Colour, chemistry and crustaceans

Life on earth is full of colour. This short blog aims to uncover why colour exists chemically and why shrimps turn pink!

Evolutionarily, it is just a question of natural selection and for some flora and fauna, colour is breathtaking. For others it’s just not.

Vibrant colours – not so much the bee though

Colour depends on many things-

  • Biology-how we see things and interpret them
  • Chemistry of why something is that colour (molecules and stuff)
  • The physics of how light reflects or is absorbed into something

This whole blog actually started because of the colour of some shrimps I was cooking. Crustacean muscle is grey in colour- but when you cook it, it turns pink. So I sat in my kitchen pondering this, then did a bit of digging.

In fact, if you start to look at other crustaceans- like some lobsters, this common colour change can also be seen. Blue in the sea they become red when they cook.

What is more, they don’t turn grey afterwards following cooking when they cool down. So what is happening is permanent.

Why is that?

Shrimp are little crustaceans that feed off of plankton and other things off of the bottom of the sea have something about the chemistry of their muscle that causes the change.

Before starting I needed to understand why and how things exhibit colour.

What is colour?

You know that if you pass a beam of white light through a prism, or water droplets it separates into its separate colours-(see rainbows as an example in nature) and white light is made up of a variety of many different frequencies of light-

Yosemite national park - waterfalls make the best rainbows
Yosemite national park – waterfalls make the best rainbows

So, if white light is all around us , why if white light is beamed off of an object like an orange, is that object not white?

Selective absorption:

White light contains all the different colour frequencies- colours of the rainbow in fact- and when it shines that light can be transmitted, reflected or absorbed

When the lightwaves strike the object (red, orange, yellow, blue)- some of them are absorbed (called resonance)

[Resonance: molecules in an object resonate at the same frequency as the lightwave, the light wave is absorbed. ]

An orange for example, looks orange in white light because most of the frequencies within the light is absorbed into an orange except for one: ORANGE

The orange light is reflected by the skin of the fruit.

With citrus on my mind I drew the infographic below; lemons this time.

In a nutshell:

An object is a specific colour within a beam of white light- because it is reflecting that colour . The remaining colours from the white light are absorbed.

The molecules in each pigment/colour found in any object absorb diverse light frequencies in white light, and that’s to do with their chemistry (resonance).

In animals, pigments occur, exist and are obtained from carotenoids or melanins that are consumed or you are genetically programmed to have.

And if you are devoid of colour- i.e: white feathers, albino, that means a lack of pigment prevents frequencies of light being absorbed and all light is reflected

Absorption of light is a useful adaptation in plants. Plants are green (mostly) so they absorb all light except the green light- which if you look at the volume of frequencies being absorbed is ALOT.

This enables plants to gain a lot of energy from sunlight which is their means of growing ; but that’s for another lesson (photosynthesis).

Passionflower: Original watercolour by franscienceart
Passionflower: Original watercolour by franscienceart


The colour of a pigment is such because it is reflecting that colour light (and absorbing all the others. )

So why is absorbance known as being subtractive in nature?

Subtractive colours exists where the energy of the light is absorbed– by pigments. Absorption of the light by the pigment is technically it taking away light from the spectrum, which is why it is known as subtractive

Pigmentation absorbs different light frequencies

So if an object reflects off all wavelengths and absorbs none, my eye will see it as white.

If I make a mess in my paint palette and mix all the colour pigments together, I will be mixing all different molecules with a lot of different capacities of colour absorption.

As a consequence most light will be able to be absorbed and nothing will be reflected- the resulting absence of colour will be black

So back to my shrimp. Why do they turn from bluish/grey to pink?

Grey/blue colour is caused by a special pigment called β-crustacyanin (crusta- for crustacean and cyan is a blue colour), which is not red but along the blue spectrum

Lobsters are blue because that chemical is reflecting blue light and absorbing all other colours. Shrimp are grey and contain a similar pigment.

Citation: Ertl NG, Elizur A, Brooks P, Kuballa AV, Anderson TA, Knibb WR (2013) Molecular Characterisation of Colour Formation in the Prawn Fenneropenaeus merguiensis. PLoS ONE 8(2): e56920. doi:10.1371/journal.pone.0056920

However, the red colour found in cooking is down to another protein (a Carotenoid protein) called Astaxanthin.


