Keratella tropica -

K. tropica is native to southern France and Spain ^[2]. The species occurs worldwide in tropical and subtropical freshwater ^[3], but can thrive in temperate regions during the summer. It is a free-swimming species that can tolerate brackish water ^[4].

First observation in Belgium

In 1974, K. tropica was observed for the first time in Belgium in the Donk Lake (Overmere) ^[5]. In the same year, the species was also present in the Watersportbaan (Ghent) ^[6].

Spreading in Belgium

In 1983, scientists reported that K. tropica has an invasive character in Belgium during hot summers ^[7]. In the spring and summer of 2002, 2003 and 2004, the diversity of rotifers in the Scheldt estuary was investigated. The species was observed every July and August, most of the time in the headwaters upstream of Ghent. A few specimens of this rotifer were found in Antwerp – in the brackish part of the Scheldt estuary ^[4].

Spreading in neighbouring countries

In the Netherlands, this rotifer was first found in August 1959 in the freshwater tidal zone at the mouth of the Rhine, near the Biesbosch ^[8]. Later, in 1976 and 1977, the species was reported from the ‘Hollands Diep’ south of Rotterdam and in the ‘Reeuwijkse Plassen’ between Rotterdam and Amsterdam, respectively ^[6].

This rotifer also occurs in fresh water in France. However, the species is native to southern France and Spain ^[2]. In the south of France, the species was found in the provinces of Landes, Gironde (southwest) and the Dombes (north of Lyon) ^[9]. In Spain, the species occurs in artificial reservoirs spread over the entire country. This observation is mentioned in a study performed in 1972-1975 and 1987-1988 ^{[10, 11]}. The species is also observed in natural water bodies ^[12]. Nowadays, the animal is considered a widespread general freshwater species in Europe ^[4].

Introduction most likely occurred via the ballast water of ships. Apart from international transport, local shipping traffic may also have played a role in the spread of this alien species ^[13]. In the Scheldt, the introduction may have happened via transport over the channels of the Rhine or the Meuse ^[4].

Aquatic birds – like the mallard – can play a role in the spreading of rotifers in two ways. On the one hand, the resting eggs of rotifers can be eaten and released – with faeces – at a different location. On the other hand, rotifers or their resting eggs can stick to the legs or feathers and be transported to other areas ^[14]. The locations where K. tropica are found, correspond with the migration routes of birds that overwinter in (sub)tropical areas and breed in cold and temperate regions ^[9].

Transport via the ballast water of ships or spreading with migrating birds can cause a rapid expansion of this species to several freshwater bodies and estuaries ^{[4, 9]}.

Additionally, this rotifer can quickly reproduce through sexual and asexual reproduction. An entire new population can result from the asexual reproduction of only a single resting egg ^[15].

Like most rotifers, K. tropica occurs mainly in fresh water. However, the species can survive in brackish water. In the Scheldt, the animal is observed at a salinity of up to 2.62 PSU. When the species was present, the water temperature varied between 18.5 and 23.6°C. These are unusually warm conditions for the Scheldt. In our regions, this species is only present in large numbers during the summer or in places where the water is heated by warm cooling water ^{[4, 6, 8]}. Global warming may facilitate the development of this species in our regions.

This rotifer thrives in nutrient-rich environments like estuaries, such as the Scheldt ^{[16, 17]}.

During the summer, especially in August, K. tropica makes up about 8% of the total density of all rotifers in the Scheldt estuary. This can potentially affect the food web of the estuary ^[4], although there is currently no evidence that can confirm this.

Rotifers are very small organisms, only visible through a microscope. They have a head (located in the front), middle part (trunk) and back part (foot) (figure 1). The head has a ciliated crown, the corona, used to move and feed by filtering particles from the water. The mouth is located in the centre of this structure, followed by the mastax, a hard structure used to chew food particles. The shape of the mastax is quite species-specific and can be used as an identification feature. Some species are enclosed by a shell or ‘lorica’, while in others this feature is absent ^[18].

Figure 1: General body plan of rotifers (Source: VLIZ).

K. tropica is about 165 µm large (spines included) (figure 2). The animal is surrounded by a shell, which can be used for the identification of the species. It exists of several plates, one – the small posterior plate – is characteristic of this species. The shell has six anterior spines near the head and two posterior spines ^[4]. Their length varies between individuals. One of these spines (on the righthand side) is always longer than the others ^[19] (figure 3). K. tropica has no foot ^[2].

Figure 2: Keratella tropica (Source: Azémar et al. 2007) ^{[3, 4]}.

Figure 3: Variation in size of the right stalk in Keratella tropica (Source: Azémar et al. 2007) ^[4].

Research showed that female rotifers detect the presence of a predator through certain substances (kairomones) that the predators release. As a reaction to this, the offspring – in asexual reproduction with amictic eggs (see later) – develop spines that function as a defence against predators. Scientists call this ‘phenotypical plasticity’, which means that the ability to develop these spines is present in the genetic material (DNA), but the development itself is determined by the environment ^{[2, 20]}.

K. tropica belongs to the group of Monogonata. This name refers to the presence of only one (mono) gonad. In rotifers, there are always more females than males present. The males are much smaller than the females. In the absence of males, females develop their eggs without fertilisation. This is called parthenogenesis or ‘virgin birth’. In this reproduction method, ‘amictic’ eggs are produced, meaning that each chromosome is present twice (2N). Females can also produce ‘mictic’ eggs, that contain only one copy of each chromosome (N), although the situation here is more complex. Unfertilised mictic eggs develop into males, which can fertilise other mictic eggs ^{[2, 15]}. A fertilised mictic egg develops into a resting egg. This egg will only develop during better conditions – sometimes only after a couple of months or years – and will always produce female individuals. These resting eggs are very suitable to bridge adverse environmental conditions and can be transported to other areas by the currents (figure 4).

