Acartia (Acanthacartia) tonsa -

The origin of this copepod species is unknown. However, before its introduction in Europe, the species occurred in the Indo-Pacific region and along the east coast of the United States ^[2].

First observation in Belgium

In 1952, this copepod was observed for the first time near Lillo, in the Sea Scheldt [3].

Spreading in Belgium

A few years after the first sighting in the Sea Scheldt, the species was spotted in the sluice-dock of Ostend in 1960-1961 [4]. In the course of the 1980s, this copepod was dominant there [5]. In the Scheldt estuary, the species has been dominant in the saltwater section since the 1960s, especially during the summer [6, 7].

Spreading in neighbouring countries

On the 8th of April 1925, specimens of this species were found in the brackish water section of the Canal of Caen à la Mer, near Ouistreham (NW France) [8]. In the following years, this species was found in great numbers in other European countries. In the early 1930s, its presence was reported in the German Weser River (between Bremen and Bremerhaven) and in the Dutch Zuiderzee. The high abundance in which this exotic species was repeatedly observed, suggested that it already occurred in these places before this time. To confirm this suspicion, old collections of plankton samples from the Dutch Zuiderzee were analysed again. The oldest samples in which the species was found date from August and September 1916. Older material, collected before June 1912, didn’t contain any Acartia (Acanthacartia) tonsa. Unfortunately, no samples were available for the period between June 1912 and August 1916. As a result, the exact year of introduction couldn’t be determined [9].

Since the end of the 1970s, this species was found in the brackish waters along the European coast from Normandy (France) to the Gulf of Finland (Baltic Sea) [10, 11]. Further south, the species reached the Tagus estuary in Portugal in the 1980s, and since 1998 it has been detected in the Guadalquivir estuary (Southern Spain), as well as in the surrounding ponds [12].

Since the early 1970s, the species can be found in the Black Sea and since 1985 also in the Mediterranean [13].

Most likely the introduction took place via transport in the ballast water of ships ^[8].

As the species is well adapted to high water temperatures, Acartia (Acanthacartia) tonsa is often one of the dominant planktonic species during the summer months ^{[6, 14]}. In our estuaries and coastal areas, many small food particles can easily be eaten by grazers, such as Acartia (Acanthacartia) tonsa  ^[15]. Furthermore, this copepod can tolerate reduced oxygen concentrations, which often occurs in locations with little or no water flow ^[14].

Part of the success of this non-native species is due to the production of resting eggs, also known as diapause eggs. These resting eggs have a solid protective wall and are formed under unfavourable environmental conditions – such as a decrease in temperature – after which they sink to the bottom. When conditions improve, the resting eggs hatch and develop into active swimming organisms ^[16]. In the presence of sufficient food and a temperature above 20 °C, an egg can develop into an adult organism within ten days ^[17].

In addition, resting eggs can be transported with ballast water ^[2]. Adult copepods can also be brought to new locations with ships’ ballast water ^[8] or be spread locally by the prevailing currents.

Acartia (Acanthacartia) tonsa actively swims in the water column and is part of the so-called zooplankton. It is a typical estuarine species, but it can tolerate a wide range of salinities. Thus, this exotic species can thrive in coastal marine environments and (near fresh) brackish lakes ^[18].

This species is most active at temperatures above 20 °C, which favours the distribution of these copepods in warmer areas – and in waters where the temperature is raised artificially by industrial activities [8]. At moderate to low temperatures, the distribution of the species appears to be limited to somewhat less saline environments, with salinities of less than 33 PSU ^[10]. By way of comparison, our North Sea has a salt concentration of 35 PSU.

Seasonally, as the water gets warmer, this exotic species of copepod occurs in large numbers. In both the Eastern and Western Scheldt, such an effect has been demonstrated: in the summer, Acartia (Acanthacartia) tonsa replaces another species of copepod, namely Eurytemora affinis affinis ^{[6, 14]}.

Copepods are microscopically small organisms. The female individuals of the species Acartia (Acanthacartia) tonsa measure about 1.5 mm, while the males can measure up to 1 mm ^[19].

