Euplana gracilis -
The natural distribution range of the polyclad flatworm Euplana gracilis is likely the Atlantic coast of North America, where the species is found from Florida to Prince Edward Island (Canada) [2,3].
First observation in Belgium
Euplana gracilis was first observed in the port of Zeebrugge in 2023 [4].
Distribution in Belgium
The occurrence of this flatworm has so far only been demonstrated in the port of Zeebrugge [4].
Distribution in neighbouring countries
Euplana gracilis was first observed in IJmuiden in 2002, where the worm was already abundant just behind the locks (brackish water) [2]. In 2004, the species was also found in the Veerse Meer, and a year later in the northwestern part of the Eastern Scheldt (Burghsluis, Zierikzee) [5].
Further away, the species was also reported in Port Phillip Bay (Victoria) in southern Australia in 1980, an occurrence that is likely due to an introduction [6].
The introduction of this flatworm is likely related to international shipping, with the species hiding among the fouling community on ship hulls [2].
The extensive native range of this flatworm suggests that the species likely possesses a broad temperature tolerance. Additionally, the worm thrives in both marine environments and brackish water [2].
The tolerance to variations in temperature and salinity makes it likely that this species will conquer (or has already conquered) other areas in the Netherlands and Europe [2]. However, the biology of polyclads remains largely unknown to this day. There is a lack of knowledge about whether or not a larval stage occurs in their development, feeding habits, distribution, and seasonality, which hampers the analysis of potential distribution patterns [7]. While the most common developmental mode in polyclads is ‘direct development’ (where the embryo develops directly into a form resembling the young adult), there are also many species that develop indirectly through a planktonic larval stage [8].
There is no known data on the effects or potential measures related to the occurrence of Euplana gracilis.
Euplana gracilis is slightly smaller than a centimeter and has a mottled brown-gray color. The body is elongated and tapers slightly towards the rear. Tentacles are absent. The specimens from IJmuiden have two larger tentacular eye spots, each seemingly consisting of two fused eye spots, and in front of these, in the longitudinal direction of the body, two irregular rows of about four smaller cerebral eye spots each. The literature provides conflicting information regarding the pattern of eye spots on the head of this flatworm. Based on the material from IJmuiden, it could not be determined whether the eye spot pattern is variable or if multiple species have been grouped under the name Euplana gracilis [2].
Polyclad flatworms generally have a very cryptic lifestyle. Many species avoid light and tend to seek shelter in narrow openings [2]. This lifestyle may explain the lack of evidence for ‘successful colonisations’ by flatworms outside their natural habitat [9], as they can easily thrive unnoticed in new areas [2].
A rare study investigating the feeding habits of Euplana gracilis showed that the flatworm is a micropredator that, according to this study, exclusively feeds on tube-building amphipods [10].
[1] World Register of Marine Species (WoRMS) (2024). Euplana gracilis Girard, 1853. https://www.marinespecies.org/aphia.php?p=taxdetails&id=157977 (2024-10-18).
[2] Faasse, M.; Ates, R. (2003). De Nederlandse polyclade platwormen (Platyhelminthes: Turbellaria: Polycladida): 2. De uit Amerika afkomstige Euplana gracilis (Girard, 1850). Het Zeepaard 63(2): 57-60. [https://vliz.be/nl/imis?module=ref&refid=34124]
[3] Hyman, L.H. (1941). The polyclad flatworms of the Atlantic coast of the United States and Canada. Proc. U.S. Natl. Mus. 89(3101): 449-495. [https://www.vliz.be/en/imis?module=ref&refid=393690]
[4] Dumoulin, E. (2024). Persoonlijke mededeling.
[5] Faasse, M.; Ligthart, M. (2007). De Nederlandse polyclade platwormen (Platyhelminthes: Turbellaria: Polycladida): IV. Stylostomum ellipse en Imogine necopinata (en Euplana gracilis). Het Zeepaard 67(2): 44-47. [https://vliz.be/nl/imis?module=ref&refid=110005]
[6] Prudhoe, S. (1982). Polyclad turbellarians from the southern coasts of Australia. Rec. South Aust. Mus. 18(16): 361-384. [https://www.vliz.be/en/imis?module=ref&refid=394256]
[7] Tosetto, L.; McNab, J.M.; Hutchings, P.A.; Alonso Rodriguez, J.; Williamson, J.E. (2023). Fantastic flatworms and where to find them: Insights into intertidal polyclad flatworm distribution in southeastern Australian boulder beaches. Diversity 15(3): 393. [https://www.vliz.be/en/imis?module=ref&refid=393674]
[8] Rawlinson, K.A. (2014). The diversity, development and evolution of polyclad flatworm larvae. EvoDevo 5(1): 9. [https://www.vliz.be/en/imis?module=ref&refid=393688]
[9] Faubel, A.; Gollasch, S. (1996). Cryptostylochus hullensis sp. nov. (Polycladida, Acotylea, Platyhelminthes): a possible case of transoceanic dispersal on a ship’s hull. Helgol. Meeresunters. 50: 533-537. [https://vliz.be/nl/imis?module=ref&refid=25069]
[10] Janiak, D.S.; Adams, J.N.; Rubinoff, B.; Osman, R.W. (2017). Predator-prey interactions of the polyclad, Euplana gracilis, and the amphipod, Apocorophium lacustre, in the Chesapeake Bay. Est. Coast. 40(2): 513-521. [https://www.vliz.be/en/imis?module=ref&refid=393683]
VLIZ Alien Species Consortium (2024). Euplana gracilis. Introduced alien species of the Belgian part of the North Sea and adjacent estuaries anno 2024. Flanders Marine Institute (VLIZ). 4 pp.