Neomysis americana -

Native to the Atlantic Coast of North America – from Newfoundland in the north to Florida in the south –  Neomysis americana is the most common opossum shrimp ^[2]. The species lives in coastal waters and estuaries, but can be found at depths of several hundred metres ^[2-4].

First observation in Belgium

In Belgium, Neomysis americana was first observed during the hyper- and epibenthic monitoring campaigns for benthic organisms in the Sea Scheldt of the Research Institute for Nature and Forest (INBO) in the summer of 2012 ^[5,6]. The species was presumably already present earlier but went unnoticed, as it was found in large numbers in 2012 ^[6].

Spreading in Belgium

So far, the known distribution of Neomysis americana remains limited in Belgium. For the time being, the species was only observed in the Sea Scheldt. It mainly thrives in the parts with relatively high salinity (mesohaline zone), while it does not occur in parts with lower salinity (oligohaline zone) and freshwater areas ^[5-7].

Spreading in neighbouring countries

In September 2010, Neomysis americana was observed for the first time before the coast of Schiermonnikoog, one of the Wadden islands in the north of the Netherlands. This was the first observation of the species in European seas ^[4]. In 2017, the species was found to be common in the Seine estuary in France ^[8,9].

Presumably, Neomysis americana ended up in our regions via the ballast water of ships. Through the same transport method, this species reached the Atlantic Coast of Argentina, Uruguay and southern Brazil ^{[4, 10]}.

Neomysis americana has a wide tolerance to different salt concentrations and temperatures but generally occurs in waters with salinity below 28 psu ^{[11, 12]}. However, it prefers a salinity of 4 psu. Its preference for brackish water means that the species can be very common in estuaries ^{[2, 11]}. To compare: the salinity of the North Sea is on average 35 psu. Other studies indicate a preference for intermediate temperatures and salinities ^[13], which could explain why the species mainly occurs in temperate climates.

Neomysis americana is an opportunist and omnivore. In winter, it feeds in its original habitat mainly on the detritus originating from cordgrass (Spartina) species ^{[2, 14, 15]}. The presence of common cordgrass (Spartina townsendii) in the Scheldt estuary (mainly in the western Scheldt, more limited in the Sea Scheldt near Doel) ^[16] offers opportunities for the establishment of Neomysis americana. Today, the species is found in high numbers (up to 65 individuals per m²) in sediment samples ^[17].

The opportunistic nature ^[2] of the species in combination with its broad salinity and temperature tolerance ^{[11, 12]} contribute to the further spread of the species beyond its natural habitat.

In warmer regions, egg-bearing females remain present throughout the year, because reproduction, in contrast with temperate regions, can take place the entire year. This has also facilitated the colonisation of South American estuaries ^[15]. Due to the small temperature range of juveniles ^[13], the species can’t occur in tropical or cold climates. Because of climate change, the species may continue to spread in the future ^[13]

As Neomysis americana has only been found in Europe since 2010, not much is known about its impact on the local ecosystem or economy and it is unclear whether it will settle permanently in our regions ^[4]. However, in South America the species has been present in estuaries in Argentina, Uruguay and Brazil since the 1960s ^{[11, 18]}. This allowed the studying of this species there for a long time. It has been discovered that Neomysis americana now plays an important role in the food web of these estuaries. It forms an important food source for the juveniles of almost any fish species, which seem to gather during the winter in places where Neomysis americana is also abundant ^{[2, 15]}.

Neomysis americana is very similar to the native Neomysis integer. However, the latter has a broader salinity tolerance and occurs less at sea. From hyper- and epibenthic samples in the Sea Scheldt, it was concluded that N. integer occurs more inland. On the other hand, N. americana is sometimes even more abundant than N. integer during the spring at the brackish sampling points ^[19]. N. americana can also be confused with the native Acanthomysis longicornis (not yet found in the Sea Scheldt) ^[4].

N. americana is 10 to 12 mm long, which means that it is on average 5 mm smaller than the native N. integer. Detailed descriptions can be found in the scientific literature ^{[4, 20]}. The most distinctive feature is the occurrence of smaller spines in between the larger spines along the whole length of the telson. Females have a brood pouch in which they can carry eggs. In this pouch, the complete development of the eggs takes place ^[21]. This is why Mysida are often called ‘opossum shrimps’ ^[11]. The species can lay eggs 2 to 3 times a year ^[22].

N. americana exhibits a daily vertical migration. It lives in the sediment during the day and comes to the surface at night. They do this to escape predators during the day and feed at night ^{[3, 11, 23]}

[1]          World Register of Marine Species (WoRMS) (2024). Neomysis americana (S.I. Smith, 1873). [http://www.marinespecies.org/aphia.php?p=taxdetails&id=157807] (2024-10-18).

[2]          Jumars, P.A. (2007). Habitat coupling by mid-latitude, subtidal, marine mysids: import-subsidised omnivores. Oceanogr. Mar. Biol. Ann. Rev. 45: 89-138. [http://www.vliz.be/nl/catalogus?module=ref&refid=111467]

[3]          Sato, M.; Jumars, P.A. (2008). Seasonal and vertical variations in emergence behaviors of Neomysis americana. Limnol. Oceanogr. 53(4): 1665-1677. [http://www.vliz.be/nl/catalogus?module=ref&refid=297809]

[4]          Wittmann, K.J.; Vanagt, T.J.; Faasse, M.; Mees, J. (2012). A New Transoceanic Invasion? First Records of Neomysis americana (Crustacea: Mysidae) in the East Atlantic. The Open Marine Biology Journal 6: 5. [http://www.vliz.be/nl/catalogus?module=ref&refid=219539]

[5]          Speybroeck, J. (2018). Persoonlijke mededeling

[6]          Soors, J.; de Beukelaer, J.; Bezdenjesnji, O.; Buerms, D.; Lefranc, C.; Speybroeck, J.; Van de Meutter, F. (2022). Two new alien crustacean invaders Grandidierella japonica (Stephensen, 1938) and Neomysis americana (S.I. Smith, 1873) in Belgium. Bioinvasions Records 11(3): 747-757. [https://www.vliz.be/nl/imis?module=ref&refid=355791]

[7]          Vlaams-Nederlandse Scheldecommissie (2014). Scheldemonitor Dataportaal. [http://www.scheldemonitor.be/dataportal ] (2014-07-22).

