Growing between the tides around the Andaman and Nicobar Islands, in the tropical waters of the Indian Ocean, are clusters of what look like tiny, green mushrooms. In fact, this is a type of seaweed, or algae – each one made from a single, gigantic cell. In 2021, scientists named them Acetabularia jalakanyakae, also known as the “mermaid’s wineglass”, because of its umbrella-shaped cap. Prof Felix Bast, a phycologist from the Central University of Punjab, was inspired by Hans Christian Andersen’s classic tale, The Little Mermaid, to give this new species a mythical twist. “I first thought to name it syreni, which is Latin for mermaid,” says Bast. “Then I changed my mind. Why go with Latin? This is from India, and I am Indian.” So he landed instead on the Sanskrit word for mermaid, jalakanyakae. With funding from the Indian government, Bast had been working for several years surveying marine algae along the coasts of the Andaman and Nicobar Islands, the capital of which, Port Blair, lies 740 miles east of the city of Chennai. But it was only when he returned to the islands on holiday with his sister last year that he spotted something strange on the shoreline of Port Blair. At first glance, the algae looked like a common species, Acetabularia acetabulum. On closer look, however, Bast realised the shape of the circular caps, the size of a small coin, wasn’t quite right. He took some back to his lab in a Ziploc bag, and with a scanning electron microscope and DNA sequencing showed this is indeed a distinct species, the fifth Acetabularia from India. Acetabularia algae are some of the largest-known single-celled organisms, beaten only by a couple of other marine seaweeds. The giant cell is shaped into three sections: the root (or rhizoid); stem; and cap, which has the ability to regenerate. “If you chop off one cap, it can regrow,” says Bast. In the root, there’s a single, giant nucleus, housing the algae’s DNA. In the 1930s, the German scientist Joachim Hämmerling conducted groundbreaking experiments with Acetabularia and showed for the first time that a cell’s nucleus controls how an organism grows. He cut off the root of one Acetabularia species, grafted it to the stem of another, and the cap grew back in the shape of the root species. Instructions for growth were evidently coming from the root. Thanks to Acetabularia, Hämmerling anticipated – by several decades – the discovery of messenger RNA, a critical molecule that shuttles the genetic code from DNA inside the nucleus to the rest of the cell. Individual mermaid’s wineglass alga are either male or female. To reproduce, the giant nucleus splits into multiple copies which drift up the stem to the cap and burst into the sea as tiny gametes. The gametes are mobile and they have light-sensitive eyespots. “Just like a human eye,” says Bast. They swim towards the light and the sea surface where they’re more likely to encounter a gamete of the opposite sex, compared with floating about in three-dimensional open water. “The surface is two dimensional, like a sheet of paper, it’s much easier,” he says. Male and female gametes fuse, then they swim away from the light, which leads them down to the dark seabed to settle and grow into a new wineglass. Mermaid’s wineglass algae might grow in the relatively unpolluted waters of the Andaman and Nicobar Islands, but they face a global threat. The ocean is absorbing more anthropogenic carbon from the atmosphere and consequently becoming more acidic, which puts species like the mermaid’s wineglass algae at risk. As Bast explains, more than half of the dry weight of this algae is calcium carbonate, which melts in acidified seawater. “Any organism, be it animal or a plant, with calcium carbonate is highly prone to ocean acidification,” says Bast. “So, that will be having a tremendous impact on species like Acetabularia because the calcium carbonate will simply dissolve into the acid.”