Body-Snatching Barnacles and Skeleton Shrimp: 8 Curious Coastal Crustaceans

They might be the best known examples, but crustaceans can be much stranger than the typical rockpool-dwelling crab. Here we look at some of the more bizarre creatures in this subphylum, such as the...

• Corpse-Carrying Phronima
• Buoyant Barnacles
• Wood-Eating Gribbles
• Remipedes, the Centipedes of the Sea
• Sneaky Skeleton Shrimp
• Body-Snatching Barnacles
• Sparkling Sea Sapphires
• Crushing Coconut Crabs

Corpse-Carrying Phronima

Phronima may not seem too bad at first - they resemble semi-transparent shrimp, albeit with a set of sharp claws. They also tend to carry around a "barrel" as both protective shelter, swimming aid and in the case of females, nursery (they're sometimes called pram shrimp as a result.) Here's where it gets weird - this barrel is the hollowed out remains of another creature called a salp.

Salps are tubular, gelatinous zooplankton that filter food from the water. There's little they can do when a Phronima swims up and hollows them out, turning their remains into a mobile home. To increase the creepiness factor a little more, researchers believe that a small amount of the salp is left alive... which helps maintain the grisly structure of the "barrel!"

Buoyant Barnacles

Pretty much everyone's familiar with barnacles, but did you know that they're actually crustaceans? Even stranger, did you know that there's a species of barnacle that floats through the water rather than remaining attached to a rock?

The buoy barnacle (Dosima fascicular) attaches to drifting particles or objects in the water when young, then starts to produce a spongy cement. This results in a growing "float" (similar to polystyrene packaging) that allows the barnacle to sail the seas - and sometimes entire colonies of the barnacles will form around the initial float!

Wood-Eating Gribbles

You might not expect to find marine creatures specializing in eating logs, but the gribble has an appetite for wood - and specialized enzymes to help it digest the tough material.

Despite their cute name, these crustaceans are adept at destroying wooden piers and damaging boats. They have a particularly robust enzyme that breaks up the polysaccharides in tough plant material like cellulose, reducing it down to useful simple sugars. This enzyme can function in harsh conditions (up to seven times saltier than seawater) and researchers believe it may be extremely useful in the production of biofuel!

Remipedes, the Centipedes of the Sea

Did you know that there's a group of venomous crustaceans?

The remipedes live in the salt or brackish water of coastal caves and aquifers. Though not directly related, they strongly resemble centipedes and supplement their diet of detritus with live prey (usually smaller crustaceans) that are captured and killed with a poisonous bite.

Victims of the remipede are injected with a mixture of toxins similar to that of a rattlesnake, combined with various enzymes and paralytics. Crustacean prey will find itself liquefied inside its exoskeleton, ready for the remipede to slurp up at its leisure!

Sneaky Skeleton Shrimp

Skeleton shrimp resemble strange, slender creatures - if you can pick them out, that is. Many of these crustaceans are well adapted ambushers, hiding themselves amidst strands of seaweed that they clutch with their back legs. There they wait until small prey to passes by - though they'll also help themselves to detritus and the odd bit of plant matter as opportunity permits.

If it wasn't for the marine environment you could easily mistake a skeleton shrimp for a preying mantis, waiting motionlessly with hooked raptorial arms at the ready. The similarities don't end there either - the females of some species consume the males after mating!

Body-Snatching Barnacles

Welcome to the real body horror section of crustaceans. The rhizocephalan barnacles parasitize crabs - and they do it in a horrifyingly insidious fashion.

The process begins when a female rhizocephalan larva finds a crab and infiltrates its shell. Once inside, the larvae begins to grow filaments throughout the hosts body, changing its behavior and preventing it from molting or reproducing. It also grows an external egg sac (where a female crab would normally carry eggs) and, if the host is male, castrates its victim. The crab will serve the barnacle as a zombie-turned-babysitter for the rest of its existence.

The male rhizocephalan larva adds another weird dimension to this parasitism - it will inject itself into the external egg sac and exist only as a group of sperm-producing cells, parasitizing the female while fertilizing any eggs she produces.

Sparkling Sea Sapphires

On a lighter note, the sea is also home to sea sapphires - tiny crustaceans that flicker between invisibility and twinkling displays of light.

Different species of sea sapphire glimmer with different colors, and they can make a patch of ocean seem like it's filled with jewels. It's only the male that shines - which suggests that the light-show might actually be a mating display to attract females.

The effect is accomplished by layers of guanine crystals sandwiching cytoplasm in an otherwise transparent body. Depending on the species, these crystals are spaced slightly differently and thus reflect a specific color of light... but only when the rays hit the crustacean from the correct angle. If the sapphire moves so light's hitting from a different angle, the twinkling stops and the sapphire fades back into invisibility!

Crushing Coconut Crabs

Coconut crabs are big - they can have a leg span of around a meter, which isn't that impressive compared to the spindly legs of a Japanese spider crab (which can reach over 3.5m from pincer to pincer) but coconut crabs are bulky with it. Not only do they crush coconuts in their claws, researchers investigating the average strength of these crustaceans found that they could lift objects as heavy as a 10-year-old human child.

Coconut crabs are actually hermit crabs, but they only borrow shells while they grow. Once mature, these crustaceans rely on their tough exoskeleton for protection and are quite capable of turning an unwary animal into dinner - as well as climbing trees to crack coconuts, they'll kill birds, rodents and other coconut crabs. They're also prolific scavengers, and it's thought that the body of Amelia Earhart may have been consumed by the crustaceans after her plane went missing over the Pacific Ocean.

Adding to the weirdness, adult coconut crabs leave the sea and spend almost all of their time on land - they lose the ability to swim and can even drown if they remain submerged for a prolonged period of time. Females briefly return to deposit eggs in the surf, then resume their land-dwelling habits - an existence that can go on for as long as 60 years!

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Sources and Further Info:

• Meet Phronima, the barrel‑riding parasite that inspired the movie Alien
• Weird and Wonderful: The barrel amphipod devours its host
• Buoy barnacle (Dosima fascicularis)
• Enzyme from wood-eating gribble could help turn waste into biofuel
• First venomous crustacean found
• Meet the skeleton shrimp
• Creepy crab parasites: rhizocephalan barnacles
• Larvae of Rhizocephalans with a Kentrogonid Type of Development and their Significance in the Taxonomy and Phylogeny of Parasitic Crustaceans
• Sea sapphire: the most beautiful animal you’ve never seen
• Coconut crabs: The bird-eating behemoths thriving on isolated tropical islands