The neuroanatomy of the Hawaiian bobtail squid juvenile bacterial light organ

Abstract Background Recent studies have shown that symbiotic bacteria can have drastic effects on host neurobiology, but few simple, accessible models currently exist in which to study these interactions. Hawaiian bobtail squid ( Euprymna scolopes ) participate in a binary symbiosis with the bacterium Vibrio fischeri , a population of which resides in a specialized hindgut-derived organ called the light organ. Upon colonization by V. fischeri , the light organ undergoes transcriptional changes that suggest neurons are among the cell types impacted by the initiation of symbiosis, but the nascent light organ’s innervation has remained uncharacterized. Results The light organ-associated nervous system (LONS) in hatchling E. scolopes is a remarkably complex segment of the peripheral nervous system. The LONS is largely plexiform and originates from two primary nerves connected by a local commissure. The abundance of synapsin-like immunoreactivity indicates that the lobe plexus is highly interconnected. We also highlight a small number of serotonin-like immunoreactive neurites innervating the anterior appendages that are poised to be directly impacted by symbiont-driven post-embryonic development. Finally, we present evidence that a limited but morphologically diverse population of neurons reside within the light organ and are often located near internal symbiont-interacting structures. Conclusions Our results show that the LONS is structurally and molecularly complex and exhibits traits characteristic of an immature nervous system, suggesting that it may undergo substantial post-embryonic refinement. This initial characterization of the LONS provides a foundation from which to investigate how beneficial bacterial symbionts affect host peripheral neurobiology in a tractable model system.