Incisocalliope aestuarius -

The native distribution of Incisocalliope aestuarius extends across estuaries, between Delaware and Georgia, along the United States East Coast ^[2].

First observation in Belgium

The first Belgian observation of this amphipod took place in the Zeeschelde near the Dutch border and dates from October 1996 ^[2]. The specimens were initially wrongly identified as Pleusymtes glaber ^[3]. As it is unlikely that this species can survive in the Zeeschelde, the collected specimens were reconsidered, and it turned out to be the amphipod Incisocalliope aestuarius ^[2].

From new analyses of old samples ^[4], it appeared that Incisocalliope aestuarius was already present in the brackish part of the Western Scheldt in 1991, but was also wrongly identified as Pleusymtes glaber ^[2]. In the period between 1988 and 1991, in the eastern part of the Dutch Western Scheldt – between Saeftinghe and the Belgian border – a high density of specimens were found and identified as Pleusymtes glaber ^[5]. As these specimens were not re-examined, it cannot be said with certainty that these have also been misidentified ^[2].

Spreading in Belgium

In 2003, the known range of Incisocalliope aestuarius in the Scheldt estuary was from Baarland (30 km upstream of the estuary) to Doel (5 km upstream of the Belgian-Dutch border) ^[2]. In the Western Scheldt, this amphipod is mainly observed in muddy puddles between stones, just above the low waterline, attached to hydroid polyps (mainly polyps of the species Hartlaubella gelatinosa). In the Western Scheldt, these polyps attached themselves to non-indigenous Japanese oysters Crassostrea/Magallana gigas. There were also a few observations of Incisocalliope aestuarius on stones at depths of 5 to 9.5 m ^[2].

In the Zeeschelde, the species remains rare, only found below the low waterline. Here the species seems to be less dependent on the presence of hydroids. After 2002, the species has been found a few times in hyper- and epibenthic samples from downstream Antwerp, and with greater regularity in samples taken from the vicinity of Doel ^[6].

Spreading in neighbouring countries

In 2015, Incisocalliope aestuarius was recorded for the first time in French waters, in the Gironde estuary ^[7]. Since 2016, established populations of this amphipod have also been found annually in German North Sea harbors ^[8]. In the same year, the species was observed in the Netherlands in the Grevelingenmeer ^[9] and in the Dutch part of the Ems-Dollard estuary ^[10].

Incisocalliope aestuarius may have reached the Scheldt by attaching itself to ship hulls or via their ballast water ^[2]. Because juveniles of amphipods do not disperse efficiently, the European range of this species is likely to remain limited ^[2]. This is also demonstrated by the fact that this species is only found in Belgium and the Netherlands (Western Scheldt and North Sea Canal) in Europe.

The spread of Incisocalliope aestuarius in the Scheldt is mainly favoured by the presence of artificial hard substrates, such as stones for reinforcement of the banks and quay walls. This amphipod is found in association with hydroids that have settled on these hard substrates. As this habitat is hardly used by indigenous species, Incisocalliope aestuarius hardly experiences any competition in our area ^[2].

In its area of origin – the American east coast – Incisocalliope aestuarius can be found in brackish water areas with salinity levels ranging from 10 to 33 PSU. In addition, this exotic species can tolerate temperatures between -2 and 29 °C. Its ability to tolerate large differences in salinity (euryhaline) and temperature (eurytherm) allows it to thrive in a wide range of habitats and makes it a potential invader of European estuaries ^[2].

Incisocalliope aestuarius attaches itself to the vertical parts of hydroid colonies. The structure of the mouthparts would suggest that this exotic species lives as a parasite in these colonies. More particularly, this amphipod is found attached to Hartlaubella gelatinosa, a species of hydroid which occurs almost exclusively on the non-native Japanese oysters Crassostrea/Magallana gigas. Meanwhile, the latter has established itself deep into the Scheldt estuary on the numerous artificial hard substrates, upon which the settlement of Hartlaubella gelatinosa and later Incisocalliope aestuarius became possible ^[2].

The absence of Incisocalliope aestuarius in the port of Walsoorden (Dutch Western Scheldt) suggests that this species prefers fast-flowing waters, such as those in the channel. However, it cannot be ruled out that other unfavourable conditions in the harbour are at the root of its absence ^[2].

It is not known what effects this species has on its environment.

