Porrhomma frasassianum spec. nov. from a sulfidic cave, Italy (Araneae: Linyphiidae)

Abstract. Porrhomma frasassianum spec. nov., a blind troglobitic spider from the sulfidic Grotta del Fiume – Grotta Grande del Vento cave system (Italy), is described and illustrated. The relationship between this new species and other European species of Porrhomma is discussed, its distribution in the Frasassi cave system is analysed and the spider species cohabiting with the newly described species are listed.

femur II only with one dorsal spine (at 0.40); femur III and IV with no spines. Patella I-IV with a long spine. All tibiae with two dorsal spines, I and II with one additional retrolateral spine. The specific, prolateral spine on tibia I is absent. Metatarsi with no spines, I-III with trichobothrium (positions: I = 0.57; II = 0.60; III = 0.50).
Pedipalp: Cymbium 0.32 mm long, retrolaterally with an extended paracymbium and a characteristic embolic division: the embolus is longer than in other European species, more slender and wavy on its convex margin ( Fig. 1a-c).

Description of female
Size, colour and characteristics of the legs identical with the male.
Of the 15 species of Porrhomma in Europe (Nentwig et al. 2021) 12 also occur in Italy and 5 of them are considered troglophilic (Pantini & Isaia 2021). Porrhomma frasassianum is closest to P. rosenhaueri (L. Koch, 1872), a cave-adapted species known particularly from Central Europe, England and to date only from northern Italy (Lombardia, Trentino-Alto Adige, Veneto). The two species differ particularly by the shape of prosoma (Bourne 1977), the number of spines and subtle details of the genital morphology.
Distribution. Known only from the type locality.  (Sarbu et al. 2000) and it is noteworthy that although P. frasassianum is present in very large numbers (total 114 = 22 )), 64 ((, 28 jj) it has so far only been found in two locations: the Ramo Sulfureo section and the Rinoceronte Lake area in the same cave system. The shortest cave passages from here to the surface habitats are 400m long. Specimen number increases rapidly in the proximity of the thermal sulfidic waters, where the walls of the passages are covered by gypsum crusts. Collembola, which probably represent the food of these spiders, also occur in high densities in areas located close to the sulfidic lakes and streams where they feed on chemoautotrophic microbial biofilms. However, in the deep sulfidic sections of the Grotta Sulfurea, a different cave, located at a distance of only 150 m from Ramo Sulfureo and in which there are the same ecological conditions, P. frasassianum has not been found so far.