Many people wonder if starfish bleed when injured, a natural question given how different these marine creatures look from humans and other familiar animals. Exploring the internal workings of a starfish reveals a biology uniquely adapted to its marine environment, which differs significantly from what we typically associate with blood and circulation.
What We Mean By Blood
In most animals, particularly vertebrates like humans, “blood” refers to a specialized fluid circulating within a closed system of vessels. This fluid, often red due to iron-rich hemoglobin, contains red blood cells that transport oxygen from respiratory organs to tissues. It also carries white blood cells for immunity, platelets for clotting, and plasma, which transports nutrients, hormones, and waste products. This system delivers vital substances and removes metabolic byproducts, maintaining overall function.
How Starfish Internally Circulate
Starfish do not possess blood in the conventional sense, nor do they have a centralized heart or closed blood vessels. Instead, they primarily rely on a unique hydraulic system known as the water vascular system. This system is a network of fluid-filled canals and tube feet, which plays a role in locomotion, feeding, and gas exchange. Seawater enters this system through a porous plate on their upper surface called the madreporite, then circulates through various canals to facilitate movement.
Beyond the water vascular system, starfish also have a separate hemal system. This system consists of channels that contain a colorless coelomic fluid, which is distinct from blood as it lacks hemoglobin and does not transport oxygen. While not a true circulatory system for oxygen, the hemal system is thought to aid in the distribution of digested nutrients throughout the body. The coelomic fluid within this system also contains specialized cells, called coelomocytes, which perform immune functions and clear foreign materials.
When a Starfish is Injured
Given their unique internal systems, a starfish does not “bleed” in the same way a vertebrate would. If a starfish sustains an injury, any fluid that exits its body would be either seawater from its water vascular system or coelomic fluid from its hemal system. These fluids do not carry oxygen, meaning the loss of these fluids does not equate to the oxygen-carrying blood loss seen in vertebrates.
Starfish possess mechanisms to cope with injuries, including the ability for coelomocytes to form a clot and seal wounds, preventing further fluid loss and infection. Starfish also have significant regenerative capabilities. They can often regrow lost arms, and some species can even regenerate an entire body from just a portion of an arm, provided a part of the central disk is present. This process can take several months to over a year.