Yellow boxfish are absolutely cute and incredibly fun to watch, but they come with some surprising survival tactics. Their bright yellow color and seemingly aimless swimming might make them stand out, but they’ve got unique strategies up their sleeves. Instead of relying on speed or spiky defenses, these fish have adopted a different approach to stay safe in the wild.
Distinguishing Features
Yellow Boxfish are bright yellow with distinctive black spots and a unique boxy shape, making them fairly easy to spot. However, they are quite shy, so finding them can be a bit of a challenge!
As they mature, their black spots gradually fade, and their vibrant yellow color shifts to a darker, more earthy mustard tone. Additionally, their box-like shape elongates into a more rectangular form, and their tail fin becomes more prominent as they grow...So, while they’re adorable when they’re young, they don't keep that same fresh look as they age!
Behavior and Defense Mechanisms of Yellow Box Fish
Here’s where it gets interesting:
when threatened, a Yellow Box Fish can inflate its body to look more intimidating and release a cloud of potent toxin called ostracitoxin (or pahutoxin). This powerful toxin can quickly kill any nearby fish. What’s more, this toxin can also harm other Yellow Box Fish, which contributes to their solitary nature.
Pahutoxin works by causing a haemolytic effect - basically, it bursts the red blood cells of predators and clings to their gills, leading to a slooow, suffocating death.This extreme defense protects Yellow Boxfish from predators like sharks and groupers but can harm any gilled animals in water with just ten parts per million of the toxin.
The Yellow Boxfish has more than just toxins up its sleeve. Its boxy body is covered in rigid bony plates with openings for its mouth, eyes, fins, and tail. Any predator attempting to bite a Yellow Boxfish is in for a rough time. If they manage to avoid the toxin, they’ll likely end up with a mouthful of hard, bony plates!
You might think being box-shaped is a disadvantage in the streamlined world of water. Sure, Boxfish might look clumsy with their small fins doing all the swimming, but new research reveals it’s not about stability or speed for these fish. They actually use their unique shape and their instability to their advantage. Their tail fins act as rudders, allowing them to make sharp turns in any direction while hovering. They can even swim sideways, diagonally, and upside down!
Feeding and Diet
The Yellow Box Fish has a small mouth, but it’s perfectly suited for its varied diet. While they primarily graze on marine algae scraped from coral, they’re also known to enjoy a mix of tubeworms, sponges, crustaceans, mollusks, and small fish. If it can fit between their puckered lips, it’s on the menu!
Breeding
The exact age at which Yellow Boxfish reach sexual maturity isn’t well documented, but they usually start breeding once they’re large and developed enough. These fish are typically solitary but may form small harems during breeding season, with one male up to four females.
Yellow Box Fish lay their eggs in open water, usually near the surface. The eggs are scattered by ocean currents, which helps spread them out and reduce the risk of predators finding them all. The eggs hatch into planktonic larvae that drift with the currents until they settle into suitable habitats as they grow.
Where to Find the Yellow Boxfish on Koh Tao
Juvenile Yellow Box Fish can be spotted at several dive sites around Koh Tao, especially in areas with plenty of cracks and crevices where they can hide and seek shelter :
Hin Wong Bay, White Rock, Shark island, Aow Leuk, Southwest Pinnacle, Sail Rock (etc)
Next time you spot a boxfish, remember that what once seemed like a clumsy fish is actually a master of fluid dynamics, using its unique body to navigate with precision. So tiny, but such a redoubtable toxic terror!