In shrimp the red Astaxanthin protein that gives you the red colour is not working properly because its bound up by another protein. It is bound to β-crustacyanin.

As mentioned, normally shrimp are a blueish/grey colour. Upon boiling, the blue β-crustacyanin is altered by the heat so that the red/ pink astaxanthin is set free and released.

So now the pigment present in my cooked shrimp ( astaxanthin) is available to reflect red light and absorb the rest of the light spectrum. The blue β-crustacyanin no longer works and can’t bind the red pigment and itself cannot reflect light or anything else.

The blue-black to pink-orange color change on cooking of lobster, due to thermal denaturation of an astaxanthin–protein complex, α-crustacyanin, in the lobster carapace

On the origin and variation of colors in lobster carapace – now published in Physical Chemistry Chemical Physics

Astaxanthin is more common than you think and the major carotenoid found in crustaceans and is seen in their blue, purple and yellow colours. In larger animals, astaxanthin is found in octopus, which they obtain by eating plankton. Many fish accumulate carotenoids, in fact salmon accumulate astaxanthin in muscle.

So now you know. Crustaceans have colour created by the plankton they eat and this colour is temperature regulated.

The differences in the colours between crustaceans is due to the chemical nature of the pigments in the carotenoids they consume and how the crustacean biology handles them.

Pigments themselves exist as different colours because their chemistry enables them to absorb light at different frequencies.

This can be nicely summarized by looking at flamingos.

Flamingos and their feathers are white naturally but eating shrimp and blue green algae provide them with the carotenoids that gives them their distinctive colour.

So, it appears to be true; you are what you eat……….

Big tick energy: how a tiny flea created a revolution in British art | Art and design | The Guardian

In 1664, scientist Robert Hooke drew a flea and created the first great work of British art. Without it, perhaps, there would be no Stubbs, Constable and Hirst
— Read on

this article highlights how there was no divide between science and art. This sort of thinking revolutionised both science and art alike.

Chemistry in tiny, tiny bites: Ionic Bonds.

Does chemistry bring you out in a cold sweat? Then I hope these small bitesize chemistry bits will help. Today I’m covering ionic bonds.

I have written this blog for a friend of mines daughter- who is crazily bright- but hates chemistry with a passion and JUST. DOESN’T. GET .IT.

Here goes…you know who you are.

Opals- a hydrated form of silica

It starts with matter. Everything around us. And what is crazy is that by changing the atomic structure of the matter that surrounds us we can change its physical properties.

Matter is; wool and metal and sugars and fats and basically everything. These “things” are made from building blocks, that in their simplest, purest forms are elements.

How else would a cow that eats only grass produce milk that we drink? (Clue:Change chemistry of the grass to convert it into fat and sugar)

One form in which we find pure elements or simple combinations of elements in the earth are minerals and crystals. These help us understand what and how the earth is made up. Crystals themselves have these amazing flat planes that reflect light and this is due to a highly ordered internal arrangement of atoms.

Crytals and chemical bonding
Sulphur crystals-

What makes some crystals hard, or very twinkly, or crumbly, is the way their atoms are arranged in a uniform manner and how they are bonded, namely: metallic, molecular, covalent and ionic.

Crystals therefore become an easy way of describing chemical bonding.

An aquamarine the size of your fist- an example of covalent bonds

Diverse crystals are made up using different types of bonding of their atoms which gives them different properties for example : Diamonds: made from covalent bonds: [covalent (where atoms share electrons and are very hard to break (BFFs) and hard as ..diamond)].

diamonds -covalent bonds
Every colour diamond on the planet- some glow .

Ionic bonds in crystals

An ionic crystal is also known as a salt. (Little crystals of table salt for example.) 90% of all minerals we find in the earth are ionic compounds. So to describe ionic bonding we can use a goto example: Table salt.

Ionic solids (think of salt), unlike covalent solids (think diamond) -crumbles easily, is soluble in water and conducts electricity- figure out why if you can.

Ionic solids aka crystals aka salt (take your pick)- are made from a highly ordered and repeating lattice structure, but the geometry of the lattice depends entirely on what types of ions (elements with a charge) you have and their ratio.

