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Symbionts dominate planetary diversity and three primary symbiont diversification processes have been proposed: co-speciation with hosts, speciation by host-switching, and within-host speciation. The last mechanism is prevalent among members of an extraordinary marine symbiosis in the Indian River Lagoon, Florida, composed of a host mantis shrimp, Lysiosquilla scabricauda , and seven host-specific commensal vasconielline "yoyo" clams (Galeommatoidea) that collectively occupy two distinct niches: burrow-wall-attached, and host-attached/ectocommensal. This within-host symbiont radiation provides a natural experiment to test how symbiont coexistence patterns are regulated in a common ancestral habitat. The competitive exclusion principle predicts that sister taxa produced by adaptive speciation (with distinct morphologies and within-burrow niches) are most likely to coexist whereas the neutral theory predicts no difference among adaptive and non-adaptive sister taxa co-occurrence. To test these predictions, we engaged in (1) field-censusing commensal species assemblages; (2) trophic niche analyses; (3) laboratory behavioral observations. Although predicted by both models, the field census found no mixed-niche commensal assemblages: multi-species burrows were exclusively composed of burrow-wall commensals. Their co-occurrence matched random assembly process expectations, but presence of the single ectocommensal species had a highly significant negative effect on recruitment of all burrow-wall commensal species ( P  < 0.001), including on its burrow-wall commensal sister species ( P < 0.001). Our stable isotope data indicated that commensals are suspension feeders and that co-occurring burrow-wall commensals may exhibit trophic niche differentiation. The artificial burrow behavioral experiment yielded no evidence of spatial segregation among burrow-wall commensals, and it was terminated by a sudden breakdown of the host-commensal relationship resulting in a mass mortality of all commensals unattached to the host. This study system appears to contain two distinct, superimposed patterns of commensal distribution: (1) all burrow-wall commensal species; (2) the ectocommensal species. Burrow-wall commensals (the plesiomorphic condition) broadly adhere to neutral theory expectations of species assembly but the adaptive evolution of ectocommensalism has apparently led to ecological exclusion rather than coexistence, an inverse outcome of theoretical expectations. The ecological factors regulating the observed burrow-wall/ectocommensal exclusion are currently obscure but potentially include differential recruitment to host burrows and/or differential survival in "mixed" burrow assemblages, the latter potentially due to changes in host predatory behavior. Resampling host burrows during commensal recruitment peak periods and tracking burrow-wall commensal survival in host burrows with and without added ectocommensals could resolve this outstanding issue., Competing Interests: Jingchun Li is an Academic Editor for PeerJ., (©2024 Harrison et al.)

Photosymbioses between heterotrophic hosts and autotrophic symbionts are evolutionarily prevalent and ecologically significant. However, the molecular mechanisms behind such symbioses remain less elucidated, which hinders our understanding of their origin and adaptive evolution. This study compared gene expression patterns in a photosymbiotic bivalve ( Fragum sueziense ) and a closely related non-symbiotic species ( Trigoniocardia granifera ) under different light conditions to detect potential molecular pathways involved in mollusc photosymbiosis. We discovered that the presence of algal symbionts greatly impacted host gene expression in symbiont-containing tissues. We found that the host immune functions were suppressed under normal light compared with those in the dark. In addition, we found that cilia in the symbiont-containing tissues play important roles in symbiont regulation or photoreception. Interestingly, many potential photosymbiosis genes could not be annotated or do not exhibit orthologues in T. granifera transcriptomes, indicating unique molecular functions in photosymbiotic bivalves. Overall, we found both novel and known molecular mechanisms involved in animal-algal photosymbiosis within bivalves. Given that many of the molecular pathways are shared among distantly related host lineages, such as molluscs and cnidarians, it indicates that parallel and/or convergent evolution is instrumental in shaping host-symbiont interactions and responses in these organisms.

Background: Photosymbiotic associations between metazoan hosts and photosynthetic dinoflagellates are crucial to the trophic and structural integrity of many marine ecosystems, including coral reefs. Although extensive efforts have been devoted to study the short-term ecological interactions between coral hosts and their symbionts, long-term evolutionary dynamics of photosymbiosis in many marine animals are not well understood. Within Bivalvia, the second largest class of mollusks, obligate photosymbiosis is found in two marine lineages: the giant clams (subfamily Tridacninae) and the heart cockles (subfamily Fraginae), both in the family Cardiidae. Morphologically, giant clams show relatively conservative shell forms whereas photosymbiotic fragines exhibit a diverse suite of anatomical adaptations including flattened shells, leafy mantle extensions, and lens-like microstructural structures. To date, the phylogenetic relationships between these two subfamilies remain poorly resolved, and it is unclear whether photosymbiosis in cardiids originated once or twice., Results: In this study, we establish a backbone phylogeny for Cardiidae utilizing RNASeq-based transcriptomic data from Tridacninae, Fraginae and other cardiids. A variety of phylogenomic approaches were used to infer the relationship between the two groups. Our analyses found conflicting gene signals and potential rapid divergence among the lineages. Overall, results support a sister group relationship between Tridacninae and Fraginae, which diverged during the Cretaceous. Although a sister group relationship is recovered, ancestral state reconstruction using maximum likelihood-based methods reveals two independent origins of photosymbiosis, one at the base of Tridacninae and the other within a symbiotic Fraginae clade., Conclusions: The newly revealed common ancestry between Tridacninae and Fraginae brings a possibility that certain genetic, metabolic, and/or anatomical exaptations existed in their last common ancestor, which promoted both lineages to independently establish photosymbiosis, possibly in response to the modern expansion of reef habitats.

