In December 2020, the IUCN listed the sunflower sea star, Pycnopodia helianthoides, as critically endangered. This species, which can measure nearly one meter in diameter, was common from Alaska to Mexico. It has lost more than 90% of its population in just a few years.
This figure says a lot about an animal we think we know because we have seen it on a postcard. So we completely revised our old article. The unverifiable figures are gone, and recent research has arrived.
Here is what we know. And what we still don’t know.
How many species of sea stars are there?
There are nearly 1,900 current species of sea stars, spread across 36 families and about 370 genera, according to the census by Christopher Mah and Daniel Blake published in 2012 in PLoS ONE based on the global asteroid database from the WoRMS registry. They form the class Asteroidea, within the echinoderms.
No relation to fish.
A sea star has neither a backbone nor fins. Its closest relatives are sea urchins and sea cucumbers. Its skeleton is made up of small calcareous plates embedded in the skin, which explains its rough texture to the touch. But the group occupies all ocean levels, from the tidal zone down to about 6,000 meters deep, according to the same study. The richest family is the Goniasteridae. Many species still await description.
Our previous version mentioned 40 families and 350 genera: we aligned the figures with the reference publication, which remains more honest than a total copied from site to site without date or author.
How does a sea star live without a brain?
A sea star does not have a centralized brain. A nerve ring around the mouth, connected to a nerve running along each arm, coordinates its movements, and a network of canals filled with seawater powers hundreds of small tube feet located under the arms.
This hydraulic circuit is called the water vascular system. It propels the animal forward and keeps it attached to the rock. The tube feet, or podia, adhere to the surface by a chemical secretion and then detach on demand. At the tip of each arm, a small rudimentary eye, the ocellus, perceives light and large shapes like a reef face. That is enough to orient itself. To eat, many species push their stomach out through the mouth and digest the prey on site, outside the body.
Yet, the animal has no fixed front or back, since any arm can lead the movement.
Jellyfish, another group without a brain, pose the same kind of puzzle, and we gathered some examples in our 10 amazing things to know about jellyfish. The comparison ends there.
Can a sea star really regenerate?
Yes, most sea stars can lose an arm and regrow it. Some go much further: in Linckia, a detached arm can regenerate an entire animal, as documented by the Florida Museum of Natural History with its “comet” stars, a long arm followed by four small arms growing back.
The animal can also drop an arm itself when a predator grabs it. This is called autotomy. This reflex is among the defense mechanisms of marine animals we have listed elsewhere. In most species, a fragment must retain part of the central disk to survive. However, regrowth is energy costly, and for months part of the reserves go into the new arm rather than growth or reproduction.
Its duration varies depending on the species, temperature, and food.
So we will not give a universal timeframe, lacking reliable figures that apply to 1,900 species. The beautiful time-lapse videos show a result, rarely the real time.
What is sea star wasting disease?
Sea star wasting disease is an epidemic that appeared in 2013 on the Pacific coast of North America. Sick animals develop lesions, lose their arms, then disintegrate. Several species were affected, and the sunflower star paid the heaviest price.
A study conducted by Oregon State University and The Nature Conservancy, published in December 2020, compiled more than 61,000 surveys from 31 datasets. Measured decline: 90.6%. Approximately 5.75 billion individuals are believed to have died. The IUCN classified the species as critically endangered the same year. In the United States, NOAA proposed in March 2023 to list it as a threatened species under the Endangered Species Act, and its file still presented it as “proposed” when consulted on October 3, 2026.
And the cause long resisted researchers.
In August 2025, a team from the Hakai Institute, the University of British Columbia, and the University of Washington identified in Nature Ecology & Evolution a bacterium, Vibrio pectenicida (strain FHCF-3), as the agent of the disease in the sunflower star. NOAA reminds that the role of marine heatwaves remains under study. This matters for what comes next. Fewer sunflower stars means more sea urchins, and an overabundance of sea urchins decimates kelp forests.
Why is the crown-of-thorns starfish hunted on the Great Barrier Reef?
