# How Octopuses Change Colour: answers and explanations

Testmigo Practice Test 4, Listening Part 4

Source: Written by Testmigo.

## Answer key

| Q | Answer | Proof |
| --- | --- | --- |
| 31 | mate | vivid displays to impress a potential mate |
| 32 | hormones | don't use hormones at all |
| 33 | rest | as much energy as the octopus uses at rest |
| 34 | pigment | just one visual pigment |
| 35 | checkerboard | placed on a checkerboard of blue and yellow squares |
| 36 | blue | reacted most quickly to blue light |
| 37 | papilla | called a papilla, P, A, P, I, double L, A |
| 38 | sac | That's sac, S, A, C |
| 39 | muscles | like the spokes of a wheel, are the radial muscles |
| 40 | plates | stacks of extremely thin plates |

## Why these answers are right

**31. mate.** The lecturer says some species put on vivid displays to impress a potential mate. The summary turns vivid into striking and potential into possible, so the person the display is aimed at is a mate.

**32. hormones.** After saying fish colour cells may respond to hormones, to nerves or to both, the lecturer states that cephalopod chromatophores don't use hormones at all and are wired to the brain. Do not rely on matches don't use, so the answer is hormones.

**33. rest.** The 2024 study estimated that running the system flat out would cost nearly as much energy as the octopus uses at rest. Nearly becomes almost and uses becomes consumes in the summary, and the gap after at needs rest.

The why and the trap for questions 34 onwards are free with an account on the [answers page](https://testmigo.com/ielts/listening/how-octopuses-change-colour/answers).

## Transcript

**Announcer:** Part 4. You will hear a lecture about how octopuses and their relatives change the colour of their skin.

**Announcer:** First, you have some time to look at questions 31 to 40.

**Announcer:** Now listen carefully and answer questions 31 to 40.

**Lecturer:** Good morning, everyone. Today's topic is how octopuses, squid and cuttlefish change the colour of their skin.

**Lecturer:** I'll begin with what colour change is for and the puzzles it raises, and I'll finish with the cross-section of skin on your handout.

**Lecturer:** So, what's it for? Mainly camouflage. These are soft-bodied animals, and blending into the background keeps them alive.

**Lecturer:** But colour is also a way of sending messages. Some species put on vivid displays to impress a potential mate, and a sudden flash of dark and pale can startle a predator that's come too close.

**Lecturer:** What really sets cephalopods apart is speed: a cuttlefish can switch from sandy brown to yellow with dark markings in under a second.

**Lecturer:** That's because of how their pigment organs, the chromatophores, are controlled. In fish, for instance, colour cells may respond to hormones, to nerves, or to both.

**Lecturer:** Cephalopod chromatophores don't use hormones at all. They're wired to the brain, and the common octopus devotes over half a million nerve cells to running them.

**Lecturer:** This comes at a price. A 2024 study measured the oxygen used by pieces of octopus skin as their chromatophores expanded and contracted.

**Lecturer:** Scaling up, the researchers estimated that running the whole system flat out would cost nearly as much energy as the octopus uses at rest. Not while it's hunting, mind you: just sitting still.

**Lecturer:** Now, the first puzzle. You'd assume that matching a background requires colour vision, and yet most cephalopods are thought to be colour-blind.

**Lecturer:** Their eyes seem to have just one visual pigment, while most humans have three kinds of cone. With one pigment you can judge brightness, but not colour.

**Lecturer:** In one experiment, cuttlefish were placed on a checkerboard of blue and yellow squares, matched in brightness for a cuttlefish eye.

**Lecturer:** On black and white squares they show a bold, blotchy pattern. On the blue and yellow version they didn't, so the squares apparently looked alike.

**Lecturer:** So how might they match colours? One idea, proposed in 2016 by researchers at Berkeley and Harvard, involves their odd pupils, which can be shaped like a U or a W.

**Lecturer:** Because a lens focuses each colour at a slightly different distance, those pupils might let them extract colour information from the blur. But that's still a hypothesis.

**Lecturer:** The other possibility, and I'll admit it's my favourite, is that the skin itself senses light.

**Lecturer:** In 2015, a team at the University of California, Santa Barbara, shone light on pieces of skin cut from the California two-spot octopus. Although they were cut off from the eyes and brain, the chromatophores expanded.

**Lecturer:** They tried different wavelengths. You might expect red or yellow to work best, as those are the colours of the pigments themselves, but in fact the skin reacted most quickly to blue light.

**Lecturer:** That peak, at around four hundred and eighty nanometres, matches the light-sensitive protein, or opsin, in octopus eyes, and the team found opsin in nerve cells in the skin.

**Lecturer:** Another study that year found similar molecules in squid and cuttlefish skin. Nobody claims the skin can see, but it may contribute to the patterns these animals make.

**Lecturer:** Right, let's turn to the cross-section and work down from the surface.

**Lecturer:** At the top is a raised bump, called a papilla, P, A, P, I, double L, A. Octopuses and cuttlefish raise and flatten these to change their skin from smooth to bumpy.

**Lecturer:** There's nothing rigid inside a papilla. The muscles that raise it also hold it up, rather like the muscles of your tongue.

**Lecturer:** Below that is the chromatophore layer. At the centre of each chromatophore is a tiny elastic sac of pigment. That's sac, S, A, C, not the kind you carry your shopping in.

**Lecturer:** Around the sac, like the spokes of a wheel, are the radial muscles. When the nerves fire, the muscles contract and pull the sac open, spreading the colour.

**Lecturer:** When they relax, the elastic sac shrinks back to a tiny dot.

**Lecturer:** Underneath the chromatophores are the iridophores. These hold stacks of extremely thin plates, which reflect light to give those shimmering greens and blues.

**Lecturer:** In at least some species, those plates turn out to be made of protein.

**Lecturer:** And deepest of all are the leucophores, which reflect whatever light falls on them: white under white light, blue under blue light. That's very useful for matching the surroundings.

**Lecturer:** So, a colour-blind animal whose skin may sense light for itself. Next week, we'll look at squid that make their own light.

**Announcer:** That is the end of Part 4. You now have 30 seconds to check your answers.

[Practice How Octopuses Change Colour](https://testmigo.com/ielts/listening/how-octopuses-change-colour)