Starfish

Starfish, or sea stars, are marine invertebrates belonging to the class Asteroidea. These animals are found in every ocean on Earth, from warm tropical zones to frigid polar regions. They live on the seabed at many depths, ranging from shallow intertidal zones to the deep abyssal zones. There are approximately 1,900 known species of starfish. They are characterized by a central disc and arms that radiate outward. While many species possess five arms, others have much higher counts. For example, the Antarctic species Labidiaster annulatus can have over fifty arms.

The anatomy of a starfish is complex and highly specialized. Their body wall consists of several layers, including an epidermis and a thick dermis. Inside the dermis, they have an endoskeleton made of calcium carbonate structures called ossicles. These ossicles are honeycomb-like lattice structures composed of calcite microcrystals. They can appear as flat plates, granules, or spines on the aboral, or upper, surface. Some species possess pedicellariae, which are scissor-like ossicles used to displace organisms or even catch prey. Other structures, like the umbrella-like paxillae, protect delicate gills in species that live buried in the substrate.

A defining feature of the starfish is its water vascular system. This is a hydraulic system of fluid-filled canals used for locomotion, food manipulation, and gas exchange. Water enters the system through the madreporite, a sieve-like structure on the upper surface. From there, water moves through a stone canal into a ring canal around the mouth. Radial canals then branch into each arm, leading to tiny bulb-shaped organs called ampullae. When the muscles in the ampullae contract, water is pushed into the tube feet, or podia. These tube feet use adhesive chemicals rather than suction to grip the seafloor. This allows the starfish to move in a wave-like motion across the substrate.

The digestive system of a starfish allows them to be opportunistic predators. Most starfish feed on benthic invertebrates. Some primitive species, like Astropecten, swallow prey whole and digest it in a cardial stomach. In more advanced species, the starfish can perform stomach eversion. This means they push their cardiac stomach out through their mouth to engulf and digest food externally. Once the food is partially digested, it moves into the pyloric stomach. Here, long tubes called pyloric caeca secrete enzymes and absorb nutrients. This specialized process allows them to consume prey that might be difficult to swallow.

Starfish possess a unique way of sensing their environment without a centralized brain. Instead, they have a complex nervous system featuring a nerve ring around the mouth and radial nerves in each arm. They can perceive touch, light, temperature, and chemical changes in the water. At the tips of their arms, they have eyespots containing simple ocelli, which are light-sensitive cells. Their tube feet and spines are also highly sensitive to touch and chemicals. This chemical sensitivity allows them to detect the scent of food sources. If one arm detects an attractive stimulus, it can temporarily override the other arms to direct movement.

The history of starfish is reflected in an ancient but sparse fossil record. Evidence suggests they have existed since the Ordovician period, about 450 million years ago.

Finding these fossils is difficult because starfish tend to disintegrate after they die. Most fossils consist only of hard parts like spines or ossicles. Despite their ancient history, starfish remain vital to modern ocean ecosystems. Some, such as the ochre sea star, act as keystone species. This means they have an outsize impact on the structure of their environment. Others, like the crown-of-thorns starfish, can be destructive predators of coral reefs.

Starfish are also known for their incredible ability to undergo regeneration. Many species can regrow damaged parts or even entire lost arms. They can also shed arms voluntarily as a method of defense against predators. This biological resilience is paired with diverse reproductive strategies, as they can reproduce both sexually and asexually. Because of their symmetrical and appealing shapes, they have also entered human culture. They are frequently used in art, design, and logos. In some cultures, they are even collected as curios or eaten.