Balanus glandula -

Balanus glandula was originally described by Darwin off the coast of Baja California (Mexico), where the species is one of the most common barnacles of the Balanidae family (Crustacea, Cirripedia). The species has a natural distribution along the coasts of the north-eastern Pacific, as far as Alaska ^[2-4]. Over the past half-century, Balanus glandula has successfully navigated and colonised the coasts of Argentina ^{[5, 6]}, Japan ^[7] and South Africa ^{[8, 9]}. Balanus glandula is expected to spread along European coasts ^[10].

First observation in Belgium

The first European sighting of this invasive barnacle species occurred in July 2015 in a biofouling community on the research vessel ‘RV Belgica’, when it was in dry dock ^[10].

The ‘RV Belgica’, with its homeport of Zeebrugge (Belgium), is mainly active in the southern North Sea, but every year in the summer she makes a longer expedition to the southern European waters. Therefore, at the time of discovery, the status of Balanus glandula in Belgian waters could not yet be confirmed ^[10].

Spreading in Belgium

In October 2016, the species was often found on buoys along the Belgian coastline. During a targeted search, from December 2016 to March 2017, the species was found on the outer harbour walls of both Ostend and Zeebrugge and on numerous breakwaters spread between Oostduinkerke and Zeebrugge ^[10].

In October and November 2017, Balanus glandula was found on all breakwaters along the entire Belgian coastline and part of the coastline of the Netherlands (e.g. Cadzand) ^[11].

Spreading in neighbouring countries

To date, Balanus glandula has only been documented in the Netherlands (Cadzand) ^[11].

Balanus glandula was probably introduced via ballast water or biofouling in the port(s) of Zeebrugge and/or Ostend.

Balanus glandula lives in the mid to high intertidal zone of rocky shores and is also found on various types of artificial hard substrates or floating objects such as buoys ^[10] and plastic litter ^[12]. This exotic species can survive in a wide range of environmental conditions and can be found both on exposed cliffs and in sheltered bays ^[3]. The species can thrive in water with poor circulation, low salinity and mild wave action ^[3]. Balanus glandula can withstand longer periods of dehydration (up to 13 days) ^[13]. Moreover, the climate of north-western Europe is suitable for this invasive barnacle. It can also grow to a larger size and is physically more robust than other native barnacle species ^[10]. Its reproductive capacity and physical robustness can promote its spread and success in Europe.

The densities of Balanus glandula in the intertidal zone may be influenced by differences in the degree of exposure and the amount of time the barnacles are submerged (submersion time) ^[11]. Experimental studies in an intertidal area in Argentina showed the highest densities of both juvenile and adult individuals in the high intertidal ^[14]. Substrate type also appears to significantly influence population density, with individuals showing a preference for soft-rough substrate compared to soft-smooth substrate ^{[11, 14]}. In addition, competition (for space) with other barnacle species and exposure to wave action may play a role in the occurrence of this barnacle species.

This recently introduced barnacle in the North Sea will most likely compete with other intertidal barnacles living on artificial hard substrates along the Belgian and European coasts. This may lead to a change in the local barnacle biodiversity and can potentially affect biodiversity in the southern North Sea.

The calcareous shell of Balanus glandula consists of six side plates and a calcified base plate. Four calcareous plates, two terga and two scuta close the top of the shell. A characteristic feature of Balanus glandula is the dark spot in the middle of the scuta, where the black inner surface of the tissue covering the terga and scuta is visible from the outside. This is because the inner surface of the scuta is locally thinner and forms a relatively deep hollow ^{[2, 10]}.

[1]         World Register of Marine Species (WoRMS) (2020). Balanus glandula Darwin, 1854. [http://www.marinespecies.org/aphia.php?p=taxdetails&id=394848] (2020-11-17).

