![[BlueTrend Mini Classroom] Feeling Down? Just Drop an Arm! The Ocean's Most Dramatic Creature — And Its Surprising Enemy: The Seagull!](/_images/60a2cd42ddf1-1440w.webp)
The Editor says: Have you ever seen a creature escape by shedding its own limb? That's exactly what sea stars do! When startled, a sea star will detach one of its arms at the point where it meets the body in order to flee — and the severed arm can then grow into an entirely new sea star. Beyond this remarkable self-repair ability, the sea star's natural predators have been declining in the wild due to fishing pressure, causing sea star populations to grow and potentially tipping the balance of natural ecosystems. A prime example is the Crown-of-Thorns sea star, often called the "coral killer," which has become a long-term focus of scientific concern. Read on and join BlueTrend to discover the wonders of the sea star!
Sea Star Basics
True to its name, the sea star looks just like a star. It is an echinoderm with a body arranged in five-fold radial symmetry. Sea stars possess a complex water vascular system — controlled through a sieve-like plate near the center of their upper surface called the madreporite — which regulates water flow in and out of the body and connects to numerous tiny tube feet on the underside, allowing them to move, absorb water, and grasp food.
The underside reveals the ambulacral grooves lined with tube feet, as well as the mouth. photo credit：李承錄。
Despite their rigid texture, sea stars can move their arms with surprising flexibility, weaving between rocks and reef. The sea star family is enormous — approximately 1,600 species have been identified — and they inhabit environments ranging from tropical to polar waters, from intertidal zones to depths of 6,000 m on the ocean floor.
Sea Stars' Remarkable Regenerative Ability
Sea stars possess extraordinary regenerative powers. A lost arm can regrow rapidly, and remarkably, the detached arm itself can develop into an entirely new sea star. Sea stars cleverly exploit this ability as a defense mechanism: when threatened by a predator, they will deliberately shed an arm to facilitate their escape.
These two sea stars appear to have originated from a single individual that split apart. photo credit：李承錄。
The Sea Star's Feeding System
What we usually see on top of a sea star is actually its anus — the mouth is located on the underside of the body (facing downward).
The sea star's mouth is on its underside! photo credit：李承錄。
Don't let a sea star's small, cute appearance fool you — it is actually carnivorous. Its diet includes coral, crustaceans, other echinoderms, and more.
But how does it eat a tightly clamped bivalve? When hunting shellfish, a sea star uses its tube feet to pry open the two hard shells. It then everts its own stomach directly out of its mouth, enveloping the prey and slowly digesting it with gastric juices secreted on the spot. Once the nutrients are absorbed, digestion continues internally, and any remaining waste is expelled through the anus.
Smaller sea star individuals are usually generated from a detached arm. photo credit：李承錄。
Natural Predators of the Sea Star
Despite their hard exterior, sea stars do have a number of enemies capable of breaking through their defenses. Besides the Harlequin Shrimp — which we introduced previously as a sea star hunter that saws off arms and drags them back to its den for a leisurely meal — large triton's trumpet snails, frog shells, crab, and pufferfish are also known to happily feast on sea stars!
BlueTrend shares: Another predator rarely seen in Taiwan is the seagull, which takes advantage of low tide to swoop down and snatch up sea stars. They stuff their beaks completely full in one go, as if terrified the food will be stolen — a sight that's both entertaining and a little alien-like!
A greedy seagull chomping down on a sea star — doesn't it look like something out of a sci-fi film? photo credit：Edward Palm。
Managing Editor: Jenny Tsai
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