Starfish, also known as sea stars, are marine invertebrates belonging to the class Asteroidea. They are not fish, but echinoderms, related to sea urchins and sea cucumbers. These creatures are characterized by a central disc and typically five arms, though some species can have many more. They are found exclusively in saltwater environments.
General Marine Environments
Starfish inhabit marine waters across the globe. They are found in all the world’s oceans, from warm, tropical regions to frigid, polar waters. This global presence highlights their adaptability to diverse oceanic conditions.
Starfish primarily live on the seabed, known as benthic animals. They occupy a range of depths, from shallow intertidal zones to the deep-sea floor, reaching down to at least 6,000 meters (20,000 feet). The greatest concentration and diversity of sea star species are found in coastal areas, particularly in temperate and tropical regions. These environments offer a combination of food availability, suitable substrate, and stable conditions.
Specific Habitat Zones
Starfish inhabit various distinct zones within the marine environment. The intertidal zone, the area between high and low tide marks, is a common habitat for many species. Here, starfish often cling to rocky surfaces or seek refuge in crevices and under seaweed during low tide to avoid drying out. They are also found in tide pools, which are depressions that trap seawater when the tide recedes.
Below the low tide mark, the subtidal zone extends to the edge of the continental shelf. Starfish are abundant in these areas, often found on rocky and soft substrates. Coral reefs, complex ecosystems in tropical and subtropical waters, provide shelter and abundant food for various sea star species. Some species, like the Crown-of-thorns starfish, are coral predators that can significantly impact reef health.
The deep sea, including abyssal zones, hosts many starfish species. These deep-sea dwellers are adapted to extreme pressure and perpetual darkness, often relying on organic matter that sinks from the surface. Seagrass beds, found in coastal areas, offer shelter and food for starfish and other invertebrates. These beds are important nursery grounds for many marine species. Kelp forests in temperate coastal waters similarly provide a rich habitat and food source, with sea stars sometimes preying on kelp-eating urchins to maintain the ecosystem’s balance.
Environmental Factors
Starfish are sensitive to the environmental conditions of their habitats. Their strict requirement for saltwater stems from their inability to regulate their internal salt and water balance. They cannot survive in freshwater and are rarely found in estuarine environments where salinity fluctuates widely.
Starfish can tolerate a broad range of temperatures, which contributes to their global distribution. Some species thrive in warm tropical seas, while others are adapted to the frigid waters of polar regions. However, extreme temperature fluctuations can negatively affect their feeding and growth, and some species are vulnerable to high temperatures. For example, some intertidal species can modify their body temperature by absorbing seawater to stay cool.
Starfish inhabit diverse substrate types, including rocky, sandy, muddy, and gravelly seabeds. Their tube feet, equipped with suction cups, allow them to grip tightly to rocky surfaces, resisting wave action. Some species, such as sand-sifting starfish, require soft, sandy substrates where they can burrow to find food.
Water depth and pressure also define starfish habitats. While many species are found in shallow coastal waters, others are adapted to the pressures of the deep ocean, living thousands of meters below the surface. Light levels vary significantly across these depths, from sunlit shallow waters where algae thrive to the complete darkness of the abyssal zones. Deep-sea starfish have adapted to life without sunlight, consuming detritus or preying on other deep-sea organisms.