Crabs are all crustaceans that belong to the Phylum Arthropoda of the Animal Kingdom. Their body is divisible into head, thorax, and abdomen.
Crabs are found everywhere that is in the land, sand, rocks, in the world’s oceans, in freshwater lakes, rivers, streams, and even in the stagnant water bodies.
Almost all of the crabs are nocturnal, but a very few species are diurnal as well. They come out and hunt for their prey only at night time. Although, you may find some of the crabs coming out of their dark habitats during the day time as well, but that’s very rare and exceptional.
Moreover, their nocturnal nature also saves their life as the majority of their predators remain active during the daytime.
The whole body of the crab is generally covered with a thick exoskeleton, and have a single pair of pincers (also called claws) to catch, hold, and snatch the muscles of its prey.
Their thick exoskeleton is made up of a protein called chitin and calcium carbonate. It is often made of separate plates connected by a thin membrane which creates “joints” and allows a crab to move more easily.
Many of the crabs are social creatures, preferring to live and forage together in groups. You will easily find many crabs living in or near the estuaries or rocky shorelines.
A lot of crabs live only in the subtidal zones, which means that they live in a habitat in those areas where the seabed is below the lowest tide.
Other crabs can live in the intertidal zone, which means that they live in those areas where the ocean meets the land between high and low tides.
Crabs are nocturnal creatures
Yes, crabs are nocturnal creatures but some of the species are diurnal as well. They are nocturnal because of their adaptation in order to maintain their daily activity of the day-night cycle.
Whereas, many of them like a few species of marine hermit crabs and others are diurnal. Some can be active at both the day and night time as well, mostly at the night time.
Fiddler crabs inhabiting the intertidal zone must adapt their activity to both the day-night cycle and the cycle of the tides so they can adjust itself to being both diurnal and nocturnal depending on the circumstances. Due to this they can exhibit both specific diurnal and nocturnal habits.
So, if you ever hear someone saying that all crabs are nocturnal, then just stop them by saying that they are wrong. Tell them that there are crabs like the Fiddler crabs that can remain very active during the day.
Fiddler crabs are very active during the day and return to their burrows at night and during high tide, plugging the entrance with mud or sand.
Majority of the crabs being nocturnal is actually their type of adaptation that have made them become more active at night time to hunt, mate, or avoid heat and predators.
It is very important to note that the majority of the crabs are strictly nocturnal and will often hide around during the day time in their dark burrows, or inside the rocky shores.
They won’t move around much during the day unless they’re being distracted by predators or watered away by tides.
So, that’s why you’ll only occasionally see them out and about during the day but those crabs are the exception, and not the rule. The nature’s rule is that crabs are nocturnal and not diurnal.
Why do crabs come out at night?
You will find almost the majority of the crabs coming out at night. To see a many of them you can choose to go to the beach after the late evening with your flashlight and soon you’ll find them scampering across the sand.
The land crabs are mostly active at night time as this is the time when the temperature on land is low. And so, they remain dormant during the hottest hours of the day.
If you want to see a water crab at night then you often find them lying in the cool, moist, dark places near the coastline of the oceans, or on the sand and mud near the streams, rivers, lakes, etc.
Such cool and moist places are their favorite kind of places where their gills won’t get dried out and where they can have better shelter.
They also come out at night time in order to adjust their body’s biological cycle that is their “biological clock” to adapt themselves in accordance to their day-night rhythm.
At night they become active and go searching for their food. They are omnivorous in feeding. Omnivores ones, for example, hermit crabs, can feed on anything they can grab and eat like small fishes, insects, ants, molluscs, worms, other crustaceans, or even sometimes small plants.
Some marine crabs are seen feeding primarily on algae, phytoplankton, and other aquatic plants as well. Some small marine species can even feed on fungi, bacteria, and detritus, depending on their availability and the crab species.
At night, as they become more active they start to use all of their sensory bits and antennas to smell, touch, and search for the food. They have sensors on their antenna and on the setae (hairs) on their legs for smelling and finding their food.
A majority of them are social creatures, so you will often find them at nighttime not only in a mood of searching for food, but also climbing, and meeting each other socially.
Hermit crabs, for example, have been extensively noticed for acting with each other in very standard and social ways, by crawling over each other, smelling each other, and even by having “feeler fights” or “pushing contests” as well.
All of these happens at nighttime when they are the most active due to their nocturnal nature.
What do crabs do during the day?
Most of the crabs will sleep during most of the day and will only become active after its dark that is when the sun has set.
These crabs will spend most of their day time hiding themselves and will only come out if distracted or after being washed away by tides.
During the day, they become lazy and sleepy, and so will spend most of the timing hiding in rocky crevices and deep burrows. But, some marine species usually become active at low tide, both at day and night time.
They dig their own burrows in sand or loose soil. During the day, the animal stays hidden to reduce water loss from heat.
Some crabs will roam freely in the deep ocean beds in large groups, during the daytime as well. As it is already dark in the deep ocean bed, so they don’t even care if its night or day. They follow a different kind of tidal cycle to maintain their daily rhythm.
