|Topics in this section on physiological ecology include gas exchange, considered here, and TRAIL-FOLLOWING, TEMPERATURE EFFECTS, and HEAT-SHOCK PROTEINS considered in other sections.|
Research study 1
West-coast whelks Nucella spp. generally live in clean, well-aerated water, but the question1 arises as to whether turbid water might have an effect on their gas-exchange abilities. This is investigated at Friday Harbor Laboratories, Washington in N. lamellosa and N. canaliculata. Turbid2 waters are created by adding silt to seawater and comparing rates of oxygen uptake in this medium with those in clean seawater medium. The author uses 11 gastropod species in the study, including 5 from the west coast, of which 2 are the Nucella species listed above.
Results3 show non-significant effects of sediment on oxygen uptake for both species (see graph). The results of the broader study are similarly inconclusive, with about half of each taxonomic category being affected by sediment, the other half, not, and with no clear pattern being evident. Overall, the author concludes that there is no correlation between the structure of the ctenidial complex and the effect of sediment on gas exchange.
NOTE1 the research question posed by the author is much more interesting than indicated here. The broad intent is to test C.M. Yonge’s idea that differences in habitat distribution and species diversity between vetigastropods and caenogastropods, including whelks, are caused by differences in their respective abilities to take up oxygen in turbid waters. Primitive vetigastropods as, for example, keyhole limpets Diodora aspera, have paired bipectinate ctenidial systems (2 rows of filaments) thought to be prone to clogging; thus, individuals of this species tend to live in clear water on hard substrata. Advanced vetigastropods as, for example, Calliostoma spp., have a single bipectinate ctenidium, thought to be less prone to clogging. Caenogastropods have abandoned the complex bipectinate ctenidium entirely and have a single monopectinate type. The idea is that these different ctenidial morphologies may have permitted a broader range of habitats to be occupied, thus leading to greater species diversity
NOTE2 created by adding 0.5g silt (4-37µm particle diameter) to 275ml seawater. The author does not explain the rationale for testing this particular degree of seawater turbidity, but does note that the level used is higher than that normally encountered by the snails in their natural habitats. During an oxygen-consumption test the sediment is kept in suspension by a magnetic stirrer
NOTE3 the author uses an unusual method to determine live mass, namely, subtracting the mass of soft tissues after dissection from the overall live mass including shell of the retracted snail. No effort appears to have been made to correct for differential amounts of sea water contained within the mantle cavity. The problem, whether methodological or something else, is convincing the reader that an average Nucella is comprised of only 0.5-3mg live tissues. However the error arose (perhaps the axis scale is actually logorithmic and not converted from logarithmic values), it should not affect the statistical conclusions, given that all snails are treated in the same way
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