From toxic killers to giant deep-sea monsters, the ocean is home to some truly bizarre crustaceans – but not all of them are what they seem. Some of its members have evolved terrifying survival mechanisms, while others have grown to monstrous proportions.
But before we dive into the ocean's most bizarre residents, we have to address a bit of a biological identity crisis.
It turns out, the term 'crab' is used loosely – and not all the crabs we think of as crabs are actually crabs. So, at the risk of saying the word 'crab' too many times, what is a true crab?
A true crab has 10 jointed limbs, with the front pair formed into claws. The remaining eight are used to walk or paddle. The tail is short, tucked underneath the crab’s body, and the exoskeleton is broad and hard, protecting the crab’s inner organs.
Weirdest crabs in the world
Xanthid crabs
Some of the deadliest species of crabs in the world, xanthid crabs are a diverse group of brightly coloured crustaceans, which are highly toxic. They can be found in a wide range of colours, including yellows, reds, oranges and blacks – colours that serve as a warning sign to other species. Unlike pufferfish, cooking the crab doesn’t destroy the poison – and, be warned, there’s no antidote.
Another fun fact for you: the xanthid crab is the only species of crab with asymmetrical reproductive organs.
Japanese spider crabs
The biggest crab in the world is believed to be the Japanese spider crab, with a leg span of nearly 4m and clocking in on the scales at a whopping 13.6kg. Despite their enormously long legs, their bodies are surprisingly small by comparison.
Plus, they can live for up to 100 years, making them the crab with the longest documented life span. Throughout their life, they will grow by occasionally shedding their heavy shells. This process leaves them vulnerable to the elements, losing mobility for hours while their new shell hardens.
Porcelain crabs
Porcelain crabs are weird, as they aren’t actually crabs. In fact, they are what’s known as 'false crabs' and instead evolved from squat lobsters, developing their crab-like shape through an evolutionary process known as carcinisation. Other species to go through this process include king crabs and coconut crabs, both of which have evolved from hermit crabs.
Unlike true crabs, porcelain crabs only have three pairs of walking legs – their fourth pair is very small, tucked away and only really used for grooming. They also have a long, filamentous antennae.
Hermit crabs
Hermit crabs are also imposters in this group, as they aren’t true crabs either. Their misnomer comes from the fact that they – like true crabs – have ten legs, and their front claws and legs function very similarly to those of true crabs.
The 'hermit' in their name refers to their habit of carrying their home on their backs, as they occupy discarded shells of other animals to protect their abdomen, which lacks a protective exoskeleton.
Boxer crabs
Clutching a small sea anemone in each claw, boxer crabs take their name from this peculiar method of defence. Also known as the 'pom pom crab', these crabs wear their anemone gloves aloft, which also helps them gather tiny food particles and debris. It’s a mutually beneficial agreement for the anemone, which gets a free ride across the ocean floor – giving them access to more food and oxygen than they would get otherwise.
Spiny spider crabs
Referred to as a 'living sea urchin', the spiny spider crab decorates itself with seaweed, shells and other sea debris to create a clever camouflage. Their legs are disproportionately long, with a span of over 50cm seen in adult males.
Devil crabs
Also known by the name 'toxic reef crab', the devil crab’s name reflects its lethal powers. They accumulate lethal neurotoxins from their diet, making them toxic to eat – even when cooked. They’re easy to spot, with bright blotches of red and brown across their shells, providing a warning sign to potential prey.
Spanner crabs
The name of this crab species is intriguing. It is known as 'frog crab' and 'spanner crab' interchangeably, because of its unusual appearance, frog-like shape and unique behaviour. Unlike most other species of crab, which walk sideways, the spanner crab only walks forwards and backwards. Its front claws are shaped like spanners, used to pry open shells of molluscs.
Zebra crab
These tiny crustaceans are usually no longer than a couple of centimetres in length, covered in a bold black-and-white pattern of zebra stripes. They are parasitic, feeding exclusively on venomous sea urchins, which they then inhabit, living among their spines.
Top images: Ken Usami/Getty Images
