Which of these is a fish: starfish or jellyfish?

While both starfish and jellyfish have the word "fish" in their names, neither organism is actually a fish in the biological sense. This can be surprising at first, especially because common names often don’t reflect scientific classifications. To determine which of the two is a true fish, we need to look at what defines a fish and compare that with the characteristics of starfish and jellyfish.

A fish, in biological terms, is a vertebrate animal that lives in water, has gills throughout its life, and usually has fins and scales. Fish are members of the subphylum Vertebrata under the phylum Chordata, meaning they possess a backbone or spinal column. Examples of true fish include salmon, trout, tuna, and goldfish. These animals breathe through gills, swim using fins, and have a skeleton made of either bone or cartilage.

Now, let’s examine the starfish. Despite its name, a starfish is not a fish at all. It belongs to the phylum Echinodermata, which includes sea urchins and sea cucumbers. Starfish, more correctly called sea stars, are marine invertebrates. They do not have a backbone, gills, or fins. Instead of blood, they have a water vascular system that helps them move and feed. Starfish typically have five arms (though some species have more), and they move slowly using tiny tube feet on the underside of their arms. They have no centralized brain but have a nerve ring that coordinates their movements. Their body symmetry is radial, which is very different from the bilateral symmetry seen in true fish.

Similarly, the jellyfish is also not a fish. It is a member of the phylum Cnidaria, which includes sea anemones and corals. Jellyfish are soft-bodied, gelatinous creatures with a bell-shaped, umbrella-like body and tentacles that can contain stinging cells (called cnidocytes) used to capture prey. They have no bones, no brain, and no heart. Instead of a complex nervous system, jellyfish have a simple nerve net that allows them to respond to stimuli. Like starfish, jellyfish are invertebrates and do not possess the basic anatomical features that define true fish.

In conclusion, neither the starfish nor the jellyfish qualifies as a true fish. Both are invertebrates—animals without backbones—and belong to entirely different phyla than true fish. The use of the term "fish" in their names is a remnant of older naming practices based more on appearance or habitat than scientific accuracy. The correct answer to the question "Which of these is a fish: starfish or jellyfish?" is: neither is a fish. If we go strictly by scientific classification, neither organism meets the criteria that would place them in the same group as trout, tuna, or goldfish. Understanding these distinctions is an excellent reminder of the importance of scientific naming and classification systems, which help us accurately understand the diversity of life in our oceans.