How spots are formed

According to Study.coms post on patterns in nature, pattern is defined as, “a consistent form, design, or expression that is not random”.  The patterns that are favoured by natural selection will become common across a species, but how do these patterns come about?

Six spot burnett moth Zygaena filipendulae showing spots

Alan Turing explored this problem in “The Chemical basis of morphogenesis” in 1952.  He discovered that chemicals moving across embryonic cells cause patterns.  There are activators (which increase the amount of two chemicals which mix together to create dark pigment).  And there are inhibitors (which decrease the concentration of the two chemicals, prohibiting production of pigment.)  When this occurs in alternating areas you see spots.   The colour is produced by melanophore or chromatophore cells.  This is known as the Turing Model, and has been explored repeatedly since the 1950s, notably by Jeremy Green in Developmental Biology in 2015. For more on the Turing Model click here.

Why are spots formed?

As discussed in my overview of pattern in nature, patterns are formed for a variety of reasons.  Some are used to protect from predation and camouflage.  Some are used to hide predators.  Some trick potential predators into avoiding an animal which resembles another species that has dangerous traits or an unpleasant flavour (Batesian mimicry).  Patterns can be used to attract mates or frighten off competition looking to steal your mates or territory.  So why do fish wear spots?  Below are some examples.

Wasp beetle showing Batesian mimicry.  It is harmless but resembles a wasp which puts off potential predators.

Trout and spots

The Brown trout Salmo trutta are covered in beautiful black and red spots, outlined in white.

Brown trout Salmo trutta

There are various reasons for this design.  The spots become brighter during mating which suggests they play a role in mate selection, and fish flash their colourful spots at potential competitors for females in lateral and frontal displays (Tim Travers 2014).

Close up of Brown trout pattern

They also help camouflage the fish, breaking up their outline from above and blending in with both reflected light and the speckled stones of shallow stream beds.

Brown trout swimming, seen from above

Tim Travers also suggests that the trout’s environment has led to the spots.  Evolving in crystal clear cold and glacial water, markings are visible to potential mates and rivals and therefore worth investing in.

Brown trout Salmo trutta sketch

Salmon and spots

Another spotted fish is the Salmon, Salmo salar.

Life cycle of the Salmon

Compared to similar fish in the Salmo family, such as Brown or Sea trout, the Salmon is relatively clear of spots.  However, they appear on the youngest fish, the parr, as red dots on the flanks and black dots on the back and dorsal fin.

Salmon parr

The next life stage, the smolt, has less spots, only a few on the back and gill cover.

Salmon smolt

The final stage sees the adult female hen salmon with lots of dark spots, and the male, in breeding colours, sporting red and brown and black dots.

Hen salmon

Male, or cock salmon also develop spots at mating time.  These are a complex network of red and black dots, overlaid on other blotches of colour.  Some anglers call the pattern a “tartan” because of its complexity.

Cock salmon

For more on the lifecycle of salmon look at my earlier blog or the Atlantic Salmon Trust’s page.

Salmon spots on wild vs farmed fish

There’s interesting research that suggests farmed salmon are up to 7 times more spotty than wild salmon.  As these patterns are laid down during the early smolting stage and are stable throughout a lifetime, seeing how spotty a river fish is can be used to figure out whether it is wild or an escaped animals (How does a salmon get its spots 2018 Jorgenson, Solberg and Glover).  The same consistency of pattern allows researches to identify individual fish, and could lead to less invasive and stressful forms of research on wild populations (FishBio 2018).

Puffer fish and spots

Pufferfish and box fish are often covered in little spots.  I could not find research pointing to what function these markings serve, they could be for communication, warning, mate choice, but are probably not a form of camouflage.  Perhaps it relates to the toxicity of some species and their ability to puff up by inhaling water if attacked?

Starry puffer Arothron stellatus

Sometimes these can be concentric circles of colour. In Diffusiophoresis-enhanced Turing patterns 2023, Benjamin Alessio explains how these spots are formed.  Based on the Turning model, diffusiophoresis shows how moving molecules bring other nearby molecules along with them.  If these molecules are colour-producing chromatophores, you  get different areas of colour.  This research was done on the Ornate box fish, a close relative of the puffer fish.

Turbot

Turbot are another spotted fish.  They are members of the flat fish family and have replaced their scales with skin and bony bumps called tubercles.  Turbot are able to change colour, literally changing their spots.  The skin has chromatophores producing black, red and yellow pigment.  Although these cells are in the same position, they can be turned on and off, allowing the fish to change colour in a matter of minutes or in some species, seconds.

