Cuttlefish Facts: The World's Most Unique Marine Animal
What is a cuttlefish?
Many people have never heard of the cuttlefish, but this unique sea creature is my favorite animal. A cuttlefish is a cephalopod related to squids. Like squids, they have 10 tentacles and are molluscs that have lost their hard outer shells. Unlike squids, however, they have a broad internal shell called a cuttlebone that helps them to stay afloat in water, and they have broader bodies and move more slowly than squids. Cuttlefish have many fascinating attributes, like their capability to camouflage themselves and their unusually high intelligence for invertebrates. They are among the most cunning predators on the planet, and yet are very sociable and inquisitive creatures. The cuttlefish embodies both the strangeness and the beauty of nature.
Cuttlefish Are Biologically Unique
Cuttlefish are quite quirky, as far as biology goes. They are molluscs, like clams, but they have their shell on the inside (the shell is called a cuttlebone, and is made of the mineral aragonite). The cuttlebone allows them to control the ratio of liquid to gas inside their bodies, so they can float.
Cuttlefish swim by flapping the skirt-like fin that runs around their body and controlling their buoyancy; in times when they need to move more quickly, they suck water in through their gills and squirt the water out of their siphon, a straw-like organ beneath the tentacles, to move by jet propulsion.
Cuttlefish have big, dark red eyes with a distinctive w-shaped pupil; these eyes are extremely well-developed and have no blind spot because the cuttlefish's optic nerve is behind the retina.
Cuttlefish are colorblind, but they can see contrasts in light caused by polarization. To focus on things, a cuttlefish will shift the entire lens in its eye to get an accurate image. Even before it is born, the cuttlefish can use its eyes to spot suitable prey to begin hunting when it hatches.
Another quirk of the cuttlefish is their greenish-blue blood, because instead of using hemoglobin to transport oxygen through the bloodstream, they use a different protein called hemocyanin, which contains copper. Since hemocyanin carries a lot less oxygen than hemoglobin, cuttlefish have to pump blood very quickly through the bloodstream, and so they have three separate hearts to do the job.
Any one of these things by itself makes the cuttlefish quite an interesting (some would say weird) animal. But the most fascinating thing about cuttlefish is their skin.
Cuttlefish Camouflage Themselves to Capture Prey and Avoid Predators
Even though it is colorblind, the cuttlefish is a genius at camouflage. It will change its color, pattern, texture, and even its shape to mimic anything in its surroundings.
So how do cuttlefish change colors? Cuttlefish skin contains several layers of pigment producing cells (chromatophores) above a layer of light-reflecting cells (leucophores), and there are about 200 of these cells per square millimeter. In terms of computer produced images, this would be around 359 DPI, about the resolution of a typical inkjet printer.
Yellow pigment cells are closest to the surface, and below them are red and orange producing cells, and beneath them brown and black, and at the bottom are green and blue cells called iridophores. The pigment cells are surrounded by tiny bands of muscle, like the rays around a sun.
Cuttlefish can control the contraction of these muscles individually with signals from the brain, producing a specific color on a specific part of the skin. Different intensities of pigment will come through based on the amount of contraction of these muscles, and the amount of light the leucophores are reflecting.
Colors can also be combined using multiple pigment cells, and the cuttlefish can create flashing colored lights on its body by combining the contraction of its pigment cells with the contraction of its leucophores.
Camouflage can be used for communication with other cuttlefish, or to hypnotize prey and avoid predators. A cuttlefish's color may reflect its mood; if the cuttlefish suddenly flashes to black, it may be feeling angry, or scared and projecting a terrifying image to scare away perceived predators. With a simple sequence of color, it may be communicating an entire story to other cuttlefish. It's amazing how expressive these animals are.
Camouflage is also used to a great extent in hunting. Some cuttlefish will create shimmering light all over their bodies to hypnotize prey into coming closer. Other cuttlefish will use their camouflage to sneak up on prey, slowly moving towards the unlucky crab or fish while pretending to be a rock over here, or a piece of seaweed over there, until they suddenly snatch the prey out of the water with their two feeding tentacles and consume it. Some will burrow into the sand and dig underneath their prey and some will blend in with the water and strike from above. Each species has a different hunting style, so you will see all sorts of tactics employed by the cuttlefish.
Can a cuttlefish camouflage itself in a living room?
Cuttlefish Are Extremely Intelligent
Cuttlefish have the largest brain to body ratio of all invertebrates. In marine biology labs, cuttlefish are sent through mazes and other experiments designed to test their ability to learn.
The first thing people usually notice about cuttlefish is that they display a huge degree of dexterity, being able to throw rocks around with their tentacles, and changing their shape to quickly resemble nearby objects.
Notably, one experiment had a shrimp stuck in a glass jar, and the cuttlefish was supposed to figure out how to open it. Not all of the cuttlefish succeeded, but many did, grabbing the lid and twisting it open with their tentacles, showing that the cuttlefish is capable of problem solving.
Many studies have concluded that cuttlefish are capable of spatial learning, have great navigational abilities, and a strong predatory instinct, and most impressively they are capable of observational learning.
As a by-product of their intelligence, cuttlefish are very curious, social animals. If you see them in the aquarium, they will come up to see you, too. Sometimes they will even tap on the glass to get your attention! Cuttlefish can remember faces throughout their lifespan, and in captivity they will often have a favorite keeper that they swim over to greet.
Cuttlefish are very resourceful animals and will often try to take things from their surroundings to use as tools to hunt or to play with. Cuttlefish have actually been caught using sticks to pry crabs out of their shells! One cuttlefish will hold the crab down while another will pry the shell open, and both hunters will enjoy their snack.
In the video below, a mischievous cuttlefish tries to take a pipe from a diver.
Owning a Pet Cuttlefish
Cuttlefish are amazing animals, but they are not recommended for the beginning aquarium owner. Only a few dwarf species are used as pets, and even dwarf cuttlefish need plenty of room to move around; a highly oxygenated tank around 20-26 degrees Celsius is recommended for most species, and one cuttlefish will need about 40 gallons or more to be comfortable. The tank may need to be prepared for three months before you even bring your cuttlefish home.
Cuttlefish like live food, preferably crabs, shrimp, or small fish, which can become very expensive, and it is hard to train them to eat 'dead' food, since they enjoy hunting for their prey. Do not put other fish in with your cuttlefish, because they will become a midnight snack.
Another thing that can make cuttlefish difficult to take care of is their intelligence. They have been known to open their tanks and climb out, so remember to keep the water level low enough so they can't jump up and escape. Similarly, don't put in anything that can be used as an escape tool, like heavy rocks or sticks.
Cuttlefish can be tricky to take care of, but for the advanced aquarium owner it is a fun and rewarding experience to keep one. Their sociability makes them a more interesting pet than most fish, and their intelligence and camouflage abilities will amaze you every day. Someday I would love to have one as a pet and a friend.
Would you ever want to own a cuttlefish as a pet?
© 2018 Melissa Clason