Ammothea hilgendorfi - Pacific brown-banded sea-spider

The Pacific brown-banded sea-spider originates from coastal areas in the tropical and northern parts of the Pacific Ocean ^[2]. This species lives in the tropical and temperate littoral zone of Southeast Asia (e.g., Japan) ^[3] and the west coast of America (e.g., California, Mexico) ^[4].

First observation in Belgium

In 2013, this alien species was first identified from a sample taken from the Hinder Bank, approximately 30 km from the coast. Two specimens were present in the coarse-grained sediment sample collected at a depth of 31 meters ^[5].

Spreading in Belgium

Since 2022, over 1,000 specimens of the Pacific brown-banded sea-spider have been found on the breakwaters of Knokke (across all life stages), indicating that this species has an established presence there ^[6,7]. High densities have frequently been measured, reaching over 400 individuals per square meter ^[7].

Spreading in neighbouring countries

In Europe, Ammothea hilgendorfi was first recorded along the British coast near Southampton ^{[8, 9]}, from where it slowly spread further. This sea spider was reported for the first time in the Venetian Lagoon (Mediterranean) between 1979 and 1981 ^[10-12]. During four sampling periods across eight locations, 29 additional specimens were found in the lagoon between 1991 and 2017, which, according to the authors, suggests a widespread and established presence. In fact, the Pacific brown-banded sea-spider was the only sea spider species found in the Venetian Lagoon, aside from a single occurrence of a small Anoplodactylus pygmaeus, whereas seven species previously thrived there ^[13]. The first North Sea observations date back to 2010 and were recorded in the Blackwater estuary (Essex), UK ^[14]. In the same year, the Pacific brown-banded sea-spider was also found in Turkey (Mersin Bay – Southern Turkey) ^[15].

In August 2013, the first observations of Dutch specimens in the Eastern Scheldt near Zierikzee were confirmed ^[16]. A total of 17 specimens were found in two sediment samples. One male specimen carried egg sacs, and two females contained eggs in an advanced state of development. This, along with the large number of specimens found, indicates that the species likely arrived in the Eastern Scheldt earlier and has already been established ^[17].

In 2019, the species was first observed along the French Atlantic coast in southern Brittany (Étel River estuary), where up to 70 individuals were found in a single sampling ^[18]. The sea spider was later observed at several other locations in both North and South Brittany ^[19].

It is unclear how the species ended up in the Eastern Scheldt. Most likely, its introduction to England occurred via international shipping ^[2]. From there, it is possible that a few specimens were transported to the Netherlands through the import of shellfish or recreational boating ^[2].

In Poole Harbor (southern England), the Pacific brown-banded sea-spider was found to be capable of spreading from pontoons in a marina to nearby rocky shores within five years, establishing locally 'superabundant' populations ^[14]. In the Netherlands, samples also indicate high local densities ^[16]. Further spread in the North Sea is therefore certainly to be expected.

Since the species does not have a dispersive phase in its life cycle, its spread is expected to be relatively slow, unless human activities (such as shipping or shellfish transport) further facilitate its distribution ^[20].

The Pacific brown-banded sea-spider lives on sediment, mussel and oyster aggregations, and seaweed, but can also be found on echinoderms (such as sea cucumbers and starfish) and hydroids ^{[6, 21-23]}. It is not known whether the species exerts a negative ecological impact ^[24]. However, theoretically, there could be competition for food and space with other native (and introduced) sea spiders or other fauna ^[20].

Sea spiders (class Pycnogonida) are distinct from spiders (class Arachnida) as they are found worldwide in the ocean. Most sea spiders are only a few millimeters in size and are difficult to distinguish from one another. However, some species, including the Pycnogonum litorale, Nymphon rubrum and Ammothea hilgendorfi, can reach up to 2 cm, including their legs ^[16]. The native species Nymphon rubrum is easily distinguishable from the Pacific brown-banded sea-spider. The former has a slimmer body and is uniformly light to dark reddish-brown in color ^[13]. The Pacific brown-banded sea-spider has a noticeably more robust build with distinct brown to purplish crossbands on a primarily yellowish-brown body and legs, as well as a prominent red band on its body ^[24].

Adult Ammothea hilgendorfi exhibit sexual dimorphism ^[25]. The females are slightly larger in terms of body and leg length than the males, although the males tend to have slightly broader bodies ^[25]. The hollow legs of this type of sea spider are used for the development and storage of reproductive cells. The females produce egg sacs, which are picked up and fertilised by the males. Male sea spiders typically mate with more than one female, and vice versa, a behavior known as polyandry ^[26]. It is also the males that take on parental care. They have an extra pair of leg-like appendages under their abdomen, called ovigers, which they use to carry the fertilised egg sacs until the larvae hatch ^[27].

