Biological Strategy
Siphuncle Controls Buoyancy
Nautiluses
AskNature Team
Image: Rainy City / Flickr / CC BY - Creative Commons Attribution alone
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 theirskin.
Move in/Through Gases
Living systems must move through gases (which are less dense than liquids and solids) such as those in the earth’s atmosphere. The greatest challenge of moving in gases is that because the living system is heavier than the gas, it must overcome the force of gravity. Moving efficiently in this light medium presents unique challenges and opportunities for living systems. As a result, they have evolved countless solutions to optimize drag and increase lift so that they can stay aloft and take advantage of variable currents. Additionally, they must overcome gravity when moving from a liquid or solid into the air. The fairyfly, the smallest known insect, is a tiny wasp that must move through the air. To the wasp, air feels like a heavy liquid and to move through it, it uses special feathery oars rather thanwings.
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.
- Animals
- Mollusks
- Cephalopods
- Nautiluses
Cephalopods
Class Cephalopoda (“head-foot”): Nautilus, squid, octopus, cuttlefish
Cephalopods are unique among mollusks, and even within the animal kingdom. They are lauded for their large brains and complex behaviors and are considered the most intelligent invertebrates. Among 800 species in 45 families, all are carnivorous and live in marine ecosystems. They all have a set of arms or tentacles, but only the nautilus retains an exterior chambered shell. Many species have chromatophores, which allow them to change color for defense, camouflage, or courting. They range from the size of a fingernail to just longer than a city bus (the mysterious giant squid).
Nature’s Innovations: Animals asEngineers
This lesson, with eight short videos, examines how animals have solved engineering problems and how humans have mimicked those solutions. This lesson addresses middle and high school Next Generation Science Standards in Life Sciences and Engineering Design
View Resource
The siphuncle of nautiloids controls buoyancy by active transport of ions and osmosis between the siphuncle and shell chamber.
Introduction
The nautilus is a free-swimming mollusk related to the squid or octopus, but with a hard, multi-chambered spiraling shell. Reaching through the interior of the shell is a tubular structure called the siphuncle. The nautilus uses this organ to control the volumes of water and gases within each of its shell chambers to regulate its buoyancy.
People began X-raying the animals, and early on, it became apparent that not only is there air in the chambers, but there's water,too.
Peter Ward, University of Washington (see videobelow)
The Strategy
The movement of water into and out of the chambers is driven by osmosis, resulting from changes in the concentration of ions within the chamber fluid. Ions are actively pumped back and forth between chambers to control the movement of the fluids in the chamber. Pumping ions, usually sodium and chloride, out of a chamber makes the fluid within the chamber more dilute (more watery). This causes water to diffuse out of the chamber through the siphuncle in order to equalize the gradient.
Image: Chris 73 / Wikimedia commons / GFDL - Gnu Free Document License
In this bisected Nautilus shell, the chambers are clearly visible and arranged in a logarithmic spiral.
The movement of water out of the sealed chamber lowers the gas pressure inside the chamber. At this point, gases–typically nitrogen, oxygen and carbon dioxide–dissolved in the body fluids diffuse into a chamber through the wall of the siphuncle. Initially, the gases are dissolved, but once inside the low-pressure chamber, they start to bubble out. This is much like carbon dioxide bubbling out of a freshly opened soda bottle because the trapped gases escape, reducing the pressure inside the formerly sealed bottle. With more gas in the chamber, the overall density of the Nautilus decreases while its buoyancy increases, enabling it to float upward in the water column.
To sink, the process runs in reverse: ions are pumped into the fluid of the chamber, water flows in under osmosis, pressure increases, gases move out, and density increases. As a result, buoyancy is reduced.
Nautilus Buoyancy
In this video from Shape of Life, X-rays show the chambers hidden within a nautilusshell.
The Potential
Much like the Nautilus, submarines use ballast tanks to control their buoyancy by filling them with either air (to surface) or water (to submerge). The Nautilus’s unique strategy of using osmosis gradients to move water and modulate buoyancy could have applications for new types of underwater vehicles for transportation, surveillance, and research. Smart buoys could also sense incoming traffic and submerge until boats cleared them.
Related Content
Last Updated October 14, 2016
References
“The body of the mollusc inhabits the very last of a spiralling series of chambers inside the shell. By filling the inner chambers with a mixture of air and water, the nautilus achieves perfect buoyancy, allowing it to rise effortlessly during its nightly migration from the depths of the Pacific Ocean to the surface.” (Downer 2002: 17)
Book
Weird Nature: An Astonishing Exploration of Nature's StrangestBehavior
02/03/2002 |John Downer
Reference
“It appears plausible that the rate of buoyancy control is dependent on thediameter of the siphuncular tube for circadian and other short period adjustments duringfeeding and resting periods.” (Westerman 1971: 1)
Book
Form, structure and function of shell and siphuncle in coiled Mesozoicammonoids
26/02/1971 |Gerd Ernst Gerold Westermann
Reference
“. . . a new chamber is formed by the secretion of a body fluid between the animal and the inner wall of the living chamber and that it is only when the septum and the new siphuncular tubes are sufficiently strong to withstand the hydrostatic pressure of the sea that the liquid within the chamber is pumped out.” (Denton and Gilpin-Brown 1966: 723)
Journal article
On the buoyancy of the pearlynautilus
Journal of the Marine Biological Association of the United Kingdom |12/05/2009 |E. J. Denton, J. B. Gilpin-Brown
Reference
