Ramisyllis kingghidorahi, Aguado & Ponz-Segrelles & Glasby & Ribeiro & Nakamura & Oguchi & Omori & Kohtsuka & Fischer & Ise & Jimi & Abstract, 2022

Aguado, M. Teresa, Ponz-Segrelles, Guillermo, Glasby, Christopher J., Ribeiro, Rannyele P., Nakamura, Mayuko, Oguchi, Kohei, Omori, Akihito, Kohtsuka, Hisanori, Fischer, Christian, Ise, Yuji, Jimi, Naoto & Abstract, Toru Miura, 2022, Ramisyllis kingghidorahi n. sp., a new branching annelid from Japan, Organisms Diversity & Evolution (New York, N. Y.) 22 (2), pp. 377-405 : 395-397

Ramisyllis kingghidorahi , a new branching species

All our biological and ecological observations, morphological analyses, internal anatomy observations, and phylogenetic and genetic distance analyses consistently support ◂ Fig. 10 Scanning electron microscopy images of Ramisyllis kingghidorahi n. sp., posterior-most regions and epithelium details. A–D Posterior ends. Arrow in C and D points to heavily ciliated anus. E– G Minute crests on the dorsal surface of midbody segments. Arrows point to crests laterally located on the dorsal surface. H Dorsal surface of posterior segments. I Clumps of cilia on dorsal surface of proventricular segments. Arrows pointing to pores in H. Scale bars: 100 µm A, B, I, 50 um C, G, 5 µm D, E,4 µm F, and 3 µm H

recognition of R. kingghidorahi n. sp. as a new species, closely related to R. multicaudata . The morphological differences, though clear and consistent among studied specimens, are subtle, and might not be easy to detect by a non-expert eye. This morphological semi-stasis between two clear phylogeographic lineages might be explained by a strict conserved niche ( Cerca et al., 2020, 2021) like the Petrosia canal system in which these animals live.

The asymmetrical branching body with unpaired branching may have been inherited from the last common ancestor of both species of Ramisyllis , which was probably already adapted to live inside a sponge canal system. Including S. ramosa in a phylogenetic analysis has not been possible. Hence, the reconstruction of ancestral features must consider two possible scenarios: (1) S. ramosa joining R. multicaudata and R. kingghidorahi in a monophyletic group and (2) S. ramosa not being sister to R. multicaudata and R. kingghidorahi . Under the second scenario, all observed morphological similarities would need to be understood as convergencies resulting from independent adaptations likely related to life in symbiosis with sponges.

Phylogenetic topologies reveal sister group relationships between both species of Ramisyllis , with a clear divergence in COI and 16S (which show faster mutations rates than the nuclear 18S and 28S). The very high COI distances between both species of Ramisyllis agree with previous studies delimitating species in syllids and annelids in general ( Aguado et al., 2019; Álvarez-Campos et al., 2017; Kvist, 2016; Lobo et al., 2016; Nygren et al., 2018), as well as those found for the 16S ( Bastrop et al., 1998; Gunton et al., 2020; Miglietta et al., 2010; Radashevsky et al., 2016) and ITS2 markers ( Vivien et al., 2015). Conversely, the 28S distance was 1 order of magnitude lower but still significant since this is a slow evolving gene and the obtained distances were similar to those among other syllids ( Aguado et al., 2019). In 18S (also a slow evolving gene), distances were lower still. The conserved nuclear gene 18S has previously been used in annelid species delimitation studies as a phylogenetic marker ( Cerca et al., 2020, 2021) but is known to usually underestimate the number of species ( Tang et al., 2012). Interestingly, Ramisyllis species highly differ from the other syllids in their 18S sequences as revealed by the long branch in the phylogenetic analysis (Fig. 2a) and the clear differences in 18S alignments ( Aguado et al., 2015a). This is probably due to expansion regions particularly variable in syllids ( Aguado & Bleidorn, 2010), though its significance is still unknown. The 28S branch is also quite long, although the comparison in this case is less meaningful due to the lower number and quality of the available sequences of Syllinae .

The mt-genome of R. kingghidorahi n. sp. resembles that of R. multicaudata in gene order, AT and GC skews, codon usage bias, and secondary structure of most tRNAs ( Aguado et al., 2015a). However, it strongly differs from most available syllid mt-genomes ( Aguado et al., 2016, 2015a), particularly in having a strongly modified gene order and highly derived nuclear ribosomal sequences.