Yes, a starfish, more accurately known as a sea star, is indeed an animal. This marine invertebrate belongs to the phylum Echinodermata, placing it firmly within the animal kingdom. While its common name might suggest otherwise, sea stars are distinct from fish and possess a unique biology that sets them apart.
What Makes an Organism an Animal?
Animals are broadly defined by several fundamental characteristics that distinguish them from other life forms like plants and fungi. All animals are multicellular organisms. They are also heterotrophic, obtaining their nutrition by consuming other organisms.
Animal cells are eukaryotic, possessing a nucleus and other membrane-bound organelles, but they uniquely lack cell walls, which are present in plants and fungi. Most animals exhibit motility at some stage of their life cycle. These characteristics collectively define the diverse group of organisms classified as animals.
Starfish: Meeting the Animal Criteria
Sea stars fully meet the criteria that define an animal. They are multicellular organisms with specialized cells and tissues. Like other animals, sea stars are heterotrophic, acquiring nutrients by consuming other organisms. They are primarily carnivores, preying on mollusks, crustaceans, and even small fish, or acting as scavengers.
Sea star cells are eukaryotic, containing a nucleus and other organelles, and they do not possess cell walls. Despite their often-stationary appearance, sea stars are motile creatures. They move using hundreds of small tube feet, which allow them to crawl across the seabed.
The Unique Biology of Sea Stars
Sea stars belong to the phylum Echinodermata, a name derived from Greek words meaning “spiny skin,” referring to their often-bumpy or spiny outer surface. A distinctive feature of adult sea stars is their radial symmetry, typically five-fold, with arms radiating from a central disc. While their larvae are bilaterally symmetrical, they undergo metamorphosis to develop this pentaradial symmetry as adults.
Sea stars possess a water vascular system, a hydraulic network of fluid-filled canals used for locomotion, feeding, and respiration. Water enters this system through a sieve-like plate called the madreporite, located on their upper surface, and is then channeled to the tube feet. These tube feet extend and retract through hydraulic pressure, enabling the sea star to grip surfaces and move.
Sea stars exhibit a feeding mechanism where many species can evert their stomach through their mouth to digest prey externally. This allows them to consume prey larger than their mouth opening, with digestive enzymes breaking down tissues before the liquefied food is absorbed.
Sea stars also possess an ability to regenerate lost arms. This regeneration process can take months to over a year to complete.