Rare Nudibranch Sighting in Koh Tao

In 2015, during my first full-time year of travel, I ended up on the island of Koh Tao in southern Thailand. Friends I’d met in Australia had recommended it as one of the most popular places for scuba diving in the country. I quickly fell in love with the tight-knit community, the laid-back pace of life, and the seemingly endless diving I did in those first few months. Fast-forward ten years: I’ve lived on Koh Tao for more than five years total and now make it a habit to return at least once a year to reconnect with friends and log as many dives as possible.

On a trip in 2023, I dedicated much of my free time underwater to searching for nudibranchs — colorful, shell-less sea slugs found in oceans around the world. Known for their striking patterns and shapes, they breathe through feathery structures on their backs, which gives them their name: nudi for “naked” and branchia for “gills.”

I was absolutely thrilled when I spotted this vibrant nudibranch clinging to a hydroid at 21 meters deep at the Tanote Bay dive site. As far as I can tell, it’s an undescribed species in the genus Trinchesia (also sometimes called Tenellia), commonly referred to as “sp. 15” by marine biologists. In this naming convention, sp. means the exact species hasn’t been formally described, and the number is a placeholder used before it’s given a scientific name.

The structure it’s crawling on is a hydroid — a small, plant-like animal related to jellyfish and corals. Hydroids often grow in delicate, branching colonies and are easily mistaken for sea plants. They’re worth paying attention to on a dive, as some nudibranchs, including Trinchesia sp. 15, are closely associated with specific hydroid species and are rarely found anywhere else.

I love spotting undescribed species like this as it is a reminder of how much of the ocean is still unexplored and poorly understood. Every observation adds to the collective knowledge that helps scientists understand marine biodiversity and sometimes, those photos can even play a part in officially identifying and naming a new species!