Synidotea laticauda -
Synidotea laticauda originates from the San Francisco Bay Area and the neighbouring estuaries on the west coast of the United States [4]. This isopod is one of the most common species there [5]. In our regions, the species mainly occurs in shallow brackish water [4], on rocky jetties and buoys between a fouling community of polyps – including the (non-native) rope grass hydroid Calyptospadix cerulea – bryozoans and oysters [6].
First observation in Belgium
This isopod was observed for the first time in Belgium in a soil sample taken on the 27th of September 2005, in the Sea Scheldt near the Doel Nuclear Power Station [7]. However, the species was misidentified as Synidotea laevidorsalis [8]. It is quite possible that this isopod was already present in the study area before 2005 [7].
Spreading in Belgium
S. laticauda has a limited distribution in the Scheldt estuary and only occurs near Doel and Kallo. In 2005, three individuals were found in two different soil samples, but most individuals (>100) were collected in 2007 from filters that sieve the cooling water of the Doel Nuclear Power Station [7]. At this location, the species is still abundant [8]. Since 2013, it has also been found annually in the Sea Scheldt near Doel [9]. Recently, this isopod was found in the Western Scheldt [10].
Spreading in neighbouring countries
S. laticauda has been present in Europe since 1975 in the Gironde estuary in France [11]. Originally, the species was confused with the native isopod Idotea emarginata. Only in 1991 was the isopod recognized as a non-native species [12], but – due to the taxonomic uncertainty at the time – wrongly identified as S. laevidorsalis. After re-examining the samples, it turned out to be S. laticauda [3].
A 1991-1994 study in the Guadalquivir estuary in southern Spain found more than 1,000 S. laticauda individuals per sample. Again, these individuals were initially identified as S. laevidorsalis [13]. In the adjacent estuary of the Guadiana River, the species was also found in 2009 and 2010 [14].
In 2016, S. laticauda was observed in the Port of Brunsbüttel, in the Elbe Estuary, Germany [15].
In Europe, S. laticauda seems to be always observed near estuaries with international ports. Therefore, it is suspected that shipping is responsible for the introduction of S. laticauda to Europe. Possibly, the species got transported attached to ship hulls or – more likely – carried in the ballast water [7, 16, 17].
In its area of origin (San Francisco Bay, United States), S. laticauda mainly inhabits warmer waters with a reduced salinity [4]. In Belgium, the species was only found near Doel and may benefit from warm water discharge from the cooling towers of the nuclear power station located there [7]. In recent years, the species seems to be moving further away from Doel, at least during the summer months [18, 19].
This isopod is an opportunistic omnivore. As a predator, it feeds on amphipods taken from the water column with its legs. As a scavenger, it consumes dead oysters, crabs (including the blue crab Callinectes sapidus in America) and conspecifics. Finally, as a herbivore, it feeds on Sea lettuce (Ulva sp.) and to a lesser extent on Cordgrass (Spartina). Bryozoans and polyps are also part of this isopod’s diet [17].
Very little is known about the ecology of this isopod in our waters. Its numbers fluctuate seasonally, being more common during the winter months than during the summer [5, 16]. In its area of origin, the species occurs mainly on artificial substrates such as buoys, pontoons and ropes for oyster farming [16]. S. laticauda is sporadically found in soil samples [7, 16] and soil grinders [5, 12, 18]. The latter suggests that this isopod also swims in the water column, just above the bottom [12]. The species has also been found swimming between vegetation [20].
S. laticauda lives in brackish water but has a wide salinity tolerance. The species has been found in the Gironde estuary at salinities between 0.1 and 24 PSU, and it is most abundant between 1 and 10 PSU [12]. Fresh water (0 PSU) appears to be lethal to S. laticauda, but it can tolerate salinities of up to 35 PSU (seawater).
The species can tolerate water temperatures between 5°C and 25°C without any problems. It has a high mortality rate (>65%) at water temperatures above 30°C [21]. In addition, the species does not tolerate strong environmental changes, such as the input of fresh water via rivers or rainfall [17].