Ionic compounds form different shapes because the way atoms intersperse with each other isn’t just random. Smaller atoms fit into spaces between bigger atoms, attractive forces influence everything and so ionic bonds make lattices that can be different shapes; tetrahedral, octahedral etc .

Ionic bonds as salty salt crystals

There are many types of salt- but we’re going for the table salt you put on your chips. And that is made from 2 elements Sodium (Na) and Chlorine(Cl).

Sodium (Na) by itself it’s a crazy little metal, you stick it in water and it fizzes like a bath bomb, but combine it with…..oh, I dunno chlorine (Cl)- which is also quite a toxic gas and………poof!

Voila: you have sodium chloride (NaCl)- or table salt, ( Halite crystal ).

Na+(sodium ions), are essential in nervous conduction for example- and yet we don’t explode. Combine Sodium with a hydroxide and you get Sodium hydroxide- or NaOH, commonly known as caustic soda. Great for dissolving pretty much anything (Good for drains).

Get your head round the fact that chemicals aren’t bad things they are just the things that are around us.

Well the premise for all of this toing and froing with sodium is that in nature it exists a lot in its ionic form (like NaCl). And these rules can apply to ALL elements. I’m just using sodium as an example.

What is an ionic bond?

Let’s see if you know these facts. If you do- we can move on. If not- learn them.

All elements exist with a central proton and neutron and a shell of electrons

If you know this then continue……

Electrons carry negative charge

If you know this then continue……

This shell of electrons changes with each element and there’s a pattern to it

If you know this then continue……

As the element gets bigger, the number of electrons increases and matches the size of the protons- so big atomic mass=loads of electrons.

If you know this then continue……

The electrons come in layers- like the circles of hell. They sometimes follow a trend, (unless you’re a crazy metal), but it’s the ones on the outside that can have the biggest influence

If you know this then continue……

If you can’t remember or work out how many electrons are in the outer shell, then look to the placement of the element in the periodic table. Group 1 has, yes you’ve guessed it, 1 electron in its outer shell.

Now here we go…..

Atoms have electrons in their outer shells- in this example either 1 (a group 1 metal) or 7 (a group 7 element)
Odd electron numbers mean that you are not as stable as you could be.

More electrons means more negative charge and more attraction.

With respect to your outer shell, if you have very few electrons in your outer shell, you are not very attractive, sort of weak. What you ‘want’ is to fill your ‘shell’ of electrons. Normally you can hold 8 electrons in your outer shell, so in order for the element (that only has 1 electron) to find peace, you either lose an electron or gain another 7. (It’s easier to loose 1 than gain 7).

So far so good?

Now, like I said, sodium is a group 1 metal. What about chlorine?

If you sneak a peak at the periodic table you would see that it belongs in group 7. What this means is that it has seven electrons in it’s outer shell. And what this means is that for it to be stable it will require 1 electron from somewhere. So, technically, any element that has one electron kicking about in its outer shell would seem an appealing thing.

They don’t want to share though. nuh uh.

Picture the scene, sodium and chlorine approach, they are attracted to one another and all of a sudden, whoop, the electron from Na is suddenly ‘donated’ to Chlorine. Its a win-win situation. Chlorine now has its shell filled and Na has lost the electron. Because of this loss of negativity, Na becomes Na+, that’s how you can tell its ionic (has charge). Cl becomes Cl-.

electrons ions ionic bonding
Donation of electrons from an atom with one electron in its outer shell

The charged ions stick around each other as opposites attract. There will be problems however if other more attractive ions turn up- but for now if it’s just them, they remain in this relationship.

Learn this: + ions or cations tend to be metals. – Ions or anions tend to be non-metals.

A salt (such as table salt (NaCl), has a balanced number of ions, 1 Cl- to 1Na+. Therefore overall there is NO CHARGE on the element

ionic bonds
Elements from group 1 and 7

When it crystallises, it becomes what we put on our chips.

This is an ionic bond- related topics: Acids and bases, titrations, crystals the world.

Youtube video for one of the most ‘fun’ ways of explaining chemistry

Interestingly- each mineral and ion we find in the group has its own colour

colours of crystals- like emerald and sapphires is dictated by the types of ions it has in it

The chemistry of colour and why colour is what it is, is for another time…….

I hope you found this helpful- please follow me if you want more updates….