Location: Indo-Pacific Ocean
Size: 6–10 cm
Fun Fact: They are hermaphrodites
The Persian carpet worm is a visually stunning flatworm found throughout the shallow warm waters of the Indo-Pacific Ocean. Known for its vibrant patterns and fascinating behaviour, this invertebrate gets its nickname from its beautiful colours that resembles the designs of traditional Persian carpets and the way it swims through water.
Appearance and Movement
Persian carpet worms are soft-bodied, flat invertebrates that grow up to 10 cm in length. Their bodies are oval-shaped and flat, allowing them to glide gracefully over rocks and coral reefs. They come in various shades, they have a dark base colour usually black to brown with swirling purple and pink patterns on top.
When crawling, they move in a slow, rippling fashion using cilia (tiny hairs) on their undersides. They don’t have eyes but at the front of their body they have cells that are sensitive to light, so they are able to make out shadows which allow them to sense their surroundings. They lift up the front of their body so it’s easy to tell which end is which. If they need to swim, they lift up from the rock and create wave-like motions with their body—almost like a ribbon in the water.
Lifespan
Interestingly, the lifespan of the flatworm can vary significantly depending on its size. The larger flatworms—those around 10 cm—can live up to 30 years in ideal conditions. However, smaller individuals often have much shorter lives, sometimes only lasting a few months. This contrast highlights the role that size, environment and predation risk play in their longevity.
Reproduction: Penis Fencing
One of the most fascinating behaviors of the Persian carpet worm is its mating ritual, which you might be surprised to hear is both a battle and a reproductive strategy.
These worms are hermaphrodites, meaning they have both male and female reproductive organs. When two individuals meet to mate, they engage in a behaviour known as “penis fencing.” In this ritual, both worms rear up and attempt to stab each other with one of their two penises. Their goal is to inject sperm into the other while avoiding being impregnated themselves.
The worm that succeeds in stabbing the other becomes the “father,” while the one that is stabbed assumes the “mother” role, carrying and eventually laying the fertilised eggs. This strategy allows each worm to maximise its reproductive output while minimising the energy costs of gestation.
I was fortunate to witness this firsthand during a dive, it was amazing and one of my most memorable experiences in the water, I was lucky to have my camera that day.
Mimicry and Defense
In nature, bright colours often signal danger—many toxic species show off their toxins with vibrant colours. The persian carpet worm has taken advantage of this through Batesian mimicry, imitating the appearance of toxic nudibranchs (sea slugs) that are avoided by predators.
Although the Persian carpet worm is not known to be toxic itself, its bold colouration tricks predators into thinking it is—giving it a visual defense mechanism without needing actual toxins. It’s a clever evolutionary hack that helps this slow-moving creature survive in the competitive reef ecosystem.
Feeding and Diet
As a relatively small and soft-bodied predator, the Persian carpet worm feeds on tiny invertebrates such as small crustaceans. Its feeding method is both efficient and unique.
When it detects prey, the worm positions itself over the animal and drapes its body like a living rug (fitting, given its name). Its mouth, located on the underside of its body, then engulfs the prey. This technique traps the food, preventing escape, and allows for easy digestion.
Colour Variation
With their beautiful appearance, unusual mating behaviour, and deceptive defense strategies, Persian carpet worms are a perfect example of the weird and wonderful diversity found in the ocean. While often overlooked compared to larger marine animals, creatures like the Persian carpet worm are essential components of reef ecosystems—and a reminder of how much more there is to discover beneath the waves.