Very little is known about the impact of S. laticauda in our regions or other areas where it has been introduced [17]. However, along the east coast of the United States – where the species is also non-indigenous – high densities of the isopod have been observed (in September). This corresponded to greatly reduced numbers of native amphipods and the estuarine mud crab Rhithropanopeus harrisii (also native there) [16]. This suggests that high numbers of S. laticauda affect the ecosystem [17].
Like most isopod species, S. laticauda carries its fertilized eggs in a brood pouch until all larval stages are completed. The brood pouch is formed by some plates attached to the first five pairs of legs, sliding over each other like roof tiles [22]. Because of the colouration of its dorsal side, this isopod is well camouflaged. It has little claws on the legs that increase its grip on surfaces. The species has seven pairs of paddle-shaped legs that allow it to swim just above the bottom between the vegetation [12, 20].
Female individuals of S. laticauda are smaller than males and grow up to 1.3 cm, while males grow to 2.3-3 cm [3, 16]
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[3] Poore, G.C.B. (1996). Species differentiation in Synidotea (Isopoda: Idoteidae) and recognition of introduced marine species: A reply to Chapman and Carlton. J. Crust. Biol. 16(2): 384-396. [http://www.vliz.be/imis/imis.php?module=ref&refid=206436]
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[8] Soors, J. (2011). Persoonlijke mededeling
[9] Van Ryckegem, G.; Van Braeckel, A.; Elsen, R.; Speybroeck, J.; Vandevoorde, B.; Mertens, W.; Breine, J.; De Regge, N.; Soors, J.; Dhaluin, P.; Terrie, T.; Van Lierop, F.; Hessel, K.; Froidmont, M.; Van den Bergh, E. (2014). MONEOS – Geïntegreerd datarapport: INBO: toestand Zeeschelde 2013. Monitoringsoverzicht en 1ste lijnsrapportage Geomorfologie, diversiteit Habitats en diversiteit Soorten. Rapporten van het Instituut voor Natuur- en Bosonderzoek, INBO.R.2014.2646963. Instituut voor Natuur- en Bosonderzoek (INBO): Brussel. 137 pp. [http://www.vliz.be/nl/catalogus?module=ref&refid=241088]
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[13] Cuesta, J.A.; Serrano, l.; Bravo, M.R.; Toja, J. (1996). Four new crustaceans in the Guadalquivir river estuary (SW Spain), including an introduced species. Limnetica 12(1): 41-45. [http://www.vliz.be/imis/imis.php?module=ref&refid=206422]
[14] Nuño, C.; Peg, M.; Mellado-Díaz, A.; Sánchez-González, J.R.; Toro, M. (2018). First record of Synidotea laticauda Benedict, 1897 (Crustacea: Isopoda) in the Guadiana Estuary (SW Iberian Peninsula). Limnetica 37(2): 173-179. [http://www.vliz.be/nl/catalogus?module=ref&refid=300967]
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[17] Boyd, S.G. (2008). An ecological assessment of the non-indigenous isopod, Synidotea laticauda, in Delaware Bay. MSc Thesis. Rutgers, State University of New Jersey: New Brunswick. 82 pp. [http://www.vliz.be/imis/imis.php?module=ref&refid=208361]
[18] Soors, J. (2019). Persoonlijke mededeling
[19] Waarnemingen afkomstig van Waarnemingen.be, e.i.v.N.S.v.e.d.S.N. (2019). Synidotea laticauda (Benedict, 1897) [https://waarnemingen.be/species/643529/] (2019-07-09).
[20] Cough, R. (2009). Guide to marine invaders in the Gulf of Maine: Synidotea laevidorsalis Asian isopod. Salem Sound Coastwatch: Salem. 2 pp. [http://www.vliz.be/imis/imis.php?module=ref&refid=208491]
[21] Boyd, S.; Bushek, D. (2006). The potential impact of the Asian isopod, Synidotea laevidorsalis (Miers 1881), on the Delaware Bay, USA, New Jersey Water Resources Research Institute Annual Technical Report, FY 2006. New Jersey Water Resources Research Institute: New Jersey, USA: pp. 1-4. [http://www.vliz.be/en/imis?module=ref&refid=300985]
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VLIZ Alien Species Consortium (2020). Synidotea laticauda. Non-indigenous species in the Belgian part of the North Sea and adjacent estuaries anno 2020. Flanders Marine Institute (VLIZ). 6 pp.