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Aquatic anthropogenic noise is on the rise, with growing concern about its impact on species that are sensitive to low-frequency sounds (e.g. most fish and invertebrates). We investigated whether the reef fish Halichoeres bivittatus living in both noisy and quiet areas had differing levels of baseline stress (measured as whole-body cortisol) and whether they would exhibit a physiological stress response when exposed to boat noise playbacks. While the playback experiments significantly increased cortisol levels in fish from our experiment compared to baseline levels, there were minimal pairwise differences across treatments and no difference in baseline stress for fish living in noisy vs. quiet areas. These results may be explained by low overall auditory sensitivity, habituation to a fairly noisy environment (due to biological sounds), or that boat noise simply may not represent an immediate threat to survival in this species. These findings contrast recent studies that have shown elevated stress responses in fishes when exposed to boat noise and highlights that inter-specific differences must be considered when evaluating potential impacts of anthropogenic noise on marine life.

At an intertidal site on the island of Oahu, Hawaii, the abundance of the invasive barnacle Chthamalus proteus was negatively correlated with abundance of the native limpet Siphonaria normalis. As the substratum at this site was a conglomerate of materials, we hypothesized that the observed pattern was the result of differences in substratum preferences, and/or negative interactions between the 2 species, or both. We used manipulative field experiments to test these hypotheses, and then generalized our findings with surveys across several sites that differed in sub- stratum type and abundances of barnacles and limpets. The limpet recruited in highest numbers to light-colored substrata. The barnacle settled in highest numbers on rough substrata, with a trend toward higher settlement on dark-colored substrata. Surprisingly, in experimental plots, recruitment of the barnacle was positively correlated with the density of the limpet. In contrast, limpet densities increased in experimental plots from which barnacles had been removed. Thus the pattern at this site appears to result from a combination of substratum preferences and interactions between the 2 spe- cies. These experiments and surveys of other intertidal sites on Oahu suggest that (1) limpets and bar- nacles have little effect on each other at low densities, (2) at higher densities limpets facilitate settle- ment of barnacles, but barnacles have a negative effect on limpets, and (3) substratum type can mediate the density-dependent interactions between these 2 species by affecting their settlement patterns.

The distribution of the blue mussel Mytilus edulis spans a steep gradient of environmen- tal stress between intertidal and subtidal habitats, and can provide insight into population-scale pat- terns and underlying processes of variation in physiological tolerances. I conducted a 7 wk reciprocal transplant of mussels between the upper intertidal and subtidal portion of their natural depth distri- bution, and then assayed their tolerance to either aerial exposure or submerged hypoxia in the labo- ratory. Intertidal mussels transplanted back to the intertidal had a tolerance to aerial exposure 50% higher than subtidal mussels that remained in the subtidal. That difference can be explained by changes in mussel physiology induced by ambient conditions, since the tolerance of mussels trans- planted between the intertidal and subtidal became similar to mussels occurring naturally in each habitat. The tolerance of mussels to submergence in hypoxic water —an environmental stress exacerbated by anthropogenic factors that experimental mussels had not encountered in the field — followed a pattern qualitatively similar to the aerial exposure assay: intertidal mussels had higher survivorship in hypoxic water than those from the subtidal, and an elevated tolerance was induced by transplanting subtidal individuals to the intertidal. These results suggest that the response of mussels to a change in environmental conditions could vary between the center and edge of their depth distribution as a function of stress previously encountered.

Biogenic habitat can structure marine communities by serving as both a complex sub- strate and food source for adult and recruiting organisms. We investigated the role played by bio- genic habitats (Mytilus edulis and Crepidula spp. beds) in influencing the subtidal distribution (kilo- meters scale) of the mud crab Dyspanopeus sayi in Narragansett Bay, New England (USA). In field surveys, D. sayi were 1 to 2 orders of magnitude more abundant on M. edulis and Crepidula spp. beds than at sites lacking these habitats, and a laboratory experiment confirmed that D. sayi will consume both of these species. In a field-based, substrate-choice experiment, modules containing rocks had higher D. sayi recruit densities than those containing M. edulis or Crepidula spp. This indicates that structural complexity of those biogenic substrates, rather than their availability as prey, is the primary factor influencing patterns of D. sayi recruitment. Densities of recruit and adult D. sayi on recruitment modules were higher at bare sites than at sites with biogenic habitat, where modules represented islands of structure in otherwise simple habitat. This suggests small-scale habitat selection by D. sayi. Adult D. sayi occupied modules indiscriminately at bare sites, but exhibited a preference for modules containing mussels at sites with biogenic habitats. The importance of structure for adult and recruit D. sayi, reinforced by adults' response to prey presence, likely explains this organism's association with hard, biogenic habitats in the studied system.