Marine Flatworms: Magic Carpets of the Seas

Ask any lay person about the group of animals called flatworms, and chances are they will respond along two extremes — from “eeks” to tapeworms to “oh, the Persian carpet flatworm is particularly exquisite”. We learn about flatworms in school while studying the taxonomy of invertebrate animals, but they also appear in detail in the “diseases” section. Does “liver fluke”, an infection caused by parasitic worms, ring a bell?

Common bilateral ancestor

When flatworms first evolved about 550 million years ago, they were novel in a particular aspect — their symmetry. They were neither irregularly shaped like sponges nor radially symmetrical like jellyfish. Instead, they exhibited a new kind of symmetry called bilateral symmetry; their bodies mirror one another when divided into two halves (left and right). Flatworms and other bilateral animals we see today, including humans, are thought to have evolved from this common bilateral ancestor.

I had not given flatworms much thought until I saw one for the first time while diving on Havelock Island in my early days working at a dive school. It looked like just another bright pink surface on the reef. Coralline algae, I thought, until it vanished into a crevice in the reef, its movements smooth as butter. I never found out its name, but this was my first introduction to the world of marine flatworms. I quickly learnt that marine flatworms are wonderfully bizarre creatures, not just because of how they reproduce.

What are flatworms?

And how do we distinguish flatworms from all other worms? Many different kinds of unrelated animals are categorised as worms — earthworms (Phylum Annelida), arrow worms (Phylum Chaetognatha), roundworms (Phylum Nematoda), and a host of larvae of various insects (like flies and beetles). Flatworms, of which there are over 20,000 species, are of the phylum Platyhelminthes. Platyhelminth flatworms are interesting. They have soft bodies without any segments. While they are bilaterally symmetrical, they lack a body cavity, have no respiratory or circulatory systems to house, and just one opening that serves as mouth and anus. To breathe, flatworms take in oxygen by diffusion through their outer membranes, which through further diffusion, travels internally to every corner of the body. The body of a flatworm is extremely slim and dorso-ventrally flattened (from above and below) like the thinnest layer of cheese you can slice with a knife.

Flatworms don’t often make their way into our conversations despite their abundance, possibly because most of them are parasitic (their hosts range from fish, snails, and livestock to humans) or are terribly small (less than 1 mm in length). The one group of free-living, non-parasitic flatworms we are more likely to come across and even be dazzled by are marine flatworms (Polycladida).

Marine flatworms

Marine flatworms (800 known species) occur across the world, in cool temperate waters and in the warmer tropics. They inhabit deep seas, tidepools, coral reefs, mudflats, and other marine habitats. You are most likely to stumble upon a marine flatworm during an intertidal walk or while snorkelling or diving in a coral reef. The keyword here is “stumble”. While marine flatworms seem ubiquitous, they can be quite cryptic in their habits. Even the most vibrantly coloured, showy ones, with bright pink stripes (Persian-carpet flatworm) or golden-yellow specks (yellow-spotted flatworm), camouflage seamlessly as they slither over bright coral colonies.

Art in motion

Nothing, however, can prepare you for the mesmerising spectacle of a marine flatworm mid-swim. Flatworms prefer to creep over surfaces using tiny cilia on their undersides, but occasionally, they take to the water column like ornate carpets come to life. Flatworms rapidly undulate (create a wave-like motion) the edges of their bodies to propel them through the water.

Flatworms, with their unbelievably skinny bodies, might seem like gorgeous two-dimensional art moving in a three-dimensional world. And yet, they have an elaborate branched digestive system and room for food. They are voracious carnivores with a broad preference for smaller animals ranging from crustaceans and molluscs to worms and tunicates. After latching on, flatworms extrude their tubular mouth parts to break down and slurp up the insides of their prey, avoiding having to swallow the animal to eat it. After feeding, any undigested material is regurgitated out the same way.

Dramatic reproductive behaviour

The most intriguing aspect of a marine flatworm’s life is its mode of reproduction. Flatworms are hermaphrodites. Each individual has both male and female reproductive organs. They can release spermatophores and produce eggs (when inseminated by another flatworm). Inseminated flatworms lay eggs in thin sheets, often continuing to guard and care for them until they hatch. The sperm donor flatworm plays no role in parental care. Flatworms often duel with their two-pronged extendable penises. During a duel, each flatworm extends its penis and tries to inject sperm, often in swift stabs, without getting inseminated itself. To inject sperm successfully, a flatworm does not have to aim at a specific body part as the sperm can be absorbed from anywhere on the recipient flatworm’s body. Penis fencing, as this behaviour is called, is an elaborate and dramatic event where each flatworm tries to overpower the other while trying to defend itself, sometimes somersaulting to escape a stab. A laboratory study conducted in Japan on blue-dot margined flatworms found that during most of the penis fencing duels, both flatworms had received sperm!

Evolutionary beauties

Flatworms are often referred to as primitive life forms. As hermaphrodites, they belong to an old lineage of invertebrates with relatively simple internal biological machinery. While “primitive” could be interpreted as “older”, it does also come with the connotation that these are “less advanced” creatures, lower down the evolutionary tree, with humans and other vertebrates relatively higher up. For argument’s sake, let’s look at each organism as having evolved traits, organs, and behaviours to adapt to the challenges in its environment and not rank them on a scale of sophistication or complexity. Then, the next time we come across a marine flatworm, we might feel like thanking it for bringing bilateral symmetry into the world!