Monocorophium sextonae -

Monocorophium sextonae original area of occurrence is uncertain. Some scientists suspect that this mud shrimp originates from New Zeeland, as the species was first discovered in this region (before 1921) ^{[3, 4]}.

The species is suspected to be non-indigenous, based on observations in Plymouth (England). Although Monocorophium sextonae was not found there before 1911, it suddenly became very abundant 20 years later (in 1937) ^[5].

First observation in Belgium

Monocorophium sextonae was – under its former name Corophium sextonae – first observed in Belgium during the summer of 1993 in samples taken off the west coast ^[6].

Spreading in Belgium

The species is common on wrecks in the North Sea during autumn and winter ^[7] and can reach densities of more than 4,000 individuals per m² ^[8]. In 2009, it was observed in the port of Zeebrugge ^[9], albeit in low abundance ^[10]. Furthermore, in the study area, Monocorophium sextonae can also be found in the Dutch Western Scheldt ^[11].

Spreading in neighbouring countries

Monocorophium sextonae was described in 1937 based on specimens collected near Plymouth, Great Britain. The species was already abundant here in 1934. Another specimen was collected in 1930 near Lisbon (Portugal) in the Tagus estuary ^[5].

From Britain, it probably spread naturally to Ireland, where it was first discovered in 1982 ^[12]. Currently, it can be found in the southern and western British Isles with a northern extension to Scotland and Ireland ^[13].

In the Netherlands, Monocorophium sextonae was first found in 1952 in IJmuiden. In 1953, Monocorophium sextonae was found on the brown algae of the Himanthalia group between Zandvoort and Noordwijk, in 1956 on the Uithuizerwad in the Wadden Sea and in 1960 on the eggs of the whelk Buccinum between Katwijk and Wassenaar ^[4]. In 2000, the species was commonly observed in the southwest of the Netherlands on hard substrates in places with increased salinity, such as Lake Grevelingen and the Eastern and Western Scheldt estuary ^[11].

Also, in other European countries –  like Italy (1950) ^[3], Norway (1985) and Germany (1997) ^[14] – the species was reported.

Since the exact area of origin of Monocorophium sextonae is unknown, it remains uncertain whether it is a non-indigenous species to Europe ^{[4, 15]}.

Assuming it is non-native, it was probably introduced to Europe via fouling on ships’ hulls ^[3], via ships’ ballast water or via the import of oysters. This shrimp lives in self-made tubes and attaches itself to a solid substrate (e.g. ships’ hulls) ^[14]. Once introduced, the species can spread further via natural flows ^[14].

Following its introduction into Plymouth, England, the rise of Monocorophium sextonae was accompanied by a decline in the abundance of the amphipod Crassicorophium bonellii. Competition for food and space may have been at the roots of this decline ^[13]. Therefore, some authors think that this invader may compete with the indigenous species Crassicorophium bonellii. Both species filter their food from the water column and are bottom dwellers that live in tunnels of their own making ^[12]. Others think that the success of the species was due to its higher tolerance to elevated temperatures. According to them, Monocorophium sextonae would differ enough from Crassicorophium bonellii to allow these two species to live together ^[12].

Monocorophium sextonae seems to have a high tolerance for variations in salinity ^[3]. Most likely, the temperature will play an important role in the further spread of this species ^[13]. Otherwise, little is known about the factors that might affect its spread.

Not much is known about the effect of Monocorophium sextonae on the ecosystem within the study area. The potential effects are estimated to be low ^[16] or insignificant ^[13] although it should be noted that the role of this mud shrimp in the decline of Crassicorophium bonellii in Plymouth (UK) is still under discussion ^[12] (see above).

Monocorophium sextonae live in homemade tunnels of mud on kelp and other large algae, sponges, corals and artificial substrates ^[12]. Furthermore, the species is found in soft sediments, down to a depth of about 50 metres ^[14].