[1]          World Register of Marine Species (WoRMS) (2024). Ammothea hilgendorfi (Böhm, 1879). [http://www.marinespecies.org/aphia.php?p=taxdetails&id=134607] (2024-10-18).

[2]          Faasse, M. (2013). Further dispersal of the sea-spider Ammothea hilgendorfi (Böhm, 1879) in the North Sea to The Netherlands. BioInvasions Records 2(4): 287-289. [http://www.vliz.be/nl/catalogus?module=ref&refid=298895]

[3]          Nakamura, K.; Child, C.A. (1991). Pycnogonida from waters adjacent to Japan. Smithsonian Contributions to Zoology, 512. . Smithsonian Institution Press: Washington. 74 pp. [http://www.vliz.be/nl/catalogus?module=ref&refid=300155]

[4]          Liu, R. (2008). Checklist of marine biota of China Seas. Science Press: Beijing. ISBN 978-7-03-023722-4. 1267 pp. [http://www.vliz.be/nl/catalogus?module=ref&refid=210242]

[5]          ICES Advisory Committee on the Marine Environment (2014). Report of the ICES Working Group on Introductions and Transfers of Marine Organisms (WGITMO), 19-21 March 2014, Palanga, Lithuania. CM Documents - ICES. CM 2014/ACOM:32. ICES: 259 pp. [http://www.vliz.be/nl/catalogus?module=ref&refid=298896]

[6]           Flandroit, F.; Simon, L.; Caulier, G. (2024). Description of limb anomalies resulting from molt irregularities in Ammothea hilgendorfi (Pycnogonida: Ammotheidae). Arthropoda 2(2): 156-168. [https://www.vliz.be/en/imis?module=ref&refid=393373]

[7]           Flandroit, A., Simon, L., Decoux, B., Geerinckx, N., Eeckhaut, I. & Caulier, G. (submitted). Characterization of the first invasive population of sea spiders (Pycnogonida).

[8]          Bamber, R.N. (1988). The itinerant sea spider Ammothea hilgendorfi (Böhm) in British waters. Proc. Hampsh. Field Club Archaeol. Soc. 41: 269-270. [http://www.vliz.be/nl/catalogus?module=ref&refid=298901]

[9]        Bamber, R.N. (2010). Sea-spiders (Pycnogonida) of the north-east Atlantic: Keys and notes for identification of British species. 2nd. Edition. Synopses of the British Fauna, N.S. 5. . Field Studies Council: Shrewsbury. ISBN 978-185153-273-5. 249 pp. [http://www.vliz.be/nl/catalogus?module=ref&refid=199646]

[10]          Marchini, A.; Ferrario, J.; Sfriso, A.; Occhipinti-Ambrogi, A. (2015). Current status and trends of biological invasions in the Lagoon of Venice, a hotspot of marine NIS introductions in the Mediterranean Sea. Biological Invasions 17(10): 2943-2962. [http://www.vliz.be/nl/catalogus?module=ref&refid=298897]

[11]          Occhipinti-Ambrogi, A.; Marchini, A.; Cantone, G.; Castelli, A.; Chimenz, C.; Cormaci, M.; Froglia, C.; Furnari, G.; Gambi, M.C.; Giaccone, G.; Giangrande, A.; Gravili, C.; Mastrototaro, F.; Mazziotti, C.; Orsi-Relini, L.; Piraino, S. (2010). Alien species along the Italian coasts: an overview. Biological Invasions 13(1): 215-237. [http://www.vliz.be/nl/catalogus?module=ref&refid=298898]

[12]          Krapp, F.; Sconfietti, R. (1983). Ammothea hilgendorfi (Böhm, 1879), an adventitious pycnogonid new for the Mediterranean Sea. Mar. Ecol. 4(2): 123-132. [http://www.vliz.be/nl/catalogus?module=ref&refid=245149]

[13]           Mizzan, L. (2018). New data on the presence and diffusion of the exotic sea-spider Ammothea hilgendorfi (Böhm, 1879) in the Lagoon of Venice (Italy). Boll. Mus. civ. St. nat. Venezia 69: 43-46. [https://www.vliz.be/en/imis?module=ref&refid=393368]

[14]        Bamber, R.N. (2012). Anthropogenic spread of the immigrant seaspider Ammothea hilgendorfii (Arthropoda: Pycnogonida: Ammotheidae) in UK waters. Marine Biodiversity Records 5(78): 1-5. [http://www.vliz.be/nl/catalogus?module=ref&refid=298902]

