Despite their common name, jellyfish are not actually fish. The term “jellyfish” can be misleading, as these marine animals belong to a completely different biological group than true fish. This article clarifies the biological differences that distinguish jellyfish from fish.
Understanding Jellyfish
Jellyfish are marine invertebrates, meaning they lack a backbone. They belong to the phylum Cnidaria, a group that also includes sea anemones and corals. Their bodies are primarily composed of a gelatinous substance called mesoglea, making them about 95% water. This gives them their characteristic translucent, jelly-like texture and umbrella-shaped bell.
Jellyfish move through the water by pulsating contractions of their bell, though their horizontal movement is largely dependent on ocean currents. They possess a simple nerve net for sensing their environment and coordinating movements, but they lack a centralized brain, complex organs, or a heart. Their tentacles are armed with specialized stinging cells called nematocysts, which they use to capture prey and for defense.
Understanding Fish
Fish are aquatic vertebrates, meaning they possess a backbone. They belong to the phylum Chordata and are characterized by features adapted for life in water. Most fish have a streamlined body shape, which helps them move efficiently through water.
A defining characteristic of fish is the presence of gills, which they use to extract oxygen from water. They also have fins, used for movement, balance, and steering, along with a body covered in scales for protection. Fish have a more complex internal anatomy compared to jellyfish, including a brain, heart, and a circulatory system.
Why Jellyfish Are Not Fish
The fundamental difference between jellyfish and fish lies in their biological classification. Fish are vertebrates with a vertebral column, while jellyfish are invertebrates lacking this internal skeletal structure. This distinction is central to how their bodies are organized and function. Fish use specialized fins for propulsion and steering, whereas jellyfish move by rhythmically contracting their bell-shaped bodies.
Respiration also highlights a difference; fish breathe using gills to extract oxygen from water, while jellyfish absorb oxygen directly through their body membranes. Fish have complex organ systems, including a brain, heart, and circulatory system, which are absent in jellyfish. Jellyfish rely on a simple nerve net for sensory input and coordination.