Phylogenetics and species delimitation of the recluse spider, Loxosceles rufescens (Araneae: Sicariidae) populations invading Bangkok, Thailand

The Mediterranean recluse spider, Loxosceles rufescens, has been discovered for the first time inhabiting human dwellings in Bangkok, Thailand. Expeditions across 39 localities revealed five establishments with L. rufescens populations. The highest density was recorded in a storage house on Yaowarat Road, located in the heart of Bangkok's Chinatown, where 315 individuals were found, including adults, juveniles, and spiderlings. This medically significant spider's presence in such a densely populated urban area raises concerns about potential envenomation risks. Thirteen specimens of L. rufescens were extracted for DNA and sequenced for molecular phylogenetic analyses. COI and ITS2 markers were used to investigate relationships within L. rufescens and across available Loxosceles species sequences. Results indicate COI is superior for resolving species-level genetic clusters compared to ITS2. Surprisingly, L. rufescens individuals from the same house were found in significantly distant COI lineages, suggesting mtDNA may not be suitable for studying intra-specific phylogeography in this case. Species delimitation methods ABGD and ASAP demonstrated promising results for both COI and ITS2, while bPTP and GMYC tended to overestimate species numbers. ITS2 exhibited high sequence similarity in L. rufescens, suggesting potential utility as a barcoding marker for identification of this globally distributed species. Genetic distance analyses revealed a potential barcoding gap (K2P) of 8–9 % for COI and <2 % for ITS2 in Loxosceles. This study contributes valuable sequence data for the medically important genus Loxosceles and highlights the need for integrative approaches in understanding its evolution and spread. The findings have important implications for pest management strategies and public health in urban environments.