Notomithrax ursus facts for kids
Quick facts for kids Notomithrax ursus |
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| Scientific classification | |
| Kingdom: | |
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Majidae
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| Genus: |
Notomithrax
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N. catharus
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| Binomial name | |
| Notomithrax ursus (Herbst, 1788)
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The Notomithrax ursus, also known as the hairy seaweed crab, is a type of spider crab. It belongs to the family called Majidae. These crabs are known for their unique way of blending into their surroundings.
Contents
What Does It Look Like?
Body Features
This crab has a hard, rounded, and somewhat triangular shell, called a carapace. This shell is covered in hair-like parts that come in different sizes. The shell is a bit longer than it is wide. Its surface has many bumps and spines, especially towards the front and sides.
Around the edges of the shell, there are nine sharp spines that point outwards. The front of the crab's head has two spines that form a V-shape, covered with small bristles. Some of the bristles on its body are short and hooked, while others are long and straight.
Both male and female crabs have seven segments on their underside, called the abdomen. The male's abdomen is narrower.
Legs and Claws
The hairy seaweed crab has three pairs of walking legs. These legs are longest at the front and get shorter towards the back. It also has a pair of swimming legs at the very back. At the front, it has a pair of claws called chelipeds. These claws are orange or red with white tips. The legs are long and thin, covered in brown hairs and rows of short, hooked bristles.
Master of Disguise
This crab is a "decorator crab." This means it attaches pieces of seaweed or algae from its environment onto the hooked hairs on its body. This clever trick helps the crab blend in perfectly with the ocean floor. It's like wearing a natural camouflage suit! Hairy seaweed crabs regularly update their disguise, replacing about 10-20% of their cover each day. Because of this, they move quite slowly, just like many other spider crabs.
Where Do They Live?
Global Home
The hairy seaweed crab is found in the southern parts of the world. It is native to both New Zealand and south-east Australia.
New Zealand Locations
You can find the seaweed crab all along the coastlines of New Zealand. This includes both the North and South Islands. They also live on Stewart Island and the Chatham Islands.
What Do They Eat?
A Crab's Diet
Crabs are usually scavengers, meaning they eat whatever food they can find. Hairy seaweed crabs are omnivores, which means their diet includes both plants and other animals.
Scientists have studied what these crabs eat by looking inside their stomachs. They found that algae, like red and brown seaweed, was the most common food. It was found in almost all the crabs they checked! Other common foods included small sea creatures like isopods, amphipods, and decapods (which can be small fish, crayfish, or even other crabs). They also ate things like clams, sponges, and bryozoans, but less often. Since seaweed crabs move slowly and aren't very strong, they usually eat meat from animals that are already dead.
Diet Differences
Interestingly, female crabs sometimes ate more brown algae and unknown substances than male crabs. However, the overall amount and variety of food eaten didn't seem to be very different between the sexes.
The type of food also changed with the crab's size. Smaller male crabs ate more snails, isopods, and amphipods. Larger males, on the other hand, ate more algae and decapods. Similarly, smaller female crabs ate more isopods, while larger females ate more algae. These differences likely happen because a crab's size affects what it can easily catch and eat. Both soft-bodied and hard-bodied animals were eaten, along with many kinds of algae. Seaweed crabs use their claws to tear their food apart. Larger, older crabs can process food better because they have stronger claws and bigger mouths.
Food Storage Camouflage
There's also a cool idea that the seaweed or algae the crab uses for camouflage isn't just for hiding. It might also be a snack! The crab might choose certain types of algae for its disguise because they are also good to eat. This means their camouflage can double as a food supply. A crab can eat up to 4.1% of the algae it's wearing if there's no other food around. They might even pick algae for their "suit" based on how much energy, vitamins, or nutrients it contains.
How They Find Food
The hairy seaweed crab uses its claws (chelipeds) in many different ways to catch and eat its prey.
- They can use their claws to pull off creatures like chitons and limpets from rocks.
- They can also open up clams and snails.
- To catch small arthropods, young crayfish, and worms, they poke around with their open claws and quickly snap them shut when they touch prey.
- Claws are also used to tear larger prey into smaller pieces. For example, they can rotate a brittlestar with their claws to tear off its arms.
- They even have a technique called "gravel-scrubbing." For this, the crab picks up pieces of gravel with a claw and moves them along its inner mouthparts. This helps it collect tiny particles of food and micro-organisms from the gravel.
Staying Safe from Predators
Seaweed crabs need their camouflage because they can't really defend themselves from bigger sea animals. Their predators include creatures like sea otters, sea turtles, lobsters, rock crabs, and octopuses. Smaller crabs might also be eaten by fish with strong jaws.
Studies have shown that seaweed crabs that are decorated with camouflage are much less likely to be eaten than crabs that have been cleaned. These crabs also tend to be active at night (nocturnal), which might help them avoid predators. Sometimes, seaweed crabs will even hide among sponges. Sponges can offer some protection because they sometimes have natural toxins that predators don't like.
Other Interesting Facts
Scientists have studied the digestive system of the hairy seaweed crab. They found that spider crabs have very well-developed gut systems among crustaceans. Seaweed crabs have adapted to eat large pieces of food. They have special "teeth" inside their stomachs to break down what they eat. They also have hair-like structures called setae that help with digestion. These setae help hold food as it's torn apart and ground into smaller pieces. Some setae act like filters, separating particles during digestion so that digested liquid can be absorbed more easily. Others might help filter and move particles along, or even close gaps to keep unwanted particles out.
As mentioned before, the way seaweed crabs decorate themselves is not just for camouflage, but also for storing food. Studies have shown that these crabs eat their camouflage materials most often when food is hard to find. However, it's still an important food source at other times too, as they eat it even when other food is available. They mostly eat their decorative algae at night, taking it from their bodies rather than their legs. Both eating and replacing their camouflage help them keep their disguise in good shape.
