Macrocheira facts for kids

Quick facts for kids Macrocheira
Macrocheira kaempferi
Scientific classification
Unrecognized taxon (fix):	Macrocheira

Macrocheira is a genus of deep-sea crabs belonging to the superfamily Majoidea, commonly known as spider crabs. The name Macrocheira comes from ancient Greek words meaning "large hands" or "long claws."

Today, this genus contains only one living species: the famous Japanese spider crab (Macrocheira kaempferi). It holds the world record for having the largest leg span of any living arthropod on Earth! Scientists have also discovered fossils of an extinct species named Macrocheira longirostra, which lived millions of years ago during prehistoric times.

What Is the Genus Macrocheira?

A genus is a scientific way to group closely related animals or plants. In biological taxonomy, organisms are classified into levels such as family, genus, and species.

The genus Macrocheira belongs to the crustacean group known as decapods. The word "decapod" means ten-footed, which describes crabs, lobsters, and shrimp.

Scientific Classification and Discovery

Dutch zoologist Wilhem de Haan first described and named the genus Macrocheira in 1839. He studied specimens brought back to Europe from Japan by naturalists and explorers.

The genus is placed within the superfamily Majoidea. Animals in this group are often called spider crabs or decorator crabs because of their long, slender legs and their habit of blending into their surroundings.

Living and Extinct Species

Scientists recognize two main species within the genus:

• Macrocheira kaempferi: The living Japanese spider crab, found in the deep waters around Japan.
• Macrocheira longirostra: An extinct fossil species that had a slightly different shell structure and lived during the Miocene epoch.

Physical Characteristics and Anatomy

Crabs in the genus Macrocheira are famous for their giant body proportions. Their bodies are built specifically for surviving under the heavy water pressure of the deep sea.

Record-Breaking Size and Leg Span

The Japanese spider crab is the undisputed giant of the crab world. While some land crabs like the coconut crab are heavier, no arthropod beats Macrocheira in reach.

• A fully grown adult can have a leg span of up to 3.8 meters (12.5 feet) from the tip of one claw to the tip of the other.
• The central body shell, called the carapace, can grow up to 40 centimeters (16 inches) across.
• Adult crabs can weigh around 19 kilograms (42 pounds), which is about the same weight as an average five-year-old child!

Exoskeleton and Shell Armor

Like all arthropods, Macrocheira does not have an internal skeleton made of bones. Instead, it has a tough outer shell called an exoskeleton.

• The shell is made of a strong material called chitin combined with calcium carbonate.
• The carapace is rounded and covered in bumpy knobs and small spikes that help protect it from predators.
• The crab is usually a mottled orange or reddish-orange color with white spots along its long legs.

Walking Legs and Powerful Chelae

The ten limbs of Macrocheira serve different purposes. The front pair of legs are called chelipeds, which end in pincers or claws (chelae).

• In male crabs, the claws are much longer and more powerful than in female crabs.
• The other eight legs are extremely long and slender, walking sticks that allow the crab to move slowly across uneven underwater terrain.
• The tips of the legs curve downward, functioning like grappling hooks on rocky sea bottoms.

Sensory Systems in the Deep Ocean

Living in dark ocean waters requires specialized senses. Macrocheira relies on several adaptations to navigate and find food:

• Compound eyes sit on short stalks on the front of the head, giving a wide field of view.
• Small sensory antennae pick up chemical signals and smells in the water to detect food from far away.
• Tiny sensory hairs on the walking legs can feel vibrations and taste the seabed as the crab walks.

Habitat and Deep-Sea Ecology

Members of the genus Macrocheira make their home in the cold, dark waters of the Pacific Ocean.

Geographic Range

The living species is native to the waters surrounding the Japanese archipelago. They are most commonly found in:

• Sagami Bay
• Suruga Bay
• Tosa Bay
• The coastal waters off the Izu Islands

Depth and Ocean Environment

These crabs are true deep-sea creatures, though they move between different depths depending on their age and the season.

• Adult crabs typically live at depths between 150 meters (490 feet) and 300 meters (980 feet).
• Some have been discovered at depths greater than 600 meters (2,000 feet), where sunlight cannot reach.
• The water temperatures at these depths are very cold, usually hovering between 10°C and 13°C (50°F to 55°F).
• In the springtime, adults migrate into shallower waters, around 50 meters (160 feet) deep, to mate and lay eggs.

Diet and Hunting Habits

Despite their intimidating size and long claws, giant spider crabs are not aggressive hunters. They are gentle omnivores and scavengers that play a vital cleanup role in ocean ecosystems.

What Do Spider Crabs Eat?

The diet of Macrocheira consists of whatever organic material settles on the ocean floor. Their typical foods include:

• Dead fish and decaying marine animals that drift down from above.
• Slow-moving mollusks like clams, snails, and mussels.
• Small marine invertebrates such as sea urchins and marine worms.
• Marine algae and underwater plant matter.