The crown-of-thorns starfish, or acanthaster, is a starfish that feeds on living coral. During outbreaks, it can devastate large areas of reef. That is why the Great Barrier Reef Marine Park Authority conducts control campaigns, by direct injection of substances such as vinegar.
According to the GBRMPA, it can reach nearly one meter in span. Its spines are venomous. We detailed the method in our article on vinegar against the crown-of-thorns starfish, this coral-eating starfish. The contrast with the sunflower star is striking. On one side, a species we eliminate to save the coral; on the other, a species whose absence damages kelp forests and that we seek to bring back.
However, these control campaigns address the urgency without resolving the origin of outbreaks, whose causes remain debated by researchers and likely vary from one reef sector to another.
The Great Barrier Reef simultaneously endures bleaching episodes, and the crown-of-thorns starfish adds additional pressure on already weakened reefs. Which of the two weighs more? Impossible to decide from here.
Can you touch a starfish while diving or snorkeling?
The safest for the animal is not to touch it and never take it out of the water. It breathes partly through tiny skin outgrowths, and repeated handling exposes it to stress it would very much prefer to avoid.
If your dive club mentions it at the briefing, that is the instruction to follow. In practice, if a starfish is in your path, stabilize your buoyancy and keep your distance. The tidal flats exposed at low tide attract families. That is where starfish most often end up in a bucket or a child’s hand, for a photo that lasts much longer than expected. We gathered the arguments in why you should never touch marine life.
The photo is enough.
Thus, the real cost of good practice is just a few seconds of patience, a bit of stabilization, and a photo taken without contact, which does not change your dive.
FAQ
Is a starfish a fish?
No. It is an echinoderm, a cousin of sea urchins and sea cucumbers. It has neither a backbone nor fins.
How long does a starfish live?
It depends greatly on the species, and solid data are rare. We removed from our previous version the figure of 35 years, due to lack of verifiable source.
Does a starfish cut in two give two starfish?
Sometimes, depending on the species and the cut. Generally, a fragment must retain part of the central disc to survive, except in genera like Linckia, where a single arm can regenerate. Never test this on a wild animal.
Why is the sunflower starfish critically endangered?
An epidemic of wasting disease that appeared in 2013 caused its population to drop by about 90%. The IUCN classified it as critically endangered in 2020. A bacterium, Vibrio pectenicida, was identified as the disease agent in 2025.
Sources
- Sunflower Sea Star (NOAA Fisheries, fact sheet accessed in 2026)
- Sea star listed as critically endangered following research by Oregon State University (Oregon State University, 2020)
{“@context”:”https://schema.org”,”@type”:”FAQPage”,”mainEntity”:[{“@type”:”Question”,”name”:”Une étoile de mer est-elle un poisson ?”,”acceptedAnswer”:{“@type”:”Answer”,”text”:”Non. C’est un échinoderme, cousin des oursins et des holothuries. Elle n’a ni colonne vertébrale ni nageoires.”}},{“@type”:”Question”,”name”:”Combien de temps vit une étoile de mer ?”,”acceptedAnswer”:{“@type”:”Answer”,”text”:”Ça dépend beaucoup de l’espèce, et les données solides sont rares. On a retiré de notre ancienne version le chiffre de 35 ans, faute de source vérifiable.”}},{“@type”:”Question”,”name”:”Une étoile de mer coupée en deux donne-t-elle deux étoiles ?”,”acceptedAnswer”:{“@type”:”Answer”,”text”:”Parfois, selon l’espèce et la découpe. En général, un fragment doit garder une partie du disque central pour survivre, sauf chez des genres comme Linckia, où un bras seul peut repartir. Ne le teste jamais sur un animal sauvage.”}},{“@type”:”Question”,”name”:”Pourquoi l’étoile de mer tournesol est-elle en danger critique ?”,”acceptedAnswer”:{“@type”:”Answer”,”text”:”Une épidémie de dépérissement apparue en 2013 a fait chuter sa population d’environ 90 %. L’UICN l’a classée en danger critique en 2020. Une bactérie, Vibrio pectenicida, a été identifiée comme agent de la maladie en 2025.”}}]}
Photo: Frédéric Ducarme, Wikimedia Commons, CC BY-SA 4.0
