Introduction to polychaetes (bristle worms)
The polychaetes are segmented worms (Phylum Annelida), as are earthworms (Oligochaeta). The polychaetes can be distinguished by having paired, lateral appendages, parapodia, on each segment. The parapodia are usually divided into dorsal notopodia and ventral neuropodia. Each lobe is equipped with cuticular setae (bristles) of different size and shape, and in some species the parapodial lobes are supported by a chitinous acicula. Also gills, if present, are often associated with the parapodia. In front of the mouth is the prostomium, which may be equipped with eyes and various appendages, such as cirri and palps, but not parapodia or setae. Behind the prostomium and surrounding the mouth is the peristomium, which also sometimes bears long appendages, called tentacles or cirri. Behind the peristomium is the first chaetiger, i.e. segment bearing setae (=chaetae). The posteriormost segment bearing the anus is called the pygidium. It usually bears a pair of caudal cirri and sometimes is also shaped differently from other segments. Many polychaetes have an eversible proboscis, sometimes equipped with jaws and small cuticular paragnaths, which may be important for identification.
The class Polychaeta is one of the most speciose marine invertebrate taxa, containing more than 700 species in British and German waters (Hartmann-Schröder, 1996), about 600 species in Norway (Holthe & Brattegard, 2001) and almost 500 species in Danish waters (Kirkegaard, 1992, 1996). Classification remains unstable, new species are still described, and there are no recent identification keys covering all polychaete groups of northern European waters. Polychaetes often comprise the majority of individuals in a sample of marine macrofauna. Polychaetes are found in all oceans and seas, and many species are tolerant of very low salinities, a few species living in freshwater. They also occur at all depths, in soft bottoms, on hard substrates, and some are pelagic. Polychaetes are often difficult to identify, partly because many species are morphologically similar, and partly because they often break up during collection so only fragments remain in the sample. Furthermore many species are very small (<20 mm in length and <2 mm in diameter), so a microscope is needed for identification. A complete front end is necessary for identification of most species, and in some cases an intact posterior end is also needed, or the number of segments with a certain kind of setae, gills or parapodial lobes needs to be counted. Hence even the laboratories that routinely identify polychaetes for marine monitoring programs have to give up identifying a polychaete to species level, and official "species lists" contain some taxa identified only to genus or even family level. Molecular techniques, are under development and may prove invaluable tools for species level identification of polychaetes in the future.
Polychaetes display a variety of habits. Some crawl on the surface of hard or soft bottoms, others bury deep into the sediment, building permanent or temporary tubes. Certain families are characterized by building calcareous tubes and remain inside these tubes their entire lives. Such species often have a "crown" of tentacles surrounding the mouth. Making observations on living worms may be helpful for identification. Most polychaetes have planktonic larvae, and the number of segments with parapodia etc. gradually increases as the juvenile worm grows. New segments are added in front of the pygidium, and hence the smallest segments are at the posterior end of the worm.
Planktonic larvae of polychaetes can be introduced with ballast-water. Small, tube-building species are often part of fouling communities and can be introduced as hull-fouling or associated with aquaculture species. Some species actually bore in the shells of molluscs, including commercially cultured species.
Several species of Polychaeta have been introduced to Nordic waters. Some are only considered introduced in some Nordic countries, but native or cryptogenic in others. In addition several species are considered cryptogenic in neighbouring countries (Wolff, 2005), though they have so far been considered native in those Nordic countries where they occur. These species are mostly very small (<2 mm in diameter) and therefore have been overlooked or misidentified previously.
List of introduced polychaetes in Nordic waters and their distribution in northern European waters (from Brattegard & Holthe, 1997; Wolff, 2005; Gollasch & Nehring, 2006; Främmande Arter 2006; Jensen, 2009).
| Species | Family | Status | Distribution in northern European waters |
| Alkmaria romijni | Ampharetidae | cryptogenic; alien in Norway | DK, NO, SE, DE, NL, PL |
| Aphelochaete marioni | Cirratulidae | cryptogenic | DK, DE, NO |
| Boccardiella ligerica(syn. Polydora redeki) | Spionidae | alien in the Baltic | NL, DE, FI, SE? |
| Ficopomatus enigmaticus | Serpulidae | alien | DK |
| Marenzelleria arctia | Spionidae | alien | SE, FI |
| Marenzelleria neglecta | Spionidae | alien | SE, FI, DE, PL, ES, LV? |
| Marenzelleria viridis | Spionidae | alien | DK, SE, DE, UK, NL, NO? |
| Microphthalmus similis | Hesionidae | cryptogenic | DE, NL, DK, NO |
| Neanthes succinea | Nereidae | cryptogenic | DK, DE, NL |
| Neanthes virens | Nereidae | cryptogenic in the Netherlands | DK, DE, NL, NO, SE |
| Proceraea cornuta | Syllidae | cryptogenic | NL, DK, NO |
| Scolelepis korsuni | Spionidae | alien? | NO |
| Tharyx killariensis | Cirratulidae | cryptogenic | DE, DK, NO |
A number of additional species have been introduced to the Netherlands and the U.K, mostly with oysters. These can be expected to spread into Nordic waters in the future. The list includes (from Eno et al., 1997; Wolff, 2005):
Clymenella torquata (Leidy, 1855), Goniadella gracilis (Verrill, 1873), Hydroides dianthus (Verrill, 1873), Hydroides elegans (Haswell, 1883), Hydroides ezoensis Okuda, 1934, Janua brasiliensis (Grube, 1872), Pileolaria rosepigmentata (Uchida, 1971), Polydora hoplura Claparède, 1870, Sabellaria spinulosa (Leuckart, 1849).
Brattegard, T. and Holthe, T. 1997. Distribution of marine, benthic macro-organisms in Norway. Utredning for Direktoratet for naturforvaltning, Trondheim, 380 pp.
Eno, N.C., Clark, R.A. and Sanderson, W.G. 1997. Non-native marine species in British waters: a review and directory. Joint Nature Conservation Committee, Peterborough, UK, 136pp.
Fauchald, K. and Rouse, G. 1997. Polychaete systematics: Past and present. Zoologica Scripta 26(2): 71-138.
Främmande arter i svenska hav (2006). http://www.frammandearter.se/
Gollasch, S. and Nehring, S. 2006. National checklist for aquatic alien species in Germany. Aquatic Invasions 1(4): 245-269.
Hartmann-Schröder, G. 1996. Annelida, Borstenwürmer, Polychaeta. Tierwelt Deutschlands 58: 1-648.
Jensen, K.R. 2009. National reports 2008 – Denmark. In: ICES 2009. Report of the Working Group on Introduction and Transfers of Marine Organisms (WGITMO), 11-13 March 2009, Washington D.C., USA, pp. 41-63.
Kirkegaard, J.B. 1992. Havbørsteorme I. Danmarks Fauna 83: 1-416.
Kirkegaard, J.B. 1996. Havbørsteorme II. Danmarks Fauna 86: 1-451.
Wolff, W.J. 2005. Non-indigenous marine and estuarine species in The Netherlands. Zoologische Mededelingen 79(1): 1-116.