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Bulletin de la Société royale belge d’Entomologie/Bulletin van de Koninklijke Belgische Vereniging voor Entomologie, 153 (2017): 89–93
A long-expected finding: the first record of the isopod
Eluma caelatum (Miers, 1877) from Belgium
(Crustacea: Isopoda: Armadillidiidae)
Pallieter DE SMEDT 1,2, Pepijn BOERAEVE², Gert ARIJS2, Oscar FRANKEN3, Oliver MECHTHOLD2,
Stijn SEGERS2 & Matty P. BERG3
1 Forest & Nature Lab, Ghent University, Geraardsbergsesteenweg 267, B-9090 Melle, Belgium (Gontrode)
(e-mail: Pallieter.desmedt@ugent.be)
2 SPINICORNIS, Mispeldonk 2, B-2820 Bonheiden, Belgium (e-mail: info@spinicornis.be)
3 Section Animal Ecology, Department of Ecological Science, Vrije Universiteit, Amsterdam, De Boelelaan
1085, NL-1081 HV Amsterdam, Nederland
Abstract
Since the discovery of Eluma caelatum (Miers, 1877) in the Netherlands close to the Belgium boarder
in the mid 1990’s, various attempts have been made to find this isopod species in Belgium. However,
these attempts have been unsuccessful until a joint excursion of Belgian and Dutch isopod researchers
searched for this enigmatic species once more in autumn 2016. They discovered the first Belgian
specimens of E. caelatum at the nature reserve “Zwin” near Knokke in West-Vlaanderen. One month
later, another population was found near Sint-Laureins in Oost-Vlaanderen, also close to the Dutch
border. Possibly E. caelatum colonised Belgium from the Netherlands since there are no connections
with southern populations in France. In Belgium and The Netherlands the species is bound to coastal
areas with a synantropic character, like levees of dikes and ditches while it is more of a forest species
in southern Europe. Therefore, it is unclear if the species is an accidental import from the south or if
the species is native at the edge of its distribution range.
Keywords: Eluma caelatum, Isopoda, woodlouse, distribution, Belgium
Samenvatting
Sinds de ontdekking van Eluma caelatum (Miers, 1877) in Nederland, dichtbij de Belgische grens,
midden jaren 1990 werd er getracht de soort te vinden in België. Echter zonder succes totdat een
gezamenlijke excursie van Belgische en Nederlandse onderzoekers nog eens op zoek ging naar deze
pissebed in de herfst van 2016. Ze vonden de eerste Belgische exemplaren in het “Zwin” bij Knokke
in West-Vlaanderen. Een maand later werd een tweede populatie ontdekt in Oost-Vlaanderen bij Sint-
Laureins dicht bij de Nederlandse grens. Mogelijk heeft de soort België gekoloniseerd vanuit
Nederland omdat er geen connecties zijn met zuidelijke populaties. In België en Nederland is de soort
gebonden aan kustgebieden met een synantroop karakter zoals dijken en slootoevers. In zuidelijk
Europe is de soort echter vooral in bossen te vinden. Daarom is het onduidelijk of de soort een
toevallige introductie is of een inheemse soort op de rand van zijn verspreidingsgebied.
Faune de Belgique / Fauna van België
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Résumé
Depuis la découverte d'Eluma caelatum (Miers, 1877) aux Pays-Bas, près de la frontière belge, au
milieu des années 90, diverses tentatives ont été faites pour trouver cette espèce d'isopode en Belgique.
Ces tentatives sont restées infructueuses jusqu'à sa découverte en automne 2016. En effet, c’est lors
d’une excursion, co-organisée par des chercheurs belges et hollandais, dans la réserve naturelle du
Zwin près de Knokke (West-Vlaanderen) que plusieurs spécimens ont été découverts pour la première
fois de Belgique. Un mois plus tard, une autre population a été trouvée près de Sint-Laureins (Oost-
Vlaanderen), localité également proche de la frontière hollandaise. E. caelatum aurait-il colonisé la
Belgique depuis les Pays-Bas étant donné qu'il n'y a aucun lien avec les populations du sud de la
France? De plus, en Belgique comme aux Pays-Bas, l'espèce est liée aux zones côtières à caractère
synanthropique, comme les digues et les fossés, alors qu'il s'agit plutôt d'une espèce forestière dans le
sud de l'Europe. Par conséquent, cette espèce a-t-elle été introduite accidentellement dans nos régions
ou est-elle à la limite de son aire de répartition septentrionale?
