Housekeeping Mutualisms: Do More Symbionts Facilitate Host Performance?

Mutualisms often involve one host supporting multiple symbionts, whose identity, density and intraguild interactions can influence the nature of the mutualism and performance of the host. However, the implications of multiple co-occurring symbionts on services to a host have rarely been quantified. In this study, we quantified effects of decapod symbionts on removal of sediment from their coral host. Our field survey showed that all common symbionts typically occur as pairs and never at greater abundances. Two species, the crab Trapezia serenei and the shrimp Alpheus lottini, were most common and co-occurred more often than expected by chance. We conducted a mesocosm experiment to test for effects of decapod identity and density on sediment removal. Alone, corals removed 10% of sediment, but removal increased to 30% and 48% with the presence of two and four symbionts, respectively. Per-capita effects of symbionts were independent of density and identity. Our results suggest that symbiont density is restricted by intraspecific competition. Thus, increased sediment removal from a coral host can only be achieved by increasing the number of species of symbionts on that coral, even though these species are functionally equivalent. Symbiont diversity plays a key role, not through added functionality but by overcoming density limitation likely imposed by intraspecific mating systems

Species and size diversity in protective services offered by coral guard-crabs

Coral guard-crabs in the genus Trapezia are well-documented defenders of their pocilloporid coral hosts against coral predators such as the Crown-of-Thorns seastar (Acanthaster planci complex). The objectives of this study were to examine the protective services of six species of Trapezia against corallivory, and the extent of functional diversity among these Trapezia species.Studies conducted in Mo’orea, French Polynesia showed the Trapezia—coral mutualism protected the host corals from multiple predators through functional diversity in the assemblage of crab symbionts. Species differed in their defensive efficacy, but species within similar size classes shared similar abilities. Smaller-size Trapezia species, which were previously thought to be ineffective guards, play important defensive roles against small corallivores.We also measured the benefits of this mutualism to corals in the midst of an Acanthaster outbreak that reduced the live coral cover on the fore reef to less than 4%. The mutualism may positively affect the reef coral demography and potential for recovery during adverse predation events through shelter of multiple species of small corals near the host coral. Our results show that while functional diversity is supported within the genus, some Trapezia species may be functionally equivalent within the same size class, decreasing the threat of gaps in coral protection caused by absence or replacement of any single Trapezia species

Variation in partner benefits in a shrimp—sea anemone symbiosis

Symbiotic interactions, where two species occur in close physical proximity for the majority of the participants’ lifespans, may constrain the fitness of one or both of the participants. Host choice could result in lineage divergence in symbionts if fitness benefits vary across the interaction with hosts. Symbiotic interactions are common in the marine environment, particularly in the most diverse marine ecosystems: coral reefs. However, the variation in symbiotic interactions that may drive diversification is poorly understood in marine systems. We measured the fecundity of the symbiotic shrimp Periclimenes yucatanicus on two anemone hosts on coral reefs in Panama, and found that while fecundity varies among host species, this variation is explained largely by host size, not species. This suggests that shrimp on larger hosts may have higher fitness regardless of host species, which in turn could drive selection for host choice, a proposed driver of diversification in this group

Ecological drivers and habitat associations of estuarine bivalves

Community composition of the infaunal bivalve fauna of the St. Lucie Estuary and southern Indian River Lagoon, eastern Florida was sampled quarterly for 10 years as part of a long-term benthic monitoring program. A total of 38,514 bivalves of 137 taxa were collected and identified. We utilized this data, along with sediment samples and environmental measurements gathered concurrently, to assess the community composition, distribution, and ecological drivers of the infaunal bivalves of this estuary system. Salinity had the strongest influence on bivalve assemblage across the 15 sites, superseding the influences of sediment type, water turbidity, temperature and other environmental parameters. The greatest diversity was found in higher salinity euhaline sites, while the greatest abundance of individual bivalves was found in medium salinity mixohaline sites, the lowest diversity and abundances were found in the low salinity oligohaline sites, demonstrating a strong positive association between salinity and diversity/abundance. Water management decisions for the estuary should incorporate understanding of the role of salinity on bivalve diversity, abundance, and ecosystem function

Benefits for plants in ant-plant protective mutualisms: a meta-analysis.

Costs and benefits for partners in mutualistic interactions can vary greatly, but surprisingly little is known about the factors that drive this variation across systems. We conducted a meta-analysis of ant-plant protective mutualisms to quantify the effects of ant defenders on plant reproductive output, to evaluate if reproductive effects were predicted from reductions in herbivory and to identify characteristics of the plants, ants and environment that explained variation in ant protection. We also compared our approach with two other recent meta-analyses on ant-plant mutualisms, emphasizing differences in our methodology (using a weighted linear mixed effects model) and our focus on plant reproduction rather than herbivore damage. Based on 59 ant and plant species pairs, ant presence increased plant reproductive output by 49% and reduced herbivory by 62%. The effects on herbivory and reproduction within systems were positively correlated, but the slope of this relationship (0.75) indicated that tolerance to foliar herbivory may be a common plant response to absence of ant guards. Furthermore, the relationship between foliar damage and reproduction varied substantially among systems, suggesting that herbivore damage is not a reliable surrogate for fitness consequences of ant protection. Studies that experimentally excluded ants reported a smaller effect of ant protection on plant reproduction than studies that relied upon natural variation in ant presence, suggesting that study methods can affect results in these systems. Of the ecological variables included in our analysis, only plant life history (i.e., annual or perennial) explained variation in the protective benefit of mutualistic ants: presence of ants benefitted reproduction of perennials significantly more than that of annuals. These results contrast with other quantitative reviews of these relationships that did not include plant life history as an explanatory factor and raise several questions to guide future research on ant-plant protection mutualisms

Interspecific co-occurrence (a) and intraspecific density (b) of five focal symbiont species from 133 surveyed corals.

<p>Pairwise co-occurrence of five focal symbiont species are shown within Trapezidae (i.e. Crabs – blue), within Alpheidae (i.e. Shrimps – pink), between the two families (Crabs and Shrimps – green). Black circles and solid line represent the observed data. Colored rectangles and dashed lines represent the 95% quantiles from 10,000 randomly simulated communities. Observed values are significantly different from the randomly simulated communities when the black circle falls outside the 95% quantiless. Species tend to avoid to avoid confamilials (with 2 of 4 comparisons demonstrating avoidance), while the crab, <i>T. serenei</i> is positively associated with both shrimp species. At the intraspecific level, pairs are more common and singlets and triplets are more rare than expected by chance. See Appendices B and E for raw data.</p

Independent effects of symbionts (<i>Alpheus</i>: A and <i>Trapezia</i>: T) on the proportion of sediment removed in the experiment.

<p>Figure shows backtransformed mean ± 95% CI, n  =  13. Grey vertical bars are give the 2.5% and 97.5 quantiles for the distribution of effects of symbionts assuming independence and calculated from equations 1, 2, and 3. The white bar in center of each grey bar is the mean from these simulations. Observed values fall within the null distributions for both the additive and substitutive designs suggesting that the effects of symbionts are independent (i.e. the data provide no evidence for antagonism or synergy between symbiont species).</p