The deep sea has a habit of keeping secrets.

For decades, researchers have explored the waters off Costa Rica, lowering equipment hundreds of meters beneath the surface into a world without sunlight. From above, these azure waters can seem familiar, supporting fisheries, tourism and coastal communities. But far below, in the cold darkness, lives a collection of animals that few people will ever see. Among them was a fish that scientists did not realize was new to science.

That fish has now been formally described as Rhinochimaera costaricana , a new species of rhinochimaera, a group of deep-sea fishes often referred to as “ghost sharks.” Despite the nickname, ghost sharks are not actually sharks — they belong to a closely related group called chimaeras, which share a common ancestor with sharks and rays but diverged hundreds of millions of years ago. The new species was identified using a combination of traditional anatomy and modern genetics in an approach known as integrative taxonomy that is becoming increasingly important as scientists work to distinguish species that can appear remarkably similar at first glance.

The discovery is based on three male specimens collected off the Pacific coast of Costa Rica between 2000 and 2023. Measuring between 30.5 and 32.6 inches (775 and 830 millimeters) in total length, these animals were carefully examined and compared with 90 specimens representing the three currently recognized species within the genus Rhinochimaera . At first glance, separating one species from another in the deep sea can be quite the challenge: many deep-sea animals have evolved similar body shapes and features that help them survive in a harsh environment characterized by crushing pressure, low temperatures and perpetual darkness. So it’s the small anatomical differences that often become critical clues. In this case, researchers measured 49 different physical characteristics of the specimens and found that the Costa Rican animals possessed a unique combination of traits that distinguished them from their relatives. They have a shorter snout, a larger and taller spine in front of the first dorsal fin, a higher first dorsal fin, a wider space between its dorsal fins and fewer tubercles, or small bumps, on its tail.

The differences sound subtle, sure, but they are exactly the kinds of features taxonomists use to determine whether a population represents a distinct species. Much like detectives piecing together evidence at a crime scene, scientists look for consistent patterns that separate one group from another. But the researchers who discovered this species and wrote an aritcle about it also analyzed a segment of DNA known as the cytochrome oxidase I, or COI, gene. Often referred to as a “genetic barcode,” this gene is commonly used to identify species and assess evolutionary relationships. The DNA sequences obtained from two specimens of R. costaricana differed from those of other Rhinochimaera species by between 3.9 and 4.7 percent. In the world of species identification, these numbers are significant as they exceed commonly accepted thresholds used to distinguish species. Multiple species-delimitation analyses also supported the conclusion that the Costa Rican specimens represent a distinct evolutionary lineage.

Together, the anatomical and genetic evidence paint a compelling picture: this is something entirely new to us. It’s beyond exciting! And it makes one wonder how much the deep is keeping from us, as the deep ocean covers the majority of our planet. Yet it remains one of Earth’s least explored environments; many regions have been sampled only sporadically, while others remain largely untouched by scientific surveys. Every expedition into deeper waters has the potential to reveal species that have gone unnoticed for centuries. We live in an era of satellites, artificial intelligence and global connectivity, yet scientists continue to encounter vertebrate species completely unknown to science!

As technology continues to advance, integrative approaches are helping scientists build a more accurate picture of life on Earth. And species cannot be protected if they are unknown. Understanding what lives in a particular region is often the first step toward effective conservation and management. Deep-sea ecosystems are increasingly influenced by human activities, including fishing, resource extraction and climate-driven environmental changes. The more we learn about the species inhabiting these environments, the better equipped we are to understand how those ecosystems function and respond to disturbance.