Feeding Behavior

Spider crabs use their long claws to carefully pick up food items from the muddy seabed. Their claws are strong enough to pry open hard clam shells and tear tough seaweed into bite-sized pieces.

Because they live in cold water where metabolism is slow, they do not need to eat massive amounts of food every single day.

Life Cycle and Reproduction

The life cycle of Macrocheira is a long and fascinating journey from a tiny drifting larva to a massive ocean giant.

Mating and Egg Laying

Mating takes place in late winter and early spring when adult crabs gather in shallower ocean zones.

• Female crabs carry thousands of tiny eggs on the underside of their abdomen until they are ready to hatch.
• The mother protects the eggs and gently fans them with water to provide oxygen.
• When the eggs hatch, microscopic larvae emerge and float up toward the sunlit ocean surface.

Larval Development Stages

Baby spider crabs do not look anything like their parents. They go through several distinct larval stages:

• Zoea stage: The tiny larvae drift as part of the plankton, feeding on microscopic organisms.
• Megalopa stage: The larva begins to develop tiny claws and legs, resembling a miniature crab.
• After several weeks of drifting, the surviving larvae settle onto the ocean floor and transform into juvenile crabs.

Molting and Growth

Because a crab's hard shell cannot expand, it must periodically shed its shell to grow larger, a process known as molting (ecdysis).

• The crab absorbs water to swell up and crack its old exoskeleton.
• It carefully climbs out of the old shell, including the coverings on its legs and eyes.
• For several days, the crab's new skin is completely soft, making it vulnerable to predators.
• The crab hides in rocky crevices until its new shell absorbs minerals from the water and hardens.
• Adult crabs molt less frequently as they reach their maximum size.

Impressive Lifespan

The Japanese spider crab is one of the longest-living arthropods in the world. Scientists estimate that healthy individuals can live between 50 and 100 years in the wild!

Camouflage and Defense Mechanisms

Surviving in the ocean requires smart defenses against large predators like giant octopuses and deep-diving sharks.

Natural Armor and Spikes

The primary defense for Macrocheira is its thick, mineral-rich shell. Young crabs have sharp spines along their legs and shell that make them uncomfortable for predators to swallow.

Decorator Behavior

Younger spider crabs often practice decorating behavior. They use small sponges, sea anemones, and algae, attaching them to the hooked bristles on their shells. This natural camouflage makes the crab look just like a rocky ocean reef.

Regrowing Lost Limbs

If a predator bites off a leg, or if a leg gets trapped under a heavy rock, the crab can release the limb through a process called autotomy. Over several future molts, the lost limb can slowly regenerate and grow back!

Fossil Record and Evolutionary History

The evolutionary history of the genus Macrocheira stretches back through millions of years of Earth's past.

Prehistoric Relatives

Paleontologists have uncovered fossilized remains of ancient spider crabs in rock formations across Japan and the northern Pacific rim.

The extinct species Macrocheira longirostra lived during the Miocene epoch, roughly 5 to 23 million years ago. These fossils prove that giant spider crabs have lived in deep Pacific waters for millions of years with very little change in their general body plan.

Living Fossils

Because the living Japanese spider crab has retained many ancient physical characteristics over tens of millions of years, scientists sometimes refer to it informally as a "living fossil."

Human Interaction and Conservation

Humans have interacted with the giant spider crab for centuries through fishing, scientific study, and public aquariums.

Cultural Significance and Fishing

In Japanese coastal communities, particularly around Suruga Bay, the spider crab is known as takaashigani (タカアシガニ), which means "tall-legged crab."

• Fishermen use specialized deep-sea trawl nets to catch the crabs during specific seasons.
• The crab is considered a culinary delicacy in parts of Japan, often served steamed or boiled with vinegar.
• To prevent overfishing, strict laws prohibit catching the crabs during their spring breeding season when they move into shallow waters.

Spider Crabs in Public Aquariums

Due to their massive size and alien-like appearance, giant spider crabs are star attractions in large public aquaria all over the world.

• Because of their calm temperament, they are often kept in peaceful multi-species deep-water exhibits.
• Aquarists feed them using long poles to deliver fish, squid, and shrimp directly to their claws.
• Many aquariums have participated in research programs to study their molting patterns and larval development.

Conservation Status

While Macrocheira kaempferi is not currently listed on the global endangered species list, marine scientists monitor their populations carefully.

Deep-sea bottom trawling and rising ocean temperatures present potential challenges to deep-sea ecosystems. Protecting their deep-water habitats ensures that these magnificent ocean giants continue to thrive for generations to come.

Fascinating Facts About Macrocheira

• Giant Reach: A full-grown Japanese spider crab could straddle a standard family car from side to side!
• Gentle Giants: Despite looking like giant underwater monsters from a science fiction movie, they move very slowly and rarely use their claws aggressively.
• Walking on Stilts: Their legs are so long that they walk with a distinct sway, looking like giant eight-legged stilts walking across the mud.
• Ancient Survivors: Their ancestors walked on the ocean floor long before the earliest human ancestors walked on land.