05 July 2007

Everybody loves algae


This snail, apparently feeding on the abundant algae (cyanobacteria) growing on the rock, is a Physa species. They are very abundant along the shallow edges of a large creek that runs thru the park near where I live. Despite the fact that they are aquatic snails, they actually breathe air via the inner roof of the mantle cavity that functions as a lung. And because of that, they need to surface frequently to take in fresh air.

Often, I find them at the boundary between water and air. They leave the water frequently and crawl on the wet surfaces of the rocks. The snail near the top of the picture below was completely outside the water, while the lower one was partially out.

Their amphibious lifestyle seems to give them the best of both worlds: their lungs enable them to leave the water to graze on the algae-covered rocks, while the proximity of their aquatic home assures that they are unlikely to run out of water.


Kevin Z said...

What is the current thought on the evolution of lungs in terrestrial snails? Are they derived from ctenidia or vice versa? Which came first terrestrial or marine?

These are things I should already know, but can't seem to pull out of the recesses of my medulla.


Terrestrial snails evolved from marine ancestors.

The "lung" of a terrrestrial snail is simply the inner roof of the mantle. The gills (ctenidia) of marine snails did not evolve into lungs.

I should write posts on these subjects.


Comment by David Campbell originally left at Conch-L:

Ctenidia are the primitive condition in gastropods. Most of the heterobranchs, including standard sea slugs and pulmonates, have lost the ancestral ctenidia. Many have evolved some substitute, e.g. the lungs of pulmonates or the secondary gills of nudibranchs.

The freshwater pulmonates in my aquarium don't seem too picky about
animal or vegetable origin as long as it's slow enough to bite. They
don't seem to like the filamentous green alga, though.

Kevin Z said...

I swear nothing eats filamentous algae. or the red crusty crap. i gave up on our dept. aquarium after 2 years. (mostly since inverts won't be till god knows when due to renovations). I tried to keep it going but it was getting too time consuming to clean.