Nemopsis bachei - American brackish water medusa

Most researchers assume that N. bachei is native to the Atlantic coast of North America. The species was discovered off the coast of Massachusetts and currently still occurs there from Woods Hole to Florida ^{[2, 3]}. In some areas, it appears only seasonally ^[4]. Some scientists question whether the east coast of North America is the actual area of origin of N. bachei ^[5]. They think it is strange that such an abundant species was discovered this late. A possible explanation could be that marine research in these areas was rather scarce at the time ^[6].

First observation in Belgium

On the 2^nd of September 1996, medusae of N. bachei were observed for the first time in the Wielingdok and Containerdok in the outer port of Zeebrugge ^[4]. Finding the polyps appeared to be more difficult. The small polyps are only noticeable at a certain light incidence. On the 13th of September 2002, in the port of Zeebrugge, polyps were observed for the first time attached to a piece of translucent plastic film ^[7].

Spreading in Belgium

N. bachei is present almost anywhere along the Belgian coast ^[8]. The species is abundant in the Connection Dock in the inner port of Zeebrugge and presumably arrived there via seawater pumped into the dock through the sea lock. N. bachei is also widely distributed in the port of Ostend and Sluice Dock ^[9]. N. bachei is found further offshore at the wind farm on the Thornton Bank ^{[8, 10]}.

This jellyfish species appears in the water column from May until December, with peak densities in June. The highest numbers of N. bachei observed in Belgium are 23 individuals per m^{3 [9]}.

Spreading in neighbouring countries

The first observation of N. bachei in Europe occurred in Scotland (1853), where it was misidentified as N. crucifera, a new species to science. In 1879, the same mistake was made in Norway. This time, N. bachei got identified as N. heteronema. 

The first observation in the Netherlands (1905) came from the Zuiderzee ^{[5, 11, 12]}.  During the summer of 1993, the jellyfish was found in the Eastern Scheldt, and in September 1996, it was found for the first time in Cadzand. In the same year, N. bachei was seen near Neeltje Jans and in the Western Scheldt, more specifically in the cooling water inlet of the Borssele nuclear power station ^[4].

In Germany, this non-native species was observed in 1942 near the island of Helgoland ^[13]. From there, N. bachei might have spread – with a few intermediate steps along the Dutch coast – into the southern part of the North Sea. In France, N. bachei was observed in 1953 at the Atlantic coast and in the Gironde estuary.

This species established itself permanently in the Netherlands, Germany, France and Norway ^{[14, 15]}. N. bachei has also been observed along the west coast of Scotland ^[4]

Way of introduction

Polyps can attach to ship hulls and spread via shipping. The jellyfish stage (medusa) and larvae of N. bachei can be carried in the ballast water of ships ^{[4, 14]}. One of these transport vectors was probably responsible for the primary introduction to Europe. Alternatively, this jellyfish could have ended up here by hitchhiking on favourable ocean currents (e.g. the Gulf Stream). However, this is not very likely, since this jellyfish has never been observed in the open ocean before ^[4].

Furthermore, the polyps can reach new areas by attaching themselves to floating objects like wood and plastic ^[7]. Polyps attached to commercially imported oysters can also facilitate spreading ^[4]. Since N. bachei is mainly observed in areas with low salinity – like estuaries and marinas – it is easy to assume that recreational yachts promote the secondary (local) spreading of the species ^[7].

N. bachei is a typical coastal species that endures – as long as sufficient food is present – many different and varying environmental conditions ^[10]. This jellyfish thrives both in brackish water with a salinity of 10 PSU and in very salty water with salinities of 45 PSU. They can even tolerate extreme salt concentrations of up to 75 PSU ^{[6, 10]}. By comparison, the North Sea has a salinity of approximately 35 PSU. When environmental conditions are temporarily unfavourable for the medusae, the species can survive as polyps ^[5].

