The nature of the symbiotic association of the red alga Harveyella mirabilis (Reinsch) Schmitz and Reinke (Cryptonemiales) and its red algal hosts Odonthalia and Rhodomela (Ceremiales) was investigated. The distribution of H. mirabilis was revised to include additional host species as well as a greater range of occurrence in the northeast Pacific. A study of procarp development confirms that H. mirabilis should be retained in the order Cryptonemiales. Based only upon morphological criteria, H. mirabilis has been defined previously as parasitic on its red algal hosts. In the present study, a new definition of parasitism has been formulated to include physiological as well as morphological aspects of parasitism. Accordingly, a red algal parasite is defined as any red alga living temporarily or permanently within or on a host, deriving benefits from it and causing it harm. The association of Harveyella mirabilis and its hosts is considered with regard to (1) the reproductive and developmental dependence of H. mirabilis on a specific host, (2) the possibility of metabolite exchange between host and Harveyella, and (3) the effects of the presence of Harveyella on its host. A field and laboratory study of the development and reproduction of H. mirabilis has revealed that the completion of its life history is dependent on the presence of a suitable host and that reproduction and development are affected by seasonal changes in environmental parameters. Initial spore germination occurs in host wounds inflicted primarily by grazing isopods and amphipods. Rhizoidal cells penetrate the walls between host cells and establish secondary pit connections with host cells. Subsequent development is characterized by rapid proliferation of the rhizoidal cells within the host, a rupturing of the host's outer wall region and the final development of a colourless reproductive pustule. Morphological and cytological studies have shown that H. mirabilis conforms to a typical "Polysiphonia-type" life history. The effects of seasonal fluctuations in seawater salinity and temperature, and changes in the hours of bright sunshine (photoperiod) on the reproduction and development of H. mirabilis have been examined over a 20-month period. Gametogenesis occurred in northeast Pacific populations in the spring and fall between a seawater temperature range of 8.5-11 C whereas tetrasporogenesis occurred in the late winter as both photoperiod and water temperature increased. H. mirabilis is physiologically dependent upon the host 0. floccosa as a source of nutrients. Liquid scintillation analysis and light microscopic autoradiography have demonstrated that H¹⁴CO₃⁻ is photosynthetically assimilated by the host and subsequently transferred to H. mirabilis. The primary flow occurs from host medullary cells to adjacent rhizoidal cells of H. mirabilis. A secondary transfer occurs from host cells dispersed in the pustule to adjacent H. mirabilis cells. Ion exchange chromatography and chemical extraction techniques were employed to separate labeled fractions of 0. floccosa and H. mirabilis after various periods of ¹⁴C-translocation. The change in radioactivity in the alcohol-soluble neutral fraction most closely paralleled the total increase in radioactivity in H. mirabilis and the corresponding decrease in 0. floccosa. To identify translocated compounds, the labeled neutral fractions were separated by paper chromatography. An increase in radioactivity was associated with an unknown substance in Harveyella which had an R glucose value similar to glucuronic and galacturonic acids in both acidic and basic solvent systems. A concomitant decrease in radioactivity was associated with both a high R glucose unknown and mannitol in 0. floccosa neutral sugars separated in both basic and acidic solvents. Cortical, medullary, and rhizoidal cells of H. mirabilis were examined by light and electron microscopy to determine the structural mechanisms involved in nutrient transfer. A membrane system in the rhizoidal cells, consisting of the plasmalemma, pinocytotic vesicles, multivesicular and concentric bodies, ER, dictyosomes, microbodies and an extensive vacuolar system may be involved in the uptake, processing and distribution of nutrients throughout these cells. Histochemical identification was made of proteins, lipids and carbohydrates associated with this vacuolar/vesicle system. Light autoradiography, supported this proposed membrane uptake mechanism. Plasma-lemmal extensions (plasmalemmavilli) of H. mirabilis medullary cells in the pustule may also be involved in nutrient uptake. The effects of H. mirabilis on the host 0. floccosa were also examined by light and electron microscopy. In host medullary cells adjacent to rhizoidal cells, changes occur in vacuolation, plasmalemmal vesiculation, ER, plastids, nuclei, pit connections and walls. Direct penetration of host cells by rhizoidal cells of H. mirabilis occasionally occurs resulting in death of the host cells. Host medullary and cortical cells dispersed in the emergent pustule show few of the degenerative responses noted in host cells adjacent to H. mirabilis, rhizoidal cells. On the contrary, host cell division and photosynthetic assimilation of H¹⁴CO₃⁻ all increase. Spherical virus-like solitary bodies (S-bodies) occur in all cells of H. mirabilis and in host cells attached to H. mirabilis by secondary pit connections. The possibility that these structures may induce the infective host response is discussed. In the concluding discussion the possible evolution of H. mirabilis considered in relation to proposed theories of the origin of red algal parasites. / Science, Faculty of / Botany, Department of / Graduate