Sea stars, correctly known as starfish, are recognizable inhabitants of the ocean floor whose inner workings defy the biology common to most animal life. These marine invertebrates, belonging to the phylum Echinodermata, thrive in environments ranging from shallow tidal pools to the deepest abyssal zones. Their survival is secured by a unique anatomy that bypasses the need for several complex systems found in other animals. Sea stars notably lack a traditional heart and a closed circulatory system.
The Direct Answer: Absence of Blood and Heart
Sea stars do not possess blood in the conventional sense, nor do they have a centralized pumping organ like a heart to drive fluid circulation. The fluid that fills their internal body cavity, the coelom, is called coelomic fluid. This fluid is essentially filtered seawater, containing specialized cells and proteins that give it a function analogous to blood in other organisms.
Coelomic fluid transports nutrients and plays a significant role in the sea star’s immune defenses. It differs from vertebrate blood because it lacks oxygen-carrying pigments like hemoglobin. The fluid is circulated throughout the body cavity by the movement of cilia, which are tiny, hair-like projections lining the internal surfaces. A vestigial system of canals known as the hemal system exists, but it is greatly reduced and does not play a major role in material circulation.
The Water Vascular System
The primary mechanism replacing a traditional circulatory system is the water vascular system (WVS), a hydraulic network unique to echinoderms. This system draws in and pressurizes seawater, using it for movement, internal transport, and gas exchange. The process begins on the sea star’s aboral (upper) surface, where seawater enters through a small, porous plate called the madreporite.
From the madreporite, water flows through the stone canal, connecting to the circular ring canal that encircles the mouth. Five radial canals branch from this central ring, extending down the length of each arm. These radial canals supply small, lateral canals that terminate in the ampullae and tube feet.
The tube feet, visible on the underside, are the external, movable parts of the WVS. Each foot connects to an internal, muscular sac called an ampulla. When the ampulla contracts, it forces water into the tube foot, causing it to extend and adhere to a surface via suction. This hydraulic action allows the sea star to achieve locomotion, grasp prey, and anchor itself.
The continuous fluid movement within the WVS also functions as an internal transport system. As seawater moves through the network, it aids in distributing absorbed nutrients from the digestive system to the body tissues. This fluid exchange sustains the metabolic needs of the sea star’s tissues, compensating for the lack of a robust blood network.
Alternative Methods for Respiration and Waste Removal
Sea stars rely on simple biological processes to manage respiration and metabolic waste. Gas exchange, the uptake of oxygen and the release of carbon dioxide, occurs through simple diffusion across specialized, thin-walled structures. The primary respiratory surfaces are the dermal branchiae, commonly called papulae or skin gills, which are delicate, finger-like projections extending from the body wall.
These papulae are extensions of the internal coelom, allowing the coelomic fluid to come into close proximity with the oxygenated seawater. Oxygen diffuses directly from the water across the thin papulae walls into the coelomic fluid, while carbon dioxide moves in the opposite direction. Gas exchange is also facilitated by the numerous tube feet, whose thin walls provide additional surface area for diffusion.
For waste management, sea stars lack dedicated excretory organs like kidneys, relying instead on diffusion and specialized cells. The main metabolic waste product, ammonia, is highly soluble and is released directly into the seawater. This excretion occurs largely through the same respiratory surfaces—the dermal branchiae and the tube feet—where ammonia diffuses out of the coelomic fluid.
Solid waste particles are handled by specialized amoeboid cells called coelomocytes, which patrol the coelomic fluid. These cells engulf foreign particles and debris before migrating to the tips of the papulae. The waste-laden coelomocytes are then expelled into the surrounding water, cleansing the sea star’s internal environment.