Many people encounter starfish, often called sea stars, and ponder whether these unique marine animals are truly alive. Their sometimes rigid, symmetrical forms and lack of obvious movement can lead to questions about their biological status. Despite their common name, these fascinating creatures are not fish, but belong to a group of marine invertebrates known as echinoderms. This article explores what defines life and how starfish meet those biological parameters.
Defining Life
Biologists define living organisms by fundamental characteristics that distinguish them from nonliving matter. Living things exhibit a high degree of organization, composed of structured cells, tissues, and organ systems. They also engage in metabolism, obtaining and using energy from their environment to power life processes.
Living organisms maintain homeostasis, a stable internal environment, even as external conditions fluctuate. They undergo growth and development, increasing in size and changing form over their lifespan. Reproduction, the ability to produce offspring, ensures the continuation of their species. Living entities respond to stimuli, reacting to changes in their surroundings. Organisms adapt, evolving over generations to better suit their environment.
Starfish: A Living Creature
Starfish meet the criteria for living organisms. Their bodies show complex organization, typically featuring a central disc and five or more arms radiating outwards, though some species can have many more arms. This radial symmetry houses a decentralized nervous system with a nerve ring around the mouth and radial nerves extending into each arm, allowing for coordinated movement and sensory processing without a centralized brain. They also possess a well-developed digestive system, including a unique ability to evert their stomach to digest prey externally.
Starfish demonstrate metabolism by feeding on a variety of prey, such as clams, mussels, oysters, and even coral. Many species use their tube feet to pry open bivalve shells, then extend their cardiac stomach into the shell to digest the soft tissues externally before reabsorbing the stomach. They maintain internal stability through a water vascular system, a hydraulic network of fluid-filled canals that aids in locomotion, food manipulation, gas exchange, and waste transportation. This system uses water pressure to operate their tube feet.
Starfish exhibit growth and development, starting as microscopic, bilaterally symmetrical larvae called bipinnaria that swim freely. These larvae undergo a complex metamorphosis, settling on the seabed and transforming into the radially symmetrical adult form. Reproduction occurs both sexually and asexually. Sexually, most species release eggs and sperm into the water for external fertilization, while asexually, many can regenerate lost arms, and some species can even regrow an entire new individual from a single detached arm or by fission of their central disc.
Starfish respond to stimuli in their environment. They can detect touch, light, temperature, and chemicals in the water using sensory cells, tube feet, and simple eyespots located at the tips of their arms. Their ability to move using hundreds of tube feet, coordinated by their nervous system, allows them to search for food and avoid predators. Over evolutionary time, starfish have developed adaptations, such as their regenerative capabilities, which serve as a defense mechanism against predators and a method of asexual reproduction, contributing to their widespread success in marine ecosystems.