One of the most striking physical features of hermit crabs is their eyes.
Invariably bright and round, the eyes are unmistakable. However, the size and prominence of the eyes don’t mean that hermit crabs have good vision.
Hermit crabs’ eye anatomy enables them to see all around. Compound eyes mounted on stalks give hermit crabs 360-degree vision, but their eyesight is restricted.
They can differentiate between colors and shades, but not necessarily different shapes. Although vision is one of their senses, scent, touch, and hearing are more important.
How Good Is Hermit Crab Vision?
Hermit crabs’ eyes are comparable to insects. Like their fellow invertebrates, hermit crabs have compound eyes, which 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 they have a much greater range of vision than their size. The multitude of lenses and cells also helps determine the difference between lights and colors.
That doesn’t mean that hermit crabs have superior vision to humans or mammals, as compound eyes aren’t designed to filter out the finer details of vision.
How Does Hermit Crab Vision Work?
As discussed, a hermit crab’s vision involves breaking a sight down through thousands of tiny lenses, which means they’re constantly aware of their surroundings.
Vision is most impactful when detecting light contrast and fast movement.
Hermit crabs can see 360 degrees, so rapid movement will often elicit a startled response. Equally, the ability to absorb and reflect light assists nocturnal eyesight.
Hermit crabs won’t focus on a particular sight, so think of hermit crabs’ vision as an early warning system where they’ll learn that something is in their periphery.
If not startled and driven to hide, hermit crabs will investigate this item with their antennae, which is what tells a hermit crab what it needs to know.
Do Hermit Crabs Recognize Each Other by Sight?
Hermit crabs live according to a strict social hierarchy. If one hermit crab asserts dominance, this order must be respected. Failure to be sufficiently subservient will result in conflict.
Ecological Research explains that hermit crabs don’t select mating partners according to size, suggesting that hermit crabs can’t recognize the shape of a tank mate. This fits in with our discussion about how hermit crabs’ vision doesn’t 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’s unclear is whether this was a conscious decision based on eyesight or through scent.
You may notice that hermit crabs respond to the sight of tankmates eating. It’s common for subservient hermit crabs to start eating once a colony leader does so, which suggests that hermit crabs have some degree of visual recognition.
How Far Can Hermit Crabs See?
Hermit crabs have long-range vision relative to their size.
Nobody can say with certainty how far hermit crabs can see into the distance. However, their eyesight is more impactful from afar than up close.
As mentioned, hermit crabs’ eyes can’t distinguish fine details, but they understand what they need to know based on movement and light.
The shape and pattern of an object cease to become relevant once it’s close. Hermit crabs decide early what they must do once something catches their eye.
Hermit crabs may not emerge in your presence until you gain their trust. Hermit crabs have learned to recognize your sound, scent, and sight of you from afar.
When 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 evolved to have superior night vision. Hermit crabs are happiest in dim lighting due to their sensitivity to light; they 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 humans.
This ignores that hermit crabs aren’t reliant on their vision anyway. As mentioned, hermit crabs trust their antennae with information far more than their eyes.
What Colors Do Hermit Crabs See?
It has been found through various experiments that hermit crabs detect different colors.
According to the Journal of Experimental Biology, wild hermit crabs are frequently attracted to shells of strong visual contrast to the native substrate.
Hermit crabs aren’t colorblind, as red, green, and blue all garnered a reaction from hermit crabs. Most notably, blue objects provoked a startled response, leading to hermit crabs hiding.
Is My Hermit Crab Going Blind?
Even if a hermit crab has two eyes, you may wonder if it’s blind because they’re clumsy.
Bumping into static objects in a tank is a natural behavior for hermit crabs, as they’re 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. They often bump into each other when playing and testing strength to determine dominance.
Hermit Crab Has Glazed Eyes
It can be common to find hermit crabs with a glazed, cloudy look in their eyes, which is a common physical symptom in hermit crabs preparing to molt.
This dullness in the eyes is often the first sign of molting, which will be accompanied by:
- 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 are starting to droop.
The lenses of hermit crabs’ eyes undergo renewal during molting. When the process is complete, the hermit crab will be bright-eyed and attentive to its surroundings.
Maintaining Healthy Eyes in Hermit Crabs
While eyesight isn’t critical to hermit crabs, this doesn’t mean you can take a cavalier approach to ocular health, as irritation can cause distress.
To ensure hermit crabs maintain healthy eyes, follow these steps:
- Maintain a balance between darkness and light – 12 hours of each.
- Ensure the tank 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 the tank daily and deep clean it every few months.
The substrate of a hermit crab tank can get 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, especially the eyes.
If mites start to irritate, hermit crabs consider that severing their eyestalks is preferable to living with an infestation. This can be avoided by keeping a habitat sufficiently sanitary.
Do Hermit Crabs Close Their Eyes?
Hermit crabs don’t have eyelids.
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.
This layer of skin doesn’t block brightness, so protection from invasive light is limited.
Can Hermit Crabs Live Without Vision?
According to the 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.
Most hermit crabs trust their antennae more than their eyes. Behavior explains hermit crabs demonstrate a flight response to visual threats.
Hermit crabs don’t have ears but detect sound through ground vibrations. A loud noise will be distracting and overpower any visual cues.
Hermit crabs’ eyesight is best described as average. Hermit crabs can see well enough to keep themselves safe, especially at night. They don’t rely heavily on their eyesight to negotiate their surroundings.