Juvenile Turbot turbot Scophthalmus maximus 

They are also remarkable in that their bodies twist as they develop, meaning the adult has one eye which has migrated 90 degrees around its body.  This allows the fish to be incredibly flat.  Adult fish flap to stir up sand, and once a sprinkling of this lies on top of the spotted fish, it is indistinguishable from the sandy bottom of the sea floor.  Turbot spots make this fish a master of camouflage.

Adult turbot Scophthalmus maximus

For more on flatfish camouflage, have a look at Changeable Camouflage: How well can flounder resemble the colour and spatial scale of substrates in their natural habitats? by D. Akkaynak et al 2017 in Royal Society Open Science.

Sketch of the Plaice Pleuronectes platessa another colour-changing flatfish species

Leopard shark spots

The Leopard shark is also known as the Zebra shark because it has stripes when it is young.  These morph into spots as the shark ages.  It is thought that these different stages allow the markings to fufill different functions.

When young, the stripes break up the outline of the baby shark.  Lots of striped sharks together are confusing for a predator meaning it is less likely to fix on one individual animal.  This is known as the Predator dilution effect.  The markings could also send a message to territorial fish, informing them that a baby shark is in no way a threat and therefore is not worth fighting.  It could even be a form of Batesian mimicry as the stripes resemble those on venomous sea snakes. (The Sandy Zebra Shark: A new colour morph in Copeia by Dahl 2019)

Zebra or Leopard shark Stegostoma tigrinum

The shark undergoes ontogenetic change before adulthood.  This means a change in appearance which is genetically linked to the age of the animal.  The adult shark emerges with distinctive spots.  Because this change occurs at sexual maturity, it’s been suggested that the spots may be involved in mate choice and breeding. (A Zebra that changes its spots by Sophie Maycock 2020).

Conclusion

This is the very tip of the iceberg when considering spotted fish.  Stingrays, Zebrafish, Triggerfish, Grouper, Guppies, Cichlid, and Surgeonfish can all be covered in dots.  It is much easier to find out what a fish with spots looks like than it is to untangle why it wears this livery.  Whether monotone or multicoloured, single spots or concentric rings and circles, one thing is certain.  The patterns on fish are stunning, and a real joy to illustrate and research.

For a whole lot more on colouration in fish, read Price et al’s overview in Pigments Patterns and Fish Behaviour in Zebrafish 2015.

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 The Glanwye Fishery is loved by anglers who catch numerous Atlantic salmon and other fish there.  It’s run by a group of fishermen who support (and are members of) the Wye and Usk Foundation.

Starting on the Glanwye map

Initially I was shown the existing map of the fishing sites on the Glanwye stretch, and we discussed how we’d like the finished piece to look.  It needed to have the river and fishing spots on, but also to include species that were often seen there, and some vignettes of the beautiful views.

Glanwye Fishery Map

I spent a couple of happy afternoons with a camera and notebook, walking the fishery and deciding on the best views to include.  I find landscape painting more challenging than illustrating plants and animals.  So wanted to be sure I chose sites that were easily recognisable, and that I’d enjoy painting them.

Photo of Church House Pool

Salmon Life cycle

Various stages of the salmon life cycle needed to be included; I worked these up first of all, and put them at the base of the map.

I learnt a lot from the client and from researching the Atlantic Salmon Salmo salar (please see my blog for more on Salmon life stages).

Salmon life stages

What species and landscapes to include on the Glanwye map?

Next came the job of deciding what species of plant and animal to include.  My client has an in-depth knowledge of the area, so could easily suggest animals and knew where they were most commonly seen.  The otter needed to be at Craig Dhu; the wagtails are much in evidence at Church House Pool.  Prevelent species such as red kite, swallow, and kingfisher could be incuded wherever was most convenient.  I added a couple of flowers I saw while researching, and then it was time to draw up the pencil rough.

The layout was a challenge as there had to be plenty of room for text, and the landscape vignettes needed to leave room for the plants and animal illustrations.  I painted up the river and mapped fields lightly, then began to work on the landscapes.

Glanwye Fishery in progress

With rivers, the main object is to try to make the scene look watery, and not to over-paint and swallow too much of the white page with unneccessary colour.  I think the view of Church House Pool worked best.

