Sea stars, commonly known as starfish, are marine invertebrates belonging to the class Asteroidea within the phylum Echinodermata. They are exclusively found in saltwater environments.
Diverse Marine Habitats
Sea stars inhabit a wide array of marine environments, showcasing remarkable adaptability to different ecological zones. Many species thrive in the intertidal zone, the area along coastlines exposed to air during low tide and submerged during high tide. Here, they can be found clinging to rocky shores, nestled in tide pools, or sometimes in mudflats and sand beaches. These species have evolved to endure the fluctuating conditions of this dynamic habitat, including changes in salinity and temperature, as well as wave action.
Moving deeper into the ocean, sea stars are abundant in the subtidal zone, which remains continuously submerged. This includes the shallow ocean floor, where they navigate sandy or muddy substrates. Some sea stars are also prominent residents of vibrant coral reefs, finding shelter among the structures and preying on other invertebrates. Beyond coastal areas, sea stars extend their reach to the deep sea, inhabiting abyssal depths down to 6,000 meters (20,000 feet) or more. These deep-sea species often adapt to environments with minimal light and extreme pressure, relying on detritus or other deep-sea organisms for sustenance.
Global Distribution
Sea stars exhibit a widespread global distribution, inhabiting all the world’s oceans, including the Atlantic, Pacific, Indian, Arctic, and Antarctic regions. This extensive geographical spread means they can be found in a broad range of temperatures, from warm tropical waters to cold polar seas.
The highest diversity of sea star species is often observed in the Indo-Pacific and Atlantic regions. While some species, like the Northern Pacific sea star, are native to specific areas such as the coasts of northern China, Korea, and Japan, others have become introduced species in new oceanic areas, such as southern Australia. The ability of different species to tolerate varying conditions allows them to occupy nearly every marine biome on Earth.
Environmental Preferences
The presence of sea stars in specific locations is largely dictated by their environmental preferences, which are crucial for their survival, feeding, and reproduction. Salinity is a primary factor. While most species prefer consistent oceanic salinity, some, like Asterias rubens or Asterias amurensis, can tolerate a range of salinities and may even survive in estuarine waters where freshwater mixes with saltwater.
Temperature also plays a significant role in their distribution. Some sea star species are adapted to cold waters, such as the polar six-rayed star found in the northwest Atlantic and Arctic regions, while others thrive in warm, tropical environments. For instance, the Northern Pacific sea star prefers cooler temperatures. Water depth influences their habitat choices, with deep-sea species adapted to high pressure and low light conditions.
The type of seafloor substrate is another important environmental consideration for sea stars. Some species prefer rocky surfaces, which allow them to cling using their tube feet, while others inhabit sandy or muddy bottoms where they can burrow or find prey. For example, the sunflower sea star can be found on various substrates, from rocky kelp forests to sand and mud flats. These specific environmental conditions collectively shape where different sea star species can successfully establish and maintain populations.