Startle behaviors are rapid, high-performance motor responses to threatening stimuli. Startle responses have been identified in a broad range of species across animal diversity. For investigations of neural circuit structure and function, these behaviors offer a number of benefits, including that they are driven by large and identifiable neurons and their neural control is simple in comparison to other behaviors. Among vertebrates, the best-known startle circuit is the Mauthner cell circuit of fishes. In recent years, genetic approaches in zebrafish have provided key tools for morphological and physiological dissection of circuits and greatly extended understanding of their architecture. Here we discuss the startle circuit of fishes, with a focus on the Mauthner cells and associated interneurons called spiral fiber neurons and we add new observations on hindbrain circuit organization. We also briefly review and compare startle circuits of several other taxa, paying particular attention to how movement direction is controlled.

The vertebrate nervous system exhibits dramatic variability in regenerative capacity across species and neuronal populations. For example, while the mammalian central nervous system (CNS) is limited in its regenerative capacity, the CNS of many other vertebrates readily regenerates after injury, as does the peripheral nervous system (PNS) of mammals. Comparing molecular responses across species and tissues can therefore provide valuable insights into both conserved and distinct mechanisms of successful regeneration. One gene that is emerging as a conserved pro-regenerative factor across vertebrates is activating transcription factor 3 (ATF3), which has long been associated with tissue trauma. A growing number of studies indicate that ATF3 may actively promote neuronal axon regrowth and regeneration in species ranging from lampreys to mammals. Here, we review data on the structural and functional conservation of ATF3 protein across species. Comparing RNA expression data across species that exhibit different abilities to regenerate their nervous system following traumatic nerve injury reveals that ATF3 is consistently induced in neurons within the first few days after injury. Genetic deletion or knockdown of ATF3 expression has been shown in mouse and zebrafish, respectively, to reduce axon regeneration, while inducing ATF3 promotes axon sprouting, regrowth, or regeneration. Thus, we propose that ATF3 may be an evolutionarily conserved regulator of neuronal regeneration. Identifying downstream effectors of ATF3 will be a critical next step in understanding the molecular basis of vertebrate CNS regeneration.

Fishes exhibit a remarkable diversity of body shape as adults; however, it is unknown whether this diversity is reflected in larval stage morphology. Here we investigate the relationship between larval and adult body shape as expressed by body elongation. We surveyed a broad range of ray-finned fish species and compared body shape at larval and adult stages. Analysis shows that the vast majority of fish are more elongate at the larval stage than at the adult stage, and that adults display greater interspecies variation than larvae. We found that the superorder Elompomorpha is unique because many species within the group do not follow the observed elongation trends. These results indicate that much of the diversity observed in adults is achieved in post-larval stages. We suggest that larval morphology is subject to common constraints across the phylogeny.

Primary mechanosensory neurons play an important role in converting mechanical forces into the sense of touch. In zebrafish, Rohon-Beard (RB) neurons serve this role at embryonic and larval stages of development. Here we examine the morphology and physiology of RBs in larval zebrafish to better understand how mechanosensory stimuli are represented along the spinal cord. We report that the morphology of RB