Housekeeping mutualisms: do more symbionts facilitate host performance?

An integrative phylogeography for inferring cryptic speciation in the Alpheus lottini species complex, an important coral mutualist

We use molecular analyses, color patterns, and records of distribution of mating pairs to reconstruct the global phylogeography of Alpheus lottini, a complex of cryptic coral-associated snapping shrimp species. Molecular data support the delineation of ancestral clades A, B, and C, and suggest five additional subdivisions within clades A and B. Clades A, B1, B2, and C exhibit color pattern differences and/or evidence of assortative mating, and thus merit species-level recognition. There is no evidence for assortative mating within clades A and B1, with likely reproductive compatibility (i.e., fertile clutches) in areas of sympatry. The clade diversity peaks in the Mariana Islands and the early branching clade C is restricted to the northern periphery of the Central and Western Pacific suggesting a Pacific origin of this group outside of the Coral Triangle. These findings underscore the prevalence of allopatric processes with possible ecological or microallopatric speciation in areas where clades overlap.

How fishes and invertebrates impact coral resilience

Increasingly intense and frequent ocean heatwaves are causing widespread coral mortality. These heatwaves are just one of the many stressors - among for instance ocean acidification, nutrient pollution and destructive fishing practices - that have caused widespread decline of coral reefs over the past century. This destruction of reefs threatens the remarkable biodiversity of organisms that depend upon coral reefs. However, recent research suggests that many of the fishes and invertebrates that inhabit coral reefs may play an underappreciated role in influencing the resistance and recovery of corals to stressors, especially those caused by global climate change such as ocean heatwaves. Unraveling the threads that link these coral inhabitants to the corals' response to stressors has the potential to weave a more comprehensive model of resilience that integrates the plight of coral reefs with the breathtaking diversity of life they host. Here, we aim to elucidate the critical roles that coral-associated fishes and invertebrates play in mediating coral resilience to environmental stressors. By integrating recent research findings, we aim to showcase how these often-overlooked organisms influence coral resilience in the face of climate change.

The Potential for Alternative Stable States in Food Webs Depends on Feedback Mechanism and Trait Diversity

AbstractAlternative stable ecosystem states are possible under the same environmental conditions in models of two or three interacting species and an array of feedback loops. However, multispecies food webs might weaken the feedbacks loops that can create alternative stable states. To test how this potential depends on food web properties, we develop a many-species model where consumer Allee effects emerge from consumer-resource interactions. We evaluate the interactive effects of food web connectance, interspecific trait diversity, and two classes of feedbacks: specialized feedbacks, where consumption of individual resources declines at high resource abundance (e.g., from schooling or reaching size refugia), and aggregate feedbacks, where overall resource abundance reduces consumer recruitment (e.g., from resources enhancing competition or mortality experienced by recruits). We find that aggregate feedbacks maintain, and specialized feedbacks reduce, the potential for alternative states. Interspecific trait diversity decreases the prevalence of alternative stable states more for specialized than for aggregate feedbacks. Increasing food web connectance increases the potential for alternative stable states for aggregated feedbacks but decreases it for specialized feedbacks, where losing vulnerable consumers can cascade into food web collapses. Altogether, multispecies food webs can limit the set of processes that create alternative stable states and impede consumer recovery from disturbance.

Epifaunal invertebrate assemblages associated with branching Pocilloporids in Moorea, French Polynesia

Reef-building corals can harbour high abundances of diverse invertebrate epifauna. Coral characteristics and environmental conditions are important drivers of community structure of coral-associated invertebrates; however, our current understanding of drivers of epifaunal distributions is still unclear. This study tests the relative importance of the physical environment (current flow speed) and host quality (e.g., colony height, surface area, distance between branches, penetration depth among branches, and background partial mortality) in structuring epifaunal communities living within branching Pocillopora colonies on a back reef in Moorea, French Polynesia. A total of 470 individuals belonging to four phyla, 16 families and 39 genera were extracted from 36 Pocillopora spp. colonies. Decapods were the most abundant epifaunal organisms (accounting for 84% of individuals) found living in Pocillopora spp. While coral host characteristics and flow regime are very important, these parameters were not correlated with epifaunal assemblages at the time of the study. Epifaunal assemblages associated with Pocillopora spp. were consistent and minimally affected by differences in host characteristics and flow regime. The consistency in abundance and taxon richness among colonies (regardless of habitat characteristics) highlighted the importance of total habitat availability. With escalating effects of climate change and other localized disturbances, it is critical to preserve branching corals to support epifaunal communities.

