Sea stars are marine invertebrates belonging to the class Asteroidea within the phylum Echinodermata. They are not fish, but are defined by their radial symmetry and a unique water vascular system that assists in movement and feeding. With over 1,900 accepted living species, this diverse group is found in all the world’s oceans, indicating broad adaptability. The specific habitat is determined by depth, the physical composition of the seafloor, and the chemical parameters of the water.
Global Presence and Depth Zones
Sea stars inhabit a vast global range, extending from the frigid polar regions to the warm, tropical waters of the equator. Their vertical distribution is equally expansive, colonizing the entire ocean floor from the water’s edge down to the deepest trenches. This distribution is often categorized into three major zones, each presenting distinct environmental challenges.
The intertidal zone, exposed to air during low tide, requires species like the common sea star (Asterias rubens) to endure dramatic fluctuations in temperature, salinity, and wave action. Moving offshore, the subtidal or neritic zone encompasses the shallow coastal waters over the continental shelf, where light penetration is high and food is abundant. This zone supports the highest density and diversity of sea stars, thriving in the stable, nutrient-rich environment.
Below this, the abyssal zone represents the deep sea, beginning at depths of approximately 4,000 meters and extending to over 6,000 meters. Starfish species adapted to this environment, such as those in the family Porcellanasteridae, must withstand immense pressure and total darkness. Deep-sea species often exhibit specialized traits, including the ability to burrow into soft mud and relying on organic material that drifts down from the surface.
Preferred Substrate Environments
The physical material of the seafloor, known as the substrate, is a primary factor in determining a sea star’s specific habitat and feeding strategy. Rocky shores and consolidated pavements are favored by active predators, which use their powerful tube feet and suckers to cling firmly against waves and currents. The ochre sea star (Pisaster ochraceus), a well-known species of the North American Pacific coast, inhabits this hard substrate, preying on mussels and barnacles.
Conversely, many species are highly adapted to soft substrates, such as sandy and muddy bottoms. Sand-sifting sea stars have specialized tube feet that allow them to glide across and burrow just beneath the surface to consume detritus and small invertebrates. Species like the mud star (Ctenodiscus crispatus) are abundant on soft mud bottoms worldwide, functioning as deposit feeders by ingesting the sediment.
Coral reefs host specialized sea stars, including the crown-of-thorns starfish (Acanthaster planci), which feeds directly on coral polyps. Studies show that its locomotion rate is significantly higher when moving across sandy substrates than when navigating complex structures like coral rubble or consolidated pavement. This suggests that the ease of movement across a substrate directly influences their foraging patterns and distribution within the reef system.
Essential Water Conditions
Starfish are almost exclusively marine organisms, and their physiology is highly dependent on stable water chemistry, particularly salinity and temperature. They lack the sophisticated osmoregulation systems found in many other marine animals, meaning they cannot tolerate wide swings in salt concentration. Consequently, they are rarely found in brackish estuaries or freshwater environments, as a salinity drop can quickly become lethal.
The salinity range for most sea stars is close to that of normal seawater, though some intertidal species, such as Asterias rubens, can temporarily tolerate a wider range, sometimes down to 16 parts per thousand (‰). Temperature stability is equally important, as these invertebrates are sensitive to thermal changes. While many tropical species thrive in a narrow range of 24–26°C, temperate species exhibit a much broader tolerance, surviving in water ranging from 0°C to 25°C.
Maintaining water quality is also paramount, as sea stars are vulnerable to contaminants and require sufficient levels of dissolved oxygen. The water’s pH level must remain stable, generally within the alkaline range of 8.1 to 8.4, to support their metabolic processes and the integrity of their calcium carbonate endoskeleton. Fluctuations in these chemical parameters can be detrimental, illustrating the specific environmental constraints that define sea star habitats.