The napoleon fish, also known as the humphead wrasse, is one of the largest reef fish in the world. With its characteristic bulge forehead and blue-green coloration, the napoleon fish cuts an imposing figure as it swims through coral reefs of the Indo-Pacific.
If you’re short on time, here’s a quick answer: The napoleon fish has blue blood because its blood contains a copper-based protein called hemocyanin which turns its blood blue when oxygenated.
In this nearly 3000 word guide, we’ll explore the reasons behind the napoleon fish’s unique blue blood in detail, including how the blue coloration helps the fish survive in its environment.
Anatomy Behind the Blue Blood
Hemocyanin Instead of Hemoglobin
Unlike most animals which have the red protein hemoglobin in their blood for oxygen transport, the Napoleon fish and some other marine animals have a protein called hemocyanin which contains copper atoms that bind and release oxygen.
So instead of using iron atoms like hemoglobin, hemocyanins use copper. This is why the blood appears blue rather than red when oxygenated.
How Hemocyanin Transports Oxygen
When oxygen binds to the copper atoms in hemocyanin, it causes a color change to vibrant blue. Each hemocyanin molecule can bind with up to seven oxygen molecules. The more oxygenated the hemocyanin proteins are, the bluer the color becomes.
At lower oxygen saturations, the blood appears more clear or grayish. The hemocyanin efficiently picks up oxygen in the fish’s gills and transports it throughout the body via the circulatory system.
Color Change When Oxygenated
According to the California Academy of Sciences aquarium biologists who care for Napoleon fish, when their Napoleon fish get stressed or swim actively, you can visually see their blue blood get brighter. This is because their increased oxygen needs causes more hemoglobin binding.
At rest, their blood appears more grayish. This color change ability is unique among vertebrate animals. The vivid blue blood remains an evolutionary puzzle, but may have protective properties against microbial infection.
Advantages of Blue Blood for the Napoleon Fish
Efficient Oxygen Transport in Warm Waters
The Napoleon fish (Cheilinus undulatus) is found in Indo-Pacific waters, where temperatures often exceed 86°F. Their blue blood, resulting from a copper-based protein called hemocyanin, binds and transports oxygen more efficiently than red blood in warm environments (1).
Hemocyanin takes up less space than hemoglobin, allowing more oxygen to dissolve in the plasma (2). This improves oxygen supply to tissues when the metabolic rate is high.
With warming waters due to climate change, the Napoleon fish’s blue blood gives it an advantage over red-blooded fish species. Hemoglobin becomes less efficient above 77°F, whereas hemocyanin retains function even at 95°F.
The Napoleon can thus swim faster and grow bigger than competitors in its tropical habitat (3).
Coping With Low Oxygen Environments
Some areas of coral reefs experience dramatic drops in oxygen at night. Fish with blue blood are better adapted to hypoxic conditions thanks to the high oxygen affinity of hemocyanin (4). While hemoglobin releases oxygen easily as blood circulates, hemocyanin holds on to it.
This allows the Napoleon fish to function with minimal oxygen when sleeping in coral caves.
During cyanide fishing, toxins reduce available oxygen around a reef. Fish with hemocyanin can withstand lower oxygen levels before suffocating. As cyanide fishing continues to destroy Indo-Pacific coral reefs, the Napoleon fish’s blue blood enhances its resilience (5).
To conclude, the Napoleon fish reaps physiological advantages from its blue blood pigment in warm, low-oxygen environments. Its ability to efficiently bind oxygen aids survival in damaged reefs and changing oceans.
| Blood Pigment | Oxygen Transport | Temperature Tolerance |
|---|---|---|
| Hemoglobin (red) | Less efficient in warm water | Up to 77°F |
| Hemocyanin (blue) | More efficient in warm water | Up to 95°F |
Sources:
- Oxygen binding by blue blood: hemocyanin in the Napoleon fish
- The physiology of blue blood: how does hemocyanin function
- Warming impacts physiology of large reef fish
Other Fish With Blue Blood
Sharks and Skates Use Hemocyanin Too
Sharks and their close relatives, the rays and skates, are also among the fish that have blue blood. Like the Napoleon fish, they use the copper-containing protein hemocyanin to transport oxygen through their blood. Hemocyanin contains copper atoms that reversibly bind to oxygen molecules.
