The vast majority of worms will prove to be harmless or at the least, a predator of other small worms and pose no threat to the larger life forms that we place in our tanks. Of course there are exceptions. While some worms can grow large and seem threatening they are in fact a vital part of the clean up crew, taking care of unseen livestock deaths and left over food.  It is this clean up service that they provide that has led to some species being labeled as fish killers as they are usually blamed for such fish losses when they are seen on the corpse of the fish doing their job of cleaning house.

           

    In the hopes of saving you some confusion as has happened to me quite a few times, I am showing a few examples of the rear sections of various worm species as they can be quite easily confused for the worm's head. A great many of the worms can be very lengthy and it is very common to break the worms while trying to remove them from rocks. Of course a good many times, I end up with the wrong half. To further mislead you, the worm, even though broken in half will still move its rear section in much the same manner as you would expect the head to do. Add a few false eye spots and a worm's hiney quickly becomes its head in appearance. Some species such as the Nemertea have a suction cup like appendage as a hold fast for their rear section which looks very much like a mouth and is often thrashed about as you would expect a head to do, but in reality the rear section is actually just in search of something to grasp onto.

     
   


    The Nemertea   ( Ribbon Worms )  Generally, nemertines are carnivorous; most feed on small invertebrates like crustaceans and annelids, but some feed on the eggs of other invertebrates, and a few live inside the mantle cavity of molluscs and feed on microbes filtered out by the host.
   Below, a Baseodiscus hemprichii
     
    Below, an unknown species
      
    Below, a Notospermus tricuspidatus
     
     Below, an unknown species
      

        
     Below, various unknown species
     


      The  Echiura  -  Commonly called "Spoon worms", are considered close relatives of the Anellids but lack the segmentation of the Anellids. Most species are found in soft sediments forming burrows. The proboscis of the worm is usually the only part of the worm that we see as it gropes around for detritus and other food particles on the substrates near its burrow.

       

    The Polynoidae  -  The polynoids known commonly as "scale worms" are slow moving predators which feed on small prey such as small crustaceans, echinoderms, polychaetes, gastropods, sponges and hydroids in spite of their strong jaws. They are famous for their commensal habits, some living with asteroids, echinoids, holothorians, sea anemone, sea grasses and corals. They are known as scale-worms as their dorsal surface is covered with enlarged scales or elytra which may be concealed by a dense coating of setae. They are medium sized. They are secretive and are mostly found occupying tight crevices beneath stones. Some species are commensals in the burrows and tubes of other invertebrates or the shells of hermit crabs.

      
      Extended Jaw Details :
     
     Below, a Harmothoinae sp.
     
    Below: Unidentified Scaleworm
     
    Below:  A scaleworm of the Acoetidae family. Creates tubes much like the "featherdusters" do and is an active predator and scavenger. Also a large worm, the specimen shown below was as thick as my finger and at least four inches long.
     

     



      The  Sigalionidae   - The Sigalionids are burrowing predators living in sand or mud. A few species inhabit tubes. Others have spinning glands and produce numerous tough fibers incorporated into the tubes. They are common in soft-bottoms. Being long-bodied and their scales being closely appended to the bodies, they appear rather pentagonal. Also members of the "Scaleworms".
    Species below is a Euthalenessa oculata
     
       Extended Jaw details:
     

   The Cirratulids  - Cirratulids are deposit feeders which gather food from the sea bottom by means of their palps. They are sluggish worms which bury themselves below the surface of sea bottoms leaving only their gills and palps visible. Some are free-living and inhabit tubes, while others are capable of burrowing through corals, shell or rock. Hence, they occur both in shallow water as well as deep sea. They inhabit mud flats or muddy areas between rocks. They are usually orange or bright red and are found among mussels and kelps. They are indirect deposit-feeders. They have no proboscis as their food is collected by their palps and carried to the prostomium.

      
     
     
     Below:  A hair worm hiding just under the sand's surface extending its feeding palps.
 

   The Sipunculida Worms  Article #2  - These animals, which are commonly called "peanut worms" because some have the general shape of shelled peanuts, are not particularly well studied. Only about 320 species have been formally described, all marine and mostly from shallow waters. While some burrow into sand and mud, others live in crevices in rocks, or in empty shells. Still others bore into rock.

     
     

    The Priapulida Worms  - The Priapulida, or Priapula are a small phylum (about 16 species known to scientists) of normally small worm like animals. They range in length from 0.5 mm for species of Tubuluchus to around 200 mm for species of Priapulus. They occur in most seas, both tropical and polar, at a variety of depths, from shallow coastal waters to as far down as 7,200 meters. The name Priapulida refers to the fact that the scientists who first named them thought they looked like a human penis, however they look more like some species of cacti and it would not be unreasonable to use the term Cactus Worms as their common name. All known species are marine and benthic (meaning they live in or on the sea floor).

     
  

                                      

   These are what give the "bristle" worms their name. Each member of this family comes armed in various rigid hair like structures that are used for defensive means. Do not attempt to handle these worms since their bristles easily penetrate human skin, depending on the species and your own personal reaction level, the resulting puncture can result in anything from a mild irritation to being very pain full. Do not attempt to rub them off as you will only drive the bristles in deeper. Instead, soak the effected area in very warm vinegar until the bristles dissolve.  A thank you to David Lee for sacrificing his finger in the name of science!
   

    The  Phyllodocidae  - The phyllodocids are slender errant worms often brilliant green, yellow or red. Most of them live in crevices and under stones. Active predators of other small worms. The phyllodocids are common shallow-water polychaetes commonly associated with hard substrates and corals rather than mud and sand.