Figure 4: Sexual and asexual reproduction in rotifers (Source: VLIZ).

[1]          World Register of Marine Species (WoRMS) (2020). Keratella tropica (Apstein, 1907). [http://www.marinespecies.org/aphia.php?p=taxdetails&id=248010] (2020-11-17).

[2]          Segers, H. (2009-2011). Persoonlijke mededeling

[3]          Segers, H. (2007). Annotated checklist of the rotifers (Phylum Rotifera), with notes on nomenclature, taxonomy and distribution. Zootaxa 1564: 1-104. [http://www.vliz.be/en/imis?module=ref&refid=140748]

[4]          Azémar, F.; Van Damme, S.; Meire, P.; Tackx, M. (2007). New occurrence of Lecane decipiens (Murray, 1913) and some other alien rotifers in the Schelde estuary (Belgium). Belg. J. Zool. 137(1): 75-83. [http://www.vliz.be/en/imis?module=ref&refid=112260]

[5]          Coussement, M. (1977). Nieuwe gegevens omtrent de Rotatoria-fauna van het Donkmeer in Oost-Vlaanderen. Natuurwet. Tijdschr. 58(3): 138-146. [http://www.vliz.be/en/imis?module=ref&refid=140153]

[6]          Leentvaar, P. (1980). Note on some Brachionidae (Rotifers) from the Netherlands. Hydrobiologia 73(1-3): 259-262. [http://www.vliz.be/en/imis?module=ref&refid=139914]

[7]          Dumont, H.J. (1983). Biogeography of rotifers. Hydrobiologia 104: 19-30. [http://www.vliz.be/en/imis?module=ref&refid=139918]

[8]          Leentvaar, P. (1961). Quelques rotateurs rares observes en Hollande. Hydrobiologia 18: 245-251. [http://www.vliz.be/en/imis?module=ref&refid=140138]

[9]          De Ridder, M. (1981). Some considerations on the geographical distribution of rotifers. Hydrobiologia 85: 209-225. [http://www.vliz.be/en/imis?module=ref&refid=139912]

[10]        Guiset, A. (1977). General distribution of planktonic rotifers in Spanish reservoirs. Ergeb. Limnol. 8: 222-225. [http://www.vliz.be/en/imis?module=ref&refid=140334]

[11]        De Manuel Barrabin, J. (2000). The rotifers of Spanish reservoirs: Ecological, Systematical and zoogeographical remarks. Limnetica 19: 91-167. [http://www.vliz.be/en/imis?module=ref&refid=140744]

[12]        Miracle, M.R. (1982). Biogeography of the freshwater zooplanktoninc communities of Spain. J. Biogeogr. 9(6): 455-467. [http://www.vliz.be/en/imis?module=ref&refid=142407]

[13]        Wasson, K.; Zabin, C.J.; Bedinger, L.; Diaz, M.C.; Pearse, J.S. (2001). Biological invasions of estuaries without international shipping: the importance of intraregional transport. Biol. Conserv. 102: 143-153. [http://www.vliz.be/en/imis?module=ref&refid=140750]

[14]        Frisch, D.; Green, A.J.; Figuerola, J. (2007). High dispersal capacity of a broad spectrum of aquatic invertebrates via waterbirds. Aquat. Sci. 69: 568-574. [http://www.vliz.be/en/imis?module=ref&refid=140746]

[15]        Thane, A. (1974). Rotifera, in: Giese, A.C. et al. Reproduction of marine invertebrates: 1. Acoelomate and Pseudocoelomate Metazoans. Reproduction of marine invertebrates, 1. Academic Press: New York: pp. 471-484. [http://www.vliz.be/en/imis?module=ref&refid=10016]

[16]        Duggan, I.C.; Green, J.D.; Shiel, R.J. (2002). Distribution of rotifer assemblages in North Island, New Zealand, lakes: relationships to environmental and historical factors. Freshwat. Biol. 47: 195-206. [http://www.vliz.be/en/imis?module=ref&refid=140126]

[17]        Van Damme, S.; Struyf, E.; Maris, T.; Ysebaert, T.; Dehairs, F.; Tackx, M.; Heip, C.; Meire, P. (2005). Spatial and temporal patterns of water quality along the estuarine salinity gradient of the Scheldt estuary (Belgium and The Netherlands): results of an integrated monitoring approach. Hydrobiologia 540(1-3): 29-45. [http://www.vliz.be/en/imis?module=ref&refid=75934]

[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]

[19]        Green, J. (1980). Asymmetry and variation in Keratella tropica. Hydrobiologia 73(1-3): 241-248. [http://www.vliz.be/en/imis?module=ref&refid=139916]

[20]        Zagarese, H.E.; Marinone, M.C. (1992). Induction and inhibition of spine development in the rotifer Keratella tropica. Freshwat. Biol. 28: 289-300. [http://www.vliz.be/en/imis?module=ref&refid=140341]

VLIZ Alien Species Consortium (2020). Keratella tropica. Non-indigenous species in the Belgian part of the North Sea and adjacent estuaries anno 2020. Flanders Marine Institute (VLIZ). 7 pp.