[8]          Massé, C.; Chouquet, B.; Dubut, S.; Durand, F.; Gouillieux, B.; Dancie, C. (2018). Premier signalement de l’espèce non indigène Neomysis americana (Crustacé : Mysidacé) dans l’estuaire de la Seine (Normandie, France). An Aod 6(1): 7-16. [https://www.vliz.be/nl/imis?module=ref&refid=396654]

[9]          Pezy, J.-P.; Raoux, A.; Timsit, O.; Dauvin, J.C. (2019). A rapidly established population of the invader mysid Neomysis americana (S.I. Smith, 1873) in the Seine estuary. Mar. Biodiv. 49(3): 1573-1580. [https://www.vliz.be/nl/imis?module=ref&refid=324926]

[10]          Carlton, J.; Geller, J.B. (1993). Ecological roulette: the global transport of nonindigenous marine organisms. Science (Wash.) 261: 78-82. [http://www.vliz.be/nl/catalogus?module=ref&refid=26986]

[11]          Da Silva Gama, A.M. (2008). Fatores ambientais e mecanismos comportamentais determinantes da ocorrência de Mysida (Crustacea) no estuário da Laguna dos Patos e região costeira adjacente. PhD Thesis. Pontifícia Universidade Católica do Rio Grande do Sul: Porto Alegre, Brasil. 82 pp. [http://www.vliz.be/nl/catalogus?module=ref&refid=297795]

[12]          Brown, H.; Bollens, S.M.; Madin, L.P.; Horgan, E.F. (2005). Effects of warm water intrusions on populations of macrozooplankton on Georges Bank, Northwest Atlantic. Cont. Shelf Res. 25(1): 143-156. [http://www.vliz.be/en/catalogue?module=ref&refid=299908]

[13]        Paul, S.; Calliari, D. (2017). Salinity and temperature tolerances of Neomysis americana (Crustacea: Mysida) sub adults. Perspectives on vulnerability, and distribution in South America. J. Exp. Mar. Biol. Ecol. 486: 373-378. [http://www.vliz.be/nl/catalogus?module=ref&refid=297805]

[14]        Zagursky, G.; Feller, R.J. (1985). Macrophyte detritus in the winter diet of the estuarine mysid, Neomysis americana. Estuaries 8(4): 355-362. [http://www.vliz.be/nl/catalogus?module=ref&refid=297815]

[15]        Viñas, M.D.; Ramírez, F.C.; Mianzan, H.W. (2005). Annual population dynamics of the opossum shrimp Neomysis americana Smith, 1873 (Crustacea, Mysidacea) from an estuarine sector of the Argentine Sea. Sci. Mar. (Barc.) 69(4): 493-502. [http://www.vliz.be/nl/catalogus?module=ref&refid=78900]

[16]        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]

[17]        Faasse, M. (2019). Persoonlijke mededeling

[18]        Calliari, D.; Cervetto, G.; GCastiglioni, R.; Rodríguez, L. (2007). Salinity preferences and habitat partitioning between dominant mysids at the Rio de la Plata estuary (Uruguay). J. Mar. Biol. Ass. U.K. 87(2): 501-506. [http://www.vliz.be/nl/catalogus?module=ref&refid=119170]

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

[20]        Zimmer, C. (1909). Die nordischen Schizopoden, Nordisches Plankton: Zoologischer Teil: 3. Crustacea. Reprinted ed. Nordisches Plankton., 3. von Lipsius & Tischer: Kiel: pp. 1-178. [http://www.vliz.be/nl/catalogus?module=ref&refid=8234]

[21]        Reesor, C.M. (2012). Temporal distribution of Morone saxatilis eggs and larvae and Neomysis americana in the Shubenacadie estuary. MSc Thesis. Dalhousie University: Halifax, Nova Scotia. 100 pp. [http://www.vliz.be/nl/catalogus?module=ref&refid=297806]

[22]        Ober, G.T.; Thornber, C.S.; Grear, J.; Kolbe, J.J. (2017). Ecological differences influence the thermal sensitivity of swimming performance in two co-occurring mysid shrimp species with climate change implications. J. Therm. Biol. 64: 26-34. [http://www.vliz.be/nl/catalogus?module=ref&refid=297803]

[23]        Calliari, D.; Cervetto, G.; Gomez, M.; Bastreri, D. (2001). Short-term variability in abundance and vertical distribution of the opossum shrimp Neomysis americana in the Solis Grande River estuary, Uruguay. Atlântica 23: 117-125. [http://www.vliz.be/nl/catalogus?module=ref&refid=297790]

VLIZ Alien Species Consortium (2024). Neomysis americana. Introduced alien species of the Belgian part of the North Sea and adjacent estuaries anno 2024. Flanders Marine Institute (VLIZ). 5 pp.