Colour and Bioluminescence in Fish (Lecture 7)

ICHPTHYOLOGY

What are the two main types of colour production in fish?

Biochromes (pigments that absorb light) and schematochromes (structural reflection).

What are chromatophores?

Cells in fish responsible for colouration; located subdermally and come in different types, overlaid in chromatophoresomes.

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What are the two types of colour change in fish

Physiological (short-term via pigment movement) and morphological (long-term via changes in chromatophore type/number).

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What controls physiological colour change?

Nervous or hormonal signals; pigment moves within chromatophores.

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What triggers physiological colour change?

Background colour, social interactions, and camouflage needs.

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What is morphological colour change used for?

Long-term changes associated with life stages, like migration or development (e.g., salmon silvering).

List 5 functions of fish colouration

UV protection, thermoregulation, intraspecific communication, interspecific signaling, and concealment.

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How does countershading work?

Dorsal surface is dark, ventral surface is light to counteract shadowing and blend in from multiple angles.

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What is obliterative colouration?

silvery, reflective surfaces that bounce light to reduce visibility in the open ocean.

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What anatomical features enable silvering

Stratum argenteum (guanine crystal layer) and iridophores in the skin.

Why is transparency useful?

Effective in variable vegetation environments; offers camouflage when colour matching isn't possible.

What is disruptive colouration?

Patterns that break up the body outline to make prey unrecognizable to predators.

Why do some fish hide their eyes or have fake ones?

to confuse predators about the true orientation and increase escape chances.

What is interspecific mimicry?

One species mimicking another for protection or predation; includes Batesian and Müllerian mimicry.

What is Batesian mimicry?

Harmless species mimics a harmful/venomous one to avoid predation.

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What is Müllerian mimicry?

Multiple venomous species share similar colours to reinforce predator avoidance.

What is facultative mimicry?

Ability to switch between mimicry forms depending on context (e.g., fangblenny, mimic octopus).

What is bioluminescence?

Chemical production of light in living organisms using luciferin, enzyme, and oxygen.

How does bioluminescence differ from fluorescence?

Bioluminescence is chemical and occurs in darkness; fluorescence requires light to be re-emitted.

What is the role of luminous bacteria in fish?

Symbiotic bacteria emit light; fish offer nutrients and oxygen. Fish can't directly control the light.

What are photophores?

Light-producing organs in fish; light may be controlled by irises, rotation, or chromatophores.

What controls bioluminescence in self-luminous fish?

Hormonal or nervous control, mechanical screening (chromatophores or photophore rotation).

Why do deep-sea fish produce blue bioluminescence?

Blue penetrates farthest; their eyes are adapted to detect only blue light.

What is counterillumination?

Bioluminescence on the ventral side to match ambient light and reduce silhouette in the twilight zone.

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What are the limitations of counterillumination?

Only effective at depth or night; requires precise intensity matching and is energetically costly.

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What are the functions of bioluminescence

Camouflage, communication, prey luring, territory defense, and courtship.

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How is red pigment used in deep-sea environments?

It appears black and is undetectable—unless by species that can see red, making prey visible.

How is bioluminescence used in predation?

To lure prey with light (e.g., anglerfish) or startle predators.

Why is colour matching important in benthic species

Helps them blend with substrates like sand, vegetation, or coral for protection or ambush.