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Kashimoto et al. introduce the giant sea anemones, which form mutualistic relationships with anemonefish., Competing Interests: Declaration of interests The authors declare no competing interests., (Copyright © 2024 Elsevier Inc. All rights reserved.)

Cnidarians (corals, hydras, jellyfish, sea anemones) are prey-devouring creatures with a simple nervous system, the function of which is largely unknown. A new study on the freshwater polyp Hydra has now uncovered the neuronal circuits that control its feeding behavior., Competing Interests: Declaration of interests The author declares no competing interests., (Copyright © 2023 Elsevier Inc. All rights reserved.)

To survive in the nutrient-poor waters of the tropics, reef-building corals rely on intracellular, photosynthetic dinoflagellate symbionts. Photosynthates produced by the symbiont are translocated to the host, and this enables corals to form the structural foundation of the most biodiverse of all marine ecosystems. Although the regulation of nutrient exchange between partners is critical for ecosystem stability and health, the mechanisms governing how nutrients are sensed, transferred, and integrated into host cell processes are largely unknown. Ubiquitous among eukaryotes, the mechanistic target of the rapamycin (mTOR) signaling pathway integrates intracellular and extracellular stimuli to influence cell growth and cell-cycle progression and to balance metabolic processes. A functional role of mTOR in the integration of host and symbiont was demonstrated in various nutritional symbioses, and a similar role of mTOR was proposed for coral-algal symbioses. Using the endosymbiosis model Aiptasia, we examined the role of mTOR signaling in both larvae and adult polyps across various stages of symbiosis. We found that symbiosis enhances cell proliferation, and using an Aiptasia-specific antibody, we localized mTOR to symbiosome membranes. We found that mTOR signaling is activated by symbiosis, while inhibition of mTOR signaling disrupts intracellular niche establishment and symbiosis altogether. Additionally, we observed that dysbiosis was a conserved response to mTOR inhibition in the larvae of a reef-building coral species. Our data confim that mTOR signaling plays a pivotal role in integrating symbiont-derived nutrients into host metabolism and symbiosis stability, ultimately allowing symbiotic cnidarians to thrive in challenging environments., Competing Interests: Declaration of interests The authors declare no competing interests., (Copyright © 2023 The Authors. Published by Elsevier Inc. All rights reserved.)

Throughout animals, embryonic cells must ultimately organize into polarized epithelial layers that provide the structural basis for gastrulation or subsequent developmental events [1]. Precisely how this primary epithelium maintains continuous integrity during rapid and repeated cell divisions has never been directly addressed, particularly in cases where early cleavages are driven in synchrony. Representing the early-branching non-bilaterian phylum Cnidaria, embryos of the sea anemone Nematostella vectensis undergo rapid synchronous cell divisions and ultimately give rise to a diploblastic epithelial body plan after gastrulation [2, 3]. Here, using live imaging of apical polarity proteins in Nematostella embryos, we demonstrate that cell polarity is established by the four-cell stage and then reiteratively lost during subsequent mitoses, correlating with transient adhesion disengagement and dramatic deformations of embryonic morphology. Intriguingly, the re-establishment of polarity and adhesion during each interphase is associated with a process of whole-embryo compaction analogous to that observed in mammals [4-7]. Because similar protein dynamics are observed in dividing epithelial cells in Drosophila melanogaster, we propose that cell-cycle-coupled oscillations in apical polarity may be conserved throughout Metazoa., (Copyright © 2017 Elsevier Ltd. All rights reserved.)

Feeney and Brooker introduce sea-anemone associated fish., (Copyright © 2017 Elsevier Ltd. All rights reserved.)

The global decline of reef-building corals is due in part to the loss of algal symbionts, or "bleaching," during the increasingly frequent periods of high seawater temperatures. During bleaching, endosymbiotic dinoflagellate algae (Symbiodinium spp.) either are lost from the animal tissue or lose their photosynthetic pigments, resulting in host mortality if the Symbiodinium populations fail to recover. The >1,000 studies of the causes of heat-induced bleaching have focused overwhelmingly on the consequences of damage to algal photosynthetic processes, and the prevailing model for bleaching invokes a light-dependent generation of toxic reactive oxygen species (ROS) by heat-damaged chloroplasts as the primary trigger. However, the precise mechanisms of bleaching remain unknown, and there is evidence for involvement of multiple cellular processes. In this study, we asked the simple question of whether bleaching can be triggered by heat in the dark, in the absence of photosynthetically derived ROS. We used both the sea anemone model system Aiptasia and several species of reef-building corals to demonstrate that symbiont loss can occur rapidly during heat stress in complete darkness. Furthermore, we observed damage to the photosynthetic apparatus under these conditions in both Aiptasia endosymbionts and cultured Symbiodinium. These results do not directly contradict the view that light-stimulated ROS production is important in bleaching, but they do show that there must be another pathway leading to bleaching. Elucidation of this pathway should help to clarify bleaching mechanisms under the more usual conditions of heat stress in the light., (Copyright © 2013 Elsevier Ltd. All rights reserved.)