Investigations of Neural Ontogeny in the Larval Oyster Crassostrea virginica and the nudibranch Berghia verrucicornis: A Histological and Immunohistochemical Approach

Abstract

Bivalves are a diverse group of molluscs that inhabit both marine and freshwater habitats. These animals play a vital role in the success of entire aquatic ecosystems by filtering the water and providing structural complexity to the benthos. Bivalves are also economically significant. Aquaculture practices rearing various bivalve species has expanded worldwide, which in turn has created a renewable food resource and millions of dollars in revenues. Due to anthropogenic and natural occurring events, bivalve populations have recently declined. In order to restore existing populations and facilitate aquaculture practices, research has shifted towards broadening our knowledge of bivalve biology including neurological morphology. Such information is vital to our understanding of how the nervous system effects and modulates behavior. This is particularly true during the larval period of development where behaviors such as setting and metamorphosis are critical for the production of viable adults. Bivalve ontological research concerned with morphologic and, more recently, immunohistochemical analyses have occurred over the past century; however, none of these studies has provided a detailed histological analysis of nervous system development. The first chapter of my thesis provides a review of previous research concerned with bivalve ontogeny from fertilization through the mature larval stage with references to research on adult bivalves where applicable. Emphasis is placed on nervous system development. In addition, studies relating to potential cues for larval settlement and subsequent metamorphosis are discussed as they relate to the nervous system. This review emphasizes the fact that a detailed investigation of bivalve nervous system development is still lacking in the extant literature. Chapter 2 provides a histological analysis of the ontogeny of the central and peripheral nervous systems during the three major larval stages of Crassostrea virginica (the Eastern oyster). C. virginica was chosen as my model organism due to it’s ecological and economic significance. This study characterizes the larval nervous system, reports the presence of novel ganglia and describes the peripheral nerves that innervate specific larval organs and tissues. Based on the facts that small cardioactive peptides (SCPs) have been documented to modulate ciliary beating, feeding and / or gut motility in adult gastropods and in a few adult bivalves and that they have been reported as present in a few larval gastropods, chapter 3 of my thesis uses immunological techniques to examine the presence and location of SCPs in the larval stages of C. virginica and Berghia verrucicornis. The location of SCPs within the central nervous system and peripheral tissues is discussed along with proposed functions of SCPs in molluscan larvae. Collectively, this thesis is a significant addition to our understanding of the bivalve larval nervous system, This new information will assist future studies in forming a connection between larval neural morphology and critical life history behaviors. As such, these findings will potentially aid in facilitating bivalve restoration efforts and advances in aquaculture techniques.