Skin Acts as Membrane

white and black tail of sea snake seen underwater

Biological Strategy

Sea snakes

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Modify Buoyancy

Buoyancy is an upward force exerted by air or liquid on a solid object that works against the object’s weight. A hawk glides through air and a duck floats on water due to buoyancy. Some living systems (such as fish eggs) can remain sedentary and therefore maintain the same buoyancy at all times. But most must adjust their buoyancy because they can’t survive long unless they change position. These living systems require strategies to not only be buoyant, but also adjust buoyancy level. They often modify buoyancy by adding or decreasing lift, or by changing their weight. For example, birds like condors that soar for a long time shift the directional orientation of their wingtip feathers to manage buoyancy. Fish alter the amount of air in their swim bladders to increase or decrease their weight, thus altering their buoyancy.

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Capture, Absorb, or Filter Gases

Oxygen, carbon dioxide, and nitrogen are all gases of particular importance to living systems. Living systems obtain these gases from water and air by trapping them, absorbing them across surfaces, or filtering them. Gases are low in density and viscosity, and when dissolved in liquids, are very low in concentration–so extracting them requires specialized strategies. As a result, living systems use the properties of gases, liquids, and solids to assist in capture, absorption, and filtering. For example, fish gills have many filaments to create a large surface area to maximize how many blood vessels come in close contact with oxygen dissolved in the water.

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Reptiles

Class Reptilia (“creeping”): Lizards, crocodiles, turtles, snakes

Reptiles retain some of the key characteristics that first enabled vertebrates to live permanently on land. They have dry skin covered in scales made of keratin that help prevent water loss. (Amphibians still need to stick close to water to keep their skin moist.) Many reptiles reproduce by laying eggs that are watertight and have a yolk, so eggs can develop on land while staying moist and nourished. Reptiles are also ectotherms, meaning they don’t produce their own body heat. This means they burn through energy much more slowly than “warm blooded” creatures of the same size. It also means that they need to keep warm to keep active, which is why you might see them spending a seemingly inordinate amount of time simply basking in the sun.

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The skin of a diving sea snake acts as a membrane to filter out CO2 from its body.

“Sea snakes, air-breathers like us, dive deep and stay under for prolonged periods while swimming for considerable distances. They deal with this problem of variable buoyancy in what must be the simplest possible way. One such snake, Pelamis platurus, typically descends about 30 meters with enough air in its lung (it has only one) to be neutrally buoyant at that depth. But it loses gas, mainly carbon dioxide and nitrogen, from its skin, whose permeability lets it act as a kind of gill and supplement its lungs. So as it swims along it gradually ascends, thus maintaining its neutral buoyancy as part of its requisite return to the surface (Graham et al. 1987.)” (Vogel 2003: 98)

Vogel S. Comparative Biomechanics: Life’s Physical World. Princeton: Princeton University Press; 2003.

Last Updated September 14, 2016

References

Book

Comparative Biomechanics: Life's Physical World, Second Edition

Princeton University Press | 17/06/2013 | Steven Vogel

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