(Photo credit: Garm and Nilsson, 2013. Royal Society)
There is more to a starfish than meets the eye. For years, scientists and marine biologists alike were puzzled as to the purpose and function of the tiny “eyes” that are located on the tips of a starfish’s arms. However, a team of scientists have recently uncovered clues as to their purpose. Here’s looking at you, Andes Garm and Dan-Eric Nilsson! These two researchers conducted observations and tests with the blue starfish (Linckia laevigata) in an attempt to “see” what the purpose of a starfish eye actually is.
But first off, let me clarify what a starfish actually is. No, it is not from another planet nor is it some freak fish that has five fins or arms. These awesome creatures are actually echinoderms (“Spiny skin”); a type of marine invertebrate. Invertebrates, unlike us, do not have a skeleton on the inside of their bodies. Instead, their organs are protected by hardened material that acts as armor. You could say that invertebrates are not as brave as us because they don’t have any backbone. Ha….ha…ha. (No more bad jokes, I promise!)
The term echinoderm applies to starfish because all of their soft organs are protected by a hardened layer of spiny, calcified skin. Sea urchins and sand dollars are also echinoderms. Anything that is a starfish applies to more than 2,000 species of marine invertebrates! The classic shape that pops up in our heads when we think of a star is five points jutting out from a central point. This is exactly how a starfish is shaped; they a central nervous system, mouth and organs all within the center of their body with five arms that radiate outward. However, this CAN be misleading because different species of starfish may have more than five arms. The sun star actually has a total of 40 arms! Whoa! If I had that many arms, I would be able to get a lot done in a short amount of time.
The crazy thing about starfish is that they don’t have a brain or even blood! The nervous system that allows them to move and function is actually spread throughout their body core as well as their arms. They filter water through their body instead of blood. Madreporites are plates with small holes in them that are located along a starfish’s body; these holes acts as sieves that draw water into a starfish’s body where it moves through multiple canals. Because starfish do not have an internal skeleton, the water that they draw in acts as a structural support for the starfish’s body and also allows them to go places. Just think of it as five water beds all attached to each other at their headboards.
If a starfish loses one of its arms, it can grow another one! One reason why this is possible is because all of the important organs that are crucial for a starfish to live can be found within their arms. This process is called regeneration and it is also how they can reproduce. In addition, starfish can simply reproduce by releasing sperm and eggs close to each other in the water.
If you have the urge to watch a good Sci-fi horror flick, forget Alien or Prometheus. Put those DVD’s back up on the shelf and watch a starfish instead! These animals are one of the few that can eat their food right outside their bodies! Each arm of a starfish has hundreds of tiny, tubed “feet” that act as miniature suction cups. These suction cups allow a starfish to grasp various surfaces as well as grab a hold of their prey. A starfish loves nothing more than to grab a hold of a clam or an oyster, pry it open and then plunge its sack-shaped stomach in to get at the oyster’s soft insides. The stomach goes to work on wrapping itself around the meat, digesting it and then retreating back into the starfish’s body. Eat your heart out, Ellen Ripley!
Starfish can live a lifespan of thirty to thirty-five years. The largest species of starfish is the sunflower star; this monster has a whopping 40 arms and can weigh up to eleven pounds. The paddle-spined sea star is one of the smallest species and is only about the size of one of your fingernails. But enough about starfish in general, let’s get back to setting our sights on how these amazing creatures can see.
The photo at the very top of this article is one “eye” that can be found on the tip of a starfish arm. Barely measuring one millimeter in width, the eye can close, or retract, into the arm if the starfish feels threatened. Each arm of a starfish has a central groove that runs from the center of the starfish out to each end of its five arms. Inside of this groove is where the feet of the starfish are located. The eyes of the starfish rest right at the very end of the groove on the tip of the arm (you can see where the eye is located via the white arrow in the picture). Each eye resembles a tiny, red lightbulb.
Anders-Garm and Dan-Eric Nilsson are pioneers when it comes to studying starfish vision in-depth. Never before had a study been conducted that intimately probed the intricacies of potential vision in any kind of echinoderm.
So FINALLY, after all this time, HERE are the five ways starfish see…and number three really does blow me away!
(1) Within each eye, there are about 150-200 ommatidia (omma-what?!) Ommatidia are tiny “units” that collect light. They are very similar to the compound eyes that can seen on a fly or other insect.
(2) Starfish eyes do not contain lenses like our eyes do. The ommatidia only collects light while the ommatidia found in insect eyes do have lenses to direct light onto underlying cells.
(3) Here it is…..It is very likely that Star fish eyes CAN SEE IN EVERY DIRECTION AT ONCE!!! (My mind is blown.) Their eyes have a wide range of vision (see number 4 below). Since they have five eyes that are on the tips of their arms, this allows them to “see” all around since the arms radiate outward in a circle from their body core.
(4) The field of vision of each eye is huge; it extends over 210 degrees horizontally and 170 degrees vertically. This is an even larger vision range than what we can see!
(5) Starfish can literally only view the world around them in shades of grey. They are color blind and can only see light and dark objects. The ommatidia, or light-detecting units in each eye catch light very slowly and can only detect objects that are either immobile or don’t move fast.
Garm and Nilsson studied these details through the eyes of blue sea stars. This species of starfish live primarily on coral reefs. Garm and Nilsson would place the starfish near a coral reef. At first, the star fish would roam around aimlessly. However, once a starfish got within a few feet of the coral, they would make a run for it. This allowed Garm and Nilsson to come to the conclusion that starfish use their eyes to navigate and find safety from predators.
Pretty cool, huh? I hope this article was an eye-opener. It just goes to show that not everyone sees the world in simple black and white. May your vision of our amazing world grow clearer and brighter as you explore and learn new things! Catch ya in the next read!
By Julie ‘Jules’ Cremer
National Geographic. “Starfish (Sea Star): Asteroidea”. www.animals.nationalgeographic.com/animals/invertebrates/starfish/. Web. Accessed 21 September 2015.
Young, Ed. “Starfish Spot the Way Home with Eyes on Their Arms”. National Geographic online. http://www.phenomena.nationalgeographic.com/2014/01/08/starfish-spot-the-way-home-with-eyes-on-their-arms/. Web. 8 January, 2014. Accessed 21 September, 2015.
Garm and Nilsson. 2013. Visual Navigation in starfish; first evidence for the use of vision and eyes in starfish. Proc Roy Soc B. http://dx.doi.org/10.1098/rspb.2013.3011. Web. Accessed 21 September, 2015.