Animals integrate different relevant sources of sensory information as they develop, reflecting changes in the connectivity and growth of distinct brain regions as they develop into adulthood and perform complex behavior. Danionella dracula is a miniature and transparent species of teleost fish closely related to zebrafish (Danio rerio), and in my PhD, we developed D. dracula as a new model system for studying the neurodevelopment of adult, ethologically relevant behaviors. We provide the first comprehensive description of social behavior for any species within the Danionella genus. Males produce bursts of pulsatile sounds and a distinct postural display–extension of a hypertrophied lower jaw, a morphological trait that is not present in other Danionella species—during aggressive, but not courtship interactions. Novel pairs of size-matched or -mismatched males were combined in resident-intruder assays where sound production and jaw extension could be linked to individuals. In both dyad contexts, resident males produced significantly more sound pulses than intruders. During heightened sonic activity, the majority of the highest sound producers also showed increased jaw extension. Residents extended their jaw more than intruders in size-matched, but not -mismatched contexts. Larger males in size-mismatched dyads produced more sounds and jaw extensions compared to their smaller counterparts, and sounds and jaw extensions increased with increasing absolute body size. These studies establish D. dracula as a sonic species that modulates putative acoustic and postural (jaw) displays during aggressive interactions based on residency and body size. This provides the foundation for further investigating the role of multimodal displays in a new model clade for neurogenomic and neuroimaging studies of aggression, courtship, and other social interactions. To investigate developmental changes in the volume of canonical sensory and sensory-related integration regions of vertebrate brains that could be relevant to the development of adult behavior, we used third harmonic generation (THG) imaging for morphometric analysis of forebrain and midbrain regions in D. dracula, which is optically accessible through its entire lifespan. Whole-brain investment in the telencephalon, a multimodal sensory integration center in the forebrain, is lowest at 30 days post fertilization (dpf) and then increases significantly at 60dpf when juveniles begin to exhibit adult morphology and in 90dpf adults that demonstrate courtship and aggressive behaviors. In contrast, investment in the midbrain optic tectum, a retinal-recipient visual integration center, is greatest at 30dpf and then decreases significantly in 90dpf adults. There are no significant changes across life stages in the relative volume of the olfactory bulbs, a direct forebrain target of olfactory receptors, and of the midbrain torus semicircularis, a secondary auditory and lateral line processing center. In sum, complete optical access in all life stages of Danionella opens the door for the investigation of correlated changes in vertebrate brain structure and behavior from larval to adult stages of development.