Banded Seaperch, Hypoplectrodes nigroruber sighted with asymmetric head pigmentation

David Muirhead photographed this Banded Seaperch, Hypoplectrodes nigroruber with a blackened head at Port Stanvac on 26^th December 2025.

He posted the photo on iNaturalist, along with these comments about the blackened head: –

“This individual has a black head on the L side but normal colours and markings on the R side. Unfortunately, I couldn’t get a photo of the R side. However, I have encountered many similar scenarios among a few other species of reef fish in SA waters (Moonlighter, Zebrafish especially),and I again don’t have a clear scientific explanation. However, I again think that these are adaptations (natural selection at work) that arise when water clarity and hence light penetration is lower than normal for long periods, accompanied by black or dark grey background benthic colour from silt.”

Further, “This fish definitely had sharply defined asymmetry of head colour, very similar to one of those zebrafish observations from years ago* during the very heavy siltation event at Lady Bay, SW Fleurieu. I see parallels with the current HAB, which has made every type of subtidal habitat in GSV darker, often almost black.”

* (See both A Most Peculiar Colour Pattern in Zebra Fish, Girella zebra and More Unusual Black-headed Fish Sightings)

After taking a quick look at https://link.springer.com/article/10.1007/s10695-025-01549-0 , I asked the Microsoft Copilot AI Assistant, “What is melanin?”

The Microsoft Copilot AI Assistant gave me “a clear, grounded explanation of melanin i.e. Melanin is a biological pigment—a complex polymer made from the amino acid tyrosine—that gives colour to skin, hair, eyes, and even some internal tissues in humans and other animals. It’s produced by specialized cells called melanocytes, which live mainly in the skin’s basal layer, hair follicles, and parts of the eye and brain.”

It also gave me lots of other information, including “melanin plays a fascinating role in colour change in marine species”.

I then asked, “What is the use of melanin in fish?”

The Assistant then gave me “a clear, evidence‑based explanation of what melanin does in fish, grounded in the sources we just retrieved”: –

🎨 What Melanin Is Used For in Fish

Melanin is one of the principal pigments in fish coloration, produced inside specialized pigment cells called melanophores, a subtype of chromatophores.

🐟 Key Functions of Melanin in Fish

1. Colour Production (Black, Brown, Grey)

• Melanin is responsible for dark colours in fish—black, brown, and grey tones.
• These colours appear when melanin granules disperse or concentrate inside melanophores:
  • Dispersed melanin → darker appearance
  • Concentrated melanin → lighter appearance

2. Camouflage & Rapid Colour Change

• Fish use melanin to change colour dynamically for camouflage, communication, or stress responses.
• Hormones such as α‑MSH (melanin‑stimulating hormone) and MCH (melanin‑concentrating hormone) regulate how melanin moves within melanophores.
• This allows fish to:
  • Darken against dark backgrounds
  • Lighten against pale backgrounds
  • Display patterns during social interactions

3. Protection from UV Radiation

• Melanin absorbs harmful UV radiation.
• While not as extensively studied as in mammals, melanin in fish contributes to photoprotection, especially in shallow‑water species exposed to strong sunlight.

4. Thermoregulation

• Darker melanin‑based colours can help fish absorb more heat, which is one of the proposed functions of fish coloration.

5. Communication & Social Signalling

• Melanin‑based patterns (bars, stripes, patches) are used for:
  • Dominance displays
  • Courtship
  • Territorial warnings
  • Stress signalling
• These patterns can be static or rapidly altered by melanin movement.

