Protopalythoa variabilis, now Palythoa variabilis, is a western Atlantic zoanthid that forms clusters of upright, gritty polyps on intertidal rock and shallow reefs from the Caribbean to Brazil. As its name says, it is variable in colour and shape. A Brazilian study found its colonies start as females and switch to male function later, a pattern called protogynous hermaphroditism.

It was described in 1898 by James Edwin Duerden, a British zoologist then working at the Institute of Jamaica on the island’s anemones and zoanthids. Like other “protopalys” it has been moved into Palythoa.

How do you identify Palythoa variabilis?

Polyps stand up from a thin shared base instead of being sunk in a thick mat, so colonies look like clumps of short columns with flat, tentacle-fringed discs on top. Colours range through brown, tan, olive and grey. Sand embedded in the column wall gives a rough, gritty feel. In the field it is best told from the mat-forming P. caribaeorum, which often grows alongside it, by growth form.

How does it reproduce?

Boscolo and Silveira sampled tagged colonies every month from June 1996 to June 1997 in the São Sebastião channel, São Paulo, Brazil. They found that both P. variabilis and P. caribaeorum are sequential protogynous hermaphrodites. In P. variabilis, 83% of polyps were fertile, 65.3% of colonies were in the female phase, eggs were released in April and May 1997, and sperm in February and March 1997. Gametes developed more or less continuously.

Most growth is still asexual: polyps bud from the base, and fragments broken off by waves reattach and grow, so a single colony can cover a large patch of rock.

Where does it live?

On rocky shores and shallow reefs in the western Atlantic, including the Caribbean and the Brazilian coast, and on the remote oceanic islands of Trindade and Martin Vaz, more than 1,000 km off the Brazilian coast. It tolerates exposure at low tide, wave surge and the warm, sometimes murky water of reef flats. Brazilian studies have also looked at the zooxanthellae it hosts.

Who was James Edwin Duerden?

The man who named Palythoa variabilis had one of the more unusual careers in marine biology. James Edwin Duerden was born in Lancashire, England, in 1869 and trained at the Royal College of Science in London in the late 1880s. In 1895 he became curator of the museum at the Institute of Jamaica in Kingston. Jamaica gave him easy access to reefs, rocky shores and mangroves, and over the next few years he worked on the island’s caves, fisheries, archaeology and, above all, its corals, sea anemones and zoanthids. His 1898 description of P. variabilis came out of that Jamaican work.

Duerden then moved to the United States, completing a doctorate at Johns Hopkins University in 1900 and teaching at the University of North Carolina and the University of Michigan. He published a long run of papers on how coral polyps build their skeletons and on the early growth of young corals, and he led a Carnegie-funded trip to study corals in Hawai‘i. In 1904 he became the first professor of zoology at Rhodes University in Grahamstown, South Africa, where for several years he was the whole department.

There his research took a sharp turn. South Africa’s ostrich-feather industry was booming, and Duerden studied the breeding and feather growth of ostriches. When the feather market collapsed around the First World War, he moved on to sheep and became a leading authority on wool, devising a scale for the crimp of wool fibres in 1927. He retired from Rhodes in 1932 and died in Leeds in 1937. It is a long road from Jamaican tide pools to wool science, but the habit of close, careful description runs through all of it, and many of the Caribbean zoanthid and anemone names he coined are still in use.

How does a zoanthid larva develop?

For a long time, almost nothing was known about how zoanthid eggs turn into new colonies. The first detailed description came from Russell Babcock and John Ryland, who in 1990 followed the embryos and larvae of a tropical Protopalythoa species from spawning to settlement. Their work is the best guide to what probably happens in P. variabilis as well, although the two species have not been compared directly.

Babcock and Ryland found that the eggs were fertilised in the water and already carried symbiotic algae when they were released. The parent colony hands its algal partners straight to its offspring, so each embryo starts life with a supply of photosynthetic cells instead of having to pick them up from the environment. The embryo divides into a hollow ball of cells, then folds in on itself to form the gut, much as in other cnidarians. After about two weeks it reaches a swimming stage called the zoanthella, and the larvae were able to settle after 17 to 19 days.

That is a long time to spend drifting. A larva that stays in the plankton for two to three weeks can be carried a long way by currents before it touches bottom. This helps to explain how zoanthids reach remote islands such as Trindade and Martin Vaz, more than 1,000 km off the Brazilian coast, and why the same species turn up across wide stretches of coastline. It also means that most of the eggs a colony releases never settle anywhere suitable. Asexual budding and fragmentation, which keep a successful clone in place, balance this gamble on the open sea.

How does it survive low tide?

Many populations of P. variabilis live high enough on the shore to be left in the air or in shallow, warm pools at low tide. For a soft-bodied animal this is a severe test. The polyps respond by closing up: the disc and tentacles are pulled down into the column, and the colony becomes a cluster of firm, gritty knobs. The sand grains and shell fragments built into the column wall make it tougher and help to shade the soft tissues inside.