View of Church House Pool

Once I’d done the landscapes it was a joy to work on the animals and plants, ; I began at the top left and worked down the piece.  The terror is always that you’ll mess up a detail towards the end, luckily I got away with it.  I particularly enjoyed painting the glinting kingfisher and the swans.

Kingfisher Alcedo atthis

Detail with Speedwell Veronica chamaedrys, Mute swans Cygnus olor & Red kite Milvus milvus

Adding the text

With all the illustration work complete, it was time to consider the text.  I traced out the letters for the titles, and painted them up in a dark purplish brown.

Glanwye Salmon fishery in progress

Then I have to admit to importing help for the remaining lettering.My mother is a fine artist and has always prided herself on her beautiful writing; it seemed the perfect time to use these qualities.

My mother (Susan Harper) helping out by doing the lettering on the Glanwye project

This completed the illustration of the Glanwye fishery.  Now all I need to do is figure out how on earth to get such a big piece of work to the client!  It was a brilliant job to work on, and I’m really pleased with the end result.

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A key resource has been the Atlantic Salmon Trust, where both visual and written references are readily available.

Salmon Parr

Salmon emerge from eggs in freshwater rivers, such as the Wye.  Until they absorb their egg sacs their known as alveolin.  After, they’re called fry.  If they escape predation and grow to about 2inches long, they are called Parr.  Sometimes they’re called or Fingerlings.

Salmon parr

A Salmon parr needs to escape predation.  So it develops camaflauging “parr marks” (bands of colour) across their backs.  These break up their shape and help the fish avoid being eaten by heron, mink, goosander, and other fish such as trout.

There is often confusion within the angling community when it comes to distinguishing between Salmon and Brown Trout (Salmo trutta) as the two species closely resemble each other.  With the young Parr, the main differences are that the salmon has a more streamlined shape, a deeper fork to its tail, a longer pectoral fin, a sharper snout and smaller mouth than a young Trout, and has no orange on the adipose fin (at the rear, to the back)

A Salmon Parr will only sport a couple of spots on its gill cover (often just one large spot), and its parr or fingerling markings are quite crisply defined.

Salmon smolt

After a couple of years of living in freshwater, the young salmon undergoes some major alterations to its body and chemistry which enable it to migrate to the salty waters of the Atlantic Ocean where the fish will feed amd grow.  These changes are called “smoltification” and occur in esturine waters where the young fish gather in shoals before setting out to the open sea.

Visually, the change from a Parr to a Smolt is primarily a loss of the stripes seen on a parr, and a change of colour as the fish becomes silvery.  This is caused by guanine crystals which form a layer in the skin and obscure the spots and fingerling markings (although these are still visible on the gill covers).

Salmon smolt

Their bodies elongate and their fins darken.  The silvery scales can be rubbed off easily.  It’s hard to distinguish between the sexes until the males became ready to breed.

Adult salmon

As adults, the Salmon will return to the freshwater of their birth to breed.  When they first appear, as “Fresh Run Salmon” both sexes are bright silver and pretty similar, with easily detachable scales.  You can often tell how long an adult salmon has been in fresh water by the birghtness of its silver colouring, and from the sea lice which die when their hosts switch from salty to fresh water.  A silvery adult with near complete sea lice in situ will have only just left the marine environment; one with less of a gleam and more degraded (or without) sea lice is likely to have been back in the river for longer.

Hen Salmon

The adult female fish will retain more of this silver colour than the male, although being in fresh water will cause her scales to darken, and she will develop a tiny kype or bump on her lower jaw.

Fresh run Hen Salmon

A female adult salmon is known as a Hen, a male is known as a Cock.

Cock salmon

Once in breeding colours it’s easy to recognise a Cock salmon.  The two main features are the change in colour and of the lower jaw. A breeding male’s colouration is often described as a tartan pattern, and means the fish is patchy with different shades of oranges, silver, and red markings.  Overall the fish is brownish-orange, pinkish-red, or yellowish in colour.  There’s plenty of variation between individuals.  The lower jaw develops a prominent hook or “kype” which is used to fight rivals who threated its teritory.  It also makes eating almost impossible; adult salmon mostly die after mating.

Cock salmon with kype

The Cock salmon will also have an enlarged adipose fin.

Salmon vs Trout

Again, there’s occasional confusion between an adult salmon and a large adult brown trout.  The Salmon have a more streamlined shape, a concave tail with a thinner neck than the trout tail, far fewer (or no) black spots below the lateral line (about half way down the fish body), and its upper jaw tends not to reach any further back than the rear of the eye.  Obviously, wiht an adult cock salmon with kype and tartan pattern, the difference us clear and immediate.