The body is sausage-shaped and consists of two parts. The most prominent part is the ‘cephalothorax’, consisting of the head and the thorax. This cephalothorax is rounded at the front, carries two pairs of antennae, several mouthparts on the head and five pairs of legs on the chest. The first pair of antennae are much longer than the second pair. The other part – the belly or ‘abdomen’ – is shortened to only one-quarter of the head and the thorax ^[19].

During the day, this species sinks to deeper water, where it is less noticeable for visually hunting fish. At night, it moves to the upper water layers to feed on all kinds of small food particles ^[20]. It swims in an irregular pattern, known as ‘hop and sink’. Because Acartia (Acanthacartia) tonsa often remains motionless in the water column, it is less noticeable to predators among other suspended particles. If a fish gets too close, it can escape due to its irregular swimming behaviour ^[21].

[1]         World Register of Marine Species (WoRMS) (2020). Acartia (Acanthacartia) tonsa Dana, 1849. [http://www.marinespecies.org/aphia.php?p=taxdetails&id=345943] (2020-11-17).

[2]         Eno, N.C.; Clark, R.A.; Sanderson, W.G. (Ed.) (1997). Non-native marine species in British waters: a review and directory. Joint Nature Conservation Committee: Peterborough. ISBN 1-86107-442-5. 152 pp. [http://www.vliz.be/nl/imis?module=ref&refid=24400]

[3]         Leloup, E.; Konietzko, B. (1956). Recherches biologiques sur les eaux saumâtres du Bas-Escaut. Mémoires de l'Institut Royal des Sciences Naturelles de Belgique = Verhandelingen van het Koninklijk Belgisch Instituut voor Natuurwetenschappen, 132. Koninklijk Belgisch Instituut voor Natuurwetenschappen: Brussel, Belgium. 100, 5 plates pp. [http://www.vliz.be/en/imis?module=ref&refid=26732]

[4]         Leloup, E.; Polk, P. (1967). La flore et la faune du Bassin de Chasse d'Ostende (1960-1961): III. Etude zoologique. Mémoires de l'Institut Royal des Sciences Naturelles de Belgique = Verhandelingen van het Koninklijk Belgisch Instituut voor Natuurwetenschappen, 157. Natuurwetenschappen, K.B.I.v.: Brussel, Belgium. 114, 3 plates pp. [http://www.vliz.be/en/imis?module=ref&refid=5289]

[5]         Tackx, M.; Polk, P. (1982). Feeding of Acartia tonsa Dana (Copepoda, Calanoida): predation on nauplii of Canuella perplexa  T. & A. Scott (Copepoda, Harpacticoida) in the Sluice-dock at Ostend. Hydrobiologia 94: 131-133. [http://www.vliz.be/nl/catalogus?module=ref&refid=3388]

[6]         Soetaert, K.; Van Rijswijk, P. (1993). Spatial and temporal patterns of the zooplankton in the Westerschelde estuary. Mar. Ecol. Prog. Ser. 97(1): 47-59. [www.vliz.be/en/imis?module=ref&refid=8467]

[7]         Tackx, M.L.; De Pauw, N.; Van Mieghem, R.; Azémar, F.; Hannouti, A.; Van Damme, S.; Fiers, F.; Daro, N.; Meire, P. (2004). Zooplankton in the Schelde estuary, Belgium and the Netherlands: spatial and temporal patterns. J. Plankton Res. 26(2): 133-141. [http://www.vliz.be/en/imis?module=ref&refid=57465]

[8]         Remy, P. (1929). Note sur un Copépode de la saumâtre du canal de Caen à la mer [Acartia (Acantartia) tonsa Dana]. Annales de Biologie Lacustre 15: 169-186. [http://www.vliz.be/en/imis?module=ref&refid=197114]

[9]         Redeke, H.C. (1934). On the occurrence of two pelagic copepods, Acartia bifilosa and Acartia tonsa, in the brackish waters of the Netherlands. ICES J. Mar. Sci./J. Cons. int. Explor. Mer 9(1): 39-45. [http://www.vliz.be/en/imis?module=ref&refid=15937]

