One of the goals of neuroscience is to classify all of the neurons in the brain. Neuronal types can be defined using a combination of morphology, electrophysiology, and gene expression profiles. Gene expression profiles allow differentiation between cells that share similar characteristics. Leveraging the advantage of Berghia stephanieae (Gastropoda; Nudibranchia), which has around 28,000 neurons, I constructed high-throughput single-neuron transcriptomes for its whole brain. I produced a single-cell dissociation protocol and a custom data analysis pipeline for data of this nature. Around 129,000 cells were collected from 18 rhinophore ganglia and 20 circumesophageal ring ganglia (brain), consisting of the cerebropleural, pedal, and buccal ganglia. Messenger RNA libraries were constructed using the 10X Genomics’ Chromium platform. After library preparation, around 1,000 cells were recovered and sequenced. The HTStream package was utilized to trim off unwanted sequences from the raw reads and remove PCR duplicates and other contamination, then the salmon alevin package was employed to construct gene-by-cell matrices containing all the transcripts for each gene in each cell. The Seurat pipeline was used to extract this expression data from the matrices, normalize it, and perform dimensionality reduction. The cells were clustered based on similarities in their gene expression profiles. The cells formed eight clusters on a UMAP graph, each having distinct marker genes. Additionally, one cluster was composed of almost exclusively cells from the rhinophore ganglia, accounting for 30% of all rhinophore ganglion cells in the sample. Cells from the rhinophore ganglia are as heteregenous as cells from the rest of the brain, with cells forming six clusters. Cell populations that express the same neurotransmitter were identified for a wide range of both small-molecule neurotransmitters and neuropeptides. In a separate project, the locomotion of Berghia was recorded over 9 days with 2 lighting regimes: LD first and DD first. The results suggest that locomotion of Berghia is governed by circadian clock and that Berghia is nocturnal. Hunger state likely plays a role in modulating this circadian rhythm.

This thesis centres around two genera of large peltospirid gastropods (Mollusca: Neomphalina: Peltospiridae) endemic to hydrothermal vent ecosystems. One is the 'scaly-foot gastropod', an emblematic species of the Indian Ocean vents with unique dermal sclerites covering the foot like roof tiles. The other was recently discovered from expeditions to the Southern and Indian oceans, lacks sclerites and possesses large opercula. As both genera and their assigned species remained undescribed, they were formally described herein which forms a basis to understanding their biology. The 'scaly-foot gastropod' from both the Central Indian Ridge (CIR) and the Southwest Indian Ridge (SWIR) were confirmed to represent a single species and is formally named as Chrysomallon squamiferum. Through molecular genetic analyses using the COI gene, genetic differentiation between SWIR and CIR populations was detected for the 'scaly-foot gastropod'. In contrast, the peltospirids with large opercula from the East Scotia Ridge (ESR) and the SWIR proved to be two distinct species within an undescribed genus. The ESR species was formally described as Gigantopelta chessoia and the SWIR species as G. aegis. The molecular genetic analyses of the COI gene, confirmed the genetic isolation of the two and consolidated their status as separate species. A 3D tomographic model of Chrysomallon squamiferum was generated to characterise the soft anatomy and morphology as well as to understand its internal anatomy and adaptation which remained little-studied. Further to the enlarged esophageal gland already known to house chemosynthetic endosymbionts, C. squamiferum was discovered to have a hypertrophied circulatory system with a gigantic, muscular heart and large ctenidium to adapt to life in a hypoxic environment and to supply the endosymbionts with necessary chemicals. Histological examinations of the sclerites and operculum showed that it was unlikely that the sclerites originated from operculum duplication. Comparisons with polyplacophoran scales revealed starkly different secretion mechanisms despite the superficial similarity, which has implications on the placement of sclerite-bearing Cambrian taxa. Overall, the results from this thesis ascertained the systematic positions of these large-sized, enigmatic peltospirids, and led to improved understanding of their ecology and evolution. The important role of larval dispersal in maintaining metapopulations across the distribution of a vent-endemic taxa is highlighted. The adaptations of vent-endemic taxa remains little-known even in well-studied species, warranting future studies on these and other species.