The New Zealand mud snail has been introduced widely around the world, and it has become highly invasive in the western United States. In its native New Zealand habitats, the mud snail can be found in a wide range of morphological forms, from relatively broad to relatively narrow shells and occurring with spines or without spines. Dislodgement in response to high water flows is affected by shell size and ornamentation, and dislodgment and re-settling may be a major form of dispersal in its introduced range. We evaluated drag coefficients of various shell shapes and sizes in the New Zealand mud snail using a drop tank. Snails from one lotic habitat and one lentic habitat in New Zealand were used; shells were mounted in a common orientation to plastic cover slips. Terminal velocity of shells falling through the drop tank was measured by timing transit between rings of infrared phototransistors to the nearest 0.0002 sec.

Results/Conclusions

Drag coefficients were significantly higher for the robust and spined shells from Lake Bruner. Drag coefficients were lower overall for shells from the Kaiapoi River, but spined shells showed higher drag coefficients than smooth shells. We could detect no significant relationship between shell size or shape and drag coefficient within populations, perhaps reflecting a limit to the sensitivity of using a drop tank to measure drag coefficients on an individual basis. These results suggest that dispersal can be affected by defensive ornamentation in this species, creating a complex relationship between biotic (predation) and abiotic (hydrodynamic drag) risk factors.