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Effect of reduced salinity conditions on the distribution and responses of the muricid intertidal snail Thais lamellosa gmelin Johannsson, Ora E.

Abstract

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.

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