The world’s oceans are home to an incredible variety of sea stars, commonly known as starfish, which display a vast range of shapes and sizes. Over 2,000 different species of these marine invertebrates exist globally, from those no bigger than a coin to giants that command impressive ocean floor real estate. Determining the single “biggest” starfish, however, requires clarifying whether that measurement refers to the widest arm span, the greatest weight, or overall bulk.
The Identity of the Largest Starfish
The species most widely recognized as the largest in terms of sheer bulk and overall mass is the Sunflower Sea Star, scientifically known as Pycnopodia helianthoides. This animal is a massive, multi-armed predator found along the northeastern Pacific coastline. A mature adult of this species can achieve an arm span, measured from arm tip to arm tip, of up to 1 meter (3.3 feet). This expansive size makes it a dominant presence on the seafloor.
In addition to its wide diameter, the Sunflower Sea Star is also the heaviest, with large specimens weighing up to 5 kilograms (11 pounds). Unlike many five-armed sea stars, Pycnopodia helianthoides is characterized by having a high number of limbs, typically possessing between 16 and 24 arms radiating from its central disc.
Ecology and Behavior of the Giant Species
The immense size of the Sunflower Sea Star supports its role as a highly active predator within its environment. It inhabits a wide geographical range, extending from the intertidal zone down to depths of 435 meters (1,427 feet) in the eastern Pacific Ocean, from Alaska to Baja California. Preferred habitats include rocky reefs and kelp forests.
The species is known to be the fastest moving sea star, capable of gliding across the substrate at speeds of up to one meter per minute. This rapid movement is powered by an estimated 15,000 tube feet located on the underside of its numerous arms. Its diet consists primarily of sea urchins, clams, snails, and other invertebrates, which it captures and digests by everting its stomach outside its body to liquefy the prey.
Comparing Other Massive Starfish
The designation of “biggest” becomes complicated when considering other metrics, as other species excel in either maximum diameter or sheer weight. The deepest-dwelling contender for the widest arm span is the slender Midgardia xandaros, which has been recorded with a diameter of up to 1.4 meters (53 inches). This fragile, deep-sea species is extremely thin and has a minimal central disc, making it the longest but not the bulkiest.
Conversely, the bulk title is challenged by the Thromidia catalai. While its diameter only reaches about 60 to 70 centimeters (24 to 28 inches), a single specimen has been documented weighing up to 6 kilograms (13 pounds). This makes it potentially heavier than the Sunflower Sea Star. Size in Asteroidea is typically measured as the distance from the center of the disc to the tip of one arm, which is then often doubled to give the total arm span or diameter.