Zoanthids also produce plenty of mucus, which slows drying and traps fine sediment on the colony surface. In the brightest, shallowest sites, colonies tend to be paler and more compact than those a little deeper, and this variation in shape and colour within a single species is one reason Duerden chose the name variabilis. When the tide returns, the polyps swell with water again and reopen within minutes to hours.

Exposure is not the only stress. Pools on reef flats can heat up quickly in the tropical sun and lose oxygen at night, and heavy rain can drop the salinity for a few hours. Species that thrive in such places are generalists, able to tolerate swings that would harm many reef-building corals. That toughness is part of why Palythoa species are so common on Brazilian shores and why aquarium specimens are so forgiving.

Does it live on sunlight or on food?

Like most shallow zoanthids, P. variabilis hosts single-celled algae in its tissues, members of the family Symbiodiniaceae. These partners turn sunlight into sugars and pass part of them to the host. A 2014 study in the journal Symbiosis examined the symbiont diversity of zoanthids in north-eastern Brazil, part of a growing body of work showing that the same zoanthid can host different symbiont types in different places.

Yet Palythoa is not just a solar-powered animal. A 2022 doctoral thesis from the Federal University of Ceará, reviewing the trophic ecology of animals on Brazil’s marginal reefs, noted that P. variabilis and P. caribaeorum appear to rely more on catching and absorbing food than on their symbionts. Their large polyps capture zooplankton, and the mucus sheet traps organic particles that the colony can take in. In turbid coastal water, where light is often limited, that ability to eat is a real advantage.

For reef-keepers, this mix explains a familiar pattern. Brown palys survive under modest light but grow much faster when fed, and colonies kept in very clean, nutrient-poor water often shrink and close. A colony fed small meaty foods a few times a week usually opens wider and spreads more quickly than one that depends on light alone.

How to find it on a rocky shore

For anyone exploring Caribbean or Brazilian shores at low tide, P. variabilis is one of the easier zoanthids to spot. Look on the lower parts of rocks and the edges of reef flats, where water drains slowly. Closed colonies look like clusters of brown or tan nipples, each a few millimetres to over a centimetre across, often mixed with turf algae and sponges. Mat-forming P. caribaeorum nearby looks more like a smooth, rubbery sheet with small pits.

Return when the tide is in, or look in pools that stay flooded, and you will see the polyps open into flat discs ringed with short tentacles. Look but do not handle: a cut finger or a splash in the eye is the most likely way for a beachcomber to meet palytoxin. Photographs taken from above, with a coin or ruler for scale, are the most useful record if you want help with identification from a local expert or an online biodiversity platform.

Why was Protopalythoa dropped?

Verrill created Protopalythoa in 1900 for zoanthids whose polyps stand free of a thick mat, which fits P. variabilis well. But when researchers sequenced zoanthid DNA, the “protopalys” did not form a separate branch; they were mixed in among mat-forming Palythoa species. Growth form turned out to be a poor guide to ancestry, and WoRMS now lists Protopalythoa as a synonym. The old name lives on in the aquarium trade and in older papers, including the 2005 Brazilian study above.

What role does it play on the reef?

On Brazilian rocky shores and reef flats, P. variabilis and P. caribaeorum are among the most abundant animals in the low intertidal zone, competing with seaweeds, sponges and corals for space. Their colonies shelter small crustaceans, worms and brittle stars between the polyps, and their mucus traps fine sediment. Because they tolerate heat, sediment and exposure better than many corals, zoanthids can become more dominant where reefs are stressed.

Why would a colony change sex?

Sex change is common in the sea, from wrasses and clownfish to many corals and anemones, and biologists usually explain it with the size-advantage idea. If an animal’s success as a male and as a female grows at different rates as it gets bigger, it pays to start as the sex that does well when small and switch to the one that does better when large. In fish, the details depend on mating systems and territories. In a sessile colony such as a zoanthid, the trade-offs are more about energy and timing.

Eggs are expensive: each one is packed with yolk and, in zoanthids, with symbiotic algae. Sperm are cheap individually but must be made in vast numbers to have any chance of meeting an egg in moving water. Why young P. variabilis colonies invest in eggs first is not settled, and the Brazilian authors did not claim to have proven a cause. One possibility is that colony size, polyp age and local conditions change the relative pay-off of each role over time, so the balance tips from female to male as a colony matures.

What the study does show is that a colony’s sex is not fixed for life, and that at any moment a population contains a mix of female-phase and male-phase colonies. Because sperm and eggs were released in different months in the São Sebastião channel, colonies in the two phases are not simply spawning side by side on the same night, a pattern that would reward further study across more years and sites. Longer records, genetic tagging of individual clones and laboratory spawning would all help to show how often a colony switches and what triggers the change. For a species so common on Brazilian shores, its sex life remains surprisingly little explored.

Is it toxic?

Treat it as toxic. Palytoxin occurs in many palys, and the amount varies. Do not touch it with bare skin, and protect your eyes.

Can you keep it in an aquarium?

Brown Atlantic protopalys appear in the trade from time to time and are hardy. Give them moderate light and flow and room to spread, and target-feed occasionally for faster growth. Our zoanthid care guide has the details, and the identification guide helps with paly versus zoa.