Adult Brown trout Salmo trutta

One more stage in the life cycle of a salmon is that on the kelt (not illustrated).  Kelts are salmon which have spawned but still remain in the river.  They regain a more silvery colour but are noticably thin, with a distended vent and frequently have red “maggots” on their gills.

A hen salmon will use her tail to scrape a nest or “redd” in the pebbles and lay eggs which are fertilized by the male, cover them over, deposit more eggs on top, cover them, and so on.  In time these will hatch into tiny salmon fry and so the cycle begins again.

For more information, please check out the Cornwall Rivers website, or teh Atlantic Salmon Trust.  For more on this project, look at my blog on the Glanwye project.

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I enjoyed the whole day enormously.  I learnt a tremendous amount, especially about insects, thanks to the endless knowledge and enthusiasm brought by Phil Ward of RWT.   Phil also happens to be the entomological recorder for the county.  I now know my reed beetle, dock beetle, and upland click beetle (a beauty).   I have got a lot better at spotting animals of all descriptions; including little froglettes which we encountered repeatedly.

How to look for freshwater life

Towards the end of this extremely interesting and enjoyable day, we took on the task of seeing what was living in the stream that runs through the reserve.  We did this by stirring up a prescribed length of stream (which related to the stream width) for exactly three minutes. We caught everything we disturbed.  Going through the finds and identifying them to family level came next.  (Further classification to genus or species was done by Phil Ward and other keen entomologists later on).

Searching for fish

We were surprised not to find any bullhead fish as they’re common in streams such as this one.  The reserves manager for RWT was keen to see if we could spot any fish at all – to no avail on this occasion, bar one tiny fry.

Freshwater life: Invertebrate variety

However, the invertebrate life we gathered was vibrant, diverse, and fascinating.  The number of mayfly nymphs was amazing.   This is encouraging.  They tend to thrive only in very clean and oxygenated streams.   We learnt that the larva can be burrowing, free-swimming, or live flattened against rocks.  Nymphs graze on plants or algae; the burrowers may feed on silt in the mud.  The adults have no mouth parts and all emerge simultaneously.  No-one is sure why, but the final stage of mayfly nymphs emerge from the water, and are able to fly, before they turn into mature adults.

We found many different caddis fly larvae; some had built pebbly homes flat on the underside of stones; others had constructed intricate turrets from tiny grains of sand,; others had cobbled together a home from pebbles, and another species favoured snippets of twig, all laid vertically together, to make their homes.  Each type of case was created by a different species of caddis fly larvae.

We also found stonefly nymphs, and alderfly larva (who differ in appearance to mayflies in having a single, feathery tail rather than three long tails).  Freshwater shrimp were in evidence too.

Follow up: Looking for Freshwater Life in a local stream

Fired up by this day out, I immediately became a family member of RWT, then I ordered a whole clutch of fold-out charts, produced by the Field Studies Council.

Thus armed, I went stream dipping a little closer to home, in the little river Edw near Erwood in Powys, and found some new organisms as well as caddis fly and mayfly nymphs.  The most fascinating were the leeches.  Only one species of leech will take human blood, the medicinal leech, and this is unfortunately now very rare (and not present in our sample).

Watching the leeches move in the Tupperware tub was a treat; the way they extended and contracted themselves was incredible, and their undulating swim was beautiful to behold.

We also found beetle and fly larvae, and large rafts of blueish springtails.

Using identification charts

Consulting the FSC Guide “The Freshwater name trail” I followed the instructions on how to calculate a biotic index for the habitat.  This is done by adding up a series of “scores” given to stream animals, then dividing the total by the number of animal types found.  Scores vary from 0 (no life) to 10 (clean and unpolluted stream).  Our score at Aberedw was 6.7, and so I realised this stream was a little more polluted than the one we examined in Tyclau Hill with RWT.

I’d expected to come across dragonfly and damselfly larvae

but a little research shows these are uncommon in streams (although you do get demoiselle nymphs and adults in running water).

Not only have I learnt lots from the biolblitz, and been able to re-visit past natural history illustrations of freshwater life, but I’ve also rekindled my passion for seeking out life in streams and ponds; I can’t describe how thrilled I am to be paddling about in streams with a guide-book and a magnifying glass again.  I tell you, this is the life!

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