and reproductive traits of the non-native amphipod Monocorophium insidiosum (Crawford 1937) were studied during their reproductive season, considering their association with the invasive polychaete Ficopomatus enigmaticus (Fauvel, 1923) in the Mar Chiquita coastal lagoon (37° 40 S, 57° 23 W). Amphipods were sampled biweekly in F. enigmaticus reef-like structures (“reefs”) and in sediment from areas without reefs. In reefs, the vertical gradient was also considered. There were differences in M. insidiosum population parameters between habitats (reef and sediment) and among layers within the reef (surface, middle and bottom). Populations in reefs overall displayed higher abundance, continuous reproductive activity and recruitment, and a male-biased sex ratio. In contrast, sediment populations exhibited lower adult densities, sporadic reproduction and recruitment, and a predominantly female-biased sex ratio. Within reefs, the surface had the highest population densities, consistent reproductive activity and a male-biased sex ratio, whereas there was no defined pattern at the bottom aside from a higher proportion of juveniles. These results suggest that the degree of population expansion of M. insidiosum into the coastal lagoon is shaped by a strong association with F. enigmaticus reefs, where M. insidiosum responds to small-scale changes in the vertical habitat structure that enhance its population reproductive potential (i.e., recruitment). This work provides evidence of different non-native and invasive species strategies in which recruitment patterns inside F. enigmaticus reefs might be a mechanism for M. insidiosum population permanence.