Everything about flatworms – the dazzling drifters of the reef!
At first glance, you might mistake them for nudibranchs — and that's no accident! Flatworms (especially marine polyclads) share the same dazzling colors and flowing movements, but they’re an entirely different group of animals.
Flatworm definition:
While nudibranchs tend to steal the show in the world of marine macro, flatworms (specifically polyclad flatworms) are no less fascinating! Though often mistaken for nudibranchs due to their flamboyant colors and undulating motion, they actually belong to a completely different phylum: Platyhelminthes (flat-bodied worms).
What are flatworms?
Flatworms, or Platyhelminthes, are simple, soft-bodied invertebrates. The ones we often encounter while diving or snorkeling are free-living marine polyclads — not to be confused with the parasitic flatworms in the same phylum. These sea flatworms are incredibly thin and move with a graceful, rippling glide over the reef.
How to recognize them?
They don’t have external gills like nudibranchs or raised cerata. Instead, their bodies are flat and leaf-like, often with a pair of small “pseudotentacles” formed by folds at the front of the body. Many species display bold warning colors — a sign they might taste bad or be toxic.
Behavior and ecology:
Flatworms are often spotted crawling on coral rubble, sponges, or algae-covered rocks. They feed on small invertebrates, and some are even known to prey on nudibranchs or colonial animals like tunicates. They're hermaphrodites, and some species engage in a curious behavior called “penis fencing” to determine which individual will carry the eggs!
Keep in mind:
Flatworms are not mollusks. They belong to a different phylum altogether. Yet their appearance and behavior make them a fun and fascinating part of the macro world — and a delight to photograph!
Habitat:
Marine flatworms can be found in shallow coastal waters around coral reefs, rocky areas, and rubble zones. They usually live on or under rocks, among sponges, or in crevices, and are more commonly seen at night or during low-light dives when they are more active.
Diet:
Flatworms are carnivores. Most feed on small invertebrates such as crustaceans, mollusks, or tunicates. Some specialize in preying on colonial animals or even eggs of other species. They often glide over their prey and engulf it using a muscular pharynx located on the underside of their body.
Identification:
Flatworms are generally very thin, with a soft, flattened, leaf-like body. They don’t have external gills or cerata like nudibranchs. Their movement is smooth and flowing, thanks to cilia on their underside and muscle contractions.
Look for:
• Bright colors and patterns (often warning signs to predators)
• A pair of “pseudotentacles” at the front of their body (folded edges, not true tentacles)
• A rippling or gliding motion
Reproduction:
Like nudibranchs, flatworms are hermaphrodites, meaning they have both male and female reproductive organs. However, they still need a partner to reproduce. Some species are famous for their bizarre mating behavior known as “penis fencing,” where two individuals duel using sharp, extendable penises — and the “winner” inseminates the other.
Flatworms lay egg capsules in hidden areas, and development varies depending on the species.
Classification / Taxonomy:
- Kingdom: Animalia
- Phylum: Platyhelminthes (flatworms)
- Class: Rhabditophora
- Order: Polycladida (free-living marine flatworms)
- Families & Genera: There are many families within Polycladida, such as Pseudocerotidae and Cotylea, which include the most colorful species seen by divers.
Size:
Marine flatworms are generally small — most species range from a few millimeters to about 8 cm, though some may reach 10–15 cm.
Lifespan:
Their lifespan is not well documented in the wild, but most marine polyclads are believed to live a few weeks to several months, depending on food availability and environmental conditions.
To keep in mind:
• Flatworms are not mollusks — they are an entirely separate animal group.
• Their coloration is often aposematic (warning colors), suggesting toxicity or distastefulness.
• While they resemble nudibranchs, their anatomy and biology are very different.