Global Diversity and Geographic Distributions of Padina Species (Dictyotales, Phaeophyceae): New Insights Based on Molecular and Morphological Analyses.

The taxonomic status and species diversity of the brown algal genus Padina (Dictyotales, Phaeophyceae) was assessed based on DNA sequences and the morpho-anatomy of specimens collected worldwide, especially from tropical and subtropical western Pacific regions. Phylogenetic analyses using chloroplast rbcL and mitochondrial cox3 gene sequences demonstrated four distinct clades for newly collected samples with high bootstrap support. Each species clade possesses a suite of morphological features that are not shared by any known species of Padina. These are P. imbricata sp. nov., Padina lutea sp. nov., P. moffittianoides sp. nov., and P. nitida sp. nov. The occurrence of these and other species of Padina clearly points to an elevated diversity of the genus in tropical/subtropical waters of the western Pacific. Phylogenetic analyses provided new insights into biogeographic characteristics of the genus, with many species in the Pacific Ocean showing shared/overlapping distributions, whereas species from the Mediterranean/Atlantic and/or the Indian Ocean tend to be confined to particular regions. Consideration has also been given to the evolutionary time frame of the genus Padina based on molecular time trees: a time tree of the concatenated data set (rbcL + cox3) revealed the estimated divergence time in the mid-Cretaceous, whereas those of cox3 and rbcL showed older estimates pointing to the periods of mid-Jurassic and Early Cretaceous, respectively.

Analysis of spatial niche structure in coexisting tidepool fishes: null models based on multi-scale experiments.

1. Fundamental and realized spatial niches were investigated through a combination of laboratory and mesocosm experiments, field observations and null model analysis in three intertidal gobiid species (Bathygobius fuscus, Chaenogobius annularis and C. gulosus). Null models based on the results of single-species experiments were used to assess interspecific spatial use and coexistence on two different scales: (i) microhabitats within a tidepool ('microhabitat' scale); and (ii) distribution among a set of tidepools ('habitat-wide' scale). 2. Patterns of microhabitat use varied from single to paired treatments, depending on paired species. Realized overlap of microhabitat use was smaller than would be expected from single-individual situations for intraspecific combinations, but not for interspecific ones. 3. Patterns of tidepool occupancy (a measure of spatial niche breadth) in the mesocosm were influenced by interspecific interactions. Two Chaenogobius species, but not B. fuscus, decreased tidepool occupancy in the hetero-specific treatments compared with the mono-specific ones. For all interspecific combinations, spatial overlap (habitat-wide scale) was significantly lower than the values expected from mono-specific situations. The results also indicated a possible trade-off between competitiveness and growth efficiency in these fishes. 4. Interspecific spatial overlap in the field was similar to that in the mesocosm experiment and the pattern of coexistence of gobiids can be explained by the results of our experiments. 5. This study demonstrates that niches of intertidal fishes may experience modifications under the influence of species interactions and that null models based on controlled experiments can greatly facilitate the deciphering of such changes in niche structure.

Testing the facilitation-competition paradigm under the stress-gradient hypothesis: decoupling multiple stress factors.

While the facilitation-competition paradigm under the stress-gradient hypothesis has received recent attention, its rigorous testing is yet to be explored. Most of the studies have considered a switch in the net interactions from competition to facilitation with increasing environmental stress as primary evidence supporting the hypothesis, though few studies examined changes in interaction along a full range of a stress gradient. Here, we have conceptualized possible variations in the patterns of change in interaction strength along such gradient. Based on this, we empirically evaluated the temporal shift in the interaction between two marine sessile animals, goose barnacles (Capitulum mitella) and mussels (Septifer virgatus), under multiple stress factors. The net effect of goose barnacles on mussel survivorship was positively related to the total stress gradient encompassing two stress factors, physical disturbance and thermal stress, while no negative value occurred even under mild conditions. When the two stress factors were treated separately, however, the net effect demonstrated apparently different patterns: monotonic increase with physical disturbance versus a quasi-asymptotic pattern (no change over a wide range) with thermal stress. These variable situations have not previously been recognized in this discipline, and the present study emphasizes the importance of an integrative and mechanistic approach to testing and deciphering the facilitation-competition paradigm.

Scaling in stream communities.

Scaling relationships between population density (N) and body size (W), and of their underlying size distributions, can contribute to an understanding of how species use resources as a function of size. In an attempt to resolve the controversy over the form of scaling relationships, an extensive dataset, comprising 602 invertebrate species, was obtained from two geographically separate stream communities (Seebach in Austria and Mynach in Wales). We analysed the temporal consistency of the N-W relationship, which was subjected to ordinary least squares (OLS), bisector (OLS(BIS)) and quantile regressions, and species-size spectra with seasonally collated data. Slopes of seasonal OLS(BIS) regressions did not depart from -1 in either community, indicating a seasonally convergent scaling relationship, which is not energetically constrained. Species-size spectra may scale with habitat complexity, providing an alternative explanation for the observed body-size scaling. In contrast to the right-skewed species-size frequency distributions of single-species assemblages, the size spectra of these benthic communities exhibited 'central tendencies', reflecting their phyletic constitution. The shape of species body-mass spectra differed between the two communities, with a bimodal and seasonally convergent pattern in the Seebach community and a seasonally shifting unimodality in the Mynach community. The body-size spectra of large, mostly insect, species (greater than or equal to 1 mm) scaled to seasonal variations in habitat complexity (i.e. fractal D), suggesting that habitat structure constrains the community organization of stream benthos.

Asymmetric coexistence: bidirectional abiotic and biotic effects between goose barnacles and mussels.

1. Species coexistence depends on the net effect of interacting species, representing the sum of multiple interaction components that may act simultaneously and vary independently depending on ambient environmental conditions. Consequently, for a comprehensive understanding of the compound nature of species interactions and coexistence, a mechanistic approach that allows a separate evaluation of each interaction component is required. 2. Two sessile filter-feeders, the goose barnacle Capitulum mitella and the mussel Septifer virgatus, coexist on moderately wave-exposed rocky shores in south-western Japan. In the upper intertidal, Capitulum positively influenced Septifer survivorship and growth through amelioration of thermal stress and of physical disturbance. On the other hand, these species are potential competitors as they have similar body sizes and modes of resource utilization. These opposite processes, facilitation and competition, are based on abiotic characteristics and biotic functions of the two species, respectively. 3. In order to quantify the bidirectional abiotic, biotic and net effects, a series of experimental manipulations was conducted involving the use of living neighbours with both abiotic and biotic effects, and artificial mimics to simulate abiotic effects without biotic effects. 4. Capitulum had strong positive abiotic effects on the mussel survivorship in most experimental periods, while the biotic effect was negligible or weakly negative, suggesting that the net effect of Capitulum on mussel survival was largely attributable to the abiotic effect. In contrast, a significantly negative biotic effect on the mussel growth rate was always present, though this was cancelled out by the larger, positive abiotic effect. In the case of Septifer, its abiotic and biotic effects on the survivorship of goose barnacles were negligible, while those on the growth rate showed temporal variation. 5. With respect to the relationship between species interaction and environmental conditions, the strength of abiotic facilitative effect of Capitulum on mussel survival increased with increasing abiotic stress, while the strength of biotic effect was negligible or weakly negative. As regards the effects of mussels on goose barnacles, our study indicated no obvious relationship. These results point to the importance of decomposing interaction for an accurate, mechanistic understanding of species relations and coexistence.