Introduction
Coastal habitats expose plants and animals to sometimes extreme environmental conditions, resulting
in a specialised subset of species. Many of these species have adapted to coastal abiotic conditions and
do not occur inland, resulting in a unique coastal fauna with species that are rare in the rest of the
country. This is certainly the case for countries with a short coastline, like Belgium. The few nature
reserves in the coastal zone are very valuable as habitats for species from different taxonomic groups,
and are therefore of high natural historical value. In Belgium, there are some woodlouse species that
can tolerate salt spray and are restricted to coastal areas, such as Armadillidium album Dollfus, 1887
(KERSMAEKERS, 1988) and Miktoniscus patiencei Vandel, 1946 (LOCK & DURWAEL, 2000). A third
species, Ligia oceanica (Linnaeus, 1767), is more common in Belgium along shores with hard
substrate, such as longitudinal embankments and is dependent on salt. In the Netherlands, a fourth
species was discovered that is mostly restricted to coastal areas: Eluma caelatum (Miers, 1877)
(TEMPELMAN & BERG, 1997; BERG et al., 2008, under its synonym E. purpurascens). This species is
known from a few former islands of the province of Zeeland (BERG et al., 2008), from over
26 locations and reaches here its northern distribution border in continental Europe. The species has
been expected to occur in Belgium (LOCK & DURWAEL, 2000) because of its close occurrence near the
Belgian border in the Netherlands (BERG et al., 2008), but previous attempts to find the species failed
(TEMPELMAN & BERG, 1997; KOEN LOCK pers. comm.). On the 22nd of October 2016, a joint
excursion of Spinicornis (Belgian Land Isopod Group) and the Dutch EIS-Isopoda survey group was
organised to look for the presence of E. caelatum in Belgium once more. Surprisingly, the species
could be easily found during a quick stop at the nature reserve “het Zwin”, near Knokke. The
experience on the habitat of this species from the sightings in the Netherlands by the last author
resulted in the relatively easy catch of this new species for the Belgian isopod fauna. Only about one
month later, a large population was discovered on a stream levee in the province of Oost-Vlaanderen,
close to the Dutch border. This raises the question if E. caelatum has been overlooked for all these
years due to lack of insight knowledge on the specific habitat of E. caelatum or if the species just
recently colonised Belgium from the Netherlands.
Belgian localities
West-Vlaanderen: Knokke-Heist, levee grassland on the inside of a dike, 31UES2489, 22.X.2016, 2ƂƂ, leg. &
det. Pallieter De Smedt ʊ Knokke-Heist, levee grassland, 31UES2489, 04.XI.2016, 1ex., leg. & det. Pepijn
Boeraeve.
Oost-Vlaanderen: Sint-Laureins, stream levee, 31UES4282, 26.XI.2016, 25ex., leg. & det. Pepijn Boeraeve.
Identification
Eluma caelatum reaches about 8-10mm (max. 15mm) when stretched and can be easily distinguished
from other Armadillidiidae-species because the eye is composed of only one large hyaline ocellus,
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instead of a group of smaller black ocelli as in most Armadillidium-species. It has a distinctive
purplish-brown colour with two broad bands of a series of elongated lighter dots across the dorsal side
of the body (Figs 1- 2). The body is densely covered with fine short hairs which gives the species a
downy appearance. The telson is triangular (Fig. 2). The species can fold itself into an almost
complete sphere with the antenna folded on the head (TEMPELMAN & BERG, 1997; BERG et al., 2008;
GREGORY, 2009).
Fig. 1. Lateral view of the first record of Eluma
caelatum for Belgium (Photo: Gert Arijs).
Fig. 2: Head and telson of Eluma caelatum (Photo:
Gert Arijs).
Distribution and habitat
Eluma caelatum has been reported in Europe from the British Isles; Western France, the Netherlands
(BERG et al., 2008), Portugal, Spain, NW-Africa, Canary Islands, Madeira and the Azores
(SCHMALFUSS, 2003). It has also been introduced in some other places in the world. As it reaches its
northern distribution border in continental Europe in the south of the Netherlands, in combination with
its occurrence in the south of Europe and northern Africa assumes that the central range lays in south
Europe.
The Belgian specimens in West-Vlaanderen were found a few centimetres from a concrete pole in the
middle of rough grassland vegetation on the inside of a dike, consisting of an Arrhenaterion trunk
community, dominated by Arrhenatherum elatius (L.) P. Beauv. ex J. Presl & C. Presl. and Elytrigia
repens (L.) Desv. ex Nevski (Fig. 3). In Oost-Vlaanderen, the specimens were found underneath dead
wood and between leaf litter on a stream levee. Both locations correspond to the habitat of E. caelatum
described from the Netherlands (BERG et al., 2008). The Belgian localities were both more or less
synantropic and less than 1,300m from the Dutch border (Fig. 4).
