title: Physiological adaptations and feeding mechanisms of the invasive purple varnish clam, Nuttallia obscurata
Creators: name: Sorber, Leesa E.
Publisher: name: Western Washington University
Year: 2013

description: <p>The Purple Varnish Clam, Nuttallia obscurata, is an example of a recent successful invader to the Pacific Northwest. Nuttallia obscurata reside in the high intertidal zone where it must adapt to daily and seasonal fluctuations in salinity and food availability. I investigated their physiological and morphological adaptations to these conditions. The acute physiological response of the invasive, N. obscurata and the native, L. staminea's excised gill tissue's respiratory metabolism (measured as oxygen consumption) in 5, 30, and 55 ppt salinities were measured in a closed system, Gilson Differential Respirometer and compared. The excised gill tissue of both species displayed the highest rate of oxygen consumption in the hyposaline (5 ppt) treatment and the lowest rate in the hypersaline (55 ppt) treatment. In addition, the excised gill tissue of N. obscurata maintained a lower overall rate of oxygen consumption in all salinity treatments than L. staminea. GLM modeling supported these results, indicating that both salinity and species were factors describing the rate of oxygen consumption. Nuttallia obscurata also must have adaptations in feeding structures or strategies to survive in the high intertidal zone where there is limited food availability during exposure at low tide. I analyzed sediment characteristics and the relative dry tissue mass of N. obscurata's gills and palps, and therefore, the gill-to-palp mass ratio, from high and mid tide zones from two field sites. Due to N. obscurata's alternate modes of feeding, I also analyzed the relative dry tissue mass of the foot and siphon tissue. Nuttallia obscurata from Birch Bay had 30% larger gill tissue and 14.8% larger siphon tissue at high tide than at mid tide potentially due to increased filter feeding during limited submersion in the expansive high intertidal zone. There was no difference in palp of foot mass from Birch Bay N. obscurata. The log gill-to-palp mass ratio placed Birch Bay N. obscurata, on the continuum of bivalve feeding strategies, closer to other known filter feeders. Nuttallia obscurata from Clayton Beach displayed no difference in the gill, palp, or siphon mass at different tide levels perhaps due to the small exposed tidal flat. There was a 9.5% increase in foot mass at high tide but there was no evidence to support that it indicates increased pedal feeding. The gill-to-palp mass ratio was 79% and 41% smaller than N. obscurata from high tide and mid, respectively, from Birch Bay. This difference is likely due to the coarse sediment grain size (0.380-2.0 mm) found at Clayton Beach, which required larger palps for increased sorting needs. The log gill-to-palp mass ratio placed Clayton Beach N. obscurata on the continuum of bivalve feeding strategies, closer to other known deposit feeders. During the course of this research, I noticed a large number (57 out of 100) of N. obscurata collected from Clayton Beach were infected with the mantle pea crab, Pinnixa faba, while a small number (1 out of 100) of the N. obscurata from Birch Bay were infected. Difference in infection rate between sites may be due to increased mud flat exposure at Birch Bay and course sediment found at Clayton Beach. A one-way ANOVA showed there was no effect of infection by P. faba on the gill-to-palp mass ratio of N. obscurata. The change in respiratory metabolism in different salinities, the varied feeding structures with tide height and sediment grain size as well as the symbiotic relationships with native organisms may well contribute information regarding N. obscurata's success as an invasive species in the high intertidal zone of the Pacific Northwest.</p>