Invasion by marine nonindigenous species (NIS) is a spread phenomenon. The tunicate Pyura praeputialis shows pronounced disjoint geographical distribution: along thousands of kilometers in wave-swept headlands on the southeastern coast of Australia, from where it appears to have originated, and exclusively along 60 -70 km inside the Bay of Antofagasta, Chile. mtDNA sequences suggested that the species invaded this rocky shore recently. We used field manipulations and juvenile P. praeputialis transplant techniques to test hypotheses regarding the capacity of the tunicate to survive and grow at different sites and tidal heights inside and outside Antofagasta, and its competitive performance for primary space (inside the Bay) against the native mussel Perumytilus purpuratus. We conclude that survival and growth of P. praeputialis showed no significant differences among sites inside and outside the Bay, and suggest that the restrictive distribution of the species in Chile is caused by a specific oceanographic retention mechanism and͞or its brief larval dispersal. We demonstrated that, inside the Bay, P. praeputialis outcompetes Perumytilus from the Mid-Low intertidal, constraining Perumytilus to the Upper MidIntertidal, modifying the local pattern of intertidal zonation. We show that predation on P. praeputialis juveniles by starfish and snails constitutes a regulatory mechanism for the setting of its low intertidal limit. Major ecological impacts caused by NIS invasions to rocky shores by aggressive primary space users may result in negative aspects, but also may contribute to biodiversity enhancement. We call attention to the need for increment manipulations and testing of ecological hypotheses regarding marine NIS.

In open ecological systems, community structure can be determined by physically modulated processes such as the arrival of individuals from a regional pool and by local biological interactions. There is debate centering on whether niche differentiation and local interactions among species are necessary to explain macroscopic community patterns or whether the patterns can be generated by the neutral interplay of dispersal and stochastic demography among ecologically identical species. Here we evaluate how much of the observed spatial variation within a rocky intertidal metacommunity along 800 km of coastline can be explained by drift in the structure of recruits across 15 local sites. Our results show that large spatial changes in recruitment do not explain the observed spatial variation in adult local structure and that, in comparison with the large drift in structure of recruits, local adult communities converged to a common, although not unique, structure across the region. Although there is no unique adult community structure in the entire region, the observed variation represents only a small subset of the possible structures that would be expected from passive recruitment drift. Thus, in this diverse system our results do not support the idea that rocky intertidal metacommunities are structured by neutral mechanisms.community structure | ecological drift | neutral theory | interaction strength | macroecology T he extent to which the structure and diversity of natural communities is determined by interactions among species, such as predation and competition that occur at local scales, and the extent to which they are the result of regional-scale processes that can be largely independent of species interactions are central questions in ecology and are the subject of intense debate (1-4). One aspect of the debate is centered on whether niche differentiation and local interactions among species play a significant role in macroscopic community patterns (e.g., species diversity, species-abundance curves) or whether, as stated by the neutral theory of biodiversity (NTB) (2), these patterns can be generated by the neutral interplay of dispersal and stochastic demography among ecologically identical species (4-10). A related topic at the center of ecological debate since the mid-1980s is that of "supply-side" ecology (11), which proposes that the supply of individuals to local communities, a process driven largely by physical factors, can be the main determinant of population and community structure, challenging the view of control by local species interactions (12-18). If recruitment were the main determinant of adult population abundance for most species in local communities, then community structure and its spatiotemporal variation would result from environmentally driven fluctuations in the supply of individuals, largely uncorrelated with local species interactions. Similarity with the NTB argument is apparent when examining large sets of species. The fact that some static, aggregated attributes of intertidal ma...