Some crabs like the yellow land crabs that live on forest land, far from the sea, usually dig burrows in moist patches of dirt soil and spend most of the day in hiding there.
Some crabs like the Sally Lightfoot Crabs can keep themselves hiding on rocky shores of the sea, where seawater can constantly flood in and spray crashing waves helping them to keep their gills moist.
You will find many small crabs like the hermit land crab, often tucked and hiding inside an empty shell for protection. They also carry that shell with them wherever they go.
Yes, it’s true that they really get inactive and sleep during the daytime, thus maintaining their daily biological rhythm.
This sleeping or inactive activity of the various crabs, mostly the land crabs, allows them to escape the drying heat of the sun, as well as predators, as well as from getting rid of dehydration.
Do crabs still come out during the day?
Yes, crabs can still come out during the day. Although they remain the most active at night, though they can also be seen at dawn and dusk, and rarely at the midday time.
But, instances of many crabs like the Ghost crab, etc. have also been seen when they come out of their dark habitats at the cold daytime and feeds on drift seaweed, fragments of shoreline plants, and small invertebrate animals as well.
Maybe they are hungry, who knows!
Sometimes they can also come out of the rock crevices at the shoreline if they are washed away due to excessive crushing waves attacking their habitats. In those cases, you will exceptionally see them lying and walking on the sandy beach to find a new home to hide again.
You may often find them during the day just after a high tide has crashed the seashore. For the next 1-2 hours after high tide is the time when you have the best chance to witness the crabs as they will be washed up to the seashore and lie in the sand.
Also during the daytime due to high or low tides, you will often find crabs generally walking around and foraging since they are not getting pushed around by tidal exchange.
Remember that crabs are most active at night. You’ll occasionally see them out and about during the day, but those crabs are the exception, and not the rule.
Can crabs see in the dark?
Yes, crabs can see at night. They do use their two compound eyes that are placed on the two eye-stalks that seem to be growing out on top of their head.
Each of the compound eyes has thousands of light-sensitive facets called ommatidia on each of their eye-stalk. This allows the crab to have a surrounding vision of around 360°.
Ommatidium (plural: Ommatidia) is each of the optical units that make up a complete compound eye of the crab.
Each ommatidium consists of a cornea acting as a lens with a transparent crystalline cone beneath through which rays converge to an image at the tip of a receptive structure, known as the rhabdom.
The rhabdom connects with various microvilli that detect light and helps to visualize things.
And so, due to the presence of thousands of ommatidia, even a small portion of light can enter inside the eyes very easily, thus allowing them to have the clearest vision in the darkest places at night.
They are so sensitive to extremely low light that even at nighttime they can clearly detect the altitude and the slightest movement of the sun and the moon for their movement and locomotion.
Thus, it is very clear that nocturnal crabs possess superpositioned compound eyes and as such their eye design is truly optimized for dim light as photons to be gathered through large apertures comprised of hundreds of lenses.
Unlike those land and shallow water crabs, those crabs that live in the deep sea beds always stay in the darkness of the deep where sunlight does not penetrate, making their eyes more sensitive to blue and ultraviolet light.
Those that live in the deep sea have eyes that are extremely sensitive to wavelengths of light of around 460-490nm. This makes it easier for them to pick up blue bioluminescent light and residual sunlight.
In nature, it’s a gift to the crabs that works out to the crab’s advantage to be nocturnal.
Which crabs are the most popular nocturnal crabs?
Crabs like the Hermit crab, Ghost crab, Coconut crab, Rainbow land crab, Vampire crab, etc. are some of the most popular species of nocturnal crabs.
Many of the nocturnal crabs are also kept as pets with proper love and care by their owners.
Hermit crabs are nocturnal and they become much active during the night time. Over the years of evolution, they have learned and adapted themselves to occupy empty scavenged mollusc shells of snails, etc. to protect their fragile exoskeletons.
Not all hermit crabs are nocturnal. Mostly the land hermit crabs are nocturnal while a few species of the marine hermit crabs are diurnal.
Ghost crabs are predominantly nocturnal and only at the nighttime, they come out searching for food. They can perfectly remain in their burrows during the hottest part of the day, and throughout the coldest part of the winter.
Coconut crabs, also called robber crabs, are the very large-sized nocturnal land crab that inhabits the southwest Pacific and Indian oceans. They only come out at night. And, during the daytime they remain inactive by hiding in a deep burrow or inside the large coconut shells sometimes.
Rainbow land crab is a very colorful land crab, but it does need a bit of water to keep it moist and happy. In the wild, they get active on the land surface at nighttime and enter into an apparently resting phase inside their burrows in the daytime. So, they are also nocturnal.
Vampire crabs are another nocturnal beautiful species of pet crabs that are semi-terrestrial in nature. These freshwater crabs have glowing yellow eyes with bright orange shells and claws. They are mostly nocturnal and so they’re much more active at night.