At the surface these snails can fill their lung with air (I’m calling it a lung though it is different in many ways to a vertebrate lung), they are air breathers after all.
Dropping to the bottom then, means they must expel this air, to gain negative buoyancy. But these snails are air-breathers and have no gill. Can they get enough oxygen from filling their lungs with water, as some freshwater limpets do. And must they then travel along the bottom to the pond edge, or a convenient big rock, and climb back to the surface to get air? Some freshwater snails and most Marine snails are gilled so wouldn’t have this problem. But these snails are, again, pulmonates, air breathers.
As I ponder this, a snail in my aquaria rises to the surface from the bottom. Did this snail climb down the sides of the tank and along the bottom, with its bubble of air, and on releasing its foot from the aquarium bottom, pop to the surface? This calls for more snail watching.
Further observations:
Some snails drop to the bottom, hang out there for a bit and then rise back to the top. Sometimes they rise very slowly. Sometimes they rise fast. Sometimes snails rise from the bottom, but do not reach the surface, they hang in the water column for awhile, then descend back to the bottom.
Can a snail change buoyancy enough when increasing its surface area by extending out of its shell? It wouldn’t seem so, that might slow its decent, but surely can’t allow a negatively buoyant individual to become positively buoyant. Do snails have another mechanism to alter buoyancy?
Buoyancy in the ocean is a well studied phenomenon. Physalia, the Portuguese man O’ War, has gas bladders that allow it to float on the surface: Physa = bladder in Greek. These jellies, which are considered colonies of little animals (zooids) connected together, have gas bladders that can be expelled, they have a connection to the outside, and can refilled this quickly, within a hour. However, it’s not just filled with air from outside; the most interesting thing here is the gases filling the float is a mix of carbon monoxide, oxygen, and nitrogen. Mostly nitrogen, but about 15% carbon monoxide. In a deep water species the amount of carbon monoxide is greater, up to 77%. That seems a bit odd since carbon monoxide is deadly to us.
Cuttlefish use a gas filled cuttlebone. Gases, mostly nitrogen, fill the small chambers within the cuttlebone. The gases within are at equilibrium with those in the cuttlefish body. Gas exchange occurs between the tissue of the animal and the cuttlebone, but it appears to be very slow.
The chambered nautilus fills its chambers with gas. The number of chambers increases as it grows, and it only lives in the very outside section of its shell. The chambers are fluid filled, and slowly gain gases. The nautilus move ions out of the fluid, water follows the ions, via osmosis, and the chambers slowly empty of fluid. Or refill, if needed I suppose. The gas is mostly nitrogen.
I suppose we should talk about fish now:
Many fish species alter their buoyancy by filling up or expelling gas from their swim bladder. The swim bladder may have a connection to the esophagus, physostome fish, or not, physoclist fish. Even in physoclist fish the bladder can be filled, or drained, pretty quickly. There is an array of capillaries, the rete mirabile, parallel to other capillaries, allowing a countercurrent exchange for efficient movement of gases. Along with the countercurrent exchange, the long, and large number of capillaries in the rete mirabile allow fast exchange. The gases, in most fish, is oxygen. To release more oxygen into the blood, fish alter the pH of their blood This, in turn, causes hemoglobin to release more oxygen, essentially what is known as the Bohr effect but is often called the Root effect in fish since there are some differences.
In a fish, the rete mirabile (retia is plural) supplies blood to the gas gland, which releases lactic acid. The gas gland is a layer of cells between the capillaries of the rete mirabile and the swim bladder. In some species lactic acid arises from glycolysis within the tissues.
By producing or holding excess carbon dioxide, more oxygen is released (its a bit more complex than this, as just the carbon dioxide aids release, as does the pH change, a fascination process).
Some fish have lost their swim bladder, others, those that live more or less permanently at a deep depth for example, have lipid filled swim bladders. Sharks, of course, have no swim bladder, but gain some buoyancy by having a large, oil soaked liver.
Back to the pesky fresh water snails. Do they exchange gases in similar ways? Do they follow their fellow Mollusks like the cuttlefish and nautilus. Being in freshwater, ion exchange like that observed in the nautilus, is probably not possible, ions are precious in freshwater. The exchange done by the cuttlefish is pretty slow. Thus, I think, the snails must follow a method similar to fish. Of course mollusks (and arthropods) have hemocyanin, not hemoglobin, as a respiratory pigment. Except for some rams horn snails which have hemoglobin. However, having oxygen biding to copper rather than iron, doesn’t alter the process much. CO2 lowers pH, decreasing the amount of oxygen held by the pigment, thus releasing oxygen too, perhaps, fill the lung.
Is this what these snails actually do?
Is there any research on this?
Notes:
The concept of countercurrent flow is accredited to the famous biologist JBS Haldane. He worked on the physiology of the kidney, and salt and water balance (two citations below).
It has been reported that Manatees control their buoyancy with their degree of farting. When I first read this I thought it was fake, but it turns out there is some evidence to support this, but it's not certain (see the Rommel and Reynolds paper below).
The fluid in arthropods and mollusks is not usually called blood, it is referred to as hemolymph or haemolymph, to use the older spelling. It is like blood, in that is carries oxygen and carbon dioxide, wastes, etc. So why isn’t it just referred to as blood? It probably should be but this fluid doesn’t carry blood cells, our has red blood cells (called that due to the packed hemoglobin that gives the color) and white blood cells, called that because the white foamy layer that forms when whole blood in separated.
Reddit users may be familiar with the term parasnailing (look for the subreddit), this is the phenomena that I have described here except that the snail may "sail" “on the water currents within aquaria.
References and further reading:
Baird MM, and Haldane JB. 1922. Salt and water elimination in man. Journal of Physiology 56:259–262.
Dillon RT. 2006. Freshwater Gastropods. pp 251 - 260 in Sturm, C.F., T.A. Pierce & A. Valdes (eds.), The Mollusks: A Guide to Their Study, Collection and Preservation. American Malacological Society, Pittsburgh, PA. 445 pp.
Denton EJ, and Gilpin-Brown JB. 1961. The buoyancy of the cuttlefish. Journal of the Marine Biology Association of the United Kingdom 41: 319-342.
Greenwald L, Cook CB, and Ward PD. 1982.The structure of the chambered nautilus siphuncle: The siphuncular epithelium. Journal of Morphology 172: 5–22.
Haldane JS, Priestley JG. 1916. The regulation of excretion of water by the kidneys: I. Journal of Physiology 50:296–303
Lenfant C, and Johansen K. 1965. Gas transport by hemocyanin-containing blood of the Cephalopod Octopus dofleini. American Journal of Physiology 209: 991–998.
Pickwell GV, Barham EG, and Wilson JW. 1964.Carbon monoxide production by a bathypelagic siphonophore. Science 144: 860–862.
Rommel S, and Reynolds JE III. 2000. Diaphragm Structure and Function in the Florida Manatee (Trichechus manatus latirostris). Anatomical record 259: 41–51.
Schmidt-Nielsen K. 2004. Animal Physiology: Adaptation and Environment 5th edition. Cambridge University Press.
Weber RE, and Hagerman L. 1981. Oxygen and carbon dioxide transporting qualities of hemocyanin in the hemolymph of a natant decapod Palaemon adspersus. Journal of Comparative Physiology B 145: 21–27.