Damselfishes alleviate the impacts of sediments on host corals

Mutualisms play a critical role in ecological communities; however, the importance and prevalence of mutualistic associations can be modified by external stressors. On coral reefs, elevated sediment deposition can be a major stressor reducing the health of corals and reef resilience. Here, we investigated the influence of severe sedimentation on the mutualistic relationship between small damselfishes (Pomacentrus moluccensis and Dascyllus aruanus) and their coral host (Pocillopora damicornis). In an aquarium experiment, corals were exposed to sedimentation rates of approximately 100 mg cm-2 d-1, with and without fishes present, to test whether: (i) fishes influence the accumulation of sediments on coral hosts, and (ii) fishes moderate partial colony mortality and/or coral tissue condition. Colonies with fishes accumulated much less sediment compared with colonies without fishes, and this effect was strongest for colonies with D. aruanus (fivefold less sediment than controls) as opposed to P. moluccensis (twofold less sediment than controls). Colonies with symbiont fishes also had up to 10-fold less sediment-induced partial mortality, as well as higher chlorophyll and protein concentrations. These results demonstrate that fish mutualisms vary in the strength of their benefits, and indicate that some mutualistic or facilitative interactions might become more important for species health and resilience at high-stress levels.

Molecular characterization reveals the complexity of previously overlooked coral-exosymbiont interactions and the implications for coral-guild ecology

Several obligate associate crabs and shrimps species may co-occur and interact within a single coral host, leading to patterns of associations that can provide essential ecological services. However, knowledge of the dynamics of interactions in this system is limited, partly because identifying species involved in the network remains challenging. In this study, we assessed the diversity of the decapods involved in exosymbiotic assemblages for juvenile and adult Pocillopora damicornis types α and β on reefs of New Caledonia and Reunion Island. This approach revealed complex patterns of association at regional and local scales with a prevalence of assemblages involving crab-shrimp partnerships. Furthermore, the distinction of two lineages in the snapping shrimp Alpheus lottini complex, rarely recognized in ecological studies, reveals a key role for cryptic diversity in structuring communities of mutualists. The existence of partnerships between species that occurred more commonly than expected by chance suggests an increased advantage for the host or a better adaptation of associated species to local environmental conditions. The consideration of cryptic diversity helps to accurately describe the complexity of interaction webs for diverse systems such as coral reefs, as well as the functional roles of dominant associated species for the persistence of coral populations.

Anomuran and Brachyuran Symbiotic Crabs in Coastal Areas between the Southern Ryukyu arc and the Coral Triangle

Parinya Limviriyakul, Li-Chun Tseng, Jiang-Shiou Hwang, and Tung-Wei Shih (2016) Symbiotic anomuran and brachyuran crabs were identified after extensive surveys of reef zones, especially from sponges, scleractinian corals, crinoids, and sea urchins, in the coastal areas of the southern East China Sea. Twenty-nine species belonging to 17 genera, 9 families, and 2 infraorders were identified (3 were identified to the generic level). More crabs belonged to the infraorder Brachyura (82.8%) than to Anomura (17.2%). Two anomuran symbionts (Allogalathea elegans and Petrolisthes virgatus) and 5 brachyuran symbionts (Tetralia glaberrima, Tetralia rubridactyla, Trapezia cymodoce, Trapezia septata, and Cymo melanodactylus) are common in this area. Two species of Anomura (Lauriea simulata, Petrolisthes virgatus) and 3 of Brachyura (Gonatonotus nasutus, Tetralia aurantistellata and Tetralia nigrolineata) were identified for the first time from waters adjacent to Taiwan. These records represent the northernmost recorded of L. simulata and T. aurantistellata. The occurrence of P. virgatus is the second in the western Pacific Ocean. This study revealed the geospatial distribution of symbiotic crabs, which connects the region from the southern Ryukyu arc to the Coral Triangle, and provides the supporting taxonomic account of symbiotic anomuran and brachyuran crab fauna inhabiting the reef zone in northern Taiwan.

