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Abstract The third iteration of the Cnidarian Model Systems Meeting (Cnidofest) was held August 14–17th, 2024 at Lehigh University in Bethlehem, PA. The meeting featured presentations from laboratories representing 11 countries, covering a broad range of topics related to cnidarian species. The research highlighted diverse topics, with sessions focused on regeneration, evo-devo, genomics, symbiosis, cell biology, physiology, neurobiology, and development. A notable shift at this meeting was the extent to which established cnidarian model systems have caught up with the classical laboratory models such asDrosophilaand vertebrates, with modern genomic, genetic, and molecular tools now routinely applied. In addition, more cnidarian systems are now being developed for functional studies by the community, enhancing our ability to gain fundamental insights into animal biology that are otherwise difficult in the complex bilaterian model systems. Together, the integration of cnidarian and bilaterian model systems provides researchers with a broader toolkit for selecting animal models best suited to address their specific biological questions. 

Abstract The second annual Cnidarian Model Systems Meeting, aka “Cnidofest”, took place in Davis, California from 7 to 10th of September, 2022. The meeting brought together scientists using cnidarians to study molecular and cellular biology, development and regeneration, evo-devo, neurobiology, symbiosis, physiology, and comparative genomics. The diversity of topics and species represented in presentations highlighted the importance and versatility of cnidarians in addressing a wide variety of biological questions. In keeping with the spirit of the first meeting (and its predecessor, Hydroidfest), almost 75% of oral presentations were given by early career researchers (i.e., graduate students and postdocs). In this review, we present research highlights from the meeting. 

The epithelial and interstitial stem cells of the freshwater polyp Hydra are the best-characterized stem cell systems in any cnidarian, providing valuable insight into cell type evolution and the origin of stemness in animals. However, little is known about the transcriptional regulatory mechanisms that determine how these stem cells are maintained and how they give rise to their diverse differentiated progeny. To address such questions, a thorough understanding of transcriptional regulation in Hydra is needed. To this end, we generated extensive new resources for characterizing transcriptional regulation in Hydra , including new genome assemblies for Hydra oligactis and the AEP strain of Hydra vulgaris , an updated whole-animal single-cell RNA-seq atlas, and genome-wide maps of chromatin interactions, chromatin accessibility, sequence conservation, and histone modifications. These data revealed the existence of large kilobase-scale chromatin interaction domains in the Hydra genome that contain transcriptionally coregulated genes. We also uncovered the transcriptomic profiles of two previously molecularly uncharacterized cell types: isorhiza-type nematocytes and somatic gonad ectoderm. Finally, we identified novel candidate regulators of cell type–specific transcription, several of which have likely been conserved at least since the divergence of Hydra and the jellyfish Clytia hemisphaerica more than 400 million years ago. 

Hydra vulgaris is an emerging model organism for neuroscience due to its small size, transparency, genetic tractability, and regenerative nervous system; however, fundamental properties of its sensorimotor behaviors remain unknown. Here, we use microfluidic devices combined with fluorescent calcium imaging and surgical resectioning to study how the diffuse nervous system coordinates Hydra 's mechanosensory response. Mechanical stimuli cause animals to contract, and we find this response relies on at least two distinct networks of neurons in the oral and aboral regions of the animal. Different activity patterns arise in these networks depending on whether the animal is contracting spontaneously or contracting in response to mechanical stimulation. Together, these findings improve our understanding of how Hydra ’s diffuse nervous system coordinates sensorimotor behaviors. These insights help reveal how sensory information is processed in an animal with a diffuse, radially symmetric neural architecture unlike the dense, bilaterally symmetric nervous systems found in most model organisms.