One of the most striking physical features of hermit crabs is their eyes. Invariably bright and round, the eyes are unmistakable. However, their size and prominence don’t mean that hermit crabs have perfect vision.
Hermit crab eye anatomy enables them to see all around. Compound eyes mounted on stalks give hermit crabs 360-degree vision. This eyesight is restricted, though. Hermit crabs can differentiate between colors and shades, but not necessarily different shapes.
Scent, hearing, and touch are more reliable than vision to hermit crabs. While eyesight is not the most important sense, excessive illumination causes distress in hermit crabs, as do certain colors and hues.
Do Hermit Crabs Have Eyes?
The eyes are one of the most prominent parts of the hermit crab anatomy. The eyes are attached to stalks, located on the head. Terrestrial hermit crabs have apposition eyes, meaning that these stalks are side by side.
Most hermit crabs have green or yellow eyes, as standard. One notable exception is the Pagurus armatus (black-eyed hermit crab). The eyes will invariably be round and bright, unless a hermit crab is preparing to molt.
Hermit crabs can see 360 degrees around them, as these stalks are flexible. This can be invaluable in the wild, as predators are all around.
Despite this, hermit crabs are not entirely reliant on their eyes. Although vision is one of the senses that hermit crabs possess, scent, touch, and hearing are all more important.
How Many Eyes Do Hermit Crabs Have?
Healthy hermit crabs have 2 functional eyes. Unlike limbs, hermit crabs do not shed their eyestalks while stressed. If a hermit crab is missing eyes, this will be a deliberate act of mutilation.
As we will discuss in due course, hermit crabs sometimes sever their own eyestalks. This will be due to discomfort caused by excessive light exposure. Hermit crabs have sensitive eyes, and a little illumination goes a long way.
Equally, hermit crabs may lose eyes in conflict. It’s rare for them to fight to wound or kill, but it can happen. If a battle escalates to this point, eyestalks are an easy target for an aggressive hermit crab.
Hermit crabs that lose their eyestalks may or may not grow them back while molting. It depends on the extent of the damage to any nerves. Even if the eyestalks do grow back, they may not contain working eyes.
How Good is Hermit Crab Vision?
Hermit crab eyes are comparable to those of insects. Like their fellow invertebrates, hermit crabs have compound eyes. This means that the eyes are broken into thousands of tiny orbs, known as ommatidia.
Each of these ommatidia contains independent cornea, lenses, and photoreceptor cells. This means that they have a considerably larger range of vision than their size. The multitude of lenses and cells also aid in telling the difference between lights and colors.
This does not mean that hermit crabs have superior vision to humans or even mammals, though. Compound eyes are not designed to filter out the finer details of vision.
A hermit crab may not recognize particular shapes by sight alone. Opinion remains split on this, with studies still ongoing. It is believed that hermit crabs cannot make out fine details, though. They cannot differentiate patterns on clothing, for example.
Hermit crab vision is good as it needs to be for them to stay alive in the wild. Coupled with other senses, hermit crabs primarily use eyesight to avoid predators. They depend more on other senses for exploration and recreation.
How Does Hermit Crab Vision Work?
As discussed, a hermit crab’s vision involves breaking a sight down through thousands of tiny lenses. This means that hermit crabs are constantly aware of their surroundings.
Vision is most impactful when detecting light contrast and fast movement. As hermit crabs can see all around them, rapid movement will often elicit a startle response. Equally, the ability to absorb and reflect light aids nocturnal eyesight.
Hermit crabs will not focus on a particular sight. Think of hermit crab vision as an early warning system. The eyes of these animals tell them something is in their periphery.
If not startled and driven to hide, hermit crabs then investigate this item with their antennae. This is what tells a hermit crab what it needs to know.
Do Hermit Crabs Recognize Each Other by Sight?
Hermit crabs must recognize and remember their tankmates. Hermit crabs live according to a strict social hierarchy. If one hermit crab asserts dominance in a habitat, this order is expected to be respected. Failure to be sufficiently subservient will result in conflict.
Ecological Research explains that hermit crabs do not select mating partners according to size. This suggests that hermit crabs cannot recognize the shape of a tank mate. This fits in with our discussion about how hermit crab vision does not allow for an assessment of shape.
Despite this, Animal Behavior explains how lobsters – a close relative of hermit crabs – avoid interacting with conspecifics they previously fought with. What is unclear is whether this was a conscious decision based on eyesight, or potentially through scent.
One thing you may notice is that hermit crabs respond to the sight of tankmates eating, though. It’s common for subservient hermit crabs to start eating once a colony leader does so. This suggests that hermit crabs have some degree of visual recognition.
How Far Can Hermit Crabs See?
Hermit crabs have long-range vision in comparison to their size. Nobody can say with any certainty how far hermit crabs can see into the distance. Their eyesight is more impactful from afar than up close, though.
This is because, as discussed, hermit crab eyes cannot distinguish fine details. Hermit crabs understand what they need to know based on movement and light. The shape and pattern of an object cease to become relevant once it is close. Hermit crabs decide early whether what they need to do when something catches their eye.
This is why your hermit crabs may not emerge in your presence until you gain their trust. Your pets have learned to recognize your sound, scent, and the sight of you from afar. By the time you reach a hermit crab, it has decided whether to hide, pinch, or welcome interaction.