The presence of the free-living medusa stage (besides the sessile polyp stage) should allow the species to cover long distances in a short period using the sea currents ^[4]. However, the jellyfish itself is very small – on average 1.5 cm – which makes it unlikely that it can cover these large distances by swimming ^[16]. The fact that polyps can attach to ship hulls helps with their dispersal (see earlier).

The impact of this species on its environment is currently unknown ^[15]. Water sports enthusiasts and swimmers should not be worried about the presence of this species of jellyfish; the cnidocytes (stinging cells) of N. bachei cannot penetrate human skin ^[17].

N. bachei has two alternating life stages: the jellyfish (medusa) and polyp stage. Both stages occur in shallow seas, close to the shore ^[4]. The free-swimming jellyfish stage is one of the stages in the life cycle. The sessile polyps produce the jellyfish, and the jellyfish then reproduce sexually. This results in free-swimming larvae, which attach themselves to a suitable substrate after a certain amount of time. From these larvae, polyps develop, which then produce the next generation of jellyfish ^[5]. The jellyfish of N. bachei originate at the top of these polyps and not on the stem as in many other jellyfish ^[11].

N. bachei has a bell-shaped body and two club-shaped tentacles. These two tentacles develop when the jellyfish’s bell reaches a diameter of 2.5-3 mm. The jellyfish continues to grow throughout its life, and the number of long tentacles continues to increase. For example, a jellyfish with a diameter of 1.3 cm can have up to 30 tentacles ^{[7, 18]}.

N. bachei mainly feeds at night. It consumes (microscopically) small crustaceans, dinoflagellates and larvae of crustaceans ^{[4, 19]}. As ambush predators, jellyfish remain stationary when they eat, so the amount of food it consumes depends almost entirely on the movement of the prey ^[20].

[1]          World Register of Marine Species (WoRMS) (2020). Nemopsis bachei L. Agassiz, 1849. [http://www.marinespecies.org/aphia.php?p=taxdetails&id=117348] (2020-11-17).

[2]          Agassiz, L. (1850). Contributions to the natural history of the Acalephae of North America: I. On the naked-eyed Medusae of the shores of Massachusetts, in their perfect state of development; II. On the beroid Medusae of the shores of Massachusetts, in their perfect state of development, Memoirs of the American Academy of Arts and Sciences., IV(II). American Academy of Arts and Sciences: Boston: pp. 221-374, 16pl. [http://www.vliz.be/imis/imis.php?module=ref&refid=206079]

[3]          Hargitt, C.W. (1901). Synopses of North-American Invertebrates. XIV. The Hydromedusae. Part III. Medusae. American Naturalist 35(415): 575-595. [http://www.vliz.be/imis/imis.php?module=ref&refid=206445]

[4]          Dumoulin, E. (1997). Het invasieachtig voorkomen in de zuidelijke Noordzee van de hydromedusen Nemopsis bachei L. Agassiz, 1849 en Eucheilota maculata Hartlaub, 1894 in augustus-september 1996 (met aanvullende data voor 1997) (Hydrozoa: Athecata, Thecata). De Strandvlo 17(4): 102-126. [http://www.vliz.be/imis/imis.php?module=ref&refid=19195]

[5]          Faasse, M.; Ates, R. (1998). Het kwalletje Nemopsis bachei (L. Agassiz, 1849), terug van (nooit?) weggeweest. Het Zeepaard 58(3): 72-81. [http://www.vliz.be/imis/imis.php?module=ref&refid=207308]

[6]          Moore, D.R. (1962). Occurrence and distribution of Nemopsis bachei Agassiz (Hydrozoa) in the Northern Gulf of Mexico. Bull. Mar. Sci. Gulf Caribb. 12(2): 399-402. [http://www.vliz.be/imis/imis.php?module=ref&refid=206265]

[7]          De Blauwe, H. (2003). Ribkwallen (Ctenophora), schijfkwallen en medusevormende hydroïden (Cnidaria: Scyphozoa, Hydrozoa) te Zeebrugge, resultaten van 5 jaar waarnemingen (1999-2003). Hans De Blauwe: Belgium. 43, 37 figs. pp. [http://www.vliz.be/imis/imis.php?module=ref&refid=41816]

