18 September 2006

Snail tracking


Taking advantage of the last days of the summer, my son and I went on a nature hike in the woods yesterday afternoon. While passing by some small, shallow pools alongside the creek, I noticed these tracks on the submerged rocks. I suspected snails and sure enough, a couple of the tracks each ended at a small roundish object that indeed was a snail about 5 mm long.


I am guessing that they are juvenile Helisoma anceps, a planorbid snail (family Planorbidae). Although they are strictly aquatic, these snails actually have lungs and are in the subclass Pulmonata along with many strictly terrestrial snails. The planorbid snails have evolved an accessory gill, the pseudobranch, that helps them extract oxygen from the water.


We may return to the creek later this afternoon to collect some of these snails with the purpose of raising them at home for a while. I am hoping to take photographs of the snails themselves.



Tim Pearce e-mailed the following comment:

"Yes, I would call those juvenile H. anceps. It does not look like Planorbella, so I would say Helisoma. Of the two species of Helisoma listed by Turgeon et al. (1998), yours is not shaped like, and is out of range for Helisoma newberryi (Burch & Tottenham, 1980), so it is most likely H. anceps.

I find it curious that the snail in the first photo is FACING the trail. Did it crawl, then turn 180 degrees when you took the photo? Or could it be following the trail of another snail? As an introduction to the literature on trail following see Karowe et al. (1993).

Burch, J.B. & Tottenham, J. L. 1980. North American freshwater snails. IV. Species lists, ranges and illustrations. Walkerana 1: 81-215, fi gs. 21-771.

Karowe, D.N., T.A. Pearce, and W.R. Spaller. 1993. Chemical communication in freshwater snails: Behavioral responses of Physa parkeri to mucous trails of P. parkeri (Gastropoda: Pulmonata) and Campeloma decisum (Gastropoda: Prosobranchia). Malacological Review, 26:9-14.

Turgeon, D. D., J. F. Quinn, Jr., A. E. Bogan, E. V. Coan, F. G. Hochberg, W. G. Lyons, P. M. Mikkelsen, R. J. Neves, C. F. E. Roper, G. Rosenberg, B. Roth, A. Scheltema, F. G. Thompson, M. Vecchione, and J. D. Williams. 1998. Common and Scientific Names of Aquatic Invertebrates from the United States and Canada: Mollusks, 2nd Edition. American Fisheries Society Special Publication 26. American Fisheries Society, Bethesda, Maryland. ix + 526 pp."


Yes, I thought the snails' positions with respect to their trails (there was another one like that) were odd too. I need to observe them while they are active.

clare said...

Yes, the snail and its trail looked odd to me too though I couldn't say why - it looked as if the trail had unfurled from the snail. Very weird. Marvellous pictures though. Surprising also (to me at least) that the trail should persist and be seen underwater.