Calyptospadix cerulea - Rope grass hydroid

The Rope grass hydroid is a cryptogenic species ^[3], meaning that its original distribution area is unknown. By 1950, this hydroid was observed at the Atlantic and Pacific Coast of the US, around India, Australia and West Africa ^[4]. Some believe this species is native to estuaries in the north of the Indian Ocean ^[5]. Others say it is a cosmopolitan of tropical and temperate waters ^[6].

The species occurs in brackish water, up to several meters deep, where it attaches itself to all kinds of hard substrates and organisms like molluscs, algae, bryozoans and tunicates ^[6].

First observation in Belgium

It is unclear which was the first observation in Belgium. In 1952, the hydroid was observed in the brackish parts of the Western Scheldt, near Doel, Lillo and Liefkenshoek, where the species was identified as Bougainvillia ramosa ^[7]. Ten years later, at the same location, scientists found established populations of the rope grass hydroid Calyptospadix cerulea and not of the other species. Therefore, suspected a later study that the observation from 1952 concerned the species Calyptospadix cerulea (former name: Garveia franciscana) ^[4]. However, there is no conclusive evidence of a misidentification since the material from 1952 was not preserved. That is why the first official observation of this species dates from 1962 ^[4].

Spreading in Belgium

Its only known distribution area in Belgium is limited to the Western Scheldt. Here, it was regularly observed near Doel and the Dutch-Belgian border between 2001 and 2005 ^[8].

Spreading in neighbouring countries

In 1920, the first individuals of the Rope grass hydroid were found in the southeast of the Zuiderzee – nowadays called IJssel Lake – in the Netherlands. These individuals were first identified as Bougainvillia ramosa ^[9]. After a new determination attempt, it was concluded that they were misidentified and actually concerned the species Calyptospadix cerulea ^[4]. Two years after the closing off of the Zuiderzee and the formation of the Ijssel Lake (1932), this species was still present ^[4]. Thereafter, the species seemed to have disappeared. Later, in 1958, was this hydroid observed near Willemstad in the Hollands Diep river, north of North Brabant ^[10]. In subsequent years, the species was present in Hellevoetsluis in the Haringvliet ^[4]. Reports of the presence of the rope grass hydroid in the Eastern Scheldt throughout the year ^[11] are disputed ^[8]. The hydroid is still present in the Scheldt and was also observed between 2000 and 2005 in the Western Scheldt, near the Dutch-Belgian border ^{[8, 12]}. In 2004, it was observed near Lauwersoog in the Dutch Wadden area ^[13], and in 2010, in the North Sea Canal between Amsterdam and the North Sea ^[8].

The Rope grass hydroid occurs, since 1946, in the estuary of the river Elbe in the west of Germany ^[4]. In the Baltic Sea, it has been present since 1950 ^[14]. Since 1978, this hydroid has been observed in the Adriatic Sea ^[6].

Introduction to new areas occurs via the attachment of adult individuals to ships and the transport of free-living larvae in the ballast water ^[5].

This cnidarian is not selective regarding the substrate to which it attaches. Suitable substrates include artificial surfaces in harbours or rocks and other organisms, such as mussels ^[6]. The species thrives in the presence of a lot of organic matter ^[6].

The Rope grass hydroid can tolerate salinities between 3.5 psu (slightly brackish) and 35 psu (seawater) ^[4] but occurs under natural conditions only in brackish water ^[15]. The distribution of the species in Belgian waters is limited to the brackish Scheldt Estuary. It has an optimal growth between 9 and 34°C ^[4] and, therefore, occurs in tropical and temperate areas ^[6].

Although the Rope grass hydroid has a sedentary lifestyle, it produces free-living larvae. Sea currents and ballast water can transport the larvae and help to spread to new areas ^{[4, 5]}.

The Rope grass hydroid attaches to ship hulls or other infrastructures, such as submerged water pumps. This could lead to biological corrosion of the material, with economic damage as a result ^[2]. Biofouling can be prevented by applying a layer of antifouling paint. However, this preventive measure has economic and often ecological consequences ^[16]. When the biofouling community gets manually removed, adult individuals may have already deposited their eggs and, thereby, still be able to reproduce. It is advised to regularly clean so that the species does not get a chance to mature enough to reproduce. To know how often this should be done, it is necessary to look at the reproduction rate of the species. An interval of one or two weeks should be sufficient to prevent the further distribution of non-native species. Nonetheless, this is a lot in comparison to the current habit of cleaning once or twice a year. It is also recommended to clean in the area of origin of the biofouling species, as wastewater from cleaning may contain eggs, which may develop new populations elsewhere ^[17].

The Rope grass hydroid forms colonies that have an orange to brown-yellowish colour. The colonies can grow up to 20 cm high and have multiple branches. The main stem – also called the hydrocaulus – is unbranched. The hydrocaulus is connected to a basal attachment system, called the hydrorhiza. With these hydrorhizae, the colony attaches to a solid substrate. The side branches of the hydrocaulus are called hydrocladia. These are subdivided into pedicels. At the end of each pedicel, one hydroid (the hydranth) is present. All hydranths within a colony have the same sex (male or female). Each hydranth has a mouth surrounded by eight to twelve tentacles. Attached to the hydrocaulus are some globular appendages. These attachments are the reproductive organs or gonophores. The hydrocladia are surrounded by a thick casing called the perisarc ^{[6, 15]}.

The tentacles of the hydroids contain nematocysts. These serve as a defence and are used to collect food. A light touch activates a nematocyst, which in turn shoots a coiled-up wire at potential prey. This wire contains paralysing poison ^[18].

