Blood comes in a variety of colors across the animal kingdom. While human blood appears red due to the presence of iron-rich hemoglobin, some animals have evolved different blood pigments to suit their environments and lifestyles.
Key Facts About Animal Blood
Here are some quick facts about blood color diversity in animals:
- Vertebrate blood contains specialized cells called red blood cells that give blood its color.
- The most common blood pigment is hemoglobin which contains iron and gives blood a red color.
- Other blood pigments include hemocyanin which has copper (blue blood) and chlorocruorin which contains iron and magnesium (green blood).
- Invertebrates like insects and crustaceans usually have colorless blood or hemolymph.
- Only a handful of animals have black blood, including some worms, leeches, insects and fish.
What Makes Blood Black?
There are a few reasons why some animals evolved to have black blood instead of the more common red blood:
- Environment – Animals with black blood tend to live in environments without light. Black blood is better camouflaged in the darkness. Examples are blood-sucking leeches and gut worms that live inside other animals.
- Chemistry – The black pigment in blood is usually hemocyanin bound to oxygen, or ferrous oxide aggregates. These chemicals can appear black or very dark at high concentrations.
- Cold temperatures – In very cold environments, blood can take on a darker color. This is seen in some Antarctic ice fish whose blood is more brownish or blackish in color.
Animals with Black Blood
Here are some of the main animal groups known to have black colored blood:
Worms
Several worm species have black blood including:
- Leeches
- Ribbon worms
- Acanthocephalans (spiny-headed worms)
- Roundworms like Ascaris
In leeches, their blood turns black after they’ve consumed a blood meal. Their blood contains hemoglobin that turns black when bound to oxygen and iron sulfide aggregates.
Insects
A few insects have black or very dark colored blood-like hemolymph such as:
- Vinegaroons
- Horseshoe crabs
- Cockroaches
Instead of hemoglobin, insects use hemocyanin to transport oxygen. This copper-based pigment can appear black at high concentrations.
Fish
There are a couple fish species with black blood including:
- Black swallower – Predatory deep sea fish
- Antarctic icefish – Lack hemoglobin and myoglobin
The black swallower has blood that turns black from hemoglobin after eating. The icefish’s blood lacks iron-containing hemoglobin and appears more brown or blackish.
Animals With Other Unusual Blood Colors
While black blood is rare, several other animals have evolved blood in non-red colors:
| Animal | Blood Color | Pigment |
|---|---|---|
| Horseshoe crabs | Blue | Hemocyanin |
| Octopus, squid, molluscs | Blue | Hemocyanin |
| Green shore crab | Yellow | Hemolymph |
| Green-blooded skink | Green | Biliverdin |
| Glass squid | Transparent | Hemocyanin |
Conclusion
To conclude, while most vertebrates possess red blood derived from hemoglobin, a select few animal species have peculiarly evolved black blood. These include certain worms, insects and deep sea fish that either inhabit lightless environments or utilize hemocyanin pigments. Beyond black blood, other blood colors like blue and green also occur in some invertebrates and reptiles. So while red blood runs through most animals with backbones, nature has painted herself an occasional canvas of rainbow-colored fluid in the animal kingdom.