[8]          Van Ginderdeuren, K.; Fiers, F.; De Backer, A.; Vincx, M.; Hostens, K. (2012). Updating the zooplankton species list for the Belgian part of the North Sea. Belg. J. Zool. 142(1): 3-22. [http://www.vliz.be/en/imis?module=ref&refid=215782]

[9]          Van Ginderdeuren, K. (2011). Persoonlijke mededeling

[10]        Vansteenbrugge, L.; Van Regenmortel, T.; De Troch, M.; Vincx, M.; Hostens, K. (2015). Gelatinous zooplankton in the Belgian part of the North Sea and the adjacent Schelde estuary: Spatio-temporal distribution patterns and population dynamics. J. Sea Res. 97: 28-39. [http://www.vliz.be/nl/catalogus?module=ref&refid=244594]

[11]        Hartlaub, C. (1911). Craspedote Medusen. I. Teil. 2. Lief.: Familie III Margelidae, Nordisches Plankton: Zoologischer Teil: 6. Coelenterata. Reprinted ed. Nordisches Plankton., 6. von Lipsius & Tischer: Kiel: pp. 137-236. [http://www.vliz.be/imis/imis.php?module=ref&refid=206440]

[12]        Tulp, A.S. (2002). Waarnemingen aan de hydromedusen Nemopsis bachei (L. Agassiz) en Eucheilota flevensis van Kampen Het Zeepaard 62(3): 89-96. [http://www.vliz.be/imis/imis.php?module=ref&refid=24928]

[13]        Nehring, S.; Leuchs, H. (1999). Neozoa (Makrozoobenthos) an der deutschen Nordseeküste: eine Übersicht. Bericht BfG, 1200. Bundesanstalt für Gewässerkunde = Federal Institute of Hydrology: Koblenz. 131 pp. [http://www.vliz.be/en/imis?module=ref&refid=120661]

[14]        ICES Advisory Committee on the Marine Environment (2006). Report of the Working Group on Introductions and Transfers of Marine Organisms (WGITMO) 16-17 March 2006 Oostende, Belgium. CM Documents - ICES. CM 2006(ACME:05). ICES: Copenhagen. 330 pp. [http://www.vliz.be/en/imis?module=ref&refid=111237]

[15]        DAISIE (Delivering Alien Invasive Species Inventories for Europe) (2018). Species Factsheet: Nemopsis bachei. [http://www.europe-aliens.org/speciesFactsheet.do?speciesId=53397#] (2018-07-01).

[16]        Vansteenbrugge, L. (2019). Persoonlijke mededeling

[17]        Dumoulin, E. (2011). Persoonlijke mededeling

[18]        Tiffon, Y. (1956). Recherches sur la faune de I'estuaire de la Gironde: 2. Présence de Nemopsis bachei (Agassiz) dans les eaux saumâtres de la Gironde (Anthomédusae). Vie Milieu (1980) 7: 550-553. [http://www.vliz.be/imis/imis.php?module=ref&refid=206417]

[19]        Purcell, J.E.; Nemazie, D.A. (1992). Quantitative feeding ecology of the hydromedusan Nemopsis bachei in Chesapeake Bay. Mar. Biol. (Berl.) 113(2): 305-311. [http://www.vliz.be/imis/imis.php?module=ref&refid=206271]

[20]        Frost, J.R.; Jacoby, C.A.; Youngbluth, M.J. (2010). Behavior of Nemopsis bachei L. Agassiz, 1849 medusae in the presence of physical gradients and biological thin layers. Hydrobiologia 645(1): 97-111. [http://www.vliz.be/en/imis?module=ref&refid=198347]

VLIZ Alien Species Consortium (2020). Nemopsis bachei. Non-indigenous species in the Belgian part of the North Sea and adjacent estuaries anno 2020. Flanders Marine Institute (VLIZ). 6 pp.