[15]           Ragkousis, M.; Abdelali, N.; Azzurro, E.; Badreddine, A.; Bariche, M.; Bitar, G.; Crocetta, F.; Denitto, F.; Digenis, M.; El Zrelli, R.; Ergenler, Y.; Fortic, A.; Gerovasileiou, V.; Grimes, S.; Katsanevakis, S.; Kocak, C.; Licchelli, C.; Loudaros, E.; Mastrototaro, F.; Mavric, B.; Mavruk, S.; Miliou, A.; Montesanto, F.; Ovalis, P.; Pontes, M.; Rabaoui, L.; Sevingel, N.; Spinelli, A.; Tiralongo, F.; Tsatiris, A.; Turan, C.; Vitale, D.; Yalgin, F.; Yapici, S.; Zenetos, A. (2020). New alien Mediterranean biodiversity records (October 2020). Mediterr. Mar. Sci. 21(3): 631-652. [https://www.vliz.be/nl/imis?module=ref&refid=331638]

[16]        Perk, F.A.; Faasse, M.A. (2014). De Zebrazeespin Ammothea hilgendorfi waargenomen in het litoraal bij Zierikzee. Het Zeepaard 74(3): 93-97. [http://www.vliz.be/nl/catalogus?module=ref&refid=241731]

[17]        Stichting ANEMOON (2013). Zeespin uit Stille Oceaan heeft Nederlandse kust bereikt. [https://www.naturetoday.com/intl/nl/nature-reports/message/?msg=19041] (2018-07-17).

[18]           Le Roux, A.; Gélinaud, G.; Le Bail, Y.; Monnat, J.-Y.; Morel, J.-Y.; Paraire, O.; Ros, J. (2020). Occurrence of Ammothea hilgendorfi (Böhm, 1879) a pycnogonid from the north Pacific, in Étel river. An Aod 8(1): 21-32. [https://www.vliz.be/en/imis?module=ref&refid=393366]

[19]           Le Roux, A.; Corbrion, P.; Gully, F.; Cochu, M.; Picard, C. (2022). Nouveaux signalements d’Ammothea hilgendorfi (Pycnogonida, Ammotheidae) sur les côtes de Bretagne. An Aod In Press: 1-12. [https://www.vliz.be/en/imis?module=ref&refid=393364]

[20]        Nederlands Soortenregister. Overzicht van de Nederlandse biodiversiteit (2018). Zebrazeespin Ammothea hilgendorfi [https://www.nederlandsesoorten.nl/linnaeus_ng/app/views/species/nsr_taxon.php?id=174045] (2018-08-09).

[21]        Ohshima, H. (1927). Notes on some pycnogons living semiparasitic on holothurians. Proc. Imp. Acad. Jpn. 3: 610-613. [http://www.vliz.be/nl/catalogus?module=ref&refid=300156]

[22]        Nakamura, K.; Fujita, T. (2004). Ammothea hilgendorfi (Pycnogonida: Ammotheidae) associated with a sea star, Coscinasterias acutispina (Echinodermata: Asteroidea) from Sagami Bay, Japan. Species Divers 9: 251-258. [http://www.vliz.be/nl/catalogus?module=ref&refid=300154]

[23]        Hedgpeth, J.W.; Haderlie, E.C. (1980). Pycnogonids, in: Morris, R.H. et al. Intertidal invertebrates of California. Stanford University Press: Stanford: pp. 636-640. [http://www.vliz.be/nl/catalogus?module=ref&refid=300157]

[24]        Eno, N.C.; Clark, R.A.; Sanderson, W.G. (Ed.) (1997). Non-native marine species in British waters: a review and directory. Joint Nature Conservation Committee: Peterborough. ISBN 1-86107-442-5. 152 pp. [http://www.vliz.be/nl/imis?module=ref&refid=24400]

[25]        Barreto, F.S.; Avise, J.C. (2008). Polygynandry and sexual size dimorphism in the sea spider Ammothea hilgendorfi (Pycnogonida: Ammotheidae), a marine arthropod with brood-carrying males. Mol. Ecol. 17(18): 4164-4175. [http://www.vliz.be/nl/imis?module=ref&refid=298903]

[26]        Andersson, M. (1994). Sexual selection. Princeton University Press: New Jersey. ISBN 9780691000572. 624 pp. [http://www.vliz.be/nl/catalogus?module=ref&refid=300152]

[27]        Arnaud, F.; Bamber, R.N. (1988). The biology of Pycnogonida. Adv. Mar. Biol. 24: 1-96. [http://www.vliz.be/en/imis?module=ref&refid=201336]

VLIZ Alien Species Consortium (2024). Ammothea hilgendorfi – The Pacific brown-banded sea-spider. Introduced alien species of the Belgian part of the North Sea and adjacent estuaries anno 2024. Flanders Marine Institute (VLIZ). 6 pp.