[2]         Pilsbry, H.A. (1916). The sessile barnacles (Cirripedia) contained in the collections of the U.S. National Museum; including a monograph of the American species. Bulletin. United States National Museum, 93. Government Printing Office: Washington. 366 pp. [http://www.vliz.be/nl/catalogus?module=ref&refid=314286]

[3]         Barnes, H.; Barnes, M. (1956). The general biology of Balanus glandula Darwin. Pac. Sci. 10(4): 415-422. [http://www.vliz.be/nl/catalogus?module=ref&refid=314279]

[4]         Newman, W.A. (2007). Cirripedia, in: Carlton, J.T. The Light and Smith manual: Intertidal invertebrates from Central California to Oregon. Fourth edition, completely revised and expanded. University of California Press: Berkeley, Los Angeles, London: pp. 457-484. [http://www.vliz.be/nl/catalogus?module=ref&refid=314283]

[5]         Spivak, E.D.; L'Hoste, S.G. (1976). Presencia de cuatro especies de Balanus en la costa de la Provincia de Buenos Aires. Distribución y aspectos ecológicos. Edición impresa a cargo de los autores: Mar del Plata. 17 pp. [http://www.vliz.be/nl/catalogus?module=ref&refid=314287]

[6]         Spivak, E.; Schwindt, E. (2014). Balanomorfos (Cirripedia: Thoracica), in: Roig-Juñent, S. et al. Biodiversidad de artrópodos argentinos, Volume 3. Universidad Nacional de Tucumán. Facultad de Ciencias Naturales: San Miguel de Tucumán: pp. 167-184. [http://www.vliz.be/nl/catalogus?module=ref&refid=314289]

[7]         Kado, R. (2003). Invasion of Japanese shores by the NE Pacific barnacle Balanus glandula and its ecological and biogeographical impact. Mar. Ecol. Prog. Ser. 249: 199-206. [http://www.vliz.be/nl/catalogus?module=ref&refid=299355]

[8]         Simon-Blecher, N.; Granevitze, Z.; Achituv, Y. (2008). Balanus glandula: From North-West America to the west coast of South Africa. Afr. J. Mar. Sci. 30(1): 85-92. [http://www.vliz.be/nl/catalogus?module=ref&refid=127526]

[9]         Laird, M.C.; Griffiths, C.L. (2008). Present distribution and abundance of the introduced barnacle Balanus glandula Darwin in South Africa. Afr. J. Mar. Sci. 30(1): 93-100. [http://www.vliz.be/nl/catalogus?module=ref&refid=127527]

[10]       Kerckhof, F.; De Mesel, I.; Degraer, S. (2018). First European record of the invasive barnacle Balanus glandula Darwin, 1854 BioInvasions Records 7: 21-31. [http://www.vliz.be/nl/catalogus?module=ref&refid=292671]

[11]       Bouwens, J. (2019). In search of non-indigenous barnacles species on man-made hard substrates in the marine environment (southern bight of the North Sea). MA Thesis. VUB, UGent, UAntwerpen: Ghent. 39 pp. [http://www.vliz.be/nl/catalogus?module=ref&refid=316806]

[12]       Rees, E.I.S.; Southward, A.J. (2009). Plastic flotsam as an agent for dispersal of Perforatus perforatus (Cirripedia: Balanidae). Marine Biodiversity Records 2: e25. [http://www.vliz.be/nl/catalogus?module=ref&refid=314376]

[13]       Carlton, J.T.; Newman, W.A.; Pitombo, F.B. (2011). Barnacle invasions: Introduced, cryptogenic, and range expanding Cirripedia of North and South America, in: Galil, B.S. et al. In the wrong place - alien marine crustaceans: Distribution, biology and impacts. Invading Nature - Springer Series in Invasion Ecology, 6: Dordrecht: pp. 159-213. [http://www.vliz.be/nl/catalogus?module=ref&refid=205520]

[14]       Savoya, V.; Schwindt, E. (2010). Effect of the substratum in the recruitment and survival of the introduced barnacle Balanus glandula (Darwin 1854) in Patagonia, Argentina. J. Exp. Mar. Biol. Ecol. 382(2): 125-130. [http://www.vliz.be/nl/catalogus?module=ref&refid=314377]

VLIZ Alien Species Consortium (2020). Balanus glandula. Non-native species of the Belgian part of the North Sea and bordering estuaries anno 2020. Flemish Institute for the Sea (VLIZ). 4 pp.