The pilidium larva is an idiosyncrasy defining the Pilidiophora. Its development is unique, and conserved even in derived pilidia; the juvenile is formed via a series of invaginations of the larval epidermis (imaginal discs), then bursts through the larval body while simultaneously consuming it in catastrophic metamorphosis. Pilidium nielseni is a lecithotrophic pilidium with two circumferential ciliary bands reminiscent of the “prototroch” and “telotroch” of a trochophore larva, the ancestral larval form of spiralians. However, pilidium nielseni represents a convergence on this larval form, not the resurgence of the ancestral larva, and typical pilidial development is conserved. In this thesis, I describe the development of pilidium nielseni, and determine it has converged on its body plan at least twice, independently.

The histology and ultrastructure of the cerebral organs have been studied in three species of monostiliferous hoplonemertean: Paranemertes penegnina Coe, Amphiporus lactifloreus (Johnston) and Tetrastemma candidum (O.F. Muller). The role of the cerebral organs in osmoregulation and behaviour has been investigated in Paranemertes. Based on the information obtained, it is concluded that the cerebral organs in these species are chemoreceptors. The structure of the cerebral organs is essentially the same in the three species studied. The cerebral organs consist of two groups of sensory cells, two groups of gland cells, and two groups of endocytic/lysosomal cells (vesicular cells), as well as ciliated cells and support cells, surrounding a ciliated, blind-ending canal. The canal is functionally divided into two channels, designated the major and minor canals. According to the orientation of ciliary basal feet in cells of the canal epithelium, the minor canal is an incurrent channel, and the major canal is an excurrent channel. The organization of cell types with respect to the direction of flow in the canal is such that along the minor canal; Type A gland cell processes are upstream from Type 2 sensory cell dendrites, and Type 2 vesicular cells are downstream from the dendrites. Similarly, in the major canal. Type B gland cell processes are upstream, and Type 1 vesicular cells are downstream, from Type 1 sensory cell dendrites. Based on this organization, and on the interpretation of cellular fine structure in the cerebral organs, it is proposed that the function of gland cells is to secrete a mucous coating over the sensory epithelium, and the function of vesicular cells is to remove this coating from the canal as the mucus is carried downstream from the dendrites by ciliary action. In gland cells, the amount of secretion product present may be regulated by autophagic breakdown of secretion granules (crinophagy), according to a variable demand for secretion in the canal. Crinophagy contributes to the amount of vesicular material (degraded secretion product) present in the cerebral organs. Although the dendrites are not innervated, dendrite sensitivity may be modulated by variation of the rate of flow through the canal, and the rate of mucous turnover across the two sensory epithelia. An efferent nerve fibre is present among the ciliated cells of the minor canal. The fibre is rare and its synapse has not been observed. It is thought that the fibre innervates a few cells which act as pacemakers, their cilia mechanically entraining the beat frequency of other cilia, thus determining the rate of flow through the canal. There is no indication that vesicular material is disposed of outside the cerebral organs. In Paranemertes and Amphiporus, but not in Tetrastemma, the cephalic blood vessel lies adjacent to the posterior glandular part of the cerebral organ, however, this association is not reflected in the internal structure of the cerebral organs. It is, therefore, unlikely that the cerebral organs in these species have an endocrine function. The function of the cerebral organs in Paranemertes has been investigated by comparing the behaviour of intact worms with the behaviour of worms from which the cerebral organs have been surgically removed. Cerebral organ removal did not affect trail following behaviour, which is associated with homing, but it abolished the response of Paranemertes to prey trails. It is concluded that the cerebral organs of Paranemertes are chemoreceptors responsible for the detection of prey. The behavioural physiology of Paranemertes has been investigated, using extracellular suction electrodes to record from the lateral nerve cords and the cerebral organ nerves. The results indicate that the cerebral organs are sensitive to prey extract and distilled water, but not to mechanical, thermal or photic stimuli. The role of the cerebral organs of Paranemertes in salinity stress tolerance has been investigated by measuring the effect of cerebral organ removal on volume regulation, and by observing the effects of hypo-osmotic media on the cytology of the cerebral organs. Removal of the cerebral organs decreases volume regulatory capacity, however, a similar change is seen in sham-operated worms, indicating that the decreased capacity for regulation is due to the operation itself and not to interference with a physiological role of the cerebral organs. Cytological changes caused by exposure to dilute sea water are similar to those seen in worms fixed in hypo-osmotic fixative. It is unlikely, therefore, that these represent a co-ordinated response of the organs to salinity stress. No exchange of material between the cerebral organs and the vascular system was observed. It is concluded that in Paranemertes, the cerebral organs are not involved in osmoregulation.