      
       
     
          
     

  
     The  Amphinomidae  - Amphinoids are also known as "fireworms" and have calcareous setae filled with poisonous secretions. When irritated, they erect their sharp setae which break off at a touch releasing their poisonous contents into the wound. Most species are found on corals, rocks and other hard substrata covered with attached organisms. They swim well and can be brilliantly colored. All but one species can be considered harmless and are great scavengers.

        
      
   Species shown below : Hermodice carunculata  - This is the one species of fireworm that you do not want in your aquarium as it is a known predator of corals.
   
     
    The below pictured specimen is a Pherecardia striata, also a member of the fireworms and is a scavenger.
         
      Genus Pareurythoe , which has very small caruncles.
       
      Eurythoe complanata  -  Probably the most commonly found worm and is a harmless scavenger, a benefit to our systems.
       
         
  
     The Hesionidae  -  The hesionids are active and carnivorous. They are commonly found in hard substrates and shallow water rarely in deep water and soft sediments. The larger hesionids are carnivores feeding on polychaetes and other small invertebrates. Some may be deposit-feeders, ingesting detritus. 
    
        
     
     
     Below, a Leocrates spp.  (member of the Hesionidae)
     

      The Myzostomida -  Quote, Leslie Harris  "There's been a lot of debate about where these belong but traditionally they've been considered polychaetes. The "legs" are parapodia; each one has a stout hook (or a few) which enable them to hold onto their hosts which are usually crinoids or other echinoderms. Some are external symbionts, others are internal & can be found in galls or in the body." Unquote.

     


     The Eunicids - The eunicids sometimes known as "rock worms" are mainly omnivores and live largely on detritus. They may be scavengers, and are found mostly in rocky habitats. Some species build tubes while others burrow into limestone. They are either errant or tubicolous frequently inhabiting coral reefs, sands and muddy substrates. These are the most commonly found worms that I encounter within live rock tunnel networks.

    A Phylogenetic analysis of the genus Eunice  -  An excellent identification guide. Down load in .pdf format.
 
       
       
     Some of the larger species, or those that grow to adult size in your aquarium may pose a threat to some sessile invertebrates and possibly some types of coral species. I have at least one large specimen in my aquarium and it has posed no threat to anything.
       
    Jaw extension , and a great visual clue as to why you do not want this in your aquarium.
       
   Below is a typical rock burrow that the larger specimens, such as the one above, will fashion by "gluing" rocks together with a mucus secretion. It is thought that these species account for new coral reef habitats as they bring together smaller rocks, thus forming a larger area that corals can settle upon, encrust and start the makings of a new reef habitat.
   


     The Syllidae  - The syllids are a large and diverse group of active worms which are mostly found creeping over sponges, ascidians, hydroids, bryozoa and algae or burrowing in the surface layers of silt and are common in protected sandbanks. They pierce the skin of these sedentary animals and pump out the juice. They are common in shallow water forms, and tend to be numerous on hard substrates and soft-bottom sediments.

      
    Species shown in the below three photos is a member of the Amblyosyllis genus ( a Syllidae )
        
     
        

    Syllidae SubFamily : Eusyllinae
     
     
     


   Syllidae SubFamily :  Autolytinae
    Specimen shown below is about 2mm in length, a very tiny worm yet its stripes are still visible from a distance.
      

   
    The Nereids  (Quote, Leslie Harris ) "Carnivores, predators, scavengers, detritivores, omnivores, herbivores - pretty much every type of feeding except filter feeding & parasitism occur in the family. Many species are switch hitters. If a tasty pod is crawling by they'll grab it but if not they'll eat mud & digest the attached bacteria, detritus, etc. "

 

      

       
       
     

     The Lumbrinerids - They are primarily soft bottom inhabitants, but can occur on hard substrates. Some construct mucus tubes. They have a well-developed jaw apparatus suitable for grasping food material. Most of them are predaceous carnivores or scavengers.

      


     The Oenonids   ( Predator of snails and clams ) - Oenonids are mostly large worms that burrow in sand and mud, but a number of species are parasitic. They resemble lumbrinerids in appearing earthworm-like, but can be distinguished from them by the presence of a flattened head. 

     
      


     The Glycerids  - The glycerids burrow in sandy or muddy substrata by means of their reversible proboscis.  The purpose of burrow construction is for prey capture. The glycerids detect the hydrostatic change within the burrow when motile polychaetes or crustaceans move across the burrow system and thus shoot out their proboscis and grab its prey.  They are mainly carnivorous.

      
     


      The Capitellidae   - The capitellids commonly known as "lugworms" resemble the earthworms and have similar habits.  They burrow in various grades of sandy mud inhabiting mucus-lined tubes. Their guts are filled with inorganic matter along with organic matter.

     


      The Dorvilleidae - Members of this family are much smaller than most of the more larger, visible families and pose no real threat to other life in our aquariums. They will appear as very small white lines on the glass of our aquariums and are very common. Each specimen that I have examined under a microscope appears to be gut loaded with algae. While a good many species are extremely small, there are of course much larger species, some of which have been reported up to three inches long.

 
     The species shown below is about 1 mm in length and is in the genus group Ophryotrocha
     


      The Spintheridae  -  An ectoparasite of sponges.  Extremely small and difficult to see when laying flat against its sponge prey. It does however leave behind its tell tale damage as shown in the below photo.  Very similar in appearance to harmless Bryozoans.