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Biology

How to See Through a Shell

The paper: Li, L., Connors, M. J., Kolle, M., England, G. T., Speiser, D. I., Xiao, X., … Ortiz, C. (2015). Multifunctionality of chiton biomineralized armor with an integrated visual system. Science, 350(6263), 952 – 956 .10.1126/science.aad1246 .

Figure 1: two chitons, Acanthopleura granulata, on a rock. Image from: en.wikipedia.org; photo credit: Hans Hillewaert.

Figure 1: two chitons, Acanthopleura granulata, on a rock. Image from: en.wikipedia.org; photo credit: Hans Hillewaert.

Introduction:

Our oceans are home to some fascinating animals. It may surprise some of you that we know very little about the majority of animals living on this planet. When we study the biology of some of these incredible lifeforms, we are able to uncover secrets that can be used to shape our own technology and medicine. Today, I am going to share some of the incredible discoveries scientists are making on an odd little ocean creature.

Meet the chiton (Figure 1). Chitons are molluscs, meaning they are related to mussels, clams, and squids. Chitons crawl around the intertidal zones and cling to structures like rocks or piers using their muscular foot. Chitons protect themselves with a plated, armor-like shell. You may have passed right by a chiton on a beach and never noticed because they look so much likes the rocks they cling to.

You certainly wouldn’t guess it at first glance, but this unassuming invertebrate is pretty incredible. The armored shell of a chiton is COVERED in hundreds of tiny eyes! (Figure 2). If you don’t find that fascinating (albeit a little weird and kind of creepy), don’t worry- there’s more.

Figure 2: A close up of a chiton shell showing the minute eyes (the dark circles). Image credit: Li et, al. 2015.

Figure 2: A close up of a chiton shell showing the minute eyes (the dark circles). Image credit: Li et, al. 2015.

There are other molluscs with a large number of eyes (Scallops, for instance, can have over a hundred eyes) but the chiton is the only known living animal to have eyes incorporated into its shell. The lenses of the chiton eyes are made of aragonite- the same non-living mineral the chiton uses to build its shell. All other species living today have eyes made of proteins, the building blocks of life, making the chiton the only living species that is able to use a mineral lens to form visual images (fossil trilobites likely had a similar visual system). Although the lenses are inorganic, the chiton has living tissue incorporated into its shell, allowing it to process the visual signals. Not only is the chiton the only living species to have eyes in its armor, but it is also the only mollusc with living tissue integrated into its mineral shell – making this one unique invertebrate!

A recent article published in Science (Li et al., 2015) examined the structural components of the eye of the chiton, Acanthopleura granulata, to investigate just how the chiton is able to use its shell for both seeing and protection.

Methods:

Uncovering the structure of an object is not always an easy task. This study used a variety of methods to determine the functionality of the chiton’s eye. If you are interested in the specific techniques used for the study, please refer to the original research article.

  1. First, scientists had to map out the overall structure of the eyes and surrounding shell
  2. The scientists then needed to determine the fine internal structure of the lens by sectioning tissue And  then determined the optical properties of the lens
  3. After the scientist had the building blocks of the chiton eye pieced together, they had to uncover how the eye functioned in real life to see what kind of image the lenses are capable of generating. To do this, the scientists experimentally measured properties of the chiton eye and then used computer models to test the visual capabilities of the eyes.
  4. Because the scientists were interested in the multifunctionality of the chiton’s shell to act as both armor and sensory tissue, they also measured the structural integrity of the eyes and surrounding parts. To do this, the scientists systematically damaged pieces of the shell as may happen in the wild (i.e. puncturing and crushing the shell).

Results:

  1. Scientists found that the eyes are located in valleys formed by the shell (see Figure 2). This may help to shield them from impacts and damage from predators.
  2. The internal structure of the lens differs from the structure of the surrounding shell. The aragonite crystals which make up the lens are larger than those of the shell and are uniformly aligned (compared to the unorganized crystals of the shell). Both of these properties reduce light scattering and help to form a clearer image.

    Figure 3: The resolution capability of the chiton eye. A. shows the plain image of the fish projected through the chiton’s lens. B. shows the image the aragonite lens is capable of producing. C. shows the likely image the chiton is able to see based on the lens structure and the spacing of light sensitive tissue.

    Figure 3: The resolution capability of the chiton eye. A. shows the plain image of the fish projected through the chiton’s lens. B. shows the image the aragonite lens is capable of producing. C. shows the likely image the chiton is able to see based on the lens structure and the spacing of light sensitive tissue.

  3. By projecting an image through the chiton’s lens, scientists were able to determine the visual properties of the eye. The scientists determined that the lens was capable of resolving clear images (Figure 3B), but because the image formation is also dependent upon other sensory structures, such as light sensitive tissue, the image generated by the chiton’s eye is probably more pixaled (Figure 3C). This is similar to issues of resolution on TVs and computer screens. Even if the picture is clear, if an image has large pixels, the image will show up fuzzy. With limitations in light sensing capabilities, the chiton is likely able to see  a 20 cm object (about the size of a fish) from 2 m away, allowing them to respond to oncoming predators in sufficient time to clamp onto a structure for protection.
  4.  When subjected to damage, the eye is more fragile than the surrounding shell, presumably due to the structural modifications of the aragonite crystal in the lens of the eyes. This structural shortcoming exemplifies the evolutionary trade-offs the animal faces (i.e. increased sensory capabilities may jeopardize shell integrity but increased shell structure limits the ability to form a clearer image).You can think of this like fortifying a castle. If you build a castle out of solid rock, it will be strong and it will be hard for anyone to break in and attack you; however, the solid walls do not allow you to see what is happening beyond your walls. If you add windows and doors to give you the ability to see outside, your castle becomes more vulnerable. Bigger windows will increase what you can see, but will also make it easier for your others to break in. This is the same trade-off the chiton must deal with when building its shell.

Significance

The chiton has evolved a novel way to alter the crystal structure of its shell in a way which allows it to see while still protecting itself. The trade-off between increasing visual capabilities and maintaining protection capabilities have resulted in interesting structural components that scientists are able to learn from. Trade-offs are inevitable in nature. If we understand how animals have evolved to maximize functionality without sacrificing structural integrity, then we can model technology after forms tweaked over millions of years of evolution rather than having to start from ground zero. Although it will be a while before we see this study put to use, material scientists are interested in using these biological results to help engineer structures that are multifunctional. This is not the first time materials scientists have turned to marine animals for inspiration. Barnacles have inspired new forms of adhesives and fish flow sensing organs are being used to help submersibles navigate. Nature provides us with an incredible diversity of forms. When we study nature’s ingenuity, we can unlock information that can help us find cures to diseases, solve ecological problems, and build better technology.

Ashley Marranzino
I recently graduated with my MS from the University of Rhode Island. I specialize in studying fish biology/ecology, and deep-sea biology but love all things to do with marine biology.

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