With an estimated 200,000 living species, Mollusca is the second most species-rich animal group. Many molluscs are economically, ecologically, or biomedically important. Additionally, the molluscan body plan is extremely varied, ranging from slugs to clams to giant squid. Because of molluscs' variation, diversity and abundance in the fossil record, there has been great interest in identifying the primitive condition of the group for nearly 200 years. The worm-like mollusc group called Aplacophora (Latin for "without a shell") has long been thought to be important to understanding early molluscan evolution. However, little is known about the evolutionary history (phylogeny) within this mostly deep-sea group. This research aims to reconstruct aplacophoran phylogeny using a combination of traditional and cutting-edge, next-generation DNA sequencing technologies. New morphological data will be collected using traditional microscopy and characters will be mapped onto the phylogeny to infer primitive features of Aplacophora as well as Mollusca as a whole.
Understanding how these animals have evolved will be of great interest to paleontologists and invertebrate biologists who have long sought to understand the early evolution of this diverse and fascinating group. Moreover, as molluscs are important models in the fields of neurobiology and parasitology, the results of this work will directly inform biomedical research.
Intellectual Merit Although closely related to animals like snails, slugs, clams, and squid, the aplacophoran molluscs are unusual, shell-less, spiny, worm-shaped animals that do not resemble other molluscs. Because of their apparent morphological simplicity, aplacophorans have been central to discussions of early molluscan evolution. Recent molecular investigations strongly supported placement of the two wormlike mollusc groups, Solenogastres and Caudofoveata, in a monophyletic taxon, Aplacophora, which is closely related to Polyplacophora (chitons). This three taxa, together called Aculifera, comprises the sister taxon of all other molluscs and thus is important to understanding the early evolution of this diverse and important phylum. However, to date, no molecular studies focused on relationships within and between the two aplacophoran clades have been conducted. Therefore, we sequenced the actively expressed genes around 20 aplacophorans (plus one chiton) and conducted phylogenetic analyses based on datasets with up to 279 genes. Our results provide further support for Aplacophora and a broad-scale phylogenetic framework for both groups. Notably, the traditional taxonomy used to classify members within Solenogastres appears to be in dire need of revision. Unfortunately, ancestral character state reconsturction analyses were ambiguous with respect to the ancestral condition of most of the characters of interest (e.g., radula [molluscan teeth-like feeding structure], spicule type, foregut gland type, etc.). During the course of this work I collected specimens of five species of animals that were previously unknown to science. Description of these animals helped to improve understanding of the biology of this little-studies group; one species appears to brood it's eggs in its mantle cavity. Another undescribed Antarctic aplacophoran was collected that was initially thought to represent a third class of aplacophoran because it exhibits morphological characters intermediate between the two groups. However, molecular data strongly indicate that this species is actually a highly unusual solenogaster. Broader Impacts This doctoral research has helped to improve understanding of early molluscan evolution through revisionary systematics of Aplacophora, morphological studies of key characters, and inference of the plesiomorphic states of key characters for all of Mollusca. Although aplacophorans are both common and relatively diverse, in the last twenty years, fewer than ten workers have performed descriptive work on the group and two of these workers are of retirement age. Therefore, training a new generation of young scientists in the morphology of these common but understudied is critical. Although this work will naturally be of great interest to malacologists and invertebrate zoologists, improved understanding of molluscan phylogeny and early evolution will inform fields ranging from paleontology to genomics to neurobiology This report was written by Kevin Kocot and edited by Ken Halanych