Metabarcoding dietary analysis of coral dwelling predatory fish demonstrates the minor contribution of coral mutualists to their highly partitioned, generalist diet

Understanding the role of predators in food webs can be challenging in highly diverse predator/prey systems composed of small cryptic species. DNA based dietary analysis can supplement predator removal experiments and provide high resolution for prey identification. Here we use a metabarcoding approach to provide initial insights into the diet and functional role of coral-dwelling predatory fish feeding on small invertebrates. Fish were collected in Moorea (French Polynesia) where the BIOCODE project has generated DNA barcodes for numerous coral associated invertebrate species. Pyrosequencing data revealed a total of 292 Operational Taxonomic Units (OTU) in the gut contents of the arc-eye hawkfish (Paracirrhites arcatus), the flame hawkfish (Neocirrhites armatus) and the coral croucher (Caracanthus maculatus). One hundred forty-nine (51%) of them had species-level matches in reference libraries (>98% similarity) while 76 additional OTUs (26%) could be identified to higher taxonomic levels. Decapods that have a mutualistic relationship with Pocillopora and are typically dominant among coral branches, represent a minor contribution of the predators' diets. Instead, predators mainly consumed transient species including pelagic taxa such as copepods, chaetognaths and siphonophores suggesting non random feeding behavior. We also identified prey species known to have direct negative interactions with stony corals, such as Hapalocarcinus sp, a gall crab considered a coral parasite, as well as species of vermetid snails known for their deleterious effects on coral growth. Pocillopora DNA accounted for 20.8% and 20.1% of total number of sequences in the guts of the flame hawkfish and coral croucher but it was not detected in the guts of the arc-eye hawkfish. Comparison of diets among the three fishes demonstrates remarkable partitioning with nearly 80% of prey items consumed by only one predator. Overall, the taxonomic resolution provided by the metabarcoding approach highlights a highly complex interaction web and demonstrates that levels of trophic partitioning among coral reef fishes have likely been underestimated. Therefore, we strongly encourage further empirical approaches to dietary studies prior to making assumptions of trophic equivalency in food web reconstruction.

Dispersal of a defensive symbiont depends on contact between hosts, host health, and host size

Symbiont dispersal is necessary for the maintenance of defense mutualisms in space and time, and the distribution of symbionts among hosts should be intricately tied to symbiont dispersal behaviors. However, we know surprisingly little about how most defensive symbionts find and choose advantageous hosts or what cues trigger symbionts to disperse from their current hosts. In a series of six experiments, we explored the dispersal ecology of an oligochaete worm (Chaetogaster limnaei) that protects snail hosts from infection by larval trematode parasites. Specifically, we determined the factors that affected net symbiont dispersal from a current "donor" host to a new "receiver" host. Symbionts rarely dispersed unless hosts directly came in contact with one another. However, symbionts overcame their reluctance to disperse across the open environment if the donor host died. When hosts came in direct contact, net symbiont dispersal varied with both host size and trematode infection status, whereas symbiont density did not influence the probability of symbiont dispersal. Together, these experiments show that symbiont dispersal is not a constant, random process, as is often assumed in symbiont dispersal models, but rather the probability of dispersal varies with ecological conditions and among individual hosts. The observed heterogeneity in dispersal rates among hosts may help to explain symbiont aggregation among snail hosts in nature.