This study doubles the known diversity of nemertean species in one region along the northeast Pacific coast by utilizing the often over-looked larval life-history stage. Prior to this work, the nemertean fauna in this region was believed to be well described; however, previous assessments were based on adult life-history stages only and significantly underestimated the real diversity. With this dissertation, we update what is known about nemertean diversity and expand upon this “life-history” approach to describe new species, identify and describe larval forms, and speculate on the phylogenetic relevance of nemertean larvae. A considerable amount of new diversity takes the form of cryptic species complexes, where existing descriptions include characteristics of several species. Micrura alaskensis, a common intertidal nemertean and an emerging model system for developmental studies, existed as a species complex consisting of five species. In this dissertation we designate a new genus, re-describe M. alaskensis, and describe four new species in this complex. In doing so we make accurate identification possible for future comparative research. The complete development of few nemertean species was known before this project began, thus few species could be identified as larvae. We have identified over 30 nemertean larvae using both embryological and DNA barcoding approaches in this work. Intriguingly, many wild-caught larvae could not be matched to species previously reported from this region and instead contribute to previously unknown diversity. This new diversity includes species previously reported only from distant geographic regions as well as species new to science. The first record of a hubrechtid on the west coast of North America and the identification of two new species in the currently monotypic genus Riserius were revealed in larval assessments. Aside from increasing known species-level diversity, we revealed novel larval types. Barcoding larvae allowed us to place larval morphotypes into a phylogenetic context and identify potentially useful larval synapomorphies for nemertean phylogenies. Our results emphasize the importance of a life-history approach to biodiversity assessments for all species with biphasic life-cycles. This dissertation includes published and unpublished co-authored material.

No description available.

Recent studies of hoplonenemertean planuliform larvae have clarified their development and provided insight into larval evolution within the phylum. However, an assessment of viviparous development using modern techniques is lacking. To help facilitate a comprehensive comparative evaluation of developmental diversity within hoplonemerteans, we have conducted a confocal laser scanning microscopy investigation of the development in Prosorhochmus americanus, one of the few viviparous hoplonemertean species. Phalloidin staining provides evidence of a modified transitory larval epidermis, and reveals that the foregut, midgut, proboscis, central nervous system, and body wall musculature form early in development, consistent with observations for planktonic and encapsulated hoplonemertean larvae. However, invaginations characteristic of these larvae were not observed. Acetylated tubulin labeling and light microscopy shows that embryos are uniformly ciliated, and some specimens possess a caudal ciliary cirrus and/or apical tuft which are characteristic of planktonic larvae. These are interpreted as vestigial structures in the non-swimming P. americanus embryos. The findings provide additional evidence that hoplonemerteans exhibit a form of metamorphosis in their life history and thus exhibit indirect development. However, a comparative assessment of larval features in P. americanus suggests an evolutionary trend towards direct development in this species.

Emplectonema gracile (Johnston 1837) is a hoplonemertean of marine intertidal hard-bottom communities and is distributed throughout the Northern Hemisphere. Although possessing a planktonic larval stage in its life history, the range of such cosmopolitan marine invertebrate species is often explained by cryptic speciation and anthropogenic transport. The purpose of this study is to test for possible cryptic species using mtDNA markers (COI and 16S rDNA) and to investigate population structure in E. gracile over a portion of its geographic range using mtDNA markers and ddRADseq nuclear SNP data. The results of both phylogenetic- and tree-based species delimitation revealed that E. gracileis a morphotype containing cryptic species. Three North Atlantic and one Pacific coast population are inferred as one species (E. gracile sensu stricto) and two Pacific coast populations (Akkeshi, Japan and Charleston, Oregon) are inferred as another species (Emplectonemasp 1), strongly confirming an earlier study and extending the range of the latter species to the Pacific coast of Japan. Anthropogenic transport is suggested as the likely mode of transport for E. gracile.Both Fst, PCA and haplotype network analyses suggest a lack of differentiation between E. gracile populations separated by large geographic distances.In contrast corresponding analyses forEmplectonemasp. 1 indicate differentiation between the two populations sampled. Further research will be necessary to reveal if rare anthropogenic transport or natural dispersal (larval transport, rafting) between geographically adjacent yet to be delimitedE. gracile morphotype populations is responsible for its seemingly disjunct distribution.