Reef Stories

Worms

Diverse & varied

Marine worms are so different

Worm Index

Worm

Bobbit Worms

Bristle Worms

Christmas Tree

Feather-Duster Worms

Flatworms

Myzostomid Worms

Horseshoe Worms

Ribbon Worms

Scale Worms

Colonial Tube-Builders

Marine worms are extremely varied ranging from filigran feather fan and Christmastree worms through agressive two meter long Bobbit worms that lunge with lightning speed toward their fish prey before crushing them with their 4 cm wide jaws. Polyclad flatworms however awake the most interest in divers.

The title image on this page really is a worm, the ‘Tornaria' larva of an Acorn worm.

Polyclad Flatworms

Flatworms live in soil, freshwater and the sea. The marine species are often very colourful and conspicuous, rivaling nudibranchs in their "artistic prowess". They are very active predators feeding mostly on shellfish and one group, on ascidians. They are often a major pest in oyster and mussel farms. The marine species have bodies that are extremely thin and delicate. For protection they rely partly on their colouration which warns of their toxicity or mimics nudibranchs that are toxic (e.g. Polyceras imitatus mimicing Phyllidia nudibranchs); and partly on escape. They can move rather quickly, being propelled by thousands of microsopic hairs on their underside while a number of species can also swim very well.

They have no eyes as such, but groups of light sensitive cells in the middle of the head region or scattered along the length of the body. This group also lacks a digestive system, anus and gills. Everything passes through the suface of their ultrathin bodies. The "mouth" can be an extendable pharynx that oozes digestive enzymes. It's not located at the "head" end of the flatworm but mostly towards the middle of the underside of the animal.

Due to the difficulty in collecting them for scientific study, many species that are seen are still undescribed. In some genera identification is through the colours and surface patterns. Other genera such as Acanthozoon and Thysanozoon while very distinct due to the well-developed surface papillae, and impossible to tell apart without microscopic examination.

One behaviour that even brought them into the newspapers when it was discovered, is penis fencing! Although each individual is both male and female, in some species it seems to be more important genetically to be a father than a mother. Penis fencing involves two individuals rapidly moving around each other in a ball with each trying to impregnate the other but avoid being impregnated themselves.

Pseudoceros Flatworms

This is the most varied of the commonly seen marine genera which is why it has been separated out here. Morphologically they are very similar, but as you can see from the images, they have a wide range of exotic colours and patterns.

Pseudoceros imitatus mimics perfectly a toxic aeolid nudibranch

Other Polyclads

Polybiceros has the largest species (Pseudobiceros hancockanus) that can reach 14cm, while P. gratus can reach over 10cm (pers.obs.). Together with the genus Maiazoon they have many species with beautifully ruffled edges looking like hems on dancing dresses.

The Persian Carpet Flatworm (Pseudobiceros bedfordi) is shown in the first few images. A pair enguaged in penis-fencing is shown in the second photo. Further on, sperm injected under the surface of the flatworm can be seen in the pair of fencing Prostheceraus sp..

Most flatworms only have pseudotentacles and only Callioplana have true tentacles.

Acanthozoon or Thysanizoon - the 2 genera with distict papillae

Maritigrella fuscopunctata on their ascidian prey

Prostheceraus sp. mating showing the lumpy injected sperm packages

Acoel Flatworms

Divers will be familiar with these as the masses if brown "spots" covering some soft corals . They are generally small, and have dull colours. Recent DNA & phyllogenetic analysis indicates that they are not related to flatworms, but should belong to their own Phyllum. They modern members of some of the first bilateral organisms in evolutionary history.

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Other groups

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