Females are, with their 5 mm in size, one millimetre larger than males. Both have a yellow-orange colour and small eyes ^[3].

[1]          World Register of Marine Species (WoRMS) (2020). Monocorophium sextonae (Crawford, 1937). [http://www.marinespecies.org/aphia.php?p=taxdetails&id=148603] (2020-11-17).

[2]          Stock, J.H. (1994). De ‘slijkgarnaal’ Corophium sextonae (Amphipoda) plotseling talrijk in de Oosterschelde. Het Zeepaard 54(4): 82-84. [http://www.vliz.be/nl/catalogus?module=ref&refid=208109]

[3]          Hurley, D.E. (1954). Studies on the New Zealand amphipodan fauna No. 7. The family Corophiidae, including a new species of Paracorophium. Trans. R.Soc. N.Z. 82(2): 431-460. [http://www.vliz.be/nl/catalogus?module=ref&refid=207782]

[4]          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]

[5]          Crawford, G.I. (1937). A review of the amphipod genus Corophium, with notes on the British species. J. Mar. Biol. Ass. U.K. 21(2): 589-630. [http://www.vliz.be/nl/catalogus?module=ref&refid=123246]

[6]          Dewicke, A. (2002). De hyperbenthische gemeenschappen van de Noordzee = Hyperbenthic communities of the North Sea. PhD Thesis. University of Ghent: Gent. 219 + 1 cd-rom pp. [http://www.vliz.be/nl/catalogus?module=ref&refid=21549]

[7]          Mallefet, J.; Zintzen, V.; Massin, C.; Norro, A.; Vincx, M.; De Maersschalck, V.; Steyaert, M.; Degraer, S.; Cattrijsse, A.; Vanden Berghe, E. (2008). Belgian shipwreck: hotspots for marine biodiversity BEWREMABI: final report. Belgian Science Policy: Brussel. 151 pp. [http://www.vliz.be/nl/catalogus?module=ref&refid=126030]

[8]          Zintzen, V. (2005). Les amphipodes tubicoles de épaves du Plateau Continental Belge. De Strandvlo 25(2): 38-49. [http://www.vliz.be/nl/catalogus?module=ref&refid=97453]

[9]          Boets, P.; Lock, K.; Goethals, P.L.M. (2012). Assessing the importance of alien macro-Crustacea (Malacostraca) within macroinvertebrate assemblages in Belgian coastal harbours. Helgol. Mar. Res. 66(2): 175-187. [http://www.vliz.be/en/imis?module=ref&refid=206987]

[10]        Boets, P. (2019). Persoonlijke mededeling

[11]        Faasse, M.; Van Moorsel, G. (2000). Nieuwe en minder bekende vlokreeftjes van sublitorale harde bodems in het Deltagebied (Crustacea: Amphipoda: Gammaridea). Ned. Faunist. Meded. 11: 19-44. [http://www.vliz.be/en/imis?module=ref&refid=37555]

[12]        Costello, M.J. (1993). Biogeography of alien amphipods occurring in Ireland, and interactions with native species. Crustaceana 65(3): 287-299. [www.vliz.be/en/imis?module=ref&refid=205695]

[13]        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]

[14]        Naylor, M. (2006). Alien species in Swedish seas: Monocorophium sextonae. Informationscentralerna för Bottniska viken, Egentliga Östersjön och Västerhave. Sweden. 2 pp. [http://www.vliz.be/en/imis?module=ref&refid=207325]

[15]        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]

[16]        Wijsman, J.W.M.; De Mesel, I. (2009). Duurzame schelpdiertransporten. IMARES Wageningen Report. Imares: Wageningen. 111 pp. [http://www.vliz.be/nl/catalogus?module=ref&refid=207323]

VLIZ Alien Species Consortium (2020). Monocorophium sextonae. Non-native species of the Belgian part of the North Sea and adjacent estuaries anno 2020. Flemish Institute for the Sea (VLIZ). 5 pp.