Thursday, 9 June 2011

Diving-Bell Spider

Diving-bell spider (Argyroneta aquatica)


It was announced on the BBC news this evening (ref. 1) that the diving bell spider (Argyroneta aquatica) can stay a surprisingly long time underwater in its 'diving-bell' bubble of air. A 'dive' of 24hr was recently reported. It seems to me quite possible that a canny spider could survive indefinitely in a submerged bubble. A submerged bubble or indeed a rubber balloon filled with air and submerged in water contains (at equilibrium) the same concentration of oxygen as in the air above the water.

                        There are some facts about the solubility of oxygen in water that are slightly counter intuitive. We all know that we drown in water, and readily conclude that there is not enough oxygen in the water to sustain our aerobic metabolism without which we suffer acidosis, cramps, and death. That is true but only part of the problem; the weight and viscosity of the water make it impossible for us to take enough of the stuff into our lungs, and out again per minute to supply the oxygen we consume in a minute; we would need gills to achieve the through-put. But it is perfectly correct that the concentration of dissolved oxygen in water is enormously less than the concentration in air.

The molar concentration of O2 in water at 20 o is close to 0.284 mmoles per litre (at STP; ref 2.) The concentration of oxygen in air is 21%, or 0.21 litres per litre of air. As the volume of one mole of an ideal gas at standard temperature and pressure is 22.4 litres, the molar concentration of pure O2  gas is close to 1/22.4 moles per litre or 0.0446 M, so when diluted with dinitrogen to 21%, its concentration will be 9.37 mM; which is 33 time the concentration of O2 in cold water. So an active fish has to 'breathe' a great deal more water than the amount of air we have to breathe to stay active. (There is much less dissolved oxygen in tepid water than in cold water, and even fish then have a hard time surviving.)

Now for the mysterious part. Notwithstanding its much lower concentration in water, the equilibrium 'activity' of dissolved oxygen is exactly the same in the two media. And if a bubble of gas is immersed in water (or if a rubber balloon is filled with air and submerged in water), the concentration of oxygen in the trapped gas (at equilibrium) will be exactly the same as in the air above the water. What might prove difficult for a submerged spider will be the diffusion rate of oxygen through the water, and into the bubble (or balloon). The bigger the bubble, the better the rate, because there is more area for the oxygen to difuse across. Stirring the water of course would make a great difference, but is not relevant in stagnant water. On the other hand, illuminated green plants generate oxygen and may help the spider survive by raising the concentration above 0.284 mM. Anyway, a canny spider goes on adding to the size of its bubble until it is satisfied that it has the capacity it requires (ref. 3). Amazing!

(The inert gasses in the bubble will remain at the same concetration as in the air; there is nothing to disturbe the equilibrium discussed above; they will diffuse into and out of the water at the same rate whether that be to and fro the air above the water or to and fro the bubble.)


1 comment:

Ian West said...

OK, I shall be the first to comment. This is elegant, and surprising until it is obvious.