Chaetotaxy of cephalothorax

The study material was initially divided into four groups based on counts of microsetae along the posterior border of the cephalothorax: 2 + 0, 2 + 1, 2 + 2 and 2 + 3 specimens. The distributions of the four groups are mapped in Figs 1a-d. It is evident from these figures that the distributions of the four groups are not random. By and large, 2 + 0 and 2 + 1 specimens occur naturally in south-eastern Denmark with nearly all western records found under introduced conditions (Figs 1a-b). By contrast, 2 + 2 and 2 + 3 (very rarely 2 + 1 and then always collected with 2 + 2 specimens) are strictly coastal except for a few specimens found in a greenhouse and railway wasteland near the coast (Figs 1c-d). Hence, specimens conform to two taxa, such that specimens with 2 + 0 (rarely 2 + 1) setae along the posterior margin of the cephalothorax can be assigned to E. tetrachelatus (88 countable specimens from 29 localities). Specimens with 2 + 2 (rarely 2 + 1 but sometimes 2 + 3) can be assigned to E. kewi (105 countable specimens from 53 localities). Therefore both species must be considered members of the Danish fauna. The presence of 2 + 1, 2 + 2 and 2 + 3 specimens also characterize British E. kewi populations (Legg 1988). Gardini (2013) observed rare instances of 2 + 1 specimens in Italian E. tetrachelatus, also in agreement with the present study. The number of preocular microsetae showed variation in both species (Tab. 1). The most frequent configuration is 2 microsetae on each side of the medial 4 macrosetae (mm4mm). This configuration was found in 50% of all E. kewi specimens counted, while in more than 80% of the E. tetrachelatus specimens. The proportion of specimens lacking one of the preocular microsetae (m4mm or mm4m specimens) is much higher in E. kewi (31%) than in E. tetrachelatus (6%). A small number of specimens was counted with only 2 microsetae (m4m), and some with 5 microsetae (mm4mmm or mmm4mm) (Tab. 1). A t-test showed that the average number of preocular microsetae is lower in E. kewi than in E. tetrachelatus (p < 0.0003). The number of microsetae has, however, little diagnostic value in separating the species, at least when identifying a single or small number of specimens since it is only the frequencies of the various configurations that differ between species. The counts and frequencies obtained in this study may not match those of populations outside Denmark. Intraspecific variation in setal configuration is not restricted to the above-mentioned species, but has also been recorded in several other members of the Chthoniidae, including many of those treated in the reviews of Zaragoza (2017) and Gardini (2013). Still, the distribution patterns of setae are important taxonomic features useful for separating species, such as in the key to Italian, Swiss Tessin, and Corsican species (Gardini 2013).

Fig. 2:

Distribution of Ephippiochthonius species. a. Ephippiochthonius kewi Gabbutt, 1966; b. Ephippiochthonius tetrachelatus (Preyssler, 1790). ● = specimens found in natural habitats; = specimens found in greenhouses, railways and harbour wastelands; ×= assumed hybrids of E. kewi and E. tetrachelatus

Tab. 1:

Number of specimens with 2, 3, 4 or 5 preocular microsetae in the first row of the cephalothorax for Ephippiochthonius kewi (n = 85) and E. tetrachelatus (n = 85). Percentage values are also given

