Local aggregations of unionoids (mussel beds) represent subpopulations constituting a riverine metapopulation and are presumed to be linked by dispersal of parasitic mussel larvae (glochidia) on host fishes. We examined movement patterns of Masu Salmon (Oncorhynchus masou masou), the obligate host fish for the freshwater pearl mussel Margaritifera laevis, during the mussel's parasitic period to determine the dispersal potential of glochidia via host fish in the Shubuto River system, Hokkaido, Japan. We conducted a mark-recapture study in a 650-m river section to describe the distance and directionality of host fish movement, and we used fyke net sampling to quantify the number of host fish moving from mainstem rivers to tributaries. Most individuals of Masu Salmon captured in the study were infected with glochidia of M. laevis. The displacement distance of infected fishes ranged from 37.5 m downstream to 512.5 m upstream (mean = 36.5 m upstream from marking location). The dispersal kernel based on a diffusion-advection model represented a leptokurtic distribution with a fat-tailed upstream bias, but most recaptured fish remained near the initial capture location. Movement of fish from mainstem rivers to tributaries was strongly affected by differences in water temperature among tributaries. Most individuals moved to cooler tributaries, but tributaries did not support populations of M. laevis. These results suggest that glochidial dispersal via host fish is important for local recruitment and upstream dispersal, but dispersal to tributaries may be a source of mortality for M. laevis.

Unidirectional water flow results in the downstream-biased, asymmetric dispersal of many riverine organisms. However, little is known of how asymmetric dispersal influences riverine population structure and dynamics, limiting our ability to properly manage riverine organisms. A metapopulation of the freshwater pearl mussel Margaritifera laevis may be sensitive to river currents because mussels are repeatedly exposed to downstream drift during floods—a parasitic life stage is the only, limited period (∼40 days) during which larvae (glochidia) can move upstream with the aid of host fish. We hypothesized that water-mediated dispersal would overwhelm upstream dispersal via host fish, and therefore, that upstream subpopulations play a critical role as immigrant sources. To test this hypothesis, we examined the effects of both up- and downstream immigrant sources on the size of target subpopulations in the Shubuto River system, Hokkaido, Japan. We found that target subpopulation size was dependent on the upstream distribution range of reproductive subpopulations and the number of upstream tributaries, which are proxies for the number of potential immigrants moving downstream. In contrast, little influence was observed of downstream immigrant sources (proximity to downstream reproductive subpopulations). These results were consistent even after accounting for local environments and stream size. Our finding suggests that upstream subpopulations can be disproportionately important as immigrant sources when dispersal is strongly asymmetric.

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A prevailing paradigm suggests that species richness increases with area in a decelerating way. This ubiquitous power law scaling, the species–area relationship, has formed the foundation of many conservation strategies. In spatially complex ecosystems, however, the area may not be the sole dimension to scale biodiversity patterns because the scale-invariant complexity of fractal ecosystem structure may drive ecological dynamics in space. Here, we use theory and analysis of extensive fish community data from two distinct geographic regions to show that riverine biodiversity follows a robust scaling law along the two orthogonal dimensions of ecosystem size and complexity (i.e., the dual scaling law). In river networks, the recurrent merging of various tributaries forms fractal branching systems, where the prevalence of branching (ecosystem complexity) represents a macroscale control of the ecosystem’s habitat heterogeneity. In the meantime, ecosystem size dictates metacommunity size and total habitat diversity, two factors regulating biodiversity in nature. Our theory predicted that, regardless of simulated species’ traits, larger and more branched “complex” networks support greater species richness due to increased space and environmental heterogeneity. The relationships were linear on logarithmic axes, indicating power law scaling by ecosystem size and complexity. In support of this theoretical prediction, the power laws have consistently emerged in riverine fish communities across the study regions (Hokkaido Island in Japan and the midwestern United States) despite hosting different fauna with distinct evolutionary histories. The emergence of dual scaling law may be a pervasive property of branching networks with important implications for biodiversity conservation.

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A checklist of the marine and estuarine fishes of the inner part of the Gulf of Nicoya, Pacific coast of Costa Rica, Central America, was compiled by examining a museum fish collection, resulting in 72 families and 274 species. Of these species, 127 (46.4%) were marine species and 147 (53.6%) were estuarine-associated species. In terms of their life history and considering the habitat type classification, 188 (almost 70% of the total) were categorized as species inhabiting soft-bottom habitats, reflecting the large estuarine environment and rich fish diversity of the Gulf despite its relatively small area in the tropical Eastern Pacific region. Furthermore, the list contains 13 threatened species of IUCN Red List, which need further research to understand their abundance and their exposure to habitat loss in the Gulf. Further detailed studies on its fish fauna and habitat are needed to better understand and conserve biodiversity within the whole Gulf.

Long-distance dispersal of freshwater mussels (order Unionoida) has been assumed to occur mainly during a parasitic larval stage (glochidia) via movements of host fishes, but its empirical evidence is largely lacking. Here, we applied a ''parasite-tag'' approach to the riverine mussel Margaritifera laevis and its obligate host fish Oncorhynchus masou masou. This method examines the relationship between the prevalence of glochidia and distance from the nearest mussel population (i.e., the putative source of glochidia), a proxy that should quantify distance moved by host fish. We hypothesized that infected fish would be found in wider habitats at the end of the parasitic period (August) than at the beginning (July) if they were functioning as effective dispersal agents. In July, the prevalence of glochidia was highest in the vicinity of mussel beds but decreased rapidly with distance from the nearest mussel bed. In August, however, infected fish were distributed diffusively across the riverine network and dispersed over 4.8 km, demonstrating substantial dispersal of glochidia by the host fish. The results of our study build upon current knowledge of mussel's dispersal ecology by providing highly needed evidence: host fish can be effective in mediating long-distance dispersal of a riverine mussel species. KeywordsMussel Á Stream Á Glochidia Á Masu salmon Á Parasitism Handling editor: Marcello Moretti Electronic supplementary material The online version of this article (

This study reports radiocarbon dates of more than 30 samples of charred residues on pottery sherds of the Incipient Jomon period. The ages of Linear-relief (Ryukisenmon) pottery were 15,300–13,700 cal BP, with great differences among the samples. The pitted decoration (Enkomon), Nail-impressed (Tsumegatamon), and pressing and dragging (Oshibikimon) types date to 13,800–12,400 cal BP. For pottery of the same type, differences among sites were large. At the Unokiminami site, the impressed cord mark (Oatsu Jomon) is the main pottery type, including Nail-impressed. The latter shows a slightly older age. Stable isotope and elemental analyses were used to ascertain the origin of charred residues on the pottery. In the data set of Jomon pottery of the oldest type, residues consisting only of cooked nuts were found. However, Jomon people, even from early times, are thought to have cooked mixed plant and animal ingredients, including marine products.