In the past, this cnidarian was often confused with Cordylophora caspia, another non-native cnidarian that tolerates a wide range of salinities. Both species can be easily distinguished. A colony of Cordylophora caspia is much smaller – only up to 10 cm – and has a more irregular shape than Cordylophora cerulea. In addition, the tentacles of Cordylophora caspia are spread over the entire body, while those of the Rope grass hydroid only surround the mouth ^[4].

[1]          World Register of Marine Species (WoRMS) (2024). Calyptospadix cerulea Clarke, 1882. [https://www.marinespecies.org/aphia.php?p=taxdetails&id=292221] (2024-10-18).

[2]          De Rincon, O.; Morris, E. (2003). Studies on selectivity and establishment of “Pelo de Oso” (Garveia franciscana) on metallic and non-metallic materials submerged in Lake Maracaibo, Venezuela. Anti-Corros. Methods Mater. 50(1): 17-24. [http://www.vliz.be/en/imis?module=ref&refid=195964]

[3]          Kerckhof, F.; Haelters, J.; Gollasch, S. (2007). Alien species in the marine and brackish ecosystem: the situation in Belgian waters. Aquat. Invasions 2(3): 243-257. [http://www.vliz.be/en/imis?module=ref&refid=114365]

[4]          Vervoort, W. (1964). Notes on the distribution of Garveia franciscana (Torrey, 1902) and Cordylophora caspia (Pallas, 1771) in the Netherlands. Zool. Meded. 39: 125-146. [http://www.vliz.be/en/imis?module=ref&refid=110855]

[5]          Cohen, A.N.; Carlton, J.T. (1995). Non indigenous aquatic species in a United States estuary: a case study of the biological invasions of the San Francisco Bay and delta. NOAA: USA. 251 pp. [http://www.vliz.be/en/imis?module=ref&refid=117462]

[6]          Morri, C. (1982). Sur la présence en Méditerranée de Garveia franciscana (Torrey 1902) (Cnidaria, Hydroida). Cah. Biol. Mar. 23(4): 381-391. [http://www.vliz.be/en/imis?module=ref&refid=63077]

[7]          Leloup, E.; Konietzko, B. (1956). Recherches biologiques sur les eaux saumâtres du Bas-Escaut. Mémoires de l'Institut Royal des Sciences Naturelles de Belgique = Verhandelingen van het Koninklijk Belgisch Instituut voor Natuurwetenschappen, 132. Koninklijk Belgisch Instituut voor Natuurwetenschappen: Brussel, Belgium. 100, 5 plates pp. [http://www.vliz.be/en/imis?module=ref&refid=26732]

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

[9]          Funke, H.C. (1922). Hydroiden, in: Redeke, H.C. Flora en fauna der Zuiderzee: Monografie van een brakwatergebied. C. De Boer Jr.: Den Helder: pp. 185-210. [http://www.vliz.be/en/imis?module=ref&refid=115198]

[10]        Den Hartog, C. (1959). Perigonimus megas, een vergeten Brakwaterpoliep. Het Zeepaard 19(1): 10-12. [http://www.vliz.be/en/imis?module=ref&refid=115242]

[11]        Oosterbaan, A. (1985). Hydropoliepen (Hydroida). Tabellenserie van de Strandwerkgemeenschap (SWG), 27. Strandwerkgemeenschap: Leiden. 22 pp. [http://www.vliz.be/en/imis?module=ref&refid=234734]

[12]        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]

[13]        Tulp, A.W. (2010). Een vondst van Pachycordyle navis (Millard, 1959) en notities over enige andere poliepen. Het Zeepaard 70(2): 42-48. [http://www.vliz.be/en/imis?module=ref&refid=145562]

[14]        Streftaris, N.; Zenetos, A.; Papathanassiou, E. (2005). Globalisation in marine ecosystems: the story of non-indigenous marine species across European seas. Oceanogr. Mar. Biol. Ann. Rev. 43: 419-453. [http://www.vliz.be/en/imis?module=ref&refid=75009]

[15]        Schuchert, P. (2007). The European athecate hydroids and their medusae (Hydrozoa, Cnidaria): Filifera part 2. Rev. Suisse Zool. 114(2): 195-396. [http://www.vliz.be/en/imis?module=ref&refid=206968]

[16]        Schultz, M.P.; Bendick, J.A.; Holm, E.R.; Hertel, W.M. (2010). Economic impact of biofouling on a naval surface ship. Biofouling 27(1): 87-98. [http://www.vliz.be/en/imis?module=ref&refid=206434]

[17]        Neves, C.S.; Moreira da Rocha, R. (2008). Introduced and cryptogenic species and their management in Paranaguá Bay, Brazil. Brazilian Archives of Biology and Technology 51(3): 623-633. [http://www.vliz.be/nl/catalogus?module=ref&refid=297322]

[18]        Ruppert, E.E.; Barnes, R.D. (1994). Invertebrate zoology. 6th edition. Saunders College Publishing: Orlando. ISBN 0-03-026668-8. 1056 pp. [http://www.vliz.be/nl/catalogus?module=ref&refid=9414]

Alien Species Consortium (2022). Calyptospadix cerulea - Rope grass hydroid. Niet-inheemse soorten in het Belgisch deel van de Noordzee en aanpalende estuaria = Non-indigenous species from the Belgian part of the North Sea and estuaria. VLIZ Alien Species Consortium (VLIZ): Oostende. 5 pp.