Species distributions

The results point to the fact that E. kewi is the most widespread species, being distributed throughout most of Denmark. It is strictly coastal (Fig. 2a), inhabiting calcareous, sandy or stony grassland along sheltered coasts and is particularly common in Central Denmark. Specimens are most commonly sifted from uprooted couch grass (Elymus repens) and from litter and debris at the base of stems. Also coastal grasslands with red fescue (Festuca rubra), tall fescue (F. arundinacea) and false oat-grass (Arrhenatherum elatius) have yielded specimens. A few records are from fissured clay at the base of coastal hills, from under objects on the beach or from leaf litter of forests edging the coast. The species is absent from exposed coasts with wind deposited acidic sand such as the coastal dunes along the Danish west coast. It prefers calcareous sand deposited by shoreline processes with the contents of calcium carbonate derived from seashells mixed in with the sand. Only one record is from an inland site, a garden centre to which it was undoubtedly introduced. The habitat of the Danish records thus agrees very well with the habitat of the type specimens in the original description of the species (Gabbutt 1966). Ephippiochthonius tetrachelatus, on the other hand, is primarily found in south-eastern Denmark at both inland and coastal sites (Fig. 2b), often in leaf litter along south facing forest edges or inside open forests. The species is also frequent under stones or logs, among stones in stone walls, usually in sunny or semi-shaded locations. More rarely specimens have been collected in wood mould inside hollow trees, in bird nests, in upwash along coasts or in garden compost heaps, stable and barn litter. Introduced specimens have been found in hothouses and garden centres, usually under objects on the floor. The species is very rare in natural habitats in western Denmark with only two known localities in Jutland. One is from a beech forest at Gråsten, south-east Jutland (JL-10456). At the onset of this study there was only one record of E. tetrachelatus from Central Jutland, a single female (JL-5608), which had been collected in upwash along the Limfjord near Stårup in 2015. Since this record appeared out of range and the habitat also fits E. kewi, an error in the recording data was suspected. Therefore, the locality was revisited in 2020, however yielding another specimen of E. tetrachelatus, thus confirming the presence of the species at the locality.

Colour differences

The two species show another morphological difference in addition to presence/absence of microsetae on the posterior border of the cephalothorax. Ephippiochthonius kewi is generally distinctly darker and with a greyish-brown cephalothorax (RGB 83–91, 64–73, 47–67) while the paler E. tetrachelatus has a yellow-brown or slightly reddish-brown cephalothorax (RGB 109–131, 75–89, 50–73) (compare Fig. 3a-d). The colour difference may be subtle when comparing paler specimens of E. kewi with E. tetrachelatus specimens, nevertheless the more greyish shade of E. kewi is distinct. In this species the individual values of red, green and blue are more similar (closer to grey) compared to E. tetrachelatus in which species the red component is more dominant. Fig. 3a shows a normal coloured E. kewi male and a relatively pale female is shown in Fig. 3b, both of which are more greyish compared to the male (Fig. 3c) and female (Fig. 3d) of E. tetrachelatus. In the author's experience the two species are reliable separated by these colour differences at least when inspecting live specimens, unless dealing with specimens exhibiting hybrid traits. Those are detected by variable chaetotaxy (see discussion below). The colour differences fade with time in alcohol-preserved material. Apparently, no previous studies have paid attention to the colour differences of these two species.

Fig. 3:

Photos of live specimens from Denmark. a. Ephippiochthonius kewi Gabbutt, 1966, male from Sdr. Lem Vig; b. Ephippiochthonius kewi Gabbutt, 1966, female from Sdr. Lem Vig; c. Ephippiochthonius tetrachelatus (Preyssler, 1790), male from Vallø; d. Ephippiochthonius tetrachelatus (Preyssler, 1790), female from Vallø

