Facts: Marine Flatworms

There are around 1000 species of marine flatworms in the order Polycladida. They may be found on coral reefs, rocky reefs, sand, and mud. In temperate waters, their colors are mainly brown and gray. Some are transparent. In the tropical waters of coral reefs, the worms are vibrantly colored and often laden with toxins. Some live permanently in cavities of sessile animals like sponges and barnacles. Some are the size of a pinhead, while others are the size of a hand. They are often confused with nudibranch molluscs. However, flatworms are flatter, faster moving, and they typically have frilly or ruffled edges. They also have no external gills. Flatworms are extremely delicate with their unsegmented, bilaterally symmetrical bodies. Their extreme fragility makes them difficult to study. Most are shaped like a leaf, although some are ribbon-like. The body of a flatworm is composed of three basic tissue layers. The gut divides into many branches as it radiates away from the mouth. The two main sensory structures usually present are the tentacles and simple eyes. The eyes are frequently near the front and scattered along the edge of the body. The eyes are only light sensitive cells held in a dark pigment cup. They allow the worms to sense the direction of light. Some species are active during the day and some species are active at night. The mouth is on the underside. It may even be toward the rear of the body. The mouth of a marine flatworm serves both for intaking food and excreting waste. Marine flatworms feed after crawling over their prey. All are carnivores. They may feed on small worms, crustaceans, or tunicates. Others may engulf small snails. Undigested remains are regurgitated. Some flatworms are cannibalistic. Some immobilize their prey with a sticky glue secreted from special glands near the mouth. Most find their food by smell by using simple sensory cells on the head. Some can detect vibrations from moving prey. They glide smoothly over the seabed, propelled by microscopic cilia on the underside. Some species swim by gracefully undulating the body. Respiration is achieved by the diffusion of oxygen into the body from the outside. Carbon dioxide escapes by the reverse process. Marine flatworms are hermaphrodites, sharing both sexes within one body. Mating occurs between two individuals and either can impregnate the other. They lay thin sheets of sticky eggs on the seabed. The eggs either hatch directly into juveniles or into free-swimming larvae. For more marine facts, click the SUBSCRIBE button!!