In the Netherlands, the species is almost exclusively found within 100m from salt water, with a
maximum of 6,300m (BERG & KROON, 1999). Most of the observations were done on sea dikes, both
on the out- or inside or along the border ditches at the base of the dike. The observation in Oost-
Vlaanderen was located about 9,500m from salt water and thus the locality furthest away from salt
water in the Low Countries. Although E. caelatum does not have to rely on salt water, it appears to be
tolerant to salt spray. It is not known why the species does not move land inwards, but maybe it cannot
compete with other isopod species that cannot tolerate salt spray.
The species has been found in Ireland for over a century, but was only relatively recently discovered in
the UK (1975) (HARDING & SUTTON, 1985) and in northern England only in 1995 (GREGORY, 2009).
HARDING & SUTTON (1985) reported 94% of the collection sites in the UK as being coastal. Two
inland sites in Ireland are close to railway lines. The species is categorised as “Naturalised” in the UK
because of its occurrence in synantropic and disturbed coastal sites (LEE, 2015). They assume the
species to be introduced via horticultural trades.
The species is reported as being a forest species in Western France were it inhabits the leaf litter layer
with a preference of chestnut tree forest (VANDEL, 1962; SÉCHET, 2004; LIVORY, 2007; SÉCHET &
NOËL, 2015). Also in northern Africa and on the Iberian Peninsula the species is found in forests, but
also in caves (VANDEL, 1962).
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Accompanying species in West-Vlaanderen were Philoscia muscorum (Scopoli, 1763), Oniscus
asellus Linnaeus, 1758, Porcellio scaber Latreille, 1804 and Armadillidium vulgare (Latreille, 1804).
This community composition corresponds with landslide levee slope communities inhabited by
E. caelatum from the Netherlands (BERG et al., 2008). Along ditches with a clay soil it occurs
together, next to the above-mentioned species with Trachelipus rathkii (Brandt, 1833), Trichiniscoides
sarsi Patience, 1908 and Trichoniscus pusillus Brandt, 1833 and Trichoniscus pygmaeus Sars, 1898. In
Oost-Vlaanderen, also Trichoniscus pusillus s.l., Trichoniscoides sarsi and Trichoniscoides albidus
(Budde-Lund, 1880) were accompanying species.
Fig. 3. Habitat of Eluma caelatum in West-
Vlaanderen. Specimens have been found at the base
of a concrete pole in the middle of the roug
h
grassland vegetation at the left side of the picture
(Photo: Pallieter De Smedt).
Fig. 4. Distribution of Eluma caelatum in Belgium.
Discussion
The status of E. caelatum in Belgium is unclear. After all, the species was relatively easy to find with
the right expert knowledge on habitat preference. We therefore assume that the species was
overlooked for some years. However, similar habitat as the location in Oost-Vlaanderen was visited
several times in the past few years without success, assuming that the number of populations is
limited. The two localities where the species was found are close to the Dutch border and no
connection can be made with southern populations (e.g. the species could not be found in suitable
areas along the Belgian coast more to the south (recent search by the authors) and in northern France
(DELASALLE & SÉCHET, 2014; inpn.mnhn.fr/)). For that reason, it sounds reasonable to assume that
the species reached Belgium from the north. It could be possible that the species is only present in
Belgium since a couple of decades. Different authors (VANDEL, 1962; BILTON, 1995; GREGORY, 2009)
assume that the species is probably not native to Western Europe, but originates from the Iberian
Peninsula or northern Africa. This is supported by the synantropic nature of the habitat in Ireland, the
UK and the Netherlands. In addition, some isolated records in Great Britain were at major ports
(GREGORY, 2009), and possibly the species was introduced via imported goods from overseas. If this
is true, we could also expect E. caelatum to occur near e.g. the large port of Antwerp or Zeebrugge in
Belgium.
In Western France, the species is hardly found at the coast but mostly in forested areas (VANDEL,
1962; SÉCHET, 2004). If forest is the natural habitat of the species, as it is on the Iberian Peninsula and
northern Africa, we could expect the French populations to be native. In northern countries like
Ireland, the UK, Belgium and the Netherlands, the distribution of the species could then be limited to
synantropic coastal areas, which have slightly higher temperatures compared to inland areas. The
suboptimal climate for E. caelatum at his northern distribution boundary could cause the species to be
restricted to coastal areas and therefore assumed to be native as well. Therefore, we could not be sure
if the species was an accidental import or a native species at the edge of its distribution range.
Nevertheless, if climate gets warmer, it could be possible that the species will colonise forest habitat in
Belgium along the coast or colonise the country from the south. Therefore, it seems worthwhile to
carefully investigate forest in the southern part of the country for the presence of E. caelatum.
93
With the discovery of E. caelatum, there are in total 35 Belgian terrestrial woodlice species with free-
living populations (DE SMEDT et al., 2016).
Acknowledgements
We would like to thank Eric Cosyns for the information on the vegetation characteristics of the collection site in
West-Vlaanderen.
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