Members of the small but growing shoal of mermaids and mermen in Brazil are getting a little worried and irate.
Until now, they've been able to slip happily into their brightly colored tails and glide away through the water without much attention from the outside world, beyond the odd chuckle or ripple of applause.
Now "mermaiding" — or "sereismo" as it's known in Brazil — is growing more popular, thanks partly to a smash-hit TV soap opera and the theme song that accompanied it.
The spread of the hobby is attracting warnings from safety agencies who believe that swimming around dressed as a glamorous sea creature from ancient mythology can involve the real and present danger of drowning.
These alarm bells have upset one person, in particular. Mirella Ferraz, 34, says she is Brazil's first professional mermaid, and a pioneer of the pastime in her country. For her, mermaiding was a "childhood dream" and a "passion" that she conceived at a time when nobody wore a tail.
Ferraz accuses a recently launched campaign that highlights the risks of "mermaiding" of spreading falsehoods and of "demonizing" the mermaid's skin. "They also say that with the tail, you can't float. It's a lie. It is buoyant," she says.
The dispute's attracting considerable interest in Brazil. The widely respected daily newspaper Folha de São Paulo this week illustrated the story with a front-page photograph of a long-haired, bearded man, sitting by a pool in a big golden mermaid tail, looking vaguely disgruntled.
He is not, in fact, a mermaid but the male variant: Triton, the half-human, half-fish son of the ancient sea god Poseidon, a notoriously bad-tempered Greek deity.
For years, tiny numbers of people have been mermaiding around the world. Interest in Brazil swelled recently, thanks to a telenovela, A Força de Querer that was broadcast nationwide last year and topped the ratings. (The show has been broadcast in English as Edge of Desire.)
The soap's theme song "Sereia" ("Mermaid") is by Roberto Carlos, a gravel-voiced 76-year-old Brazilian superstar known by his multitude of fans as "The King."
The video of Carlos' song features one of the telenovela's actresses, Ísis Valverde, swimming joyously around the ocean depths in a giant orange mermaid tail, with her locks flowing, amid shoals of tropical fish and shafts of sunlight. It's attracted more than 7 million views on YouTube.
Valverde was trained by Ferraz, the professional mermaid. Ferraz believes, thanks in part to the telenovela, more than 1,000 Brazilians are mermaiding these days.
She bases this estimate on the rising sale of tails. Statistics do not reflect the number of Brazilians who might be keen to do a little mermaiding but are deterred by the price. A decent mermaid tail can cost upwards of $1,200.
The safety campaign that's angered Ferraz was launched by Inmetro, a Brazilian government institute that monitors products for quality and safety. It issued an alert, saying research into mermaid skins found them very appealing to children, but with a "grave risk" of drowning. It points out that mermaid tails confine your legs, which makes it difficult to stand up in a pool — something Ferraz also disputes.
Inmetro's concerns are shared by others, including Dr. Renata Waksman, of the Brazilian Pediatrics Society. She says she has seen videos of girls trying to stand in mermaid tails in a swimming pool, and slipping. "Then they get frightened," she says. "That's where the danger lies, especially if there isn't an adult nearby who is really attentively monitoring what this child is doing."
Authorities in Australia have also warned of the risks of swimming with mermaid tails.
"You need to know what you are doing," says Thaís Picchi, a "mermaiding" instructor in Brazil's capital Brasilia. "Never swim with a mermaid tail alone. And you need to know how to swim."
You have to be extra careful in the sea, says Picchi: "We cannot mermaid with waves. It's not safe."
Picchi adds: "There are a lot of safety issues to be aware of. On the other hand, we don't know of any situation where someone drowned because of a mermaid tail. So why are people talking so much about it?"
There are other questions. Why not just stay on land, and save your mermaid costume for this month's carnival? Why go mermaiding at all? What's the appeal?
Picchi cites an "ancestral appeal" of mermaids, and also the Disney movie The Little Mermaid: "Everyone watched The Little Mermaid when we were kids."
She adds: "A lot of people love mermaiding because it is a fantasy, like a child's dream. They feel different. There are powerful things about mermaids. They are free, autonomous, and they take care of the ocean."
Let's hope they can also take care of themselves.