Phylogenetic diversity is a better predictor of wetland community resistance to Alternanthera philoxeroides invasion than species richness

• Highly biodiversity communities have been shown to better resist plant invasions through complementarity effects. Species richness (SR) is a widely used biodiversity metric but lacks explanatory power when there are only a few species. Communities with low SR can have a wide variety of phylogenetic diversities (PD), which might allow for a better prediction of invasibility.
• We assessed the effect of diversity reduction of a wetland community assemblage typical of the Beijing area on biotic resistance to invasion of the exotic weed Alternanthera philoxeroides and compared the reduction in SR and PD in predicting community invasibility.
• The eight studied resident species performed similarly when grown alone and when grown in eight‐species communities together with the invasive A. philoxeroides. Variation partitioning showed that PD contributed more to variation in both A. philoxeroides traits and community indicators than SR. All A. philoxeroides traits and community indicators, except for evenness index, showed a linear relationship with PD. However, only stem length of A. philoxeroides differed between the one‐ and two‐species treatments, and the diversity index of the communities differed between the one‐ and two‐species treatments and between the one‐ and four‐species treatments.
• Our results showed that in natural or semi‐natural wetlands with relatively low SR, PD may be a better predictor of invasibility than SR. When designing management strategies for mitigating A. philoxeroides invasion, deliberately raising PD is expected to be more efficient than simply increasing species number.

     

Unrecognized diversity of a scale worm,Polyeunoa laevis (Annelida: Polynoidae), that feeds on soft coral

A goal of taxonomy is to employ a method of classification based on phylogeny that captures the morphological and genetic diversity of organismal lineages. However, morphological and genetic diversity may not always be concordant, leading to challenges in systematics. The scale worm Polyeunoa laevis has been hypothesized to represent a species complex based on morphology, although there is little knowledge of its genetic diversity. Commonly found in Antarctic waters and usually associated with gorgonian corals (especially Thouarella), this taxon is also reported from the south-west Atlantic, Magellanic and sub-Antarctic regions. We employ an integrative taxonomic approach to examine the traditional morphological characters used for scale worm identification in combination with COI mitochondrial gene data and whole mtDNA genomes. Moreover, we consider P. laevis's association with Thouarella by examining data from the mMutS gene, a soft-coral phylogenetic marker. Analyses for P. laevis recovered 3 clades, two in Antarctic waters and one from the Argentina-Indian Ocean. Interestingly, genetic and morphological results show differences between specimens from South Argentina and the Antarctic region, suggesting that open ocean barriers might have limited gene flow from these regions. Bayesian phylogenetic analyses for Thouarella resulted in at least 12 lineages, although some of the lineages consist of only a single individual. Our results show different evolutionary histories for both species, confirming that association between these scale worms and their hosts is not restrictive. For both taxonomic groups, biodiversity in the Southern Ocean appears to be underestimated.     

POPULATION STRUCTURE OF A CLONAL GORGONIAN CORAL: THE INTERPLAY BETWEEN CLONAL REPRODUCTION AND DISTURBANCE

Clonality is a common feature of plants and benthic marine organisms. In some cases clonal propagation results in a modest increase in population density, while in other cases dense populations may be generated by the propagation of only a few clones. We analyzed the population structure of the clonal gorgonian Plexaura kuna across several reef habitats with a range of disturbance regimes in the San Blas Islands, Panama, and the Florida Keys, U.S.A. Using multilocus DNA fingerprinting to distinguish clones, we estimated that clones ranged in size from single individuals to 500 colonies. The number of genotypes identified on nine reefs ranged from three to 25. Population density and clonal structure varied markedly among reefs with G_O:G_E ranging from 0.03 to 1.00. On some reefs vegetative reproduction transformed P. kuna from a rare species to the numerically most abundant gorgonian. The effect of clonal propagation on P. kuna population structure was dependent on interactions between fragmentation and the reef environment (disturbance regime, substratum). We present a generalized model relating population structure of clonal species to disturbance and the mode of vegetative propagation. Disturbance promotes colony propagation and skews the size-frequency distribution of clones among P. kuna and many species that propagate via fragmentation. Propagation of these species is promoted by disturbance (disturbance sensitive), and they tend to have clones that are dispersed across local sites. Species that fragment and have dispersed clones, have high genotypic diversity in habitats with low levels of disturbance. Genotypic diversity then decreases at intermediate disturbance and increases again at the highest disturbance levels. Clonal species that do not rely on disturbance for vegetative propagation (disturbance insensitive) generally do not disperse and form aggregated clones. Among these taxa disturbance has a greater affect on individual survival than on propagation. Genotypic diversity is directly related to the level of disturbance until very high levels of disturbance, at which time genotypic diversity declines.     

Phenotypic and genetic consequences of size selection at the larval stage in the Pacific oyster (Crassostrea gigas)

The life histories of oysters in the genus Crassostrea, like those of most marine bivalves, are typified by high fecundity and low survival in nature. Rearing conditions in hatcheries however ensure optimized density, diet, and temperature. Hatcheries are becoming increasingly important for the production of juveniles in aquaculture, and their culture practices often include culling of slow growing larvae to reduce and synchronize the time taken to reach settlement. Because previous studies have found substantial genetic variation for early life developmental traits in Crassostrea gigas, these culling practices are likely to cause highly different selective pressures in hatcheries from those in the natural environment. We studied the phenotypic and genetic impact of such culling practices in a factorial cross between 10 males and 3 females subjected to progressive culling of the smallest 50% of larvae, compared with a non-culled control. Measurements were made on larval growth, survival, time taken to attain pediveliger stage and settlement success. Culling had a larger effect on the variance of these larval traits than on their means. The larvae in culled cultures were approximately 10% larger than those in controls, whereas the coefficient of variation was reduced by 30-40%. Culling also reduced the mean time to settlement by 12% and its variance by 55%. Using a multiplexed set of microsatellite markers to trace parentage, we also estimated the variance in reproductive success in a controlled experiment to quantify the consequences of intensive hatchery rearing practices. We also focused on changes in effective population size and genetic structure over time (and developmental stages). Our results show a loss of genetic diversity following removal of the smallest larvae by culling, as well as temporally varying genetic structure of the larval population. This supports the existence of genetic variability in early life developmental traits in C. gigas. Culling in hatcheries, like size-related selective pressures in the wild, are likely to have a significant genetic impact, through their effects on the timing of settlement.