Limitation of the distribution of the intertidal gastropod, Thais lamellose, by reduced salinity conditions near the mouth of the Fraser River was investigated by studying the snails' responses to and tolerances of such conditions both in the field and laboratory, and by determining its physiological capacity to control body changes in conjunction with decrease in salinity. Evidence of increased tolerance of low salinity through natural selection was sought in comparisons under very low salinities of snails transferred to Spanish Banks from populations normally experiencing low (17%o: Brockton Point) and relatively higher (24%o: Lilly Point)salinities during maximum runoff of the Fraser. Characteristics of dominant species, such as Thais lamellosa, were also discussed and related to McNaughton and Wolf's (1970) hypothesis of specialized dominants. Reactions were evaluated in terms of feeding rates, vertical/horizontal distribution, attachment and mortality. Decreases in salinity effected responses in a specific sequence: feeding decreases, animals descend from exposed surfaces, attachment strength weakens, movement lessens, detachment and then mortality increases. These alterations overlap considerably due to great variability in low salinity tolerance within each population. Immature snails and those removed from the field in summer were more tolerant than adults or those removed in the winter. Duration of exposure was critical to survival. Gradual acclimation and fluctuating conditions were thought to be responsible for the greater tolerances observed in the field as compared with the laboratory. Salinity tolerances of Brockton Point and Lilly Point snails were similar, possibly due to lower salinity conditions at Lilly Point in the past when the Fraser River emptied into the sea via Boundary Bay. Differences in movement and in vertical/horizontal distribution between the two populations were related to topographical differences in the two habitats. Lilly Point consists of extensive sand tracts in the lower intertidal where wayward snails may become lost and/or die of heat exposure and desiccation during low tides in the heat of summer; Brockton Point is rocky with interspersed mats of mussel shells. The snail is capable of detecting salinity changes and of moving to more favourable conditions subtidally. At the extreme of species distribution in Stanley Park fewer animals were found in the intertidal in June, the month of minimal salinities, than in April or July. In addition the species limit corresponds with tolerances defined in the field which suggests that salinity is directly responsible for termination of distribution rather than a biological factor acting on an animal weakened by salinity stress, although this hypothesis has not been tested experimentally. Studies of oxygen consumption and changes in dry weight indicate that metabolism fell with decreases in salinity and temperature. Differences in available energy were reflected in levels of activity rather than in changes in dry weight. Presumably such a response is important in surviving unfavourable periods. With increase in temperature males utilized gonadal material while females appeared to conserve these products. Thais lamellosa is unable to regulate the salinity of its extracellular fluids but can control its volume to some extent under salinity stress, apparently in relation to its degree of euryhalinity. This ability differed between sexes although mortality due to low salinity exposure was independent of sex. / Science, Faculty of / Zoology, Department of / Graduate

The seasonal variation in the major biochemical constituents of T. lamellosa Gmelin have been studied in relation to the reproductive cycle. Digestive gland, foot muscle and gonad were analysed for protein, glycogen, lipid and ash over a period of one year. In addition to biochemical analyses, histological sections of digestive gland and gonad were made throughout the same period. Histological data supplied information on feeding and gamete maturation. Two major periods of feeding activity occurred in April and August. Gametogenesis began in late summer and the peak spawning period was in March. Glycogen is at a maximum in the digestive gland at times of maximum feeding, but food is stored in the digestive gland in the form of lipid. Stored lipid is utilized by the animal during the winter. Glycogen is at a low level in all tissues and appears to be used primarily for lipid and yolk synthesis. The foot muscle does not store either lipid or glycogen to any appreciable extent. Under normal field conditions during the winter, the digestive gland index decreases as reserves are utilized, while the gonad size is maintained until spawning. Animals which are maintained through the same period under artificial summer conditions, show no loss in the digestive gland index, but a decrease in the size of the gonad. None of the oogonia reach maturity and the mature oocytes are resorbed. The starved animals resorb more material from the gonad than fed animals. The possible role of environmental factors in controlling reproduction is discussed. / Science, Faculty of / Zoology, Department of / Graduate