Starfish possess a remarkable physiology, thriving in marine environments without organs commonly associated with breathing. Unlike humans and fish, these invertebrates lack lungs or gills. Instead, starfish rely on unique adaptations to extract oxygen from seawater, showcasing a respiratory process tailored to their aquatic existence.
External Gas Exchange Structures
Starfish primarily absorb oxygen through specialized external structures called dermal branchiae, also known as papulae or skin gills. These thin-walled, finger-like projections extend outwards from the body wall, increasing surface area for gas exchange. Oxygen dissolved in seawater diffuses directly across their thin membranes into the starfish’s internal body cavity, while carbon dioxide diffuses out.
The tube feet, small, flexible appendages on the starfish’s underside, also contribute to gas exchange. Their thin walls allow oxygen to diffuse in and carbon dioxide to exit. While papulae are the primary sites for respiration, the collective surface area of numerous tube feet further aids this process. This dual approach ensures efficient oxygen absorption.
The Water Vascular System’s Respiratory Contribution
The water vascular system, a distinctive feature of echinoderms, plays a significant role in starfish respiration, in addition to locomotion and feeding. Water enters this hydraulic system through the madreporite, a sieve-like plate on the starfish’s upper surface. This porous structure acts as a filter, allowing seawater into the system.
From the madreporite, water flows through a stone canal to a central ring canal, which encircles the starfish’s mouth. Radial canals then branch off, extending into each arm. This continuous water flow helps maintain a concentration gradient, facilitating efficient oxygen movement across the dermal branchiae and tube feet. The water vascular system thus ensures a constant supply of oxygenated water to the exchange surfaces.
Internal Oxygen Distribution
Once oxygen diffuses across external structures, its distribution throughout the starfish’s body occurs without a centralized circulatory system. Instead, oxygen is transported by the coelomic fluid, which fills the body cavity. This fluid, analogous to blood, surrounds internal organs and carries respiratory gases, nutrients, and waste products.
Cilia, tiny hair-like structures lining the coelom and water vascular system, help circulate this fluid, ensuring oxygen reaches various tissues. As the coelomic fluid moves, oxygen diffuses directly into individual cells for metabolic processes. Carbon dioxide produced by cells then diffuses back into the coelomic fluid, exiting the body through the papulae and tube feet, completing the gas exchange cycle.