Sea stars are marine invertebrates belonging to the class Asteroidea, not actual fish. They are members of the phylum Echinodermata, sharing characteristics like radial symmetry with sea urchins and sea cucumbers. The approximately 1,900 known species exhibit remarkable diversity in size, color, and form, reflecting the vast range of habitats they occupy across the world’s oceans. Their presence spans from the warm, sunlit waters of the tropics to the frigid pressures of the abyssal plains.
Global Distribution and Fundamental Environmental Needs
Sea stars are truly cosmopolitan, inhabiting every one of the world’s major oceans, including the Pacific, Atlantic, Indian, Arctic, and Southern Oceans. This extensive global presence is strictly confined to marine environments because sea stars are obligate marine organisms. They lack the necessary internal mechanisms to regulate their body’s salt concentration, meaning their body fluids must remain at the same salinity level as the surrounding seawater.
This physiological constraint means sea stars are not found in freshwater and are rare in brackish estuarine environments, where salinity can drop rapidly. While most species thrive in typical oceanic salinity of around 35 parts per thousand (‰), some populations tolerate ranges between 12‰ and 38‰. Sea stars are fundamentally benthic animals, meaning they live on the ocean floor, utilizing a variety of substrates.
They can be found clinging to rocky surfaces, burrowing into soft mud or sand, or crawling across complex structures like coral reefs and kelp forests. Temperature tolerance is also broad across the class, ranging from species that flourish in tropical waters to those adapted to the near-freezing temperatures of polar regions.
Starfish of the Intertidal and Shallow Coastal Zones
The intertidal zone is a highly dynamic and challenging habitat for sea stars, yet it is where they are most commonly observed. Sea stars in this zone must contend with rapid fluctuations in temperature, salinity, and moisture, alongside the physical force of wave action. The ochre sea star (Pisaster ochraceus) of the North Pacific is a prominent example, thriving in this high-energy environment.
These animals have evolved specialized adaptations to survive the harsh conditions, particularly on rocky shores. Their hundreds of tube feet, powered by a hydraulic water vascular system, generate strong adhesion, allowing them to cling securely to rock faces and resist the pulling force of crashing waves. When the tide recedes, their tough, leathery outer skin helps prevent desiccation, and some species can tolerate air exposure for many hours.
In shallow coastal areas with softer substrates, such as sand or mud flats, certain sea star species have different survival strategies. These environments, often found in sheltered bays, offer less wave energy but require methods for concealment. Species like the mud star (Ctenodiscus crispatus) are adapted to soft sediments, where they burrow just beneath the surface to find food and avoid predators.
Starfish in Deep-Sea and Specialized Environments
In stark contrast to the turbulent coastal zones, sea stars are also found in the stable, low-energy environments of the deep sea. They inhabit the seafloor from the continental shelf down to the abyssal plains and deep ocean trenches, reaching depths exceeding 6,000 meters. The environmental conditions here are defined by perpetual darkness, near-freezing temperatures, and immense hydrostatic pressure.
Starfish species adapted to these depths, such as Albatrossaster richardi, belong to groups that are primarily burrowers. Some have tube feet that lack the suckers necessary for clinging to hard surfaces, instead using them for propulsion on soft sediment. Their slow-moving, stable lifestyle contrasts sharply with the adaptations needed for the intertidal zone.
Specialized Deep-Sea Habitats
Specialized deep-sea habitats host unique sea star populations. These include deep-water coral ecosystems, which provide complex three-dimensional structure on the flat abyssal plain. Some species are also found near hydrothermal vents, where they utilize a unique food web based on chemosynthesis rather than sunlight.