Tubificoides heterochaetus -

T. heterochaetus occurs along both sides of the North Atlantic Ocean and has been observed in the northeast of the Pacific Ocean. The northwest Atlantic region is a hotspot of species diversity for the genus Tubificoides. Thus, T. heterochaetus might have originated here. However, since the species was already widespread at the time of its discovery, there may be other regions where it naturally lives ^[2].

Although the species occurs at both sides of the Atlantic Ocean, it only thrives in brackish waters ^[3].

First observation in Belgium

T. heterochaetus was first observed in Belgium on the 25^th of September 1952 in the Scheldt estuary near Doel in the mud of a ditch. The species was then misidentified as Limnodrilus heterochaetus ^[4]. However, it is possible that the species has always been present in our regions but was not noticed before. Therefore is this species considered cryptogenic ^[5].

Spreading in Belgium

In Belgium, the species occurs in the brackish water zone of the Sea Scheldt, between Antwerp and the Dutch border ^[6]. Because worms belonging to the subclass Oligochaeta – to which T. heterochaetus belongs – are rarely determined to species level ^[7], it is uncertain whether this species occurs beyond the Dutch border. However, they do likely occur there since the species is very common in the Sea Scheldt near the Dutch border ^[8]. In the Yser estuary, the species has not yet been observed. However, two other species of this genus occur in the estuary, namely T. diazi and T. brownei ^[8].

Spreading in neighbouring countries

T. heterochaetus occurs widespread throughout Europe ^[9]. The worm got first described in 1926, based on individuals from the Baltic Sea. Later, the species was found in the Elbe and Weser estuary in Germany, and in Finland ^[10]. In the Elbe estuary, the species is currently found in water with salinity varying between 1 PSU to 15 PSU ^{[10, 11]}. By comparison, the North Sea has a salinity of 35 PSU.

In the former Dutch Zuiderzee – currently Lake Ijssel (In Dutch: Ijsselmeer) – this oligochaete appeared to be the most common species in 1927, although it was not reported there during a study in 1921. This indicates that the introduction of this species to the Zuiderzee must have occurred between 1921 and 1927 ^[10]. Another explanation for the absence of the species in 1921 may be the very high salinity of the Zuiderzee back then ^[12]. Genetic research on Dutch populations in Lake Grevelingen indicates that the species got introduced multiple times. Furthermore, T. heterochaetus has been observed in Europe in the Black Sea and Mediterranean Sea ^[2]. The worm was identified in the western part of the old Meuse back in 2012 ^[13]. In Denmark, the species was first observed in 2006 ^[14]. According to a report from 2014, the species also occurs in the port of Gdynia, in Poland ^[15].

It is unknown how this species got introduced to Belgian waters ^[10].

T. heterochaetus thrives in places containing a lot of organic material, such as the Scheldt estuary. It feels at home in various sediments and survives well in sand, silt, mud and clay soils. The species dominates in areas with a certain degree of pollution (eutrophication) ^{[6, 16, 17]}.

Often, T. heterochaetus is abundant in the brackish water zones of estuaries ^[3]. The species has been observed at salinities of 0.5 to 20 PSU but thrives in environments with a salinity between 2 and 14 PSU ^[17].

During the winter, this worm retreats deep into the sediment, where it is warmer. During the summer, the species can mainly be found in the upper 5 cm of the sediment. The adult individuals are equally spread over the 5 cm, while the young worms are mainly present in the upper layers ^[3].

T. heterochaetus – like most other estuarine species – is resistant to a certain degree of pollution. If there is sufficient oxygen, the species thrives in areas with increased content of organic matter ^{[3, 16, 17]}.

Where these worms occur in large numbers, they have an important influence on the structure and chemistry of the sediment-water interface. They turn over the soil to great depths and are characterized by their uninterrupted activity. These and other related worms actively pull the organic matter down into the soil. They dig through the topsoil layer with burrows, loosening the soil and promoting the growth of bacteria – the worm’s favourite meal. The process of turning around the soil is called bioturbation ^[3].

In the mudflats of contaminated estuaries, like the Scheldt, these worms are an important food source for many birds and fishes ^[3].

T. heterochaetus is a small, slim oligochaete worm measuring between 5 and 9 mm ^{[17, 18]}. The body is divided into 46 to 66 segments ^[17]. Hairy structures on the segments (brushes or chaetae) are used by the worm to move around. These brushes are important for the identification to species level ^[19].

T. heterochaetus lives freely in the soil. It is a dominant species in estuaries and creeks, at least in those affected by tidal influences ^[19]. It is most abundant from July to December when the worms reproduce. During the winter, this oligochaete moves to the warmer, deeper layers ^[6].

If for some reason T. heterochaetus loses its tail, e.g. as a result of predation by a bird or fish, it grows a new one. This process is called regeneration ^[3]. Other aquatic worms, just like the closely related earthworms, can also grow back body parts. This regeneration is made possible by their stem cells that continue to divide ^[20].

[1]          World Register of Marine Species (WoRMS) (2020). Tubificoides heterochaetus (Michaelsen, 1926). [http://www.marinespecies.org/aphia.php?p=taxdetails&id=137577] (2020-11-17).