The Galeommatoidea are a diverse but little-studied group of small bivalves, well known for the symbiotic relationships many species have with a range of invertebrate taxa. Four species collected from the Western Cape region of South Africa were examined and illustrated, providing new details on their habitat preferences, and depicting the mantle structure of live specimens for the first time. Brachiomya ducentiunus sp. nov., is described herein, and an additional record of Montacuta substriata (Montagu, 1808) is reported from South Africa. Brachiomya ducentiunus and Montacuta substriata have obligate symbiotic relationships with different burrowing echinoids, while Kellia becki (WH Turton, 1932) and Melliteryx mactroides (Hanley, 1857) are free-living. DNA data and phylogenetic analyses are provided for three of the species. [ABSTRACT FROM AUTHOR]

Active learning provides students with meaningful and introspective roles in education. However, active learning is difficult to achieve in online class settings, particularly for biology laboratories initially designed to offer direct interactions with live organisms. During the COVID-19 pandemic, a transition to online learning was required in higher education institutions, creating challenges and opportunities to reform and rethink teaching practices. In this article, we report our success in designing and implementing an at-home lab for an undergraduate invertebrate biology course. The objective was for students to learn about cnidarian photosymbiosis in a 3-week experiment using live anemones. Additionally, we tested if learning outcomes are improved when adding a learning-by-teaching experience. We divided the 19 participating students into two groups, where half created videos to teach their experiment to a hypothetical high school audience and half made video reports for their instructors. We observed that the teaching group showed better topic comprehension than the reporting group. Overall, students responded positively to this online learning experience and expressed an increased understanding of cnidarian photosymbiosis. We provide recommendations for implementing this lab with greater success and at larger scales. We argue that adding teaching scenarios in remote learning can benefit students' overall knowledge comprehension. [ABSTRACT FROM AUTHOR]

Generalists are thought to adapt to broader ecological conditions compared to less flexible specialists. However, few studies have systematically tested what ecological or life‐history traits are associated with organisms' ecological flexibility. Here, we used stony corals to test the relative effects of host traits and ecological factors on corals' flexibility to form photosymbioses with algae. We analysed data from 211 stony coral species to test if coral's geographic distribution, depth range, symbiont transmission mode or colony morphology predict coral–algal flexibility. We report a novel positive correlation between coral–algal flexibility and coral species' geographic range. Symbiont transmission mode was also a predictor of flexibility, albeit the result is less robust against sampling bias. Coral depth range and morphology did not show significant effects. We highlight that host–symbiont dispersal abilities, interactions and evolutionary histories likely contribute to the observed patterns. We urge conservation efforts to consider the ecological implications of coral–algal flexibility. [ABSTRACT FROM AUTHOR]

Coral reef growth and recovery from bleaching depend on the onset and physiological maintenance of cnidarian-dinoflagellate symbiosis. Many cnidarian hosts can acquire new symbionts (family Symbiodiniaceae) from the environment. Symbiodiniaceae can be found at significant densities within other hosts and lesser densities as free-living populations on macroalgae, in sediments, and in the water column. The mechanisms underlining host selectivity towards symbionts are still under investigation, and even less is known about the timing and community composition when hosts uptake symbionts in their natural environments. This project studied symbiont diversity in environmental reservoirs and the progression of symbiont acquisition in bleached hosts. We first characterized various Symbiodiniaceae reservoirs in an estuarine habitat of Florida Bay at Key Largo, Florida. We then deployed bleached Exaiptasia sp. anemones in the habitat and recorded their recovery for three weeks, assessing symbiont density through cell counting and diversity via next-generation sequencing (NGS). We found that environmental reservoirs exhibit higher symbiont diversity but much lower densities than in hospite living reservoirs, highlighting the potential for healthy hosts as symbiont donors. We showed that the anemones can recover from bleaching using their residual leftover symbiont cells but recover 40% faster when also uptaking new symbionts from their environment, including non-homologous strains. We also report that symbiont shuffling can be observed in disturbed anemones in as little as three days, with decreasing symbiont density but higher symbiont diversity. Our data can guide future studies towards understanding small-scale dynamics and symbiont acquisition for cnidarians recovering from bleaching. • Environmental and in hospite symbiont communities are drastically different within Florida Bay. • Bleached anemones can recover from residual symbionts within the host. • Bleached anemones can recover using environmental reservoirs in the natural habitat. [ABSTRACT FROM AUTHOR]