Habitat and distribution
Echinoderms, such as starfish, have a sensitive internal electrolyte balance that is in balance with sea water, making them impossible to exist in a freshwater environment. All of the world’s waters are home to starfish species. Tropical coral reefs, rocky coastlines, tidal pools, mud, and sand are among the habitats, as are kelp forests, seagrass meadows, and the deep-sea floor down to at least 6,000 meters (20,000 ft). Coastal locations have the largest diversity of species.
Microalgae, sponges, bivalves, snails, and other small invertebrates are all eaten by the majority of species. The crown-of-thorns starfish eats coral polyps, while other starfish species are detritivores, meaning they eat decomposing organic debris and feces. A few are suspension feeders, accumulating phytoplankton; Henricia and Echinaster are frequently seen in close proximity to sponges, benefiting from the water current they generate. Various species have been proven to be able to absorb organic materials from the surrounding water, which could make up a large part of their diet.
Special components can help with feeding and capturing. Pisaster brevispinus, a short-spined pisaster from the West Coast of America, can dig deep into the soft substrate to collect prey with a set of modified tube feet (usually clams). Grasping the shellfish, the starfish uses its adductor muscle to slowly pry apart the prey’s shell, then inserts its everted stomach into the crack to digest the soft tissues. For the stomach to enter, the gap between the valves just needs to be a fraction of a millimetre wide. Cannibalism in young sea stars has been reported as early as four days following metamorphosis.
Impact on the ecosystem
Starfish are important members of their distinct marine ecosystems. They are ecologically significant due to their vast size, various feeds, and ability to adapt to a variety of settings. Robert Paine coined the phrase “keystone species” to describe a starfish called Pisaster ochraceus in 1966. Paine discovered that predation by P. ochraceus was a major role in the diversity of organisms on the low intertidal coastlines of Washington state. When this top predator was removed from a stretch of shoreline in an experiment, it resulted in lesser species variety and the eventual dominance of Mytilus mussels, which were able to outcompete other creatures for space and resources. A study of Stichaster australis on the intertidal coast of New Zealand’s South Island in 1971 showed similar results. S. australis was discovered to have destroyed the majority of a batch of transplanted mussels within two or three months of its installation, whereas mussels in an area where S. australis had been removed expanded rapidly in quantity, overwhelming the area and jeopardizing biodiversity.
The omnivorous starfish Oreaster reticulatus appears to manage the diversity, distribution, and quantity of microorganisms on sandy and seagrass bottoms in the Virgin Islands. These starfish consume silt piles, eliminating surface coatings and algae that have adhered to the particles. Organisms that are sensitive to the disturbance are displaced by others that can quickly recolonize “clean” material. Furthermore, migratory starfish feeding creates a variety of organic matter patches, which may influence the distribution and abundance of creatures that feed on the sediment, such as fish, crabs, and sea urchins.
Starfish can have a harmful impact on ecosystems at times. Coral reefs in Northeast Australia and French Polynesia have been damaged by outbreaks of crown-of-thorns starfish. Coral cover in Polynesia fell dramatically after the arrival of migrating starfish in 2006, plummeting from 50% to under 5% in three years, according to a research. This had repercussions for the entire benthic population as well as reef-feeding fish. Asterias amurensis is one of just a handful invasive echinoderm species. Its larvae most likely came in Tasmania in the 1980s by water spilled by ships. Since then, the species has spread to the point where it now poses a threat to commercially important bivalve populations. As a result, they’re regarded pests, and they’re on the list of the world’s 100 worst invasive species, according to the Invasive Species Specialist Group.
Conspecifics, sea anemones, other starfish species, tritons, crabs, fish, gulls, and sea otters may all prey on starfish. The saponins in their body walls, which have a disagreeable flavor, constitute their first line of defense. Astropecten polyacanthus, for example, has potent toxins like tetrodotoxin in its chemical arsenal, and the slime star can spew forth vast amounts of repelling mucus. They have hard plates and spines as well as body protection. The crown-of-thorns starfish is highly unappealing to prospective predators, with sharp spines, poisons, and sometimes vivid warning colors defending it. Other species line their ambulacral grooves with spines and severely plate their extremities to protect their delicate tube feet and arm tips.
A wasting disease caused by bacteria in the genus Vibrio affects several species on occasion; however, a more widespread wasting illness causing mass mortalities among starfish arises irregularly. The most likely cause of this condition, according to a report published in November 2014, is a densovirus known as sea star-associated densovirus (SSaDV). Orchitophrya stellarum is a protozoan that infects starfish gonads and causes tissue damage. High temperatures are dangerous to starfish. Experiments have showed that when P. ochraceus’ body temperature increases above 23 °C (73 °F), their eating and growth rates drop dramatically, and they die when their temperature climbs to 30 °C (86 °F). When exposed to sunlight by a receding tide, this species possesses a remarkable capacity to absorb seawater to keep cool. To preserve the central disc and critical organs like the stomach, it appears to rely on its limbs to absorb heat.
Marine pollution affects starfish and other echinoderms. The common starfish is regarded as a marine ecosystem bioindicator. According to a 2009 study, P. ochraceus is unlikely to be as negatively affected by ocean acidification as other marine organisms having calcareous skeletons. When the pH is decreased, structures consisting of calcium carbonate are subject to dissolution. P. ochraceus was shown to be somewhat unaffected when exposed to 21 °C (70 °F) and 770 ppm carbon dioxide (far above projected levels in the next century). The nodular form of their bones, which can compensate for a lack of carbonate by producing more fleshy tissue, is most likely responsible for their survival.
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