26 July 2010

Mobility as a requirement for terrestrial life

Is there an animal, plant or fungus species, aquatic or terrestrial, that is immobile during all of its life stages? I can’t think of one. Even deeply rooted giant trees—epitomes of immovability—have pollen or seeds that are dispersed passively. If the medium around an organism is moving, there is no need for the organism itself to spend energy to move as long as it can produce small enough propagules for passive dispersal.

There are many aquatic animals, especially in the sea, that are stationary as adults. Examples that come to mind include sponges, barnacles and bryozoans. But they all have motile propagules or larvae that move actively or passively.

As far as I know, there are no terrestrial animals that are stationary as adults and which rely on the wind to disperse their potential offspring. Presumably, this is because the direction and the strength of the wind on terra firma are not as regular as those of the currents in the sea. Curiously, there are also no terrestrial or semi-terrestrial animals with stationary adults that disperse their potential offspring via running waters.

To set the record straight, I should mention that there are terrestrial animals, for example, moths, that make use of air currents to disperse pheromones, which are tiny molecules much smaller than potential offspring. Some microscopic aquatic animals, for example, bdelloids and nematodes, are also believed to be dispersed by the wind. But such animals can also move under their own power as long as they are in water, although the distances they can cover are probably much shorter than their wind-assisted dispersal ranges.

The plants that propagate only by producing shoots from their roots—are there such plants?—may also be considered to be mobile, for their roots move thru the soil. One can nevertheless imagine a stationary aquatic or terrestrial organism that never produces any motile propagules or spreading roots, but, instead, is completely replaced by an offspring. That would be a totally immobile organism. But no such organism has probably ever evolved, because its would have been a very risky lifestyle, an evolutionary dead end. One hungry predator or an environmental catastrophe, such as a falling tree, could have exterminated the one and only member of the species or the entire colony, if such an organism could have formed a colony without being mobile.

It is, therefore, not surprising that no terrestrial animal seems to have descended from marine ancestors that are immobile as adults. There are no terrestrial barnacles or bryozoans. On the other hand, mobile marine animals like snails, fishes and crabs have indeed evolved terrestrial lineages. Of course, mobility is not the only requirement for being terrestrial. Life outside the water also requires being able to obtain oxygen from the air and to resist excess water loss. Suspension or filter feeders with mobile larvae and mostly stationary adults, such as bivalve mollusks, in addition to their ineffective powers of motion, face another apparently insurmountable hurdle on land: in the absence of reliable winds and abundant nutritious particles suspended in the air, their feeding strategies are of no use.

1 comment:

Fred Schueler said...

fish, as all unioists know, were created to cart glochidia upstream.