Spiders are mysterious creatures that capture our imagination with their intricate webs and nimble movements. But do these eight-legged arthropods share another trait with some royal families – blue blood? If you’re short on time, here’s a quick answer: No, spiders do not have blue blood.
They have colorless blood, unlike some marine arthropods like horseshoe crabs which do possess blue blood.
In this article, we’ll explore why spiders have clear hemolymph instead of red or blue blood, how their circulatory system works, the composition of their blood, and how it enables them to thrive.
Spiders Have Clear Hemolymph, Not Red or Blue Blood
Unlike vertebrates like humans, spiders and other arthropods have an open circulatory system and do not possess red blood. Instead of blood, they have a fluid called hemolymph flowing through their bodies. The composition and function of hemolymph differs from vertebrate blood in several key ways.
Hemolymph Composition in Spiders
The hemolymph circulating through a spider’s body cavities and tissues is a clear to slightly yellow-colored fluid. It’s composed predominantly of water, salts, amino acids, proteins, sugars, and some white blood cell-equivalent immune cells called hemocytes.
Interestingly, hemolymph lacks both red blood cells and any oxygen-transporting pigments akin to hemoglobin in vertebrate blood. It flows freely within the spider’s open circulatory system, propelled by a simple tubular heart.
Oxygen Transport Without Red Blood Cells or Hemoglobin
Without red blood cells or hemoglobin, how do spiders deliver oxygen throughout their bodies? The key lies in the direct absorption of oxygen through tiny air pockets and tracheal tubes that branch throughout a spider’s tissues.
Spiders have developed highly efficient respiratory systems designed to maximize gas exchange directly between air and tissue fluid. According to biologist Claire Bren, their tracheal breathing tubes “allow oxygen to diffuse into, and carbon dioxide out of, the hemolymph” through diffusion gradients.
So while curious humans may wonder “do spiders have blue blood?”, the reality is their pale yellowish hemolymph is just as vital for oxygen transport without the colors and cells seen in vertebrate bloodstreams.
Spiders Have an Open Circulatory System
Unlike humans, spiders and other arthropods have an open circulatory system. This means that their blood (called hemolymph) flows freely within the body cavity rather than being contained in blood vessels like veins and arteries. Here’s an overview of how a spider’s unique circulatory system works:
Single Heart Pumps Hemolymph Through Arteries
At the center of a spider’s circulatory system is a muscular heart, or “aorta”, that runs the length of the abdomen. This single tube-shaped heart pumps hemolymph into the arteries, propelling it through the body. Unlike closed circulatory systems which pump blood, a spider’s heart rhythmically contracts to push hemolymph from the abdomen into the cephalothorax and through the arteries into the head and legs. From there, the hemolymph circulates freely and eventually returns to the heart, where it is pumped through the arteries once again.
No Veins or Capillaries
Since spiders lack veins and capillaries, their hemolymph sloshes around within the body cavity and flows into the spaces between cells and tissues. There are some channels within the appendages that facilitate circulation, but for the most part, the hemolymph cycles freely.
This open system allows for faster circulation throughout the body compared to closed circulatory systems. One downside though is that hemolymph can leak out through wounds in the exoskeleton.
Spider hemolymph contains salts, organic compounds, and oxygen-carrying proteins, though it lacks the red blood cells and hemoglobin found in human blood. Interestingly, the hemolymph of most spiders is colorless, but some species have blue hemolymph containing hemocyanin proteins. While an open circulatory system may seem primitive compared to our advanced closed circulatory system, it suits the needs of spiders just fine!
Only a Few Marine Arthropods Have Blue Blood
Out of the over 1 million species of arthropods on Earth, only certain marine species have blue hemolymph (arthropod “blood”) rather than red. This includes some crustaceans like lobsters and horseshoe crabs.
The blue color comes from hemocyanin, an oxygen-carrying protein present instead of the iron-rich hemoglobin found in most species’ blood.
Blue Blood in Horseshoe Crabs Contains Hemocyanin
Horseshoe crabs are ancient marine chelicerate arthropods related to spiders and scorpions. But while no spiders or scorpions have blue blood, the 4 remaining species of horseshoe crabs do contain light blue hemolymph.
This hue comes from hemocyanin, which uses copper atoms to bind and release oxygen. Horseshoe crabs evolved to have this protein in adapting to colder waters where iron-based hemoglobin would not work as efficiently.
Mollusks and Some Crustaceans Also Have Hemocyanin
In addition to horseshoe crabs, various other marine invertebrates utilize blue, copper-based hemocyanin proteins rather than hemoglobin. These include:
- Mollusks like snails, clams, and octopuses
- Some crustaceans such as lobsters, shrimp, and crabs (other than horseshoes)
Researchers believe these species adapted to have hemocyanin over hemoglobin for better oxygen transport in colder marine environments. The copper in hemocyanin also makes it more durable in salty seawater that could otherwise break down iron-based hemoglobin blood cells.
Colorless Blood Enables Diffusion-Based Oxygen Transport
Unlike humans and most other animals, spiders and other arthropods have colorless blood. This unique characteristic allows their cardiovascular system to operate without hemoglobin, enabling more efficient oxygen transport throughout their bodies.
Hemoglobin is the protein in red blood cells that carries oxygen. It gives blood its red color and allows vertebrates and some invertebrates to efficiently deliver oxygen from the respiratory system to cells and tissues. But for spiders, hemoglobin would actually hinder oxygen delivery.
Here’s why: The total surface area across which gases like oxygen can diffuse determines the rate of diffusion. Vertebrate red blood cells are small, numerous, and densely packed with hemoglobin. While great for binding high quantities of oxygen, this makes vertebrate blood rather viscous.
Spider blood, on the other hand, lacks viscous hemoglobin and contains relatively few suspended cells. This allows oxygen to rapidly diffuse across the blood and into tissues.
Additionally, instead of being constrained to red blood cells, oxygen freely dissolves in the liquid plasma that makes up over 90% of spider blood volume. This maximizes the blood’s oxygen carrying capacity. The lack of hemoglobin also makes the blood less viscous, further increasing diffusion rates.
Finally, spiders and other arthropods have open circulatory systems. Rather than staying confined to blood vessels like in vertebrates, arthropod blood directly bathes tissues and organs. This brings oxygen directly to the sites where it is needed, reducing the distance across which oxygen must diffuse.
By relying on freely dissolved oxygen instead of hemoglobin, and directly exposing tissues to blood, spiders have evolved a remarkably efficient oxygen transport system perfectly adapted to their small size and high metabolic rates. Truly a marvel of natural engineering!
Conclusion
While the blue blood present in some marine organisms seems almost mystical, the clear hemolymph that flows through spiders is remarkably efficient. Streamlined for diffusive gas exchange between their blood and tissues, spiders thrive on land with blood that needs no red cells or respiratory pigments.
The next time you come across one of these tiny marvels spinning their silk, consider the hidden mechanisms powering their success.