Functional diversity of mesograzers in an eelgrass-epiphyte system

Historically, small invertebrate grazers in marine plant communities have been considered to be a relatively homogeneous group in their impact on ecosystem processes. However, recent studies propose that species composition is an important agent in determining grazer effects. We used four mesocosm experiments to test the biomass-specific and density-dependent effects of common mesograzers in temperate regions (Littorina littorea, Rissoa membranacea, Idotea baltica and Gammarus oceanicus) on epiphyte and eelgrass biomass and productivity. Mesograzer species identity strongly influenced epiphyte accumulation and eelgrass growth, where Rissoa was the most efficient mesograzer (per biomass) and Gammarus had the weakest impact. Density-dependent effects varied considerably among species. Both gastropod species reduced epiphyte accumulation in direct proportion to their density, and Littorina had the strongest negative effect on epiphyte biomass. The impact of Idotea seemed to level off to a threshold value and Gammarus had no density-dependent effect on epiphyte accumulation at all. Rissoa and Idotea increased eelgrass productivity in accordance with their effect on epiphyte accumulation, whereas Littorina showed a less positive effect than could be expected by its strong impact on epiphyte biomass. Gammarus had no significant impact on eelgrass growth. Our results show that the different functional traits of superficially similar mesograzers can have important consequences for ecosystem processes in macrophyte systems

Multi-Locus Phylogeny Of Sponge-Dwelling Snapping Shrimp (Caridea: Alpheidae: Synalpheus) Supports Morphology-Based Species Concepts

Alpheid snapping shrimp are one of the most diverse groups of coral-reef fauna, and sponge-dwelling shrimp in the genus Synalpheus (gambarelloides species group) have in particular become a model system for studying the evolution of social biology and host use in marine invertebrates. Despite recent advances in understanding the evolution and systematics of Synalpheus, the taxonomy and phylogenetic relationships within this group remain challenging. More than 20 new species in the S. gambarelloides species group have been described over the past two decades, primarily within several cryptic species complexes, which has doubled the known diversity of this group in the West Atlantic. Here we construct a new phylogenetic tree describing relationships between 40 different species from the S. gambarelloides-group (119 individuals from across the Caribbean), using a combined dataset consisting of two mitochondrial loci (16S and COI), one nuclear protein-coding gene (elongation-factor 2), and 33 morphological characters. Putative conspecific specimens of Synalpheus from multiple locations across the Caribbean were always monophyletic (with one exception), providing strong support for the validity of species concepts based on morphology. Our study also provides further evidence for the monophyly of the S. gambarelloides-group in the Caribbean, resolves the molecular relationships within many recently described species complexes, and provides a new phylogenetic framework for future evolutionary studies of this group

Influence of sociality on allometric growth and morphological differentiation in sponge-dwelling alpheid shrimp

Eusocial societies are defined by a reproductive division of labour between breeders and nonbreeders that is often accompanied by morphological differentiation. Some eusocial taxa are further characterized by a subdivision of tasks among nonbreeders, often resulting in morphological differentiation among different groups (subcastes) that specialize on different sets of tasks. We investigated the possibility of morphological castes in eusocial shrimp colonies (Zuzalpheus, formerly part of Synalpheus) by comparing growth allometry and body proportions of three eusocial shrimp species with three pair-forming species (species where reproductive females and males occur in equal sex ratios). Allometry of eusocial species differed in several respects from that of pair-forming species in both lineages. First, allometry of fighting claw size among individuals other than female breeders was steeper in eusocial than in pair-forming species. Second, breeding females in eusocial colonies had proportionally smaller weapons (fighting claws) than females in pair-forming species. Finally, claw allometry changed with increasing colony size in eusocial species; large colonies showed a diphasic allometry of fighting claw and finger size, indicating a distinctive group of large individuals possessing relatively larger weapons than other colony members. Shrimp are thus similar to other eusocial animals in the morphological differentiation between breeders and nonbreeders, and in the indication that some larger nonbreeders might contribute more to defence than others. (C) 2008 The Linnean Society of London

Multitrophic functional diversity predicts ecosystem functioning in experimental assemblages of estuarine consumers

The use of functional traits to explain how biodiversity affects ecosystem functioning has attracted intense interest, yet few studies have a priori altered functional diversity, especially in multitrophic communities. Here, we manipulated multivariate functional diversity of estuarine grazers and predators within multiple levels of species richness to test how species richness and functional diversity predicted ecosystem functioning in a multitrophic food web. Community functional diversity was a better predictor than species richness for the majority of ecosystem properties, based on generalized linear mixed-effects models. Combining inferences from eight traits into a single multivariate index increased prediction accuracy of these models relative to any individual trait. Structural equation modeling revealed that functional diversity of both grazers and predators was important in driving final biomass within trophic levels, with stronger effects observed for predators. We also show that different species drove different ecosystem responses, with evidence for both sampling effects and complementarity. Our study extends experimental investigations of functional trait diversity to a multilevel food web, and demonstrates that functional diversity can be more accurate and effective than species richness in predicting community biomass in a food web context

Field experimental evidence that grazers mediate transition between microalgal and seagrass dominance

We tested the relative effects of nutrient loading, reduced predation, and reduced grazing on eelgrass community dynamics in Chesapeake Bay and found evidence supporting the mutualistic mesograzer model in which small invertebrate grazers control accumulation of epiphytic algae, buffer eutrophication effects, and thus facilitate seagrass dominance. Experimental reduction of crustacean grazers in the field stimulated a nearly sixfold increase in epiphytic algae, and reduced seagrass biomass by 65% compared to controls with grazers. Nutrient fertilization generally had much weaker effects, but an interaction with mesograzers was key in changing the sign of fertilization effects on the system: aboveground eelgrass biomass was reduced by fertilization under reduced grazing, but increased by fertilization under ambient grazing. When protected from predators in field cages, these mesograzers limited epiphyte blooms even with nutrient enrichment, and nutrients instead enhanced grazer secondary production. Crustacean mesograzers play a key role in maintaining macrophyte (seagrass) dominance in Chesapeake Bay, in buffering eelgrass against eutrophication, and in efficiently transferring nitrogen to higher trophic levels. Yet, these crustacean grazers are also highly sensitive to predator abundance. Reducing nutrient pollution alone is unlikely to restore seagrass meadows where alterations to food webs have reduced populations of algae-feeding mesograzers. Integration of both water quality and fishery management will be more effective in restoring and maintaining healthy coastal ecosystems