29 April 2006

How a slug breathes

One essential step during the evolution of terrestrial gastropods (snails and slugs) from aquatic ancestors was the development of the ability to extract oxygen from the air. Gills that need to be surrounded with water to function efficiently are useless in air. Thus, in all terrestrial gastropods gills have disappeared and the inner surface of the mantle cavity has evolved into a lung. However, the lung surface must be kept wet at all times, because gas exchange always takes place across a film of water (even in human lungs).

In pulmonate gastropods, only a single hole, the pneumostome, connects the mantle cavity to the outside world. The large pneumostome of the slug Arion subfuscus is visible in the picture below. Unless the slug is abnormally sinistral, the pneumostome is always located on the right side of the mantle (the mantle is the flap of skin covering the back of the head).

If you watch a slug long enough, you will notice that its pneumostome rhythmically opens and closes, in other words, you will witness the slug breathing. The rate of breathing of a slug is probably determined by the metabolic rate and the state of hydration of the slug as well as the ambient temperature and humidity. The series of pictures below show the opening and closing of the pneumostome (arrow) of a Megapallifera during one complete cycle that took 23 seconds. Notice how the front of the mantle puffs up when the pneumostome is fully open. I am assuming that it is the air filling the mantle cavity that makes the latter to swell.

Part 2.


Tim Pearce said...

The closing of the pneumostome would help to reduce water loss (as in the stomata of plants).

If the slug can pull air into its pneumostome, that would be a good trick - creating negative pressure inside - for a creature without a firm skeleton. Perhaps the slug uses a hydrostatic skeleton?

Another question would be to determine whether the the slug is actually forcing air in and out (as in true breathing), or whether it opens its pneumostome and relies on diffusion. One way to test this idea would be to suspend dye in the air (or smoke, for example), and carefully observe whether the air currents behave differently than simple diffusion would predict.

bev said...

Excellent photos to illustrate your piece, Aydin.