1. How Octopus and Cuttlefish Skin Changes Color: Three Cell Types and Radial Muscles
Three types of cells are involved in skin color: chromatophores, iridophores, and leucophores. Whether they are present and how they are arranged differs by species and body part [7]. Chromatophores are sacs containing pigment. Around them are dozens of radial muscles. When these muscles contract, the sacs are pulled and spread out, making the color visible [7]. Iridophores contain stacked thin plates that create color through light interference. Leucophores reflect surrounding light to create a white background [7]. This section covers color. The main focus of this article is texture change, which happens through a different mechanism.
2. How Smooth Skin Turns into Bumpy Skin with Papillae
The skin of octopuses and cuttlefish can quickly change from a flat state to a bumpy, three-dimensional state [1]. Small protrusions called "papillae" carry out this change. These are different from papillae on the human body. A 2014 comparative study examined six species of octopus and cuttlefish under a microscope and reported nine types of papillae with different shapes, sizes, and complexities [2]. These range from simple cone shapes to more complex forms.
3. Why Octopuses and Cuttlefish Copy the Texture of Their Surroundings to Hide
Papillae are thought to be used to mimic the fine texture of surrounding objects to blend into the environment. One researcher explains this three-dimensional texture as an important "additional element" for hiding against the background [6]. In front of a rough rock, even if the color matches, a flat skin would cast shadows differently than the rock. Matching the texture is thought to reduce this difference. The ability to change skin relief using skin muscles and water pressure-like forces is introduced as a characteristic specific to cephalopods (octopuses, squids, and cuttlefish) [8]. Whether it is truly "unique" is still under study.
4. How Papillae Rise Without Bones: Muscular Hydrostatic Skeletons
Papillae are thought to have a structure called a "muscular hydrostatic skeleton," where the muscles that raise them also support their shape [3]. Circular muscles and horizontal muscles work together to lift the papilla. When the muscle that pulls it in contracts, the tip is pulled down toward the body surface [3]. No hard support like a bone is needed. In cuttlefish, it was reported that simple cone-shaped papillae work with this mechanism [2]. However, the mechanism is not always the same. In the wavy papillae on the face of one cuttlefish species (*Sepia apama*), a different mechanism was found where the upper skin layer bends outward and bulges when muscles contract [3]. The mechanism of papillae seems to vary by species and location.
5. How Fast Cuttlefish Papillae Rise and Retract: Less Than 1 Second
In living cuttlefish, it is reported that one papilla can be fully raised or retracted in less than 1 second [4]. This is shorter than the time it takes to count "one, two." This number is from cuttlefish observations. It cannot be directly applied to octopuses. Descriptions for octopuses only state that similar structures exist.
6. Where the Nerves That Move Cuttlefish Papillae Are Located: The Stellate Ganglion
In cuttlefish, it is reported that the motor nerve cells that move papillae are located on the inner wall of the stellate ganglion, which is on the side of the mantle, not in the brain [4]. It is important to note that the stellate ganglion is a relay point. This does not mean the brain is not involved. Commands come from the brain and reach the papillae via this point.
7. Why Cuttlefish Papillae Stay Raised for Over 1 Hour After the Nerves Are Cut
In cuttlefish, it is reported that papillae remained raised for over 1 hour even after the nerves leading to them were cut [4]. A mechanism similar to how clams keep their shells closed, which uses little force to maintain position (a catch mechanism or latch bridge), may be involved [4][5]. However, this is only a suggestion, not a certainty. The report introduced here is about cuttlefish.
8. What Cuttlefish Use to Decide When to Raise Papillae: Vision, Not Touch
It is reported that how cuttlefish papillae appear is mainly determined by vision. Changing tactile information did not change how the papillae appeared [4]. Visual information seems to influence the state of the papillae. It does not seem they decide by checking the feel. The results introduced here are from cuttlefish.
9. How You Can Investigate Texture and Color Yourself at Home or in a Park
Look for things at home or in a park that look similar in color but feel different when touched, such as leaves and stones, or wooden boards and walls. Compare color and texture separately to see the difference. If you have the chance to see octopuses or squids at an aquarium, observe how their skin appearance changes depending on the surrounding objects. Whether you can see this depends on the situation at the time. Do not tap the tank or flash light at the glass. If you want to know more, look for aquarium explanations or books about cephalopods written in Japanese, where stories about color and texture may appear.