Osedax craigmcclaini n. sp.

Fig. 3B, C, 5A, B, 6B

Osedax sp. McClain et al., 2019, p. 7 of 14

Material examined. Holotype: SIO-BIC A13910 (GenBank COI sequence ON211944), collected from experimentally deployed alligator (Alligator mississippiensis) bones deployed at 2,034 m depth in the Gulf of Mexico, offshore of New Orleans, Louisiana, (27.312° N; 88.927° W), ROV Global Explorer dive number 16, April 12, 2019. Fixed and preserved in 95% ethanol.

Diagnosis and description. Holotype palps are pinnulated, white in preserved state; less than 1 mm long ~ 0.33 mm wide (Fig. 5B). No other body parts observed. No dwarf males observed. The rDNC diagnosis for Osedax craigmcclaini n. sp. was recovered as: ‘C’ at site 318, ‘T’ at site 333, and ‘C’ at site 462 of mitochondrial COI.

Distribution. Osedax craigmcclaini n. sp. was recovered from an alligator skeleton at 2,034 m off the Mississippi River Delta region, Louisiana, in the Gulf of Mexico (Fig. 1).

Etymology. Osedax craigmcclaini n. sp. is named for Dr. Craig McClain, an esteemed deep-sea biologist and colleague who led the experimental alligator fall project (McClain et al., 2019) and provided the Osedax specimens for this study.

Remarks. Osedax craigmcclaini belongs to Clade V, a pinnulate clade (Fig. 2). Evidence for this species was originally published in McClain et al. (2019) with COI only (GenBank Accession number MN258704), from SIO-BIC A10731. In addition, 16S (ON217799), 18S (ON220153), 28S (ON226742), and H3 (ON254807) were sequenced from the remaining the SIO-BIC A10731 DNA extraction for this study. Specimen SIO-BIC A13910 has been designated here as the holotype based on its COI sequence (ON211944) closely matching MN258704 from McClain et al. (2019) (1.2% uncorrected distance). Both sequences showed the three rDNC diagnostic bases. Based on the phylogeny shown in Figure 2, a proximate species is Osedax fenrisi Eilertsen et al., 2020, a pinnulate species collected from 2,341 m on the Arctic Mid-Ocean Ridge (Eilertsen et al. 2020). The minimum interspecific distance between the two species was 14.6% (Table 3). There are species of Osedax with smaller uncorrected COI distances, such as Osedax crouchi Amon et al., 2014 from Antarctica, which belongs to the nude palp Clade II (Fig. 2), and McClain et al. (2019) reported the new species as falling within this clade. However, this proposed placement was based on COI data only, which can be misleading (Vrijenhoek et al. 2009), and the five gene phylogeny and photographs of the holotype confirm O. craigmcclaini n. sp. as actually a member of the pinnulate Clade V. Osedax craigmcclaini n. sp. showed two unique haplotypes with seven nucleotide substitutions between them (Fig. 5B). Specimens were not observed alive, however in situ images of the alligator corpse from which O. craigmcclaini was collected show red Osedax coating the jawbone and spine (Fig. 3B, C), suggesting that living O. craigmcclaini n. sp. may have red palps.