6. Developmental & Genetic Roles

• Genes such as mc1r, sox10, and pax3 regulate melanin synthesis and melanophore development in fish.
• Mutations in these genes can produce:
  • Albino phenotypes
  • Dark morphs
  • Pattern variations

🧩 Why This Matters for Your Current Tabs

You’ve been looking at chromatophores and a fish with asymmetric head pigmentation on iNaturalist. Melanin is central to that phenomenon:

• Uneven melanin distribution can create one‑sided darkening, like the black‑headed banded seaperch you’re viewing.
• This can result from:
  • Localized melanophore activation
  • Hormonal imbalance
  • Injury or regeneration
  • Genetic mosaicism
  • Environmental triggers

If you want, we can dig into why that specific fish has a black head on one side—there are some fascinating possibilities.”

I then asked the Assistant if “We can dig into why that specific fish has a black head on one side?”

The Assistant’s response was, “What you’ve found is genuinely fascinating: a Banded Seaperch (Hypoplectrodes nigroruber) with sharp, unilateral head darkening on the left side only, while the right side appears normal.

Let’s break down the most plausible biological explanations, using what we know about chromatophores, melanin, and the environmental context described in the observation.

🧭 What Could Cause One‑Sided Blackening in This Fish?

The key clues from the (iNaturalist) page:

• The observer notes “sharply defined asymmetry of head colour”.
• They’ve seen similar cases in Moonlighters and Zebrafish during heavy siltation events.
• They link this to the current HAB (harmful algal bloom) making subtidal habitats darker, often almost black.

This gives us three strong hypotheses.

1️⃣ Asymmetric Chromatophore Activation (Most Likely)

Fish chromatophores — especially melanophores — can respond locally, not just systemically.

Why this matters:

• Melanophores sit in the dermis, and their activation can be regional, depending on local nerve input, light exposure, or stress signals.
• If one side of the fish’s head was consistently exposed to darker surroundings, that side’s melanophores could remain dispersed (dark), while the other side stays aggregated (light).

Supporting evidence from the page:

• The observer explicitly links this to darkened benthic environments during siltation and HAB events.
• They’ve seen multiple species show similar unilateral darkening under the same environmental conditions.

Mechanism:

• Fish have background adaptation pathways that adjust pigmentation to match surroundings.
• These pathways can be asymmetric if:
  • One side faces a darker substrate more often.
  • The fish habitually orients itself in a particular direction relative to light.
  • Local neural control of chromatophores differs between sides.

This is the simplest explanation that fits all the clues.

2️⃣ Injury, Regeneration, or Localized Stress Response

If the left side of the head experienced:

• abrasion
• infection
• parasite attachment
• localized inflammation

…melanophores can become hyperactive in the healing tissue.

Why this is plausible:

• Fish often show melanophore proliferation or melanin deposition around healed wounds.
• The sharp boundary could reflect the edge of a healed patch.

But:

• The observer didn’t mention scarring or texture changes.
• The pattern seems too clean and too common across species to be random injury.

So this is possible but less likely.

3️⃣ Genetic Mosaicism or Developmental Asymmetry

This is rare but real:

• A fish can have two genetically distinct cell lineages (mosaicism).
• If melanophore‑related genes differ between sides, pigmentation can be asymmetric.

Why it’s less likely here:

• The observer reports multiple individuals and species showing similar patterns during specific environmental events.
• That strongly argues for an environmental, not genetic, cause.

🧩 Putting It Together

Given the environmental context (HAB, darkened seafloor, reduced light penetration) and the observer’s history of seeing this exact phenomenon during siltation events, the most coherent explanation is:

⭐ Asymmetric background‑matching driven by local melanophore activation

The fish is likely responding to:

• darker benthic conditions
• uneven light exposure
• local neural/hormonal cues

…resulting in melanin dispersion on one side only.

This is a beautiful example of how chromatophores are not uniformly controlled — they can respond in patches, stripes, or even one side of the body.”

Further, “If you want, we can go deeper. We can explore:

• how melanophores receive asymmetric neural input
• how light detection through the skin (yes, fish can do this) drives local pigment changes
• whether Hypoplectrodes nigroruber is known for rapid colour modulation
• how HAB‑related turbidity affects fish pigmentation physiology”