Significance Sea stars inhabiting the Northeast Pacific Coast have recently experienced an extensive outbreak of wasting disease, leading to their degradation and disappearance from many coastal areas. In this paper, we present evidence that the cause of the disease is transmissible from disease-affected animals to apparently healthy individuals, that the disease-causing agent is a virus-sized microorganism, and that the best candidate viral taxon, the sea star-associated densovirus (SSaDV), is in greater abundance in diseased than in healthy sea stars.

Aim Our aim in this paper is to present the first broad-scale quantification of species abundance for rocky intertidal communities along the Pacific coast of North America. Here we examine the community-level marine biogeographical patterns in the context of formerly described biogeographical regions, and we evaluate the combined effects of geographical distance and environmental conditions on patterns of species similarity across this region.Location Pacific coast of North America.Methods Data on the percentage cover of benthic marine organisms were collected at 67 rocky intertidal sites from south-eastern Alaska, USA, to central Baja California Sur, Mexico. Cluster analysis and non-metric multidimensional scaling were used to evaluate the spatial patterns of species similarity among sites relative to those of previously defined biogeographical regions. Matrices of similarity in species composition among all sites were computed and analysed with respect to geographical distance and long-term mean sea surface temperature (SST) as a measure of environmental conditions. ResultsWe found a high degree of spatial structure in the similarity of intertidal communities along the coast. Cluster analysis identified 13 major community structure 'groups'. Although breaks between clusters of sites generally occurred at major biogeographical boundaries, some of the larger biogeographical regions contained several clusters of sites that did not group according to spatial position or identifiable coastal features. Additionally, there were several outliers -sites that grouped alone or with sites outside their region -for which localized features may play an important role in driving community structure. Patterns of species similarity at the large scale were highly correlated with geographical distance among sites and with SST. Importantly, we found community similarity to be highly correlated with long-term mean SST while controlling for the effects of geographical distance.Main conclusions These findings reveal a high degree of spatial structure in the similarity of rocky intertidal communities of the north-east Pacific, and are generally consistent with those of previously described biogeographical regions, with some notable differences. Breaks in similarity among clusters are generally coincident with known biogeographical and oceanographic discontinuities. The strong correlations between species similarity and both geographical position and SST suggest that both geography and oceanographic conditions have a large influence on patterns of intertidal community structure along the Pacific coast of North America.

I examined the settlement or recruitment of the barnacle Chthamalus anisopoma at a variety of scales: temporally, over periods lasting 12 h to 1 mo, and spatially at sites separated by <1 cm to shores separated by 10 km. Both settlement and recruitment were variable at nearly all temporal and spatial scales that I examined. The majority of variability was attributable to differences in settlement or recruitment between sample periods. When only the spatial component was considered, variability decreased with scale. Large, consistent differences in settlement between geographic locations that are at approximately the same tidal height are probably the result of differential larval supply, whereas more subtle but still consistent differences in settlement between adjacent sites may, at least in part, be due to larval behavior. Differences in settlement between close sites were observed only when settlement was relatively high. This observation may be explained as follows. First, Chthamalus settle in pits or small depressions "suitable settlement sites (SSS's)" in the surface of a substrate. Second, within an SSS, space for settlement is restricted. Third, the density of SSS's differ between sites. When settlement is high, settlement differences between locations reflect differences in the number of SSS's, as all are used. However, when settlement is low, settlement will be similar between locations, since SSS's are not limiting. In a series of field experiments using both natural and artificial substrates I found that: (1) Cypris larvae of Chthamalus settled preferentially in the presence of a number of physical and biological factors. (2) Settlement was density dependent but growth and survivorship were not, which suggests exploitative but not interference intraspecific competition. (Although Chthamalus settle gregariously they space themselves out.) (3) Variability in the settlement of Chthamalus affected the ensuing adult population even at the highest settlement densities.