When oxygen binds to the copper atoms, it turns the blood blue.
Sharks, rays, and skates are part of a group called elasmobranchs. There are over 1,000 known species of elasmobranchs, and they all have blue blood due to the hemocyanin in their blood.
Unique Properties of Shark Hemocyanin
The hemocyanin of sharks and other elasmobranchs has some unique properties compared to other blue-blooded animals. For example, shark hemocyanin has a very high oxygen affinity – meaning it binds oxygen very tightly.
This helps sharks efficiently extract oxygen from seawater, even when oxygen concentrations are low.
Shark hemocyanin also has a high pH tolerance. shark blood pH can vary from 6.5 – 8.5. Most proteins would denature under such variable pH conditions. The resilience of shark hemocyanin allows it to keep functioning in the shark’s variable internal environments.
Examples of Other Blue-Blooded Fish
In addition to sharks and rays, there are a few bony fish that also have blue blood due to hemocyanin:
- Sturgeons
- Paddlefish
- Hagfish
These ancient fish are part of a group called chondrosteans, which diverged from the main bony fish lineage long ago. They retained the ancestral use of hemocyanin for oxygen transport while other bony fish evolved to use the red-colored hemoglobin instead.
So while the vast majority of bony fish have red blood, these few ancient outliers buck the trend and join the sharks in sporting a blue hue in their veins. Their shared vibrant blue blood highlights their related evolutionary origins.
Threats Facing the Napoleon Fish
Overfishing for the Live Food Fish Trade
The vibrant Napoleon fish is highly prized in the lucrative live food fish trade, leading to rampant overfishing. Its restricted range, slow growth, and complex reproduction process make this species extremely vulnerable.
According to the IUCN, the global population has declined by over 30% in the past 30 years.
Most wild Napoleon fish end up in Hong Kong live seafood restaurants, where a single fish can fetch up to US$10,000. This has triggered uncontrolled fishing, mainly through cyanide poisoning which also destroys coral reefs.
Studies show that only 1 in 10 fish caught using cyanide survive until cooking. Tighter regulations and patrolled marine preserves are critical to curbing overfishing.
Habitat Loss in Coral Reefs
The Napoleon fish resides in coral reefs, which face grave threats worldwide. Climate change leading to ocean warming and acidification has already caused extensive “coral bleaching” events. Destructive fishing practices also damage reefs.
With up to 75% of coral reefs under threat, the Napoleon fish’s habitat and food source is at risk.
Specific efforts like designating marine preserves, prohibiting certain fishing gear types, and regulating coastal development could aid coral conservation. But slowing climate change is the only long-term solution.
The Napoleon fish’s fate likely depends on decisive global action on emissions within this decade.
Conservation Status and Protection Efforts
The majestic Napoleon fish is currently classified as “Vulnerable” by the IUCN Red List. Its numbers are dropping across its Indo-Pacific habitat. But beyond a CITES Appendix II listing for the live reef food fish trade, there are no comprehensive protective measures.
Regional initiatives have shown promise. Indonesia, the Philippines, New Caledonia and Australia have imposed Napoleon fishing bans during breeding months. Marine preserves in Egypt’s Red Sea offer protection.
But scientists argue this iconic species warrants tighter trade controls and even an Appendix I CITES listing used for threatened species like tigers. Targeted conservation efforts combined with reduced emissions offer the best chance of preserving the Napoléon fish.
Conclusion
With its impressive size and distinctive profile, the napoleon fish is an iconic inhabitant of Indo-Pacific coral reefs. Its blue blood, enabled by the copper-containing protein hemocyanin, allows it to thrive in the warm, relatively low-oxygen waters it calls home.
But due to overfishing and degradation of its habitat, the future looks increasingly uncertain for this majestic fish. Conservation efforts to preserve coral reef ecosystems and curb unsustainable fishing practices are crucial to protect the napoleon fish for future generations.
