If you’ve ever spotted a little yellow cube with fins cruising around the reef, chances are you’ve met the yellow boxfish (Ostracion cubicum). It’s one of the cutest fish you’ll ever see in the ocean. But don’t let the looks fool you — this little guy has a big and deadly secret. What is a Yellow Boxfish? The yellow boxfish is found across warm, tropical waters — especially around coral reefs in the Indo-Pacific region. That includes right here in Koh Tao! Juveniles are bright yellow with black polka dots, while adults tend to be duller with blue and grey hues. Either way, they’re unmistakable thanks to their rigid, box-shaped bodies. Built Like a Tiny Tank This fish doesn’t have the soft, bendy body of most reef dwellers. Instead, it’s got a tough, bony outer shell that gives it its cube-like shape. This “carapace” is made of fused plates, which makes the boxfish surprisingly well-armored — perfect for deterring predators and bumping through reefs with ease. How Does It Swim So Well? You might think a fish shaped like a box wouldn’t be great at swimming, but the yellow boxfish is a smooth operator. It moves with small fin flicks — mainly its dorsal, pectoral, and anal fins — and uses its tail for steering. It’s slow, yes, but super maneuverable, making it perfect for life in busy reef neighborhoods. The Secret Weapon: Toxic Slime This is where things get wild. When the yellow boxfish feels threatened or stressed, it can release a powerful toxin from its skin — a chemical called ostracitoxin (also known as pahutoxin). This toxin is deadly to nearby fish, affecting their gills and red blood cells. In closed spaces like aquariums, it can wipe out everything in its vicinity, including the boxfish itself. This defense system is rarely triggered unless the fish is seriously harassed, and takes a serious amount of energy to deploy. That’s why it’s best to observe from a distance and, as always, never try to touch or chase it. The bright yellow coloring? That’s nature’s way of saying “Don’t mess with me.” What’s on the Menu? The yellow boxfish isn’t picky. It spends its time near the bottom of the reef, nibbling on algae, sponges, small crustaceans, worms, and sometimes even molluscs. It uses its puckered mouth to pull food from cracks and crevices. This puckered mouth makes it look like the fish is going about kissing the reef which just adds to the “aww” factor. Fun Fact: Nature-Inspired Engineering The shape of the yellow boxfish inspired the design of the Mercedes-Benz Bionic Car. Yes, really. Despite looking boxy, this fish is actually super streamlined in the water. Engineers studied its form to build a car with better aerodynamics and fuel efficiency. Nature really does know best. Spotted in Koh Tao Yellow boxfish are a fun find during dives in Koh Tao. You’re most likely to see them during relaxed reef dives, especially in coral-heavy areas. They’re not super common, so spotting one is always exciting — and they make amazing photo subjects. Just remember to stay calm, go slow, and give them space. You’ll get better photos, and they’ll stay relaxed and toxin-free! Come Meet the Boxfish With UsAt Phoenix Divers Koh Tao, we love sharing marine life stories like this one. Whether you’re taking your first dive or you’re a seasoned pro, our team knows where to look to spot fascinating creatures like the yellow boxfish. Join one of our fun dives or training programs, and who knows — maybe your next dive buddy will be a little yellow cube with attitude. Ready to dive in? Contact us and let’s explore together.Get in touch