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Patterns of recruitment in marine ecosystems can reflect the distribution of adults, dispersal by ocean currents, or patterns of mortality after settlement. In turn, patterns of recruitment can play an important role in determining patterns of adult abundance and community dynamics. Here we examine the biogeographic structure of recruitment variability along the U.S. West Coast and examine its association with temperature variability. From 1997 to 2004 we monitored monthly recruitment rates of dominant intertidal invertebrates, mussels and barnacles, at 26 rocky shore sites on the West Coast of the United States, from northern Oregon to southern California, a span of 1750 km of coastline. We examined spatial variation in the dynamics of recruitment rates and their relationship to coastal oceanography using satellite‐derived time series of monthly sea surface temperature (SST). Recruitment rates showed a biogeographic structure with large regions under similar dynamics delimited by abrupt transitions. The seasonal peak in recruitment rates for both mussels and barnacles changed from a late summer–early fall peak in Oregon to winter or early spring in northern California, and then back toward summer in southern California. Recruitment rates varied greatly in magnitude across the latitudinal range. The barnacle Balanus glandula and mussels (Mytilus spp.) showed a decline of two orders of magnitude south of Oregon. In contrast, recruitment rates of barnacles of the genus Chthamalus showed a variable pattern across the region examined. The spatial distribution of associations between raw SST and recruitment rates for all species showed positive associations, indicating recruitment during warm months, for all species in Oregon, northern California, and several sites in south‐central California. By considerably extending the spatial and temporal scales beyond that of previous studies on larval recruitment rates in this system, our study has identified major biogeographic breaks around Cape Blanco and Point Conception despite considerable spatial and temporal variation within each region and among species. These large differences in recruitment rates across biogeographic scales highlight the need for a better understanding of larval responses to ocean circulation patterns in the conservation and management of coastal ecosystems.

We present findings from two sets of measurements that quantified currents around and over the full extent of a giant kelp (Macrocystis pyrifera) forest located at Mohawk Reef, Santa Barbara, California. Velocities were damped inside this 200-m 3 300-m forest, but not to the extent reported for larger (kilometer-scale) kelp beds, suggesting that alongshore currents may play a greater role in exchange than has often been assumed. Secondary flow features that bear on the performance of forest organisms were observed, including a region along the forest's outer boundary where velocities exceeded incident speeds by up to 200%. An offshore current on the order of 1 cm s 21 developed within the kelp bed, likely due to pressure gradients established across the forest coupled with topography. Wake recirculations that might have facilitated leeward retention of waterborne subsidies were not apparent. Calculations suggest that kelp beds can interact with (and thus potentially filter) substantial 1 Corresponding author (bpgaylord@ucdavis.edu;

The settlement behavior of the barnacle Chthamalus anisopoma near its upper intertidal limit was investigated using field surveys and experiments. The upper limit of Chthamalus was found to be extremely variable, ranging from 1.5 to 2.6 m above MLW. Chthamalus showed increased settlement in response to chemical cues produced by several species whose vertical distributions overlapped with its vertical distribution. Within a single species there was significantly more settlement in response to cues associated with Nertia funiculata that occurred with Chthamalus than to N. funiculata that occurred only above the vertical limit of Chthamalus. Another species that induced settlement, Acanthina angelica, is the major predator of adult Chthamalus. Acanthina may be a good settlement cue because it is a reliable indicator of the vertical distribution of Chthamalus and because (1) the clusters of Acanthina that induced settlement may move away before the new barnacles become edible; and (2) Chthamalus can develop a predator—resistant morphology in responses to Acanthina. I was also able to induce Chthamalus recruitment into areas well above their normal upper limits using a conspecific extract, indicating that conspecifics function as a strong settlement cue. The evolution of larval attraction to settlement cues should be dependent on the reliability of the cues and the consequences of not using them. Reliable cues and harsh consequences should lead to the development of "strong" (i.e., attractive) cues. However, reliable cues may be conservative; their absence may not indicate a "bad" settlement site. Therefore, a variable distribution of strong settlement cues could lead to temporal and spatial variability in a species regardless of other factors such as pre— and postsettlement physical conditions or biological interactions.

Ocean currents are expected to be the predominant environmental factor influencing the dispersal of planktonic larvae or spores; yet, their characterization as predictors of marine connectivity has been hindered by a lack of understanding of how best to use oceanographic data. We used a high-resolution oceanographic model output and Lagrangian particle simulations to derive oceanographic distances (hereafter called transport times) between sites studied for Macrocystis pyrifera genetic differentiation. We build upon the classical isolation-by-distance regression model by asking how much additional variability in genetic differentiation is explained when adding transport time as predictor. We explored the extent to which gene flow is dependent upon seasonal changes in ocean circulation. Because oceanographic transport between two sites is inherently asymmetric, we also compare the explanatory power of models using the minimum or the mean transport times. Finally, we compare the direction of connectivity as estimated by the oceanographic model and genetic assignment tests. We show that the minimum transport time had higher explanatory power than the mean transport time, revealing the importance of considering asymmetry in ocean currents when modelling gene flow. Genetic assignment tests were much less effective in determining asymmetry in gene flow. Summer-derived transport times, in particular for the month of June, which had the strongest current speed, greatest asymmetry and highest spore production, resulted in the best-fit model explaining twice the variability in genetic differentiation relative to models that use geographic distance or habitat continuity. The best overall model also included habitat continuity and explained 65% of the variation in genetic differentiation among sites.