Is A Starfish A Crustacean? (Explained)
Starfish are fascinating marine that we often call starfish, but they’re actually not fish, and for that reason, scientists prefer to call them sea stars. But do you wonder what animals they are and if they’re crustaceans? In this post, we’ll talk all about that but let’s start with a quick answer:
Starfish is not a crustacean but an Echinoderm, belonging to the phylum Echinodermata. There are many differences between them, such as their body symmetry, skeleton, feet, eating habits, breathing, circulatory system, and more.
However, this certainly doesn’t tell the whole story. Below I’ll explain more about each phylum, which in biology is a classification level just below the kingdom. Furthermore, I’ll explain the main differences between them, so you’ll notice the significant difference between starfish and the crustaceans. Read on!
Phylum Echinodermata (or Echinoderms) is a phylum that contains about 7,000 species. Interestingly, the word “echinoderms” in Greek means “spiny skin,” describing animals’ bodies covered in thin spines. Echinoderms include animals like sea stars, sand dollars, sea urchins, brittle stars, sea cucumbers, and crinoids.
They characterize not only by their spiny skin but also by the radial symmetry that allows them to sense food and predators. They don’t have heads or brains, but they have nerve rings and cells that respond to light.
All echinoderms live in the marine waters and cannot survive in freshwater or on land.
Phylum Crustacea is actually a subphylum of phylum Arthropoda. It contains more than a million different species of arthropods, making them the largest group of animals. Interestingly, the word “Arthropoda” means “jointed foot,” describing animals’ appendages such as claws, legs, antennae, and pincers.
Crustaceans include animals like crabs, shrimp, crayfish, pill bugs, barnacles, krill, remipedes, isopods, and more. Their characteristic antennae and claws are used to catch and crush food and move around the seafloor.
Most animals that belong to crustaceans live in the marine or freshwater, but some can live in moist environments on land, such as terrestrial hermit crabs, or pill bugs.
The difference between starfish and crustaceans
Now let’s talk about a few differences between these two groups of animals.
Starfish and other members of the phylum Echinodermata are radially symmetrical, meaning all their appendages originate outward from a central point. Most sea stars have five arms, but many species have much more. For instance, the sunflower sea star usually has 16 to 24 arms.
Crustaceans are bilaterally symmetrical, meaning all their appendages are arranged on opposite sides of the skeleton. They have more than four pairs of jointed appendages (their “legs”).
Sea stars have hundreds or thousands of tube feet on the bottom of their body. They’re using them in a rowing motion and move very slowly. Furthermore, they have a small suction cup at the end of each foot, which can attach to objects like rocks or corals.
Most crustaceans have actual legs, for example, crabs, and can walk either forward or sideways. Other animals, such as barnacles, can’t move, attaching themselves to their host instead.
Starfish have an internal skeleton beneath their skin (called endoskeleton), made of many small carbon carbonate plates called ossicles. Read more about it in my other article, “Do Starfish Have Bones?”. Interestingly, some echinoderms’ skeleton includes teeth, such as sea urchins, but starfish don’t have them.
In contrast to echinoderms, crustaceans have an external skeleton (called an exoskeleton or shell). These shells also consist of protein and calcium carbonate. Similarly to echinoderms, some crustaceans have teeth, such as lobsters or crabs, but some don’t.
The eating habits of these two groups are very different because of starfish’s unique way of eating. Starfish eat by extending their stomach out of their mouths. Next, they wrap it around their prey and digest enzymes directly. They mostly est small-moving animals, including some crustaceans.
Crustaceans have different feeding habits, but they depend on the species. For instance, crabs capture their prey using their claws and put it in their mouths. Other crustaceans have mouthparts with hair-like structures.
Sea stars, like other echinoderms, don’t have gills or lungs. Starfish breathe through their skin using their papulae (also called skin gills) or tube feet. They absorb the oxygen directly from the seawater and exchange gases through diffusion. They cannot breathe and survive outside of the water.
Crustaceans, however, use the same respiratory system as fish and breathe through gills. Interestingly, they can also breathe outside water if they keep their gills moist. All they have to do is take a quick dip, and they can crawl around the shore.
Both starfish and crustaceans have an open circulatory system, but there’s a significant difference between them.
Unlike crustaceans, starfish don’t have blood or a heart in their bodies. Instead, they use a water vascular system to pump the seawater through their bodies. When the water enters, it exchanges gases and nutrients but also plays an important role in starfish movements, catching food, and attaching to the rocks.
Crustaceans, on the other hand, have blood and a heart. Because of the open circulatory system, their blood is not contained within vessels, but instead, the blood goes into the heart through holes called ostia. Next, the heart pumps it out to circulate through the tissues and eventually return to the heart again.
Another significant difference between starfish and crustaceans is that starfish don’t have brains. They have a nerve ring that circulates their mouth instead. The ring connects radial nerves running the length of each arm of the starfish that can react to the stimuli like light, smell, or can possibly feel pain.
Crustaceans, however, do have a brain called a supraesophageal ganglion. The brain connects with the ventral nerve cord of ganglia, or nerve centers.
Is a starfish a mollusk?
As you probably already know, starfish are not mollusks because they belong to the phylum Echinodermata. Mollusks belong to a separate phylum called phylum Mollusca.
Mollusks are soft-bodied invertebrates, and unlike starfish, they have bilateral symmetry. The word “mollusk” means “soft” which describes their characteristic bodies very well. There are more than 110,000 species of mollusks, and they live either in the water or on land.
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