Starfish are one of the most ecologically powerful animals in the ocean. They regulate the populations of other marine species, maintain biodiversity on rocky shores and coral reefs, contribute to nutrient cycling on the sea floor, and even serve as valuable models for human medical research. Their importance stretches from the tide pools where they hunt to the genetics labs where scientists study their remarkable biology.
The Original Keystone Species
The concept of a “keystone species,” an organism whose influence on an ecosystem is far greater than its size or numbers would suggest, was literally invented to describe a starfish. In the 1960s, ecologist Robert Paine removed the ochre sea star from intertidal areas along the Washington coast and watched what happened. Without the starfish, mussels took over, crowding out other organisms, and the overall diversity of species in the area dropped sharply.
The ochre sea star keeps mussel populations in check by eating them, which frees up space on rocks for algae, barnacles, anemones, and dozens of other species. Remove the starfish, and the entire community collapses into a monoculture. This dynamic plays out along rocky coastlines throughout the Pacific, where starfish act as biological regulators that prevent any single species from dominating.
Keeping Coral Reefs in Balance
On coral reefs, starfish play a dual role that highlights just how sensitive marine ecosystems are to population shifts. The crown-of-thorns starfish feeds on coral, and at normal densities of roughly one or fewer per hectare, it actually promotes reef diversity by grazing on fast-growing corals and giving slower species room to establish. But when populations explode to 10 or even 1,000 per hectare, the same species becomes one of the most destructive forces on a reef, stripping coral cover and reducing both species and functional diversity across the entire ecosystem.
These outbreaks appear to be linked to overfishing of the starfish’s natural predators. Research on Australia’s Scott Reefs and Rowley Shoals found that common reef fish like snappers and emperors include starfish in their diets, but only when those fish populations are healthy. At fished reefs where predator numbers were low, the same fish species shifted their diets away from bottom-dwelling invertebrates. Protecting reef fish populations, in other words, helps keep starfish populations in check, which in turn protects the coral.
Recycling Nutrients on the Sea Floor
Starfish are active predators of mussels, clams, snails, and other shellfish. When they digest these prey, they break down calcium-rich shells and recycle those minerals back into the marine environment. This matters because calcium carbonate is a building block for the shells and skeletons of countless other ocean organisms.
Starfish also contribute through bioturbation, the physical mixing and reworking of ocean sediment as they move and burrow. This process oxygenates deeper sediment layers, redistributes organic material, and affects the nutrient cycling that supports microbial communities and other bottom-dwelling organisms. It’s not glamorous work, but it keeps the sea floor functioning as a living system rather than a stagnant one.
A Window Into Human Medicine
Few animals regenerate as dramatically as starfish. They can regrow entire limbs, and some species can regenerate a whole body from a single arm. That ability has made them a focus of medical research, particularly for conditions where the human body falls short: spinal cord injuries, neurodegenerative diseases, and trauma recovery.
Researchers studying nerve regeneration in starfish have found that when nerve tissue is partially removed from an arm, the animal initially loses coordination and mobility in that limb, then gradually recovers motor function as the nerve tissue regrows. The molecular signals driving that regrowth share similarities with human neural biology. One regulatory gene involved in starfish nerve repair also plays a role in maintaining human embryonic and neural stem cells. Because starfish are relatively close to vertebrates on the evolutionary tree (closer than insects or worms), these parallels are more than superficial, making starfish genuinely useful models for understanding how nerve repair might someday be triggered in humans.
Unlocking the Basics of Organ Development
Beyond regeneration, starfish larvae have become important tools for understanding how organs form in the first place. A study published in Nature Communications used the bat star to reveal the early stages of tube formation, the process by which simple tissue folds into the tubular structures that become organs like intestines, kidneys, and lungs in vertebrates. Using gene-editing tools, researchers identified a gene called Six1/2 as a key regulator of how tubes branch during development.
The finding that stood out: the cellular mechanism starfish use to build tubes, where cells proliferate and migrate simultaneously, is the same mechanism vertebrates use. This means the basic toolkit for organ development was already established hundreds of millions of years ago, at the evolutionary root of the lineage that eventually produced humans. The lead researcher noted that the same gene could eventually be used to study how organs develop cancer and how tumors become metastatic, potentially opening new avenues for cancer research within the next decade.
What Happens When Starfish Disappear
The consequences of losing starfish are not theoretical. Starting in 2013, sea star wasting disease swept through populations along the Pacific coast of North America, causing animals to literally dissolve. The sunflower sea star, a massive species capable of sprouting 24 arms and growing to the size of a bicycle tire, lost over 90 percent of its population. It is now listed as critically endangered by the International Union for Conservation of Nature and is protected under the U.S. Endangered Species Act.
Without sunflower stars, which are voracious predators of sea urchins, urchin populations in some areas have boomed. Unchecked urchins devour kelp forests, transforming rich underwater ecosystems into barren “urchin barrens” with little biodiversity. The cascade is a textbook example of what happens when a keystone predator is removed: the effects ripple outward through the entire food web.
Recovery has been slow and uneven. Populations in the Pacific Northwest and northern California are trending upward, with some sites recording the highest juvenile counts in over 20 years of monitoring. Oregon and northern California appear most promising. But southern California populations, already small before the outbreak, have seen poor recruitment and a grim outlook for recovery. The disease itself has not disappeared. It persists at low levels almost everywhere, with wasting symptoms appearing primarily in fall and early winter each year. Nearly all sightings of sunflower stars in Oregon and California have been juveniles, a hopeful sign that reproduction is occurring, though mature adults remain scarce.