Deep in the tissues of sea anemones, corals, and jellyfish are strange yellow cells which are genetically distinct from the marine animals.

More than a century after these cells were first assigned a now forgotten genus, a new paper has resurrected the name and described six new species from around the world.

"Because our team comprises scientists from seven countries, we were able to collect all of these samples, and some during the global pandemic," said lead author of the study, biologist Todd LaJeunesse from Penn State University.

"This study highlights how the spirit of scientific discovery brings people together, even in times of hardship."

First described in 1881, the yellow things were originally classified under the genus Zooxanthella by scientist Karl Brandt. Brandt also coined the term zooxanthellae, which is used colloquially to this day.

However, another scientist – a Scotsman called Patrick Geddes - was investigating these yellow cells at the same time. In 1882, without having seen Brandt's work, Geddes discovered that not only were the cells separate from the animals they were within (something Brandt had also established), but that they were beneficial, acting as mutualistic symbiotes.

The genus Geddes established to categorize these cells was Philozoon, from the Greek words for 'to love as a friend' and 'animal'. Unfortunately, Brandt came first, so the Philozoon genus was never used; Geddes switched to work in urban planning, and the newly named genus was largely forgotten.

We are now well aware that these odd microorganisms Geddes and Brandt worked on back in the day are photosynthetic dinoflagellates – single-celled algae found in symbiosis with other marine life, such as corals – under the family Symbiodiniaceae.

In the new study, researchers took a closer look at these yellow cells, using genetic data, geographical data and morphology to analyze where exactly they should sit in the genetic tree.

And after determining that these creatures need to be put into a new genus, the team pulled the genus Philozoon out of retirement for two old species and six new ones.

"We emend the genus Philozoon Geddes and two of its species, P. medusarum and P. actiniarum, and describe six new species," the team wrote in their paper.

"Each symbiont species exhibits high host fidelity for particular species of sea anemone, soft coral, stony coral and a rhizostome jellyfish."

The team wrote that Philozoon are found in shallow, temperate marine habitats around the world, including the Mediterranean Sea, eastern Australia, New Zealand, and Chile.

"Since most of the algae in the family Symbiodiniaceae have been thought to be mostly tropical where they are critical to the formation of coral reefs, finding and describing these new species in cold waters highlights the capacity of these symbioses to evolve and live under a broad range of environmental conditions," explains LaJeunesse.

"Life finds a way to persist and proliferate."

The research has been published in the European Journal of Phycology.