A new species in the Spirobranchus kraussii complex (Annelida: Serpulidae) from Australia.

The blue coral-worm Spirobranchus kraussii (Baird, 1864) (Annelida: Serpulidae) was originally described from South Africa and reported as a widely distributed Indo-Pacific species, however it is now recognized as a complex of regionally distributed species. Molecular studies led to the description of taxa within the monophyletic S. kraussii complex including S. lirianeae Brandão & Brasil, 2020 (Brazil), S. sinuspersicus Pazoki, Rahimian, Struck, Katouzian & Kupriyanova, 2020 (Arabian (Persian) Gulf), S. akitsushima Nishi, Abe, Tanaka, Jimi & Kupriyanova, 2022 (Japan), S. bakau Sivananthan, Shantti, Kupriyanova, Quek, Yap & Teo, 2021 (Singapore), and S. manilensis Sivananthan, Sivalingam, Kupriyanova, Quek, Yap & Teo, 2022 (Philippines). Spirobranchus kraussii was reported (as Pomatoleios kraussii) as an intertidal belt-forming species from mid-20th century in Queensland, Australia. Here, this species was described as Spirobranchus straughanae sp. nov. as a result of the analysis of historic specimens from the Australian Museum and newly obtained DNA sequences of cytb and 18S. Phylogenetic analyses revealed seven well-supported clades within the complex and S. straughanae sp. nov. was recovered as sister group to S. lirianeae. Genetic distances between S. straughanae sp. nov. and other species ranged from 12.8% to 25.6% (p-distance) and 17.3% to 46.6% (Kimura 2-parameter), reinforcing its status as a distinct species. Morphologically, S. straughanae sp. nov. differs from congeners in the complex by opercular characteristics, tube morphology, and glandular distribution. The talon can be used to differentiate taxa, but its structure varies significantly. Peduncular wing patterns and chaetal morphology further distinguish species within the complex. The findings underscore the importance of integrative approaches to resolution of species boundaries in the S. kraussii complex.