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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How are spots formed?

Spots in plants are caused by areas of darker pigmentation, usually red anthocyanins.  Plants sometimes have paler spots on their leaves which are caused by an absence of chlorophyll and are discussed in my blog on Variegation.  This blog will focus on a few species with spots as I could never hope to cover all the spotted plants in one blog (or one lifetime!).  The spots seen on fungi relate to their anatomy rather than the chemistry of their colour.

Orchids

Many orchids have dark spots on their lowest petal, the labellum.  The labellum is the landing pad for potential pollinators and its shape plus these markings attract insects.  Most orchids are pollinated by beetles and hymenoptera.  These insects assume they will access nectar produced at the labellum base and in the spur behind the flower, but in many cases Dactylorhiza orchids are food-deceptive, luring the pollinators in (and being pollinated) without giving them a nectar reward. (First Nature)

Diagram showing the position of the labellum

Orchids: Early Marsh Orchid

One orchid that does this is the Early Marsh Orchid Dactylorhiza incarnata ssp coccinea.  Each flower has a pattern of spots on the labellum, guiding the potential pollinators to the pollen-producing heart of the flower.

Early Marsh Orchid Dactylorhiza incarnata ssp coccinea

Plants growing in areas with less nectar-producing neighbours, such as heaths and bogs, do better with their trickery than those surrounded by flowering plants who deliver on their visual advertising. (Dactylorhiza incarnata and deceptive pollination by Lammi and Kuitunnen 1995 in Oecologia)

Orchid flower of Early marsh orchid Dactylorhiza incarnata ssp coccinea

Orchids: Common spotted orchid Dactylorhiza fuchsii

The Common spotted-orchid appears early in the year and not only has spotted flowers but also spotted leaves.  These are not always present, in plants with paler flowers these are less likely to appear.  There is variation in the leaves, both in the intensity of the colour and the number and shape of the spots.  In some plants, there will be a paler area within each spot, giving the plant a leopard-like appearance.  The spots are created by the purple anthocyanin which may also flush the stems and leaf tips red.  The spots run cross-ways to the longitudinal veins.

Common spotted orchid Dactylorhiza fuchsii

Heath spotted orchid Dactylorhiza maculata and Southern Marsh Orchid Dactylorhiza praetermissa are closely related and may also have spotty leaves and flowers.

Heath spotted orchid Dactylorhiza maculata

Orchids: Bee orchid

It would be hard to find a spottier orchid than the Cycnoches guttulatum.  This epiphytic plant attracts the Orchid bee Euglossa cybelia and glues its pollen-filled pollinium onto the insect, ensuring cross pollination with other plants of the same species.

Orchid bee Euglossa cybelia with Cycnoches guttulatum orchid

Perforated St Johns wort Hypericum perforatum

St. John’s wort is also spotted, but this is not to do with pollination.  The little “holes” that pepper the leaves of this plant are glands which contain a cocktail of hypericin and other chemicals.  These are thought to dissuade insects from eating the leaves.

The flowers also have spots, or little dots.  These are black, and appear of the edges of the petals.  You can also see them on the calyx edges, and they appear in many Hypericum species.

Slend St. John’s wort Hypericum pulchrum

Foxglove Digitalis purpurea

Nectar guides appear on flowers to guide pollinators to the centre of the flower.  Many are invisible to us as they operate within the spectrum of light that bees and butterflies use, which includes UV light.  You can see these if you shine a UV torch on open flowers at night.  However, some flowers have spotty nectar guides that are easy for us to see.   One example of this is the Monkeyflower Mimulus bolanderi, another is the Foxglove.

Foxglove Digitalis purpurea

Nectar guides radiate outward from the nectar reward and the high contrast also helps attract potential pollinators.  Foxglove are interesting as they have a guard system to help keep out insects who would like to take their nectar but would be unable to carry pollen and help in pollination.  At the throat of each flower there are a few stiff bristles which prove no problem to bees and bumble-bees but deter smaller insects.  In fact, the hairs might help bees scramble into the flower.

Flower of the Foxglove showing the nectar-guide spots

Another way foxglove select their pollinator is by having the nectar reward at the very base of the flower.  This means only long-tongued bumblebees such as the Garden bumble bee Bombus hortorum can access it.  However, some bees bypass the security system by chewing holes in the base of the flower and stealing the nectar.  For more on Foxglove pollination, check out Ray Cannon’s blog.

Foxglove Digitalis purpurea flower with White tailed bumble bee Bombus leucorum

Research proves that nectar guides are important.  When covered up, the scent and shape of flowers still attracted pollinators.  But there was a significant reduction in the percentage of those who inserted their proboscis to access nectar and then to pollinate the flower. (Dennis Hansen 2011 Proceedings of the Royal Society B: Floral Signposts)

Lamiaceae: Skullcap

The Lamiaceae or nettle and mint family also have clear spotty nectar guides on their lower petals.  This can be seen clearly in the Skullcap Scutellaria galericulata.

Skullcap Scutellaria galericulata

In cross section, you can see the position of the pollen-laden anthers, directly above the spotted landing pad on the lower corolla lip.  In many of these flowers, the weight of the visiting insect will cause the flower to rock forward and deposit pollen on its back.  (For a more detailed look at the mechanics of these levers take a look at Floral Construction and Pollination Biology in Lamiaceae by Claßen-Bockhoff in the Annals of Botany 2007)

Cross section of Skullcap Scutellaria galericulata

Lamiaceae flowers are pollinated by bees 66% of the time; but ants, hawkmoth, flies, and butterflies also play a part. (Shrishail Kulloli in Nectar guides in Lamiaceae, Current Science 2011).

Lamiaceae: White deadnettle Lamium album

The White dead nettle also has small spots on its lower corolla lip.  In this case they’re green dots against white.

White dead nettle flowers Lamium album

White dead nettle also attract pollinators with scent, oil, lipids and polyphenols. (Lamium album and nectar guides by Aneta Sulborska-Rozycka et al 2023 in Micron).

Fungi: Fly agaric Amanita muscaria

It feels wrong to put a fungus in at the end of a blog about flowers as they are a whole other kingdom.  However, one HAS to include the Fly agaric in a blog on spots.

Fly agaric Fly agaric Amanita muscaria

The distinctive white spots on the cap of this iconic fungus do not seem to have any purpose, or not that I found literature on.  They are remnants of the universal veil, a white sheath that covers the fungus’ fruiting body as it grows from the ground.  The toadstool breaks through the sac, and small warts of white tissue remain.  These may wash off in the rain.

Fly agaric Amanita muscaria cap

Fly agaric are poisonous and packed full of hallucinogenic chemicals.  However, the red colouration doesn’t seem to suggest toxicity in fungus.  The Ox tongue fungus Fistulina hepatica and Crimson waxcap Hydrocybe punicea are both red and edible (the latter is rare so please don’t eat it).

It is interesting that such a stark colouration on a toxic fungus seems to have no link to warning colouration.  Such a pattern on a beetle, fish, or butterfly would probably be a warning of toxicity.

Completed illustration of Fly agaric with materials

Conclusion

This whistle-stop tour of spotted plants and fungi has shown that the most common reason for spots on plants it as a nectar guide for pollinators.  However, some spots are glands and others, such as the warts of Fly agaric, appear by chance.

Whatever their purpose, the spots that we find on plants and fungus are both stunning and a treat to illustrate.

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