Facing the arrival of newcomers: an intertidal sea anemone approach (Hexacorallia, Actiniaria)

Abstract

In this experimental study we explore a dominance-subordination network within a key group of polyphagous opportunistic organisms. This study also includes the possible reconfiguration of this network after the arrival of exotic species of the same ecological guild. Intertidal sea anemone species have been used as an experimental model for this purpose. There are a few documented fight-specialized external morphological structures in actiniarians, including acrorhagii and catch-tentacles, which are also accompanied by a more-or-less differentiated endowment of cnidocysts. Although it is possible to identify an old immigrant in the native dominance-subordination network under study, we also explore the hypothetical arrival of an exotic population of a pantropical sea anemone species, and its effects on the current set of interactions. In this community we detected the presence of the alien species Diadumene lineata (= Haliplanella lineata, = H. luciae) of Pacific origin, which is considered to be a relatively old immigrant in European waters. Exaiptasia pallida (= Aiptasia pallida, among other synonyms), with a worldwide warm-tropical distribution, is here used experimentally as a second invasive alien. A simple Competitive Efficiency index (CEi) is proposed in order to quantify the competitive capacities of these species. The CEi of a given species is based on the percentage of conflicts won, lost, tied, and mutually avoided. Our results demonstrate how alien species can re-organize the hierarchy and dominance-subordination relationships in an established network of interactions, even considering the possible ecological substitution of natives by alien species which are ecologically equivalent (but not identical in their relationships).