Many people wonder if sea stars are a type of fish, a common misconception. While they share the ocean with fish, sea stars are fundamentally different organisms. Understanding these distinctions reveals their unique biology and place in the marine environment.
Distinguishing Sea Stars from Fish
A primary difference between sea stars and fish lies in their skeletal structure. Fish are vertebrates, possessing an internal skeleton that includes a backbone. In contrast, sea stars are invertebrates. Their bodies are supported by an endoskeleton made of small, bony plates called ossicles, embedded within their skin.
Their methods of breathing also vary significantly. Fish utilize gills to extract oxygen from water. Sea stars do not have gills or lungs; instead, they exchange gases through tiny, finger-like projections on their body surface called papulae, and through their tube feet. Oxygen moves directly from the seawater into their body fluids.
Locomotion is another distinguishing feature. Fish use fins and tails for propulsion through the water. Sea stars, on the other hand, move slowly across the seafloor using thousands of tiny tube feet located on their underside. These tube feet extend and retract using a hydraulic system, allowing the sea star to grip and pull itself along various surfaces.
Symmetry also sets these groups apart. Fish exhibit bilateral symmetry, meaning their bodies can be divided into two mirrored halves. Adult sea stars display radial symmetry, typically with five arms arranged around a central disc.
While fish have well-developed eyes, sea stars possess simple eyespots at the tip of each arm that detect changes in light and dark. They also have senses of touch and chemoreception. Sea stars do not have a centralized brain, instead relying on a decentralized nervous system.
The Unique Biology of Sea Stars
Sea stars belong to the phylum Echinodermata, a diverse group of marine invertebrates that also includes sea urchins, sand dollars, and sea cucumbers. This classification highlights their distinct evolutionary path from fish. There are over 1,900 known species of sea stars, found in all of the world’s oceans.
A defining characteristic of sea stars is their water vascular system, a unique hydraulic network of fluid-filled canals. This system plays an important role in their movement, enabling the extension and retraction of their tube feet. Water enters this system through a sieve-like plate on their upper surface called the madreporite, and its pressure powers functions like feeding and waste transport.
Sea stars are also remarkable for their ability to regenerate lost arms. If an arm is damaged or severed, they can regrow it. This regenerative process can take several months to over a year to complete. Their feeding strategies are equally distinctive; many sea stars are predators that can evert their stomach out through their mouth to engulf and digest prey, such as bivalves, externally, then reabsorbing the digested material.