Ptilohyale littoralis -

The species naturally occurs along the Atlantic coast of North America ^[2].

First observation in Belgium

Multiple specimens of this species were first found on Belgian territory on August 1, 2022, in a biofouling sample from the intertidal zone of the RT1 measurement pole (‘Blue Accelerator’), about 500 meters north of the eastern breakwater of Ostend ^[3].
 

Distribution in Belgium

In addition to the observation in 2022, these amphipods were also found in 2024 in a biofouling sample from the O6 buoy, near the RT1 measurement pole, but this time in the subtidal zone. To date, these occurrences near the entrance of the Ostend harbor remain the only observations on Belgian territory ^[3].
 

Distribution in neighbouring countries

In 2009, Ptilohyale littoralis was already abundantly present in the port of Rotterdam. In the following years, hundreds of specimens of this species were found at this location. In 2013, this amphipod was also discovered in the Eastern Scheldt (Yerseke) and Western Scheldt (Vlissingen, Borssele), though in noticeably lower concentrations compared to the port of Rotterdam ^[2]. In 2014, the species also appeared along the northern French Channel coast (Wimereux) ^[4]. In 2024, this amphipod was found on the dike of Boulogne, while no specimens could be found in the vicinity of Wimereux ^[3].

The species is likely to have been present along the European coast for much longer. For example, in 1985, an amphipod with a heavily setose second antenna was found in the Bay of Arcachon (France) ^[5]. The species was initially described as Parhyale explorator (later Ptilohyale explorator), a new species to science, however, it was later demonstrated that it was the same species as Ptilohyale littoralis ^[6].

The most likely introduction vector is transport via ballast water or hull fouling, given the initial (and abundant) occurrence in the port of Rotterdam ^[2,7]. Without additional research, it is impossible to distinguish between secondary introductions and multiple primary introductions of Ptilohyale littoralis in the Netherlands ^[2].

This amphipod is described as a brackish water/estuarine species ^[8], but it also occurs in marine environments ^[4]. The exact tolerance limits for temperature are not known.

Ptilohyale littoralis is found in both brackish and marine environments ^[2]. It inhabits the littoral zone and protects itself from predation and wave action ^[6] by hiding under rocks, among pebbles, oysters, or mussels, and beneath the Bladderwrack Fucus vesiculosus L. ^[2,4].

In amphipods, the absence of a pelagic larval stage significantly limits natural secondary dispersal, making long-distance transport primarily human-induced ^[2].

It is currently unclear whether competition occurs with native amphipods that inhabit the same habitat ^[2]. A recent study has shown that Ptilohyale littoralis may serve as prey for the non-native Asian shore crab Hemigrapsus sanguineus, indicating a potential interaction between these two species ^[10].

This amphipod reaches a length of approximately 11 mm ^[6]. In species of Ptilohyale, the posterior margins of the proximal part of the flagellum and the fifth segment (distal half only in females) of the peduncle of the antenna are densely covered with setae ^[8]. No European genus within the family Hyalidae exhibits this feature ^[2,3].

The key characteristics used to identify Dutch specimens of Ptilohyale as Ptilohyale littoralis are: (1) the number of spines on the margin of the outer branch of uropod 1 is usually three, sometimes four, and rarely five; (2) the branches of the second uropod are nearly equal in length; (3) the posterior lobe of coxa 5 is noticeably smaller than the anterior lobe; (4) the base of gnathopod 1 (in males) lacks a distinct anterodistal lobe; and (5) the posterior margins of coxae 1-4 have fairly distinct projections. For a complete description of the species, refer to specialised literature ^[2,6,8].

[1]          World Register of Marine Species (WoRMS) (2024). Ptilohyale littoralis (Stimpson, 1853). https://www.marinespecies.org/aphia.php?p=taxdetails&id=491504 (2024-10-18).

[2]          Faasse, M.A. (2014). Introduction of Ptilohyale littoralis to The Netherlands. Marine Biodiversity Records 7: 5. [https://www.vliz.be/nl/imis?module=ref&refid=234926] 

[3]          Kerckhof, F.; Kerkhove, T.R.H. (2024). De geïntroduceerde glasvlokreeft Ptilohyale littoralis (Stimpson, 1853) aangetroffen in Belgische wateren. De Strandvlo 44(3): 88-91. [https://www.vliz.be/nl/imis?module=ref&refid=395704]  

[4]          Spilmont, N.; Hachet, A.; Faasse, M.A.; Jourde, J.; Luczak, C.; Seuront, L.; Rolet, C. (2016). First records of Ptilohyale littoralis (Amphipoda: Hyalidae) and Boccardia proboscidea (Polychaeta: Spionidae) from the coast of the English Channel: habitat use and coexistence with other species. Mar. Biodiv. 48(2): 1109-1119. [https://www.vliz.be/nl/imis?module=ref&refid=299234] 

[5]          Arresti A. (1989). Parhyale explorator, a new species of talitroid amphipod from the bay of Arcachon. Bull. Mus. Natl. Hist. Nat., Sect. A Zool. Biol. Ecol. Anim. Sér. 4, 11(1): 101-115. [https://www.vliz.be/nl/imis?module=ref&refid=395741] 

[6]          Lo Brutto, S.; Iaciofano, D. (2018). A taxonomic revision helps to clarify differences between the Atlantic invasive Ptilohyale littoralis and the Mediterranean endemic Parhyale plumicornis (Crustacea, Amphipoda). ZooKeys 754: 47-62. [https://www.vliz.be/en/imis?module=ref&refid=393583] 

[7]          Davidson, I.C.; Brown, C.W.; Sytsma, M.D.; Ruiz, G.M. (2009). The role of containerships as transfer mechanisms of marine biofouling species. Biofouling (Print) 25(7): 645-655. [https://www.vliz.be/en/imis?module=ref&refid=393621] 

[8]          Bousfield, E.L.; Hendrycks, E.A. (2002). The talitroidean amphipod family Hyalidae revisited, with emphasis on the north Pacific fauna: systematics and distributional ecology. Amphipacifica 3(3): 17-134. [https://www.vliz.be/en/imis?module=ref&refid=393620]  

[9]          Fenchel, T.M.; Kolding, S. (1979). Habitat selection and distribution patterns of five species of the amphipod genus Gammarus. Oikos (Kbh.) 33(2): 316-322. [https://www.vliz.be/en/imis?module=ref&refid=393619] 

[10]        Blasi, J.C.; O’Connor, N.J. (2016). Amphipods as potential prey of the Asian shore crab Hemigrapsus sanguineus: Laboratory and field experiments. J. Exp. Mar. Biol. Ecol. 474: 18-22. [https://www.vliz.be/en/imis?module=ref&refid=393584]

VLIZ Alien Species Consortium (2024). Ptilohyale littoralis. Introduced alien species of the Belgian part of the North Sea and adjacent estuaries anno 2024. Flanders Marine Institute (VLIZ). 4 pp.