Atypus karschi Dönitz, 1887 (Araneae: Atypidae): An Asian purse-web spider established in Pennsylvania, USA

Abstract

The mygalomorph spiders of the family Atypidae are among the most archaic spiders. The genus Atypus Latreille, 1804 occurs in Eurasia and northern Africa, with a single enigmatic species, Atypus snetsingeri Sarno, 1973, known only from a small area in southeastern Pennsylvania in eastern USA. A close relationship to European species could be assumed based on geographic proximity, but A. snetsingeri more closely resembled Asian species. This study was undertaken to learn more about the genetics of A. snetsingeri, its habitat requirements and natural history. Molecular markers (CO1 sequences) were compared to available data for other atypids and showed that A. snetsingeri is identical with A. karschi Dönitz, 1887 native to East Asia. Natural history parameters in Pennsylvania were also similar in every respect to A. karschi in Japan, therefore, we propose that the spider is an introduced species and the specific epithet snetsingeri is relegated to a junior synonym of A. karschi. Cytogenetic analysis showed an X0 sex chromosome system (42 chromosomes in females, 41 in males) and we also detected nucleolus organizing regions and heterochromatin, the latter for the first time in the Atypoidea. In Pennsylvania the spider is found in a variety of habitats, from forests to suburban shrubbery, where the above-ground webs are usually attached vertically to trees, shrubs, or walls, although other webs are oriented horizontally near the ground. Prey include millipedes, snails, woodlice, carabid beetles and earthworms. Atypus karschi is the first known case of an introduced purse-web spider. It is rarely noticed but well-established within its range in southeastern Pennsylvania.

Fig 1

Phylogenetic analyses of the position of A. snetsingeri (A. karschi in Pennsylvania, USA) in the genus Atypus based on the sequences of the CO1 (A) and ITS2 (B) loci by the maximum likelihood approach. The evolutionary history was inferred using the Tamura-Nei model (A) or the Kimura 2-parameter model (B), both with a discrete Gamma distribution used to model evolutionary rate differences among sites. The models were selected based on the highest Bayesian information criterion scores of the maximum likelihood fits. The trees are drawn to scale, with branch lengths indicating the number of substitutions per site. All codon positions, including noncoding positions, were included; the analyses were based on 573 positions (A) or 345 positions (B).

Fig 2. Chromosomes of A. karschi, Pennsylvania, USA.

A, B. Male (A) and female karyotypes (B), stained by Giemsa, based on mitotic metaphase. 2n♂ = 41, X0; 2n♀ = 42, XX. Empty arrowhead–centromere of submetacentric pair. C. Male pachytene. Note heterochromatic X chromosome on the periphery of the nucleus and prominent knobs on the bivalents. Inset: Scheme of sex chromosome. Note an association of X chromosome arms. Arrow–sex chromosome. D. Male mitotic metaphase, C-banding. Chromosomes exhibit intercalar and terminal heterochromatin blocks. Inset: Magnified submetacentric chromosome containing a large block of heterochromatin (from another mitotic metaphase). Arrowhead–a large block of heterochromatin, empty arrowhead–centromere. E. Male mitotic metaphase, detection of rDNA cluster (FISH). Note chromosomes of a submetacentric pair containing a terminal rDNA cluster at long arm. Arrowhead–rDNA cluster, empty arrowhead–centromere. Scale bars: 10 μm.

Fig 3

Habitats of A. karschi in Pennsylvania, USA; (1A) suburban bushes along Essex Ave (~200 m from type locality of A. snetsingeri), (1B) fallow field at Tyler Arboretum, (1C) riparian woods, Swedish Cabin site on Darby Creek, (1D) forest, Smedley Park.

Fig 4

A. karschi and its webs in Pennsylvania, USA, (4A) adult female and male (on the left), (2B) vertical web attached to the base of a tree and (4C) to a boulder, (4D) horizontal web covered in thatch (4E) and with thatch removed, (4F) a detail of recently trimmed ivy in front of the same bushes shown in Fig 3A, with 16 purse-webs circled.

Fig 5. Boxplots showing the variation of the below-ground and above-ground lengths of excavated purse-webs of A. karschi, Pennsylvania, USA (n = 13).

The means are indicated by an x and the hollow dot indicates an outlier (less than the 25th percentile minus 1.5 × Interquartile range).

Fig 6. Study sites, counties and estimated area containing Atypus karschi in southeastern Pennsylvania, USA.

The hollow circles mark the sites described in this study and the star is the type locality of A. snetsingeri. The minimum convex polygon encloses a nearly contiguous area where vertical purse-webs have been found in a recent visual survey in the region [24], and includes the relatively few sites where a species identification to A. karschi (as A. snetsingeri) has been made through previous collections, male sightings and female excavation and examination. Sphodros species may also be found in that area as purse-web sites are revisited for species determination.