Biological Strategy
Shell Grows by AddingChambers
Chambered nautilus
Mary Hoff
Image: Klaus Stiefel / Flickr / Some rights reserved
Optimize Shape/Materials
Resources are limited and the simple act of retaining them requires resources, especially energy. Living systems must constantly balance the value of resources obtained with the costs of resources expended; failure to do so can result in death or prevent reproduction. Living systems therefore optimize, rather than maximize, resource use. Optimizing shape ultimately optimizes materials and energy. An example of such optimization can be seen in the dolphin’s body shape. It’s streamlined to reduce drag in the water due to an optimal ratio of length to diameter, as well as features on its surface that lie flat, reducing turbulence.
Modify Position
Many resources that living systems require for survival and reproduction constantly change in quantity, quality, and location. The same is true of the threats that face living systems. As a result, living systems have strategies to maintain access to shifting resources and to avoid changing threats by adjusting their location or orientation. Some living systems modify their position by moving from one location to another. For those that can’t change location, such as trees, they modify position by shifting in place. An example of an organism that does both is the chameleon. This creature can move from place to place to find food or escape predators. But it also can stay in one place and rotate its eyes to provide a 360-degree view so that it can hunt without frightening itsprey.
Modify Density
In biology, density is measured as mass per unit volume, number per unit volume, or number per unit of area. This means that density changes when mass, volume, number, or area changes. Changing density can result in both challenges and opportunities. Modifying density or managing changes in density enables living systems to adjust to their environment because density also relates to other properties, such as pressure and buoyancy. For example, as a fish swims deeper in water, the outside pressure increases. This decreases the fish’s volume and therefore increases its density; this, in turn, decreases its buoyancy. To keep from sinking, the fish adjusts to these density changes by using its fins to providelift.
Modify Size/Shape/Mass/Volume
Many living systems alter their physical properties, such as size, shape, mass, or volume. These modifications occur in response to the living system’s needs and/or changing environmental conditions. For example, they may do this to move more efficiently, escape predators, recover from damage, or for many other reasons. These modifications require appropriate response rates and levels. Modifying any of these properties requires materials to enable such changes, cues to make the changes, and mechanisms to control them. An example is the porcupine fish, which protects itself from predators by taking sips of water or air to inflate its body and to erect spines embedded in itsskin.
Physically Assemble Structure
Living systems use physical materials to create structures to serve as protection, insulation, and other purposes. These structures can be internal (within or attached to the system itself), such as cell membranes, shells, and fur. They can also be external (detached), such as nests, burrows, cocoons, or webs. Because physical materials are limited and the energy required to gather and create new structures is costly, living systems must use both conservatively. Therefore, they optimize the structures’ size, weight, and density. For example, weaver birds use two types of vegetation to create their nests: strong, a few stiff fibers and numerous thin fibers. Combined, they make a strong, yet flexible, nest. An example of an internal structure is a bird’s bone. The bone is comprised of a mineral matrix assembled to create strong cross-supports and a tubular outer surface filled with air to minimizeweight.
- Animals
- Mollusks
- Cephalopods
- Chambered nautilus
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).
As its body grows, a nautilus closes off the smaller shell space with a wall, creating a chamber that it uses to help controlbuoyancy.
Introduction
To a beachgoer, a seashell is often simply an object of beauty. To a mathematician it may be an object of intrigue or inspiration. But to the creature that made it, a shell is predominantly a protection. It protects the organism from harm in the form of predators, rocks, and other inanimate objects in its environment. But it poses a bit of a problem, too: How does an animal grow when it’s encased in a container that can’t grow with it?
Image: Hans Hillewaert / CC BY SA - Creative Commons Attribution + ShareAlike
The chambered nautilus inhabits the most recently constructed chamber of its shell and uses the other chambers to regulatebuoyancy.
The Strategy
The solution for the chambered nautilus (Nautilus pompilius) is simple and elegant. When it gets too large for its existing space, the (ultimately) volleyball-size nautilus adds on to the open end of its shell, expanding the diameter in a spiral configuration. And, in a remarkable and timely example of repurposing, it does not abandon its old space. Rather, it closes it off with a wall, creating a chamber that it uses to help stay buoyant as its body gets heavier.
Nautiluses live in the South Pacific, hundreds of meters beneath the surface of the ocean. They make their shell by mixing sugars, proteins, calcium, and other minerals, then adding the resulting crystallized material to the lip of the existing shell. But that’s not all. Every 150 days or so, a nautilus forms a membrane at its tail end that separates almost all of its body from the older portion of the shell. The one exception is a tube-shaped appendage called a siphuncle that extends back through the previously constructed chambers.
When first formed, a chamber is filled with fluid. But over time, as the growing nautilus adds bulk, the siphuncle sucks the fluid from the chamber. As a result, the shell becomes more buoyant, counterbalancing the added weight of the living animals to maintain neutral buoyancy (a condition of neither sinking nor rising).
Over the course of its life, a nautilus might add up to 30 chambers. In addition to gradually adjusting for its own increasing weight, it also can add or remove fluid from the old chambers more quickly to compensate for sudden changes such as a hefty meal or a sudden loss of part of its shell.
Over the course of its life, a nautilus might add up to 30 chambers.
The Potential
What lessons might we learn from the nautilus? The strategy of making and using closed chambers to take on and jettison a liquid already is used, and might be further applied, to regulate the position of submarines, drilling rigs, electricity-generating turbines and other manmade objects underwater.
Perhaps more universally applicable and generally beneficial, however, is this takeaway: It’s not always necessary (or even beneficial) to throw something away when it is no longer suitable for its original purpose. Rather, we might do well to consider whether an existing structure might be retained, added to, and ultimately repurposed to provide a new and valuable benefit.
Last Updated August 18, 2016
References
Journal article
Shell growth and chamber formation of aquarium-reared Nautilus pompilius (Mollusca, Cephalopoda) by X-rayanalysis
Journal of Experimental Zoology Part A: Comparative Experimental Biology |23/11/2004 |Bettina Westermann, Ingrid Beck-Schildwächter, Knut Beuerlein, Erhard F. Kaleta, RudolfSchipp
Reference
