Shell Alters Buoyancy

a green sea turtle swims through blue water in Maui

Biological Strategy

Sea turtle

AskNature Team

Image: J. Phillip Krone / Flickr / CC BY NC ND - Creative Commons Attribution + Noncommercial + NoDerivatives

Move in/on Liquids

Water is not only the most abundant liquid on earth, but it’s vital to life–so it’s no surprise that the majority of life has evolved to thrive on and under its surface. Moving efficiently in and on this dense and dynamic substance presents unique challenges and opportunities for living systems. As a result, they have evolved countless solutions to optimize drag, utilize surface tension, fine tune buoyancy, and take advantage of various types of currents and fluid dynamics. For example, sharks can slide through water by reducing drag due to their streamlined shape and specially shaped features on their skin.

Search this Function

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.

Search this Function

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.

Search this Living System

The shell of some sea turtles allows for different levels of buoyancy for juveniles and adults by changing shape.

Image: Manuel Heinrich Emha / Wikimedia Commons /

Green Turtle (Chelonia mydas) just hatched

Image: KENPEI / Wikimedia Commons /

Image: Mila Zinkova / Wikimedia Commons /

1=<a href=http://en.wikipedia.org/wiki/Green_Sea_Turtle class=extiw title=w:Green Sea Turtle>w:Green Sea Turtle</a>, ''Chelonia mydas'' is basking on <a href=http://en.wikipedia.org/wiki/Punaluu_Beach class=extiw title=w:Punaluu Beach>w:Punaluu Beach</a>

Image: Mila Zinkova / Wikimedia Commons /

[http://en.wikipedia.org/wiki/Green_sea_turtle Green turtle]

“Sea turtles sometimes swim on the surface; Jeanette Wyneken tells me that the flared, V-shaped bottom is characteristic of buoyant baby sea turtles, which are obligatory surface swimmers. With maturity and the shift to submerged swimming, the hull shape changes to one more characteristic of submarines.” (Vogel 2003:116)

Last Updated August 18, 2016

References

Book

Comparative Biomechanics: Life's Physical World, Second Edition

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

embedly preview

AskNature