Indications of hybridization

The two species are sympatric in south-eastern Denmark and only here occasional populations were found exhibiting variable chaetotaxy that exceeds the minor variability found in regions where only one species is present. This is possibly explained by hybridization from interbreeding of the two species (E. tetrachelatus × kewi). In this study, 61 specimens were considered members of nine hybrid populations, one of which is situated in Sweden (mapped in Fig. 2a-b). The chaetotaxy of the posterior row of the cephalothorax corresponds to either 2 + 0, 2 + 1, 2 + 2 or 2 + 3 for individual specimens of hybrid populations; however, 2 + 0 and 2 + 2 are still the dominant configurations (Tab. 2). This table also presents counts for non-hybrid populations of E. kewi and E. tetrachelatus for comparison. It should be noted that there is no clear distinction between hybrid and non-hybrid populations and more evidence is necessary to support the hypothesis. However, it should be stressed that presumed hybrids were only noted at localities where the two species overlap in distribution. This provides some support for hybridization being the cause for variability in chaetotaxy. It is, however, peculiar that the hybrid specimens are only found inland or, if near the coast, in habitats not occupied by E. kewi. A simple co-occurrence of the two species does not seem to be the case as yellow-brown specimens conforming to E. tetrachelatus are not present in newly collected material in which colour differences are still discernible. Specimens considered hybrids in this study seem more similar to E. kewi than E. tetrachelatus with respect to colouration. For example, specimen JL-7482 was inspected when alive and initially identified as E. kewi based on colouration, but the chaetotaxy of the posterior border of the cephalothorax (2 + 1) points to a hybrid, as does the habitat. The specimen was collected from decaying wood of an overturned beech about 1.5 km from the coast, which is a habitat of E. tetrachelatus but not of E. kewi. Likewise, three specimens collected in the city centre of Göteborg, Sweden (JL-4864) are presumed members of a hybrid population due to variable chaetotaxy. These specimens also have distinctly greyish cephalothoraxes. Fig. 4 shows a photo of one specimen compared to an E. tetrachelatus specimen, both from the Göteborg area. Thus, the colour difference allows only for separating the rare 2 + 1 specimens of E. tetrachelatus from the rare 2 + 1 specimens of E. kewi. Separating a 2 + 1 specimen of E. kewi from a 2 + 1 specimen of E. tetrachelatus × kewi in those instances where only a single specimen has been collected is not possible by colouration. Separation requires a series of specimens from the localities so that the general variability in chaetotaxy of the specimens can be determined. Some localities with hybrid populations may not have been detected during inspection of the present study material due to too few specimens being collected. However, recorded habitat may point to the species if only one specimen is available, as hybrids seem absent from coastal grassland where E. kewi is most commonly found. In Britain E. kewi is also primarily coastal, but the species has also been mapped at a few inland sites (Legg 2021). Perhaps these inland records consist of hybrid populations. Most specimens considered hybrids in this study belong to older material which have faded to such an extent during preservation that colour differences are not discernible. Thus the above-mentioned observations that hybrids resemble E. kewi in colouration may be due to chance. Thus, hybrids genetically closest to E. tetrachelatus could be intermediate in colouration or closer to the reddish-brown colour of E. tetrachelatus. There is evidence that E. kewi should be recognized as a subspecies of E. tetrachelatus, because some conditions applying to subspecies in biological classification are met: two populations living in different subdivisions of the species' range and varying from one another by morphological characteristics. Another common criterion for recognizing two distinct populations as subspecies rather than full species is the ability of them to interbreed without a fitness penalty. Whether this latter criterion is met is unknown at present. It is hoped to apply DNA-sequencing methods in a future study in order to elucidate the relationship between the two species and to look for genomic evidence supporting the occurrence of hybridization events.

Tab. 2:

Setal configuration along the posterior border of the cephalothorax in Ephippiochthonius kewi, E. tetrachelatus and specimens considered members of hybrid populations (number of specimens in categories).

Fig. 4:

Colour differences between live specimens of Ephippiochthonius tetrachelatus × kewi from Göteborg, Sweden (JL-4864) (a) and Ephippiochthonius tetrachelatus (Preyssler, 1790) from Hjuvik, Sweden (JL-7932) (b)

Chaetotaxy of genital opercula

In the original description of E. kewi both genital opercula carry 10 setae (Gabbutt 1966), thus no variation in number was mentioned. Gardini (2009) counted 10–11 setae on the anterior genital opercula and 8–10 on posterior for the male neotype and the three topotypic males of E. tetrachelatus selected for the redescription of the species. In the present study, numbers were found to vary by 1–5 setae depending on species and opercula (Fig. 5). However, the mode value is 10, i.e. this is the value most frequently counted irrespective of species and opercula. The mode is thus in agreement with Gabbutt (1966) and Gardini (2009). T-tests were performed to test if means of counts differ between species. Only means of setae counted for the anterior genital opercula of males were found to differ (p < 0.024), that is males of E. tetrachelatus have a statistically significant higher proportion of specimens having 11 rather than 10 setae compared to males of E. kewi (Fig. 12, upper left figure). Means of counted setae for the male posterior opercula did not differ between the species (p < 0.709), nor did female anterior (p < 0.778) or posterior genital opercula (p < 0.695). However, as the mode value is 10, the two species cannot be separated by counting the number of setae on genital opercula. This conclusion is restricted to the Danish material examined. Legg (1987), Legg & Jones (1988) examined British material and counted only 7–8 setae on the posterior genital opercula of E. tetrachelatus females versus 9–11 counted in this study, pointing to regional differences in this character within the large distributional area of the species.

Fig. 5:

Frequency diagrams of different setae number on anterior and posterior genital opercula for both males and females of Ephippiochthonius kewi Gabbutt, 1966 and E. tetrachelatus (Preyssler, 1790)