With the naked eye, it is difficult to distinguish Incisocalliope aestuarius from other amphipod species. Under the microscope Incisocalliope aestuarius is distinguished by its round eye shape, the bare urosoma and the absence of a spine on the dorsal side near the legs ^[2].

In contrast to native amphipods, this species is closely associated with hydroids, such as Hartlaubella gelatinosa. When collecting these hydroid colonies, this amphipod shows a typical escape reaction by crawling towards the centre of the colony. Due to its dark brown colour and light brown stripes and spots, this exotic species is well camouflaged between the hydroid colonies ^[2].

[1]          World Register of Marine Species (WoRMS) (2024). Incisocalliope aestuarius (Watling & Maurer, 1973). [http://www.marinespecies.org/aphia.php?p=taxdetails&id=148579] (2024-10-18).

[2]          Faasse, M.; Van Moorsel, G. (2003). The North-American amphipods, Melita nitida Smith, 1873 and Incisocalliope aestuarius (Watling and Maurer, 1973) (Crustacea: Amphipoda: Gammaridea), introduced to the western Scheldt estuary (The Netherlands). Aquat. Ecol. 37(1): 13-22. [http://www.vliz.be/en/imis?module=ref&refid=34073]

[3]          Ysebaert, T.J.; De Neve, L.; Meire, P. (2000). The subtidal macrobenthos in the mesohaline part of the Schelde Estuary (Belgium): influenced by man? J. Mar. Biol. Ass. U.K. 80(4): 587-597. [http://www.vliz.be/en/imis?module=ref&refid=3713]

[4]          Brummelhuis, E.B.M.; Craeymeersch, J.A.; Dimmers, W.; R., M. (1997). Het macrobenthos van de Westerschelde, de Oosterschelde, het Veerse Meer en het Grevelingenmeer in het voorjaar 1997: rapportage in het kader van het Biologisch Monitoring Programma. CEMO/NIOO: Yerseke. 41 pp. [http://www.vliz.be/en/imis?module=ref&refid=135207]

[5]          Cattrijsse, A.; Mees, J.; Hamerlynck, O. (1993). The hyperbenthic Amphipoda and Isopoda of the Voordelta and the Westerschelde estuary. Cah. Biol. Mar. 34(2): 187-200. [http://www.vliz.be/en/imis?module=ref&refid=2671]

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

[7]          Bachelet, G.; Acolas, M.-L.; Baudrimont, M.; Blanchet, H.; Daverat, F; Garabetian, F.; Labadie, P.; Legeay, A.; Leconte, M.; Lepage, M.; Lobry, J.; Maury-Brachet, R.; Nowaczyk, A.; Sauriau, P.-G.; Sautour, B.; Budzinski, H.; Chaalali, A.; David, V.; Davoult, D.; del Amo, Y.; Dessier, A.; Dévier, M.-H.; Goberville, E.; Gouillieux, B.; Gourves, P.-Y.; Jatteau, P.; Lauzent, M.; Munoz, G.; Nzigou, A.R.; Pasquaud, S.; Pierron, F.; Richirt, J.; Rochard, E.; Savoye, N.; Tapie, N. (2020). Biodiversité et fonctionnement écologique, in: Sautour, B. et al. L’estuaire de la Gironde: un écosystème altéré? Entre dynamique naturelle et pressions anthropiques. pp. 132-209 + Annexes 339-348. [https://www.vliz.be/nl/imis?module=ref&refid=396082]

[8]          Leitinger, J.; Schüler, L.; Nestler, S. (2021). Among us: first record of the non-indigenous amphipod Incisocalliope aestuarius (Watling and Maurer, 1973) in Germany. Bioinvasions Records 10(4): 875-884. [https://www.vliz.be/nl/imis?module=ref&refid=347519]  

[9]          Verduin, E.; Leewis, L.; van Haaren, T. (2018). Macrozoöbenthosonderzoek in de zoute Rijkswateren 2016: Delta (Grevelingen, Oosterschelde, Westerschelde en Veerse Meer. Eurofins Omegam B.V./Eurofins AquaSense: Amsterdam-Duivendrecht. 142 pp. [https://www.vliz.be/nl/imis?module=ref&refid=396081]

[10]          van Haaren, T. (2021). Persoonlijke mededeling. 

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