Atlantic and Indo-Pacific separation in Palythoa sibling species: phylogenomic analyses using ultraconserved elements

Life in the ocean often defies our attempts to sort it into neat boxes. This is especially true for zoantharians, a group of small, colorful, coral-like animals that live in colonies on the seafloor. To the naked eye, many of these creatures look nearly identical, a trait known as morphological plasticity, where the same genetic blueprint can produce slightly different shapes depending on the environment. Because they look so much alike, scientists have long struggled to tell where one species ends and another begins. This is not just a matter of naming things correctly; understanding whether two populations are truly separate species is essential for knowing how they evolve, how they interact with their environment, and how to protect them. When animals look the same but might be genetically different, or look different but are actually the same, the lines of their family tree become blurry.

In this context, researchers turned their attention to two specific groups of these animals: Palythoa tuberculosa, found across the Indo-Pacific, and Palythoa caribaeorum, found in the Atlantic. For a long time, these were treated as distinct species, separated by the vast distance of the Americas. However, their close resemblance and the difficulty of distinguishing them using traditional methods left scientists wondering if they were truly separate lineages or just variations of the same complex group. To get a clearer picture, a team of scientists decided to look deeper than ever before, using a powerful genetic tool called ultraconserved elements. These are specific, highly stable parts of the genome that act like reliable landmarks, allowing researchers to compare the DNA of different individuals with great precision. By examining these markers, the team hoped to finally settle the question of whether these two groups are distinct species or part of a single, evolving family.

The study brought together a diverse collection of specimens from four different locations: Brazil, the Red Sea, Okinawa, and New Caledonia. The researchers analyzed a massive dataset containing 116 specific genetic regions, totaling 35,699 base pairs of DNA, across 37 different individuals. They used two different mathematical approaches to piece together the evolutionary history of these animals. One method looked at the DNA as a single, combined story, while the other method examined how individual genes told their own separate stories, acknowledging that different parts of the genome can sometimes have different histories. The goal was to see if the animals from the Atlantic would group together on one branch of the family tree and the animals from the Indo-Pacific would group together on another, which would confirm they are separate species.

The results, however, did not show the clear separation the researchers expected. Instead of finding two distinct groups, the genetic analysis revealed a tangled mix. Animals from the Atlantic were found intermingled with those from the Indo-Pacific, and there was no clear genetic boundary separating the two regions. The data showed that the animals did not form a single, unified family tree for either the Atlantic or the Indo-Pacific populations. Further analysis of how much the different genetic regions agreed with each other showed low agreement, suggesting that the genetic history of these animals is complex and that different parts of their DNA tell different stories. This lack of a clear split indicates that increased genetic detail did not solve the puzzle of their species boundaries.

The findings suggest that these animals are not fully distinct evolutionary lineages. The lack of genetic differentiation points to the possibility that these populations are still connected, perhaps through the movement of larvae or adults over long distances. This movement could be driven by natural processes or even by human activity, such as animals hitching rides on floating debris across the ocean. It is also possible that the two groups split from a common ancestor so recently that their DNA has not yet had enough time to diverge into distinct patterns. The study concludes that P. tuberculosa and P. caribaeorum likely represent a species complex, where the boundaries between them are still forming, rather than two completely separate species.

Ultimately, this research highlights a limitation in relying solely on large amounts of genetic data to solve every biological mystery. Even with a genome-scale view, very recent divergences can remain invisible if the evolutionary process is still in motion. The study supports the idea that to truly understand these complicated relationships, scientists need to combine genetic evidence with other types of information, such as physical traits and ecological data. By acknowledging that these animals may be in the early stages of becoming separate species, or that they remain connected across vast oceans, the study offers a more nuanced view of life in the sea, one where the lines between species are not always sharp, but often fluid and in flux.