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Ocean Beauties: Marine Flatworms

Ocean Beauties: Marine Flatworms

Written by Leisure Pro Staff
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Published on August 27, 2014
marine flatworm in underwater scene Pseudoceros ferrugineus, the Fuchsia flatworm

Marine flatworms are some of the most fascinating, although simple, creatures in the world’s oceans. These small worms, ranging from a fraction of an inch to about three inches in length, are commonly found in coral reefs around the world, although some live at much deeper depths. They are usually brightly colored in blacks, pinks, blues, reds, greens, and yellows. You can spot them tucked into crevices, or moving about in the open. Their bright coloring advertises their toxicity to predators, making many species reasonably safe in open areas.

Marine Flatworms: What You Should Know

What is a marine flatworm?

Marine flatworms (or Platyhelminthes) are vibrantly colored soft-bodied invertebrates. They are free-living, meaning they do not live on the body of another creature, unlike their relatives, the dowdy flatworm which attach to other animals as a parasite. Being so small and flat, they are able to hide in tiny crevices, making them hard to spot. Marine flatworms are often called “the butterflies of the sea” because of their beautiful colors. Some pick up their color pigments from what they eat, and others by mimicking other creatures (such as nudibranchs) as a way of camouflaging.

Where are marine flatworms found?

Marine flatworms live in all of the world’s oceans, but are most common in tropical climates, and under volcanic zones. You will find them on the ocean floor, often underneath boulders and rocks, moving along quickly using tiny hair-like bristles called cilia. If disturbed, the flatworm will create waves with their body to swim away.

marine flatworm in underwater scene Polyclad Flatworms (Prostheceraeus roseus) Granada, Spain
Image via Shutterstock

Are marine flatworms poisonous?

Some are poisonous to eat and contain tetrodotoxin — the same poison found in pufferfish, blue ringed octopus, and some frog species. However, not all are poisonous, but many will mimic poisonous nudibranchs in order to survive.

Can you touch marine flatworms?

Although it is unlikely that touching a marine flatwork with harm you, it is best not to touch them. Their delicate and thin bodies can damage easily. We do not recommend touching any marine creature as you do not know what harm you can cause, even by accident.

How big do marine flatworms get?

Most are less than 1mm thick, and they are usually very small (between 10-50mm in length).

What do marine flatworms do?

Some marine flatworms are nocturnal and will hide in crevices to avoid sunlight or torchlight. Some are very bright, whereas others camouflage very well and are extremely hard to spot.

Aside from their toxins, flatworms have simple body structures. They have no respiratory or circulatory systems, so their bodies are a flat shape to allow for diffusion of carbon dioxide and oxygen. Their digestive system consists of a straightforward but many-branched intestine which allows nutrients to be spread throughout the body.

Flatworms are very fragile. When stressed, their bodies will sometimes break apart. However, flatworms have amazing regenerative abilities. Most grow new body pieces if injured. Even if the worm is cut in half, each half will regenerate, forming two whole, living flatworms.

marine flatworm in underwater scene Pseudoceros ferrugineus, the Fuchsia flatworm
Image via Shutterstock

What do marine flatworms eat?

They are carnivorous and eat things like small fish, tunicates, small crustaceans, worms, clams, and molluscs. Some even eat algae and some feed on dead organisms instead of alive ones. Most species of marine flatworms have specialized diets. In addition, some flatworm species form a symbiotic relationship with algae, in which the algae live inside the worm and photosynthesize, producing sugar for the flatworm to use as energy.

Do they reproduce?

Marine flatworms are hermaphroditic, meaning they have both male and female reproductive organs.  Some worms divide periodically, usually once every five to ten days, thus creating new worms. Other flatworms, despite being hermaphroditic, do not reproduce by themselves but perform an elaborate mating ritual with another flatworm. Once the mating ritual is complete, they lay a group of several hundred in a safe place, and after ten days, the eggs hatch. The life span of marine flatworms is generally between three weeks to three to four months long, depending on the species.

Check out this fascinating video segment from one of our favorite underwater photographers, Nick Hope, and see for yourself the beauty that they provide on the reef!

Leisure Pro Staff

Leisure Pro Staff

Marketing Director for LeisurePro