Predator-inducible defences have a strong influence on the expression of morphological traits of intertidal invertebrates. For instance, mussels exposed to predators often have thicker shells than non-exposed. On the intertidal rocky shores of Chile, the mussel Semimytilus algosus is a preferred prey of many carnivorous invertebrates, including the snails Nucella crassilabrum and Concholepas concholepas, and the crab Acanthocyclus gayi. Preliminary observations indicated that S. algosus exists as 2 morphotypes: a thick, smooth shell and a thinner, ringed shell. The thick-shell morphotype was found mostly on compact, rocky platforms, whereas the thin one was found on emergent rocks. We examined the role of invertebrate predators in determining the morphological differences observed in S. algosus as a process of defence induction. The density and size of mussel predators showed significant differences between habitats: A. gayi dominating the platforms and N. crassilabrum emergent rocks. C. concholepas did not show differences between habitats. Waterborne cue experiments demonstrated that the mussel shell thickness is increased by the presence of predators, especially A. gayi. Furthermore, in contrast to the other predators, A. gayi preferentially selects mussels of the thin-shell morphotype. We demonstrate the cause and effect connection between variation in mussel shell morphology in the laboratory and their associated spatial distribution in the field, as well as the ecological role played by predators. We propose that, at local scales, the distribution and abundance of predators in the field explain the inter-population morphological differences of the mussel S. algosus.

Predator-induced defenses have a significant influence on the expression of morphological and behavioral traits of marine species. In mussels, common responses to predators include thickening of the shell, enlargement of the adductor muscle and increases in byssus production. We hypothesize that predators with different feeding strategies have different effects on byssus production of the common intertidal mussels Perumytilus purpuratus and Semimytilus algosus in central Chile. Predators that dislodge prey mussels before killing them, such as crabs and seastars, should elicit increased byssus production in their prey compared to other predators such as whelks, which use different feeding mechanisms. Laboratory experiments with the seastar Heliaster helianthus, the crab Acanthocylus gayi, and the muricid gastropods Concholepas concholepas and Acanthina monodon showed that only A. gayi induced significant increases in byssus production, causing remarkably similar responses in both mussel species. Further experiments in which individual mussels with different attachment strengths were offered to A. gayi showed that the crabs first tried different mussels and then selected those with the weakest attachment, leading to consumption rates of weakly attached mussels that were 5 to 6 times higher than those of mussels with strong attachment to the substratum. Measurements of mussel attachment strengths in the field showed that, where A. gayi is abundant, both mussel species are more strongly attached than in habitats where this predator is scarce. While responses of mussels to crabs seem to be adaptive, the lack of a response to H. helianthus is intriguing, because it is one of the most important mussel predators in the system which can dislodge entire clumps of mussels at a time. It is possible that increased byssus production is ineffective in reducing predation by this large predator; however, this adaptive explanation requires further studies. These results highlight the predator-specific nature of many prey phenotypic responses and the importance of considering the multiplicity of predators typically present in most habitats.KEY WORDS: Byssal thread production · Tenacity · Dislodgement · Predator-prey interaction · Predator-induced defenses · Mussels · Sub-lethal effectsResale or republication not permitted without written consent of the publisher

Non-indigenous marine species have significant effects on rocky intertidal native biota and ecological processes. The tunicate Pyura praeputialis, a recent invader in the Bay of Antofagasta, Chile, has monopolized the low and mid-low rocky intertidal fringe and apparently constrained the native mussel Perumytilus purpuratus to the mid-upper fringe. We performed field experiments to determine interspecific competitive strengths and quantify survival and growth rates between these species at 2 intertidal heights: mid-low and mid-upper intertidal fringes. Our results showed that at the mid-low fringe P. praeputialis had greater competitive strength than P. purpuratus. In fact, the survival and growth rates of P. praeputialis were not significantly affected by the presence of P. pur-puratus. Further, while the survival of P. purpuratus was not significantly affected by the presence of P. praeputialis, its growth rate was affected by the degree of encroachment by the tunicate. Mussels encroached by P. praeputialis grew significantly less than non-encroached ones. At the mid-upper intertidal fringe, the survival of P. praeputialis was significantly decreased by the presence of P. pur-puratus: the tunicate is unable to grow at this intertidal fringe. At the low-intertidal fringe mussel growth rates were significantly greater than at the mid-upper fringe, while survival rates were similar. Our results support the hypothesis that in the Bay of Antofagasta the invading tunicate P. praeputialis is responsible for a major rocky intertidal ecological impact, outcompeting the native mussel from the mid-low fringe and thereby substantially modifying the zonation pattern. KEY WORDS: Invasion · Pyura praeputialis · Perumytilus purpuratus · Tunicate · Rocky intertidal · Antofagasta Bay Resale or republication not permitted without written consent of the publisher Editorial responsibility: Roger Hughes,