Multiple mutualist effects: conflict and synergy in multispecies mutualisms

Most organisms interact with multiple mutualistic species that confer different functional benefits, yet current conceptual frameworks do not fully address this complexity. A network approach considers multiple mutualistic interactions within a functional type and has been largely nonmechanistic, with little attention to the fitness consequences of specific interactions. Alternatively, consumer-resource approaches have explicitly characterized the mechanisms and fitness consequences of resource exchange, but have not been extended to functionally divergent partners. First, we merge these approaches using graphical models to define the multiple mutualist effects (MMEs) that occur when a focal species has multiple partner mutualists. This approach mirrors food web research that has been advanced by studies of multiple predator effects as well as by detailed investigations of modules nested within larger networks. Second, we define the pathways through which a focal mutualist and two or more partner species could interact, reviewing examples of MMEs that span a range from positive to negative fitness effects. Third, given the potential for nonadditivity demonstrated by the existing literature, we pose new hypotheses for species-interaction outcomes by examining factors such as the extent of overlap in rewards exchanged among partners and their resulting network topologies. Our synthesis illustrates how the consideration of MMEs can improve the ability to predict the outcomes of losses or gains of mutualisms from ecosystems.

Effectiveness of annealing blocking primers versus restriction enzymes for characterization of generalist diets: unexpected prey revealed in the gut contents of two coral reef fish species

Characterization of predator-prey interactions is challenging as researchers have to rely on indirect methods that can be costly, biased and too imprecise to elucidate the complexity of food webs. DNA amplification and sequencing techniques of gut and fecal contents are promising approaches, but their success largely depends on the ability to amplify the taxonomic array of prey consumed and then match prey amplicons with reference sequences. When little a priori information on diet is available or a generalist predator is targeted, versatile primer sets (also referred to as universal or general primers) as opposed to group- or species-specific primer sets are the most powerful to unveil the full range of prey consumed. However, versatile primers are likely to preferentially amplify the predominant, less degraded predator DNA if no manipulation is performed to exclude this confounding DNA template. In this study we compare two approaches that eliminate the confounding predator template: restriction digestion and the use of annealing blocking primers. First, we use a preliminary DNA barcode library provided by the Moorea BIOCODE project to 1) evaluate the cutting frequency of commercially available restriction enzymes and 2) design predator specific annealing blocking primers. We then compare the performance of the two predator removal strategies for the detection of prey templates using two versatile primer sets from the gut contents of two generalist coral reef fish species sampled in Moorea. Our study demonstrates that blocking primers should be preferentially used over restriction digestion for predator DNA removal as they recover greater prey diversity. We also emphasize that a combination of versatile primers may be required to best represent the breadth of a generalist's diet.

Acanthaster planci outbreak: decline in coral health, coral size structure modification and consequences for obligate decapod assemblages

Although benthic motile invertebrate communities encompass the vast majority of coral reef diversity, their response to habitat modification has been poorly studied. A variety of benthic species, particularly decapods, provide benefits to their coral host enabling them to cope with environmental stressors, and as a result benefit the overall diversity of coral-associated species. However, little is known about how invertebrate assemblages associated with corals will be affected by global perturbations, (either directly or indirectly via their coral host) or their consequences for ecosystem resilience. Analysis of a ten year dataset reveals that the greatest perturbation at Moorea over this time was an outbreak of the corallivorous sea star Acanthaster planci from 2006 to 2009 impacting habitat health, availability and size structure of Pocillopora spp. populations and highlights a positive relationship between coral head size and survival. We then present the results of a mensurative study in 2009 conducted at the end of the perturbation (A. planci outbreak) describing how coral-decapod communities change with percent coral mortality for a selected coral species, Pocillopora eydouxi. The loss of coral tissue as a consequence of A. planci consumption led to an increase in rarefied total species diversity, but caused drastic modifications in community composition driven by a shift from coral obligate to non-obligate decapod species. Our study highlights that larger corals left with live tissue in 2009, formed a restricted habitat where coral obligate decapods, including mutualists, could subsist. We conclude that the size structure of Pocillopora populations at the time of an A. planci outbreak may greatly condition the magnitude of coral mortality as well as the persistence of local populations of obligate decapods.