[10]       Brylinski, J.-M. (1981). Reports on the presence of Acartia tonsa Dana (Copepoda) in the area of Dunkirk and its geographical distribution in Europe. J. Plankton Res. 3(2): 255-261. [http://www.vliz.be/en/imis?module=ref&refid=110851]

[11]       Brylinski, J.-M. (2009). The pelagic copepods in the Strait of Dover (Eastern English Channel). A commented inventory 120 years after Eugène Canu. Cah. Biol. Mar. 50(3): 251-260. [http://www.vliz.be/en/imis?module=ref&refid=206737]

[12]       Frisch, D.; Rodriguez-Perez, H.; Green, A. (2006). Invasion of artificial ponds in Donana Natural Park, southwest Spain, by an exotic estuarine copepod. Aquat. Conserv. 16: 483-492. [http://www.vliz.be/nl/catalogus?module=ref&refid=208859]

[13]       Gubanova, A. (2000). Occurence of Acartia tonsa Dana in the Black Sea. Was it introduced from the Mediterranean? Mediterr. Mar. Sci. 1(1): 105-109. [http://www.vliz.be/nl/catalogus?module=ref&refid=1176]

[14]       Bakker, C.; Phaff, W.J.; van Ewijk-Rosier, M.; De Pauw, N. (1977). Copepod biomass in an estuarine and a stagnant brackish environment of the S.W. Netherlands. Hydrobiologia 52(1): 3-13 52(1): 3-13. [http://www.vliz.be/en/imis?module=ref&refid=3560]

[15]       Bakker, C. (1978). Some reflections about the structure of the pelagic zone of the brackish Lake Grevelingen (SW-Netherlands). Hydrobiol. Bull. 12(2): 67-84. [http://www.vliz.be/en/imis?module=ref&refid=136373]

[16]       Zilhoux, E.J.; Gonzalez, J.G. (1972). Egg dormancy in a neritic calanoid copepod and its implications to overwintering in boreal waters, in: Battaglia, B. 5th European Marine Biology Symposium. Piccin Editore: Padova: pp. 217-230. [http://www.vliz.be/en/imis?module=ref&refid=197266]

[17]       Leandro, S.M. (2006). Growth and development of nauplii and copepodites of the estuarine copepod Acartia tonsa from southern Europe (Ria de Aveiro, Portugal) under saturating food conditions. Mar. Biol. (Berl.) 150(1): 121-129. [http://www.vliz.be/nl/catalogus?module=ref&refid=260047]

[18]       Bakker, C.; De Pauw, N. (1975). Comparison of plankton assemblages of identical salinity ranges in estuarine tidal, and stagnant environments: II. Zooplankton Neth. J. Sea Res. 9(2): 145-165. [http://www.vliz.be/en/imis?module=ref&refid=13995]

[19]       Rose, M. (1933). Copépodes pélagiques. Faune de France, 26. Paul Lechevalier: Paris, France. 374 pp. [http://www.vliz.be/en/imis?module=ref&refid=111111]

[20]       Gómez-Aguirre, S. (2001). Migración vertical de Acartia tonsa y A. lilljeborgii (Crustacea: Copepoda) durante un eclipse de sol. An. Inst. Biol. Univ. Nac. Autón. Méx. (Zool.) 72(2): 167-175. [www.vliz.be/en/imis?module=ref&refid=28794]

[21]       Buskey, E.J. (1994). Factors affecting feeding selectivity of visual predators on the copepod Acartia tonsa: locomotion, visibility and escape responses. Hydrobiologia 292/293: 447-453. [http://www.vliz.be/en/imis?module=ref&refid=99129]

VLIZ Alien Species Consortium (2024). Acartia (Acanthacartia) tonsa – Copepod. Introduced alien species of the Belgian part of the North Sea and adjacent estuaries anno 2024. Flanders Marine Institute (VLIZ). 5 pp.