Most worms aren’t pretty. Earthworms may be useful and important biologically but they aren’t exactly photogenic Some are downright revolting: tapeworms, liver flukes, pin worms, round worms. There are so many sorts of worms that it has taken scientists a great deal of study to sort them all out. First, they were all thrown in one giant phylum called Vermes. Then they were separated out into flatworms and flukes (phylum platyhelminthes), nematodes and nemerteans and a number of minor invertebrate worm phyla like the kinorhynchs. The most ‘advanced’ worms have segments and get classed in the Phylum Annelida. Earthworms are the best known annelids. The other two groups are the leeches, also well known and not very pretty. Then there are the polychaete worms. Polychaete means many-haired and these segmented, many-haired marine worms are among the prettiest worms in the world.
Earthworms are oligochaetes, meaning few haired and they have conquered the land, being mainly terrestrial soil-dwellers. The Polychaete worms have stayed in the oceans and they are found from intertidal to deep ocean bottom habitats. Why are they so pretty where most worms are so ugly? It all has to do with tentacles.
If you go down to the ocean and look in the rocky intertidal zone, you are likely to find, among the snails and other molluscs, the tubes of worms. When the water is over the tubes, the tentacles of the worms stick out. When the tide goes out, the worms hide in their calcium tubes. Ok those worms aren’t that pretty. You need to go to a coral reef to see the really pretty ones. There are a whole group of polychaetes known as the Christmas Tree Worms because they are as bright and colourful as Christmas trees with their branching multicoloured tentacles.
Of course there are many not so pretty polychaetes too, although because they are all free-living, they don’t have the ‘yuck’ factor associated with parasites. The group is actually almost as diverse as the mammals: there are 17 orders in the Class Polychaeta and there are 19 orders of Class Mammalia. Basically polychaetes have two types of body plans based on lifestyle. Fourteen of the orders consist mostly of species that live sedentary adult lives: tubeworms. The other three orders are crawling, or ‘errant’ species.
The order that I am most familiar with is the Spionida. These tubeworms have a typical sedentary polychaete body plan and life style. The larvae are free-living in the plankton but the adults settle on suitable substrates and build tubes or burrow in the shells of molluscs. Their adult lives are spent tube-building and filter-feeding, sticking their tentacles out into the water and picking up particles of detritus. The basic shape is vermiform with a head end that contains the tentacles, palps, mouth and sometimes eyes. Behind the head are segments that have chaetae parapodia (short ‘legs’) for movement and then a tail section with the anus. Polychaetes have a one-way gut from mouth to anus. Most species have separate sexes although a few hermaphrodites are known.
The best known polychaetes, besides the tubeworms at the beach, are the errant bloodworms and beachworms, prized as bait animals by fishermen. These worms live in sandy or muddy substrates, are predators that actively crawl around looking for their prey and are big enough and common enough to make good bait.
Terebellids, Sabellids, Eunicids and Spionids, Phyllodocids and Flabelligerids: most of the polychaete orders are known only to specialists in the field, but these worms form an important part of the ocean benthos. They filter out detritus and keep that food source in the food chain. They aerate the bottom sediments and provide a food source for bottom dwelling fish. They clean the water as well. They are useful as well as being among the prettiest of those strange creatures we call worms.
Reference: K. Fauchald. 1977. The Polychaete worms: Definitions and keys to the orders, families and genera.