Can Hermit Crabs See in the Dark?
As hermit crabs are nocturnal, they have evolved to enjoy superior night vision. Hermit crabs are at their happiest in dim lighting. This is due to sensitivity to light, as we will discuss shortly. Hermit crabs need little illumination to see.
Every ommatidium in the eye of a hermit crab allows light into the lens. As per the British Journal of Ophthalmology, this light is absorbed and bounced around the eye. As hermit crabs have thousands of ommatidia, it stands to reason that their sensitivity to dim light is thousands of times sharper than that of humans.
All of this ignores the fact that hermit crabs are also far from reliant on vision anyway. As we have mentioned, hermit crabs trust their antennae with information more than their eyes. In the darkness, these animals are more than happy to feel their way around.
What Colors Do Hermit Crabs See?
It has been made clear through various experiments that hermit crabs detect differing colors. As per the Journal of Experimental Biology, wild hermit crabs are frequently attracted to shells of strong visual contrast to the native substrate.
Hermit crabs are not colorblind. Red, green and blue all garnered a reaction from hermit crabs. Most notably, blue objects provoked a startle response that led to hiding.
Naturally, this suggests that blue should be avoided in a habitat. When providing decoration, red or green items are preferable. Anything neutral could be considered safe, including block shades of gray or black.
Is My Hermit Crab Going Blind?
Even if your hermit crab has 2 eyes, you may wonder if it can see. Hermit crabs can be clumsy. It’s natural to worry that it could be blind. Thankfully, most actions that suggest vision difficulty are deliberate and of no concern.
Take bumping into static objects in a habitat, for example. This is a natural behavior for hermit crabs. They are using their antennae to explore their surroundings. Hermit crabs also bump into things to test their solidity. If the object is stable, it can be climbed for recreation.
This also extends to tankmates. Hermit crabs often bump into each other. They are playing, testing strength to determine a pecking order of dominance. Usually, this is harmless fun. If one hermit crab becomes vocal, though, keep an eye on things. It may escalate to fighting.
There is also the spilling of water. This is a common act in hermit crab enclosures. They are applying additional moisture to the substrate. You’ll find this behavior arises most often immediately before molting.
My Hermit Crab Has Glazed Eyes
It can be common to find hermit crabs with a glazed, cloudy look in their eyes. This does not mean the hermit crabs have cataracts or glaucoma. This is a common physical symptom in hermit crabs that are preparing to molt.
This dullness in the eyes is often the first sign of molting. It will typically be accompanied by the following behaviors:
- Lethargy and loss of interest in exercise
- Excessive digging, especially in one particular spot
- Eating and drinking more than usual
- Shedding of any weak limbs
- Eyestalks starting to droop
The lenses of hermit crab eyes undergo renewal during molting. When the process is complete, the hermit crab will once again be bright-eyed and attentive to its surroundings. Just ensure that the hermit crab is left alone to complete a molt appropriately.
If several of your hermit crabs have cloudy eyes at once, try to get a closer look. There is a small chance that your hermit crabs have an eye infection. In multiple hermit crabs, this suggests a bacterial hazard. The habitat must be cleaned urgently.
Maintaining Healthy Eyes in Hermit Crabs
While eyesight is not critical to hermit crabs, this does not mean you can take a cavalier approach to ocular health. Irritations can cause distress. To ensure your hermit crabs maintain healthy eyes, follow these steps:
- Maintain a balance between darkness and light – 12 hours of each
- Ensure a habitat is humid and the temperature is comfortable
- Provide bathing water to rinse foreign objects from the eyes
- Offer foods high in Vitamin A, such as carrots and green vegetables
- Spot clean a habitat daily and periodically deep clean it
The substrate of a hermit crab tank can get messy. It will be filled with discarded waste, uneaten food, and even small insects. Any of these can irritate the eyes of hermit crabs.
Mites, in particular, are a concern. As per the Journal of Experimental Marine Biology and Ecology, countless species of parasites attach to hermit crabs. The eyes are a popular location for such pests.
If mites start to cause significant irritation, hermit crabs consider severing eyestalks preferable to living with an infestation. This can be avoided by keeping a habitat sufficiently sanitary.
Do Hermit Crabs Close Their Eyes?
Hermit crabs do not have eyelids. This means that the only way that they can avoid a sight is by hiding within a shell. This, in itself, is only a short-term solution. Eventually, hermit crabs need to surface to eat or hydrate.
Hermit crabs have a protective layer of skin over the eye, similar to the nictitating membrane in mammals and reptiles. This offers some measure of protection from irritants, such as grit. It does not block brightness, so protection from invasive light is limited.
Can Hermit Crabs Live Without Vision?
As per Proceedings of the Royal Society, hermit crabs primarily negotiate the world by scent carried by water vapor. As water molecules enter the air, hermit crabs pick up on them. This helps them to detect food and threats quickly, taking the appropriate action.
Most hermit crabs trust their antennae more than their eyes. As explained by Behavior, hermit crabs demonstrate a flight response to visual threats. Hermit crabs would rather feel their way around than rely on eyesight.
Hermit crabs do not have ears. They pick up on sound through ground vibrations. A loud noise will be distracting and overpower visual cues.
Hermit crab eyesight is best described as average. Hermit crabs can see well enough to keep themselves safe, especially at night. They do not rely heavily on their eyesight to negotiate surroundings.