[2]          Kvist, S.; Sarkar, I.N.; Erséus, C. (2010). Genetic variation and phylogeny of the cosmopolitan marine genus Tubificoides (Annelida: Clitellata: Naididae: Tubificinae). Mol. Phylogenet. Evol. 57(2): 687-702. [http://www.vliz.be/imis/imis.php?module=ref&refid=206622]

[3]          Seys, J.; Vincx, M.; Meire, P. (1999). Macrobenthos van de Zeeschelde, met bijzondere aandacht voor het voorkomen en de rol van Oligochaeta: eindrapport OMES 1995-1998, partim Benthos. Rapport Instituut voor Natuurbehoud, 99.4. Instituut voor Natuurbehoud: Brussel. 81 pp. [http://www.vliz.be/imis/imis.php?module=ref&refid=10355]

[4]          Konietzko, B. (1953). Notes sur les Oligochètes de Belgique: 1. Eaux saumâtres du Bas-Escaut. Med. K. Belg. Inst. Nat. Wet. 29(43): 1-14. [http://www.vliz.be/imis/imis.php?module=ref&refid=20524]

[5]          Kerckhof, F.; Haelters, J.; Gollasch, S. (2007). Alien species in the marine and brackish ecosystem: the situation in Belgian waters. Aquat. Invasions 2(3): 243-257. [http://www.vliz.be/en/imis?module=ref&refid=114365]

[6]          Seys, J.; Vincx, M.; Meire, P. (1999). Spatial distribution of oligochaetes (Clitellata) in the tidal freshwater and brackish parts of the Schelde estuary (Belgium). Hydrobiologia 406: 119-132. [http://www.vliz.be/en/imis?module=ref&refid=11714]

[7]          Wijnhoven, S. (2011). Persoonlijke mededeling

[8]          Soors, J. (2019). Persoonlijke mededeling

[9]          Brinkhurst, R.O. (1981). A contribution to the taxonomy of the Tubificinae (Oligochaeta: Tubificidae). Proc. Biol. Soc. Wash. 94(4): 1048-1067. [http://www.vliz.be/imis/imis.php?module=ref&refid=206624]

[10]        Wolff, W.J. (2005). Non-indigenous marine and estuarine species in the Netherlands. Zool. Meded. 79(1): 3-116. [http://www.vliz.be/en/imis?module=ref&refid=101200]

[11]        Taupp, T.; Wetzel, M.A. (2014). Leaving the beaten track – approaches beyond the Venice System to classify estuarine waters according to salinity. Est., Coast. and Shelf Sci. 148: 27-35. [http://www.vliz.be/nl/catalogus?module=ref&refid=297674]

[12]        Van Haaren, T. (2011). Persoonlijke mededeling

[13]        Peeters, E.T.H.M.; De Lange, H.J.; de la Haye, M.A.A.; Reeze, A.J.G.; Postma, J.F. (2012). Achtergrondrapport KRW-maatlat macrofauna R8. Ecofide rapport.nummer 43b. RWS Waterdienst: Weesp. 117 pp. [http://www.vliz.be/nl/catalogus?module=ref&refid=297680]

[14]        Erséus, C.; Kvist, S. (2007). COI variation in Scandinavian marine species of Tubificoides (Annelida: Clitellata: Tubificidae). J. Mar. Biol. Ass. U.K. 87(5): 1121-1126. [http://www.vliz.be/nl/catalogus?module=ref&refid=117214]

[15]        Heyer, K. (2015). HELCOM BALSAM Project WP 4: Non indigenous species- multi disciplinary monitoring schemes to gain synergies for ballast water risk-management and environmental monitoring. Part: Testing harmonized criteria for Risk Assessments Final Report (v1). BSH - Federal Maritime and Hydrography Agency: Hamburg. 18 pp. [http://www.vliz.be/nl/catalogus?module=ref&refid=300989]

[16]        Diaz, R.J. (1989). Pollution and tidal benthic communities of the James River Estuary, Virginia. Hydrobiologia 180: 195-211. [http://www.vliz.be/imis/imis.php?module=ref&refid=206329]

[17]        Harrel, R.C. (2004). Systematic and ecological notes on Tubificoides heterochaetus from the Neches River estuary, Texas. Texas J. Sci. 56(3): 263-267. [http://www.vliz.be/imis/imis.php?module=ref&refid=206626]

[18]        Brinkhurst, R.O.; Baker, H.R. (1979). A review of the marine Tubificidae (Oligochaeta) of North America. Can. J. Zool. 57(8): 1553-1569. [http://www.vliz.be/imis/imis.php?module=ref&refid=206367]

[19]        Worsfold, T.M. (2005). Introduction to Oligochaetes. NMBAQC Benthic Invertebrate Taxonomic Workshop, November 2003. NMBAQC: Great Britain. 22 pp. [http://www.vliz.be/imis/imis.php?module=ref&refid=206620]

[20]        Scientias: Nieuws over wetenschap en technologie (2018). De regeneratie van de worm ontrafeld. [http://www.scientias.nl/de-regeneratie-van-de-worm-ontrafeld/7801] (2018-10-09).

VLIZ Alien Species Consortium (2020). Tubificoides heterochaetus. Non-indigenous species in the Belgian part of the North Sea and adjacent estuaries anno 2020. Flanders Marine Institute (VLIZ). 5 pp.