Do Starfish Have Eyes? (Explained)
Starfish, or sea stars – the term scientists prefer because the invertebrates aren’t actually fish – are generally simple animals without brains and blood, so researchers doubted these creatures have actual eyes for a long time. They used to believe that starfish navigate through the sea using their sense of smell.
So, do starfish have eyes? In this article, we’ll talk all about that but let’s start with a quick answer:
Starfish do have eyes located at the very tip of each arm. They’re less than half millimeters in size, and you can spot them as a red dot on the top of their tube feet. They can see the light, dark, and identify large structures such as coral reefs.
However, that certainly doesn’t tell the whole story. Below I’ll explain more about starfish eyes, where they’re located, and what they can see. Furthermore, I’ll explain why starfish have eyes, if all species have them, and how they regrow them. Read on!
Where are starfish eyes located?
Starfish have eyes located at the very tip of each arm that resemble arthropod compound eyes, except for the lack of true optics. If you ever have a chance and look very closely, you may spot a red or black dot on the tip of their arms – that’s the eye.
Because sea stars’ eyes are located on their arms, they have a full range of vision based on some arm bends. These animals can raise their arms and bend their tips 80°–120° upwards, which automatically raises the eyes as well.
Each eye sits on a modified tube foot – a small flexible appendage. Starfish, like other Echinoderms, use their tube feet located on the underside of their body to move around the seafloor or, like sand dollars, to pass food to their mouth.
The visual field is shaped by two rows of modified tube feed. They surround the eye and most likely block light coming from below and above. These tube feet spread out when the sea star is active and can withdraw back and hide the eye when the animal is disturbed.
Starfish eyes are very small and vary in size, depending on the species and animal’s size. The largest detected eye was 25 µm (0.025 mm) wide and 60 µm (0.06 mm) deep. This eye had about 120 photoreceptors and a similar number of pigment cells displayed in seven to eight layers along the long axis.
The amount of eyes varies depending on the sea star’s species and the number of arms. Some of the most common species have five arms, but others can have even 40! So, for example, the sunflower sea star with 24 arms has 24 eyes.
How and what can starfish see?
The starfish eye is a structure of several hundred light-collecting units, which we call a compound eye. However, unlike insects, it doesn’t have a lens to focus the light, reducing its ability to see anything but light, dark, and large structures, like coral reefs.
Starfish also don’t have the color-detecting cones like human eyes, so they cannot detect colors. They also can’t see fast-moving objects, so they probably won’t notice if something swims by very fast. Their eyes work very slowly, and the reason for that is perhaps because they move very slowly as well.
Sea stars can see in approximately 200 pixels and only about three feet (1 m) in front of them. Most of the species can also see only during the day. However, it’s enough for them to recognize a coral reef in front of them, which is essential for their survival.
Why do starfish have eyes?
One of the most obvious and simplest reasons is to find food. Starfish mostly eat shellfish like mussels, clams, oysters, but also algae and bacteria. Interestingly, particular creatures can glow at night, so starfish can locate them by seeing the light.
The following reason for starfish to have eyes is to navigate towards and through their home – coral reefs. Even though starfish vision is not great, it allows them to identify large immovable structures. This way, they can spot reefs that are an essential food source and protection.
Garm and Nilsson investigated how sea stars use their vision. They blinded several sea stars and compared their behavior with non-blinded ones by placing them one meter away from the reef and observing their movements. They found out that blinded sea stars were moving in random directions and were completely unable to find the reef. Those who could use their eyes found the reef without trouble in under half an hour.
It’s important to mention that Garm and Nilsson pointed out that sea stars could only navigate back when the reef takes up more than 30° vertically. Their behavior works only at relatively short distances, which can mean that the role of their vision is not necessarily to seek new reefs and handle major displacements but to ensure that they don’t move away from the reef.
Another reason for starfish to have eyes is that some species, like Diplopteraster multipes are bioluminescent and can send glowing signals to other starfish. This can mean that they communicate with other starfish, for example, about their reproductive state.
Starfish can regenerate their eyes
As part of the phylum Echinodermata, starfish have the availability to regrow limbs and, in some cases, entire bodies. You may also find my other blog post interesting, where I wrote about sand dollars cloning themselves.
Regrowing limbs is the way how starfish save themselves from death. So, when predators eat some of their arms or the animal is stuck under a fallen rock, it can escape and swim freely. Later, it can regrow the lost arm, together with tube feet and the eye.
Even though regeneration is primarily used to recover limbs that were eaten or removed by predators, starfish are also capable of autotomizing limbs and reproducing. It happens when starfish shed arms with part of the central disk attached. Next, the arm can regenerate into an entire starfish identical to the original one.
Do all starfish have eyes?
There are approximately 2000 species of sea stars, and each of them adapts to different conditions. Some live in shallow tropical waters, some inhabited the deep sea, and some are found even in the cold Arctic Ocean.
All animals, including humans, developed different body features to adapt and survive the environment. This is already a sign that the answer to whether all starfish have eyes is that not all sea star species have eyes. The reason is simple – because they don’t need them to survive.
One of the studied sea stars that didn’t have eyes was C. crispatus. Scientists say that the lack of eyes in this species probably doesn’t correlate with depth but rather its lifestyle. Star Ctenodiscus crispatus, just like a sand dollar, is known for its burrowing lifestyle to hide from predators, and this might be the reason why this particular sea star doesn’t have eyes.
We know much more about starfish eyes from recent research than ever before, but only a few species were studied. So, what next?
Researches have already a few interesting species in mind. For example, a large sunflower star (Pycnopodia helianthoides). This sea star, found on the Pacific coast, is a very fast and efficient predator. It’s often observed to chase down food, so it’d be great knowing whether its vision plays a significant role in its behavior.
Another interesting species is the crown-of-thorns sea star (Acanthaster planci). This starfish is responsible for destroying large areas of coral reefs in Australia and Asia. They feed on corals, and currently, the number of these sea star species causes a situation where their density exceeds the level that the available resources can sustain.
Scientists say that it’d be great to know if they use their vision to see the reef. Finding out how the crown-of-thorns sea star detects the reef would allow them to either prevent these animals from doing it or create attractive traps to catch them.
There’s still a huge gap in our knowledge but it’s fascinating that we understand these interesting creatures, and how they interact with the environment more and more.
- Garm Anders and Nilsson Dan-Eric 2014Visual navigation in starfish: first evidence for the use of vision and eyes in starfishProc. R. Soc. B.2812013301120133011 http://doi.org/10.1098/rspb.2013.3011
- Birk Marie Helene, Blicher Martin E. and Garm Anders 2018Deep-sea starfish from the Arctic have well-developed eyes in the darkProc. R. Soc. B.2852017274320172743 http://doi.org/10.1098/rspb.2017.2743
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