Starfish, also known as sea stars, are marine invertebrates with adaptations that allow them to thrive without a centralized heart. They do not possess a heart or blood to circulate fluids. Instead, their internal systems operate in a manner different from vertebrates, relying on specialized structures to perform functions typically handled by a circulatory system.
The Water Vascular System
Their water vascular system is a hydraulic network that plays a primary role in their movement, feeding, and even some gas exchange. Water enters this system through a sieve-like plate called the madreporite, located on the upper surface of the starfish. From the madreporite, water flows through a stone canal to a circular ring canal positioned around the mouth. Radial canals then extend from the ring canal into each arm of the starfish.
Along these radial canals are muscular sacs called ampullae, each connected to a tube foot. When the ampullae contract, they force water into the tube feet, causing them to extend. Relaxation of the ampullae allows the tube feet to retract. This coordinated extension and retraction of tube feet, driven by changes in water pressure, enables the starfish to move across surfaces, adhere to objects, and manipulate prey. The water vascular system also assists in the transport of oxygen and nutrients throughout the body.
Internal Transport Without a Heart
Without a heart, starfish utilize their coelomic fluid for internal transport of nutrients, gases, and waste products. The coelom is a body cavity filled with this fluid, bathing the internal organs. Cilia, hair-like structures lining the coelom, create currents that distribute dissolved substances. This fluid acts as an analog to blood in vertebrates, carrying essential materials where needed.
For gas exchange and waste removal, starfish employ specialized structures called dermal branchiae. These are finger-like projections from the body wall, providing a large surface area for diffusion. Oxygen from the surrounding seawater diffuses directly into the coelomic fluid through the thin walls of the dermal branchiae, while carbon dioxide and other waste products diffuse out. Tube feet also contribute to gas exchange.
A Simple Nervous System
Starfish possess a decentralized nervous system. Instead, their nervous system consists of a nerve ring that encircles the mouth and radial nerves that extend into each arm. Each arm can operate somewhat independently, but the nerve ring coordinates their movements.
While they do not have complex eyes, starfish have rudimentary organs called eyespots. These eyespots are located at the tip of each arm and can detect changes in light and dark. Each eyespot is composed of simple ocelli, which are clusters of pigmented cells that respond to light. This assists starfish in navigating their environment and responding to light.