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Today on Animal Fact Files we’re discussing king crabs. There are over 100 living species of king crabs. Though they look like crabs, they are not. True crabs are classified in a different infraorder. There's a phenomenon in crustaceans where non-crab-like crustaceans evolve to look like true crabs over time. This is called carcinization, and it's an amazing example of convergent evolution. King crabs differ from true crabs because they only use six of their legs for walking - true crabs use eight! Currently, king crabs are believed to be related to hermit crabs (who are also not true crabs), but this is debated. Like hermit crabs, king crabs have an asymmetrical body. In hermit crabs, this is because they need to be able to position their body inside a spiral shaped shell, but king crabs don't use a shell - at least not one they didn't grow themselves.

King crabs are some of the largest arthropods living on our planet. The biggest species can weigh over 18 pounds (8kg) and have legspans over six feet (1.8m) long! These long legs are useful for walking far without using a lot of energy. Some king crabs travel more than 6 miles (10km) a day while they seek out different feeding grounds, mating sites, and more.

King crabs live globally, but they're most abundant in the northern Pacific Ocean. They spend most of their time at the bottom of the sea floor. For this reason, they're relatively understudied. Much of the research has centered around the red king crab which is consumed by humans. King crabs living in colder regions may live in waters as shallow as 164 feet (50m) while other species live over two and a half miles (4000m) below the waves. They inhabit rocky bottom areas with places to hide, kelp forests, muddy bottom areas, and seamounts depending on their age and species. During colder months, these arthropods travel hundreds of meters up the seafloor to relatively warmer, shallower waters. They are scavengers and eat whatever decaying material they come across along with sea stars, fish, other crustaceans, bivalves, sea slugs, and more.

King crabs under five years of age are considered juveniles, and they form massive congregations called pods. These pods are believed to keep predation low, especially during molts. King crabs are most vulnerable during and right after molting. Molting is when arthropods shed their external skeleton. They are squishy and prone to death during this stage. Sea otters and fish may eat them during this time. Eggs may also be eaten by sea worms, though king crab mothers protect them.

A female king crab attracts a male with pheromones. They grab each other before mating and may remain in this embrace for up to seven days! The female molts and the male protects her, depositing spermatophores during the process. The female lays her eggs which pass over the spermatophores and become fertilized. She can have hundreds of thousands of eggs at a time. These are held on her abdomen for a year and hatch into free swimming larvae. After they hatch, she'll immediately prepare for another mate. The babies take up to four months to reach a form where they resemble the adults. At this point, they molt and settle to the seafloor to join a pod. Overall, they may live to be twenty years old, but, as we mentioned before, much of this is based on a single species and more research could reveal surprising insights into the lives of king crabs.

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