Galapagomystides aristata Blake 1985

Figures 8–10

Blake (1985:69, 71-73), Blake (1994: 119), Desbruyères et al. (1997: 80), Desbruyères et al. 2006: 216), Dreyer (2004: 55, 58, 60. 62), Gollner et al. (2015: 62), Govenar et al. (2004:179), Govenar et al. (2005:71, 72, 75), Govenar et al. (2007: 7-9, 11), Jenkins et al. (2002: 243, 245, 249-252), Rodrigo et al. (2015: 100), Tunnicliffe (1992: 340), Tunnicliffe et al. (1998: 367), Van Dover (2002: 145, 146, 148).

Diagnosis. First segment fused dorsally to prostomium. Elongated dorsal cirri on segments 1 and 2. Elongated ventral cirri on segment 2. Exaggerated hook-like joint of compound chaetae.

Material Examined. SIO-BIC A13574 (paratype transferred from USNM, used for SEM), Galapagos Rift Geothermal Vents, ~ 2,500 m depth; SIO-BIC A12104 *, SAMA E8987 *, SAMA E8988 *, SAMA E8989 *, SAMA E8990 *, SAMA E8991 – E9002 *, southern East Pacific Rise, ~ 2,200 –2,700 m depth. For locality details see Table 1. * indicates sequenced specimens .

Description. Up to 22 mm long, 1 mm wide at segment 10 for ~100 segments. Body semi-translucent and white at parapodial lobes, dorsal and ventral cirri, elongated cirri, pygidial cirri, prostomium and pygidium; deep pink/red in longitudinal center in life (Fig. 8A–D). Body brown/orange with numerous dark pigmentation speckles in preserved (formalin/ethanol) state (Fig. 8E). Rounded, lobe-like prostomium; nuchal organs not visible. Anterior dorsal edge of prostomium with paired cylindrical antennae ~ 0.2 mm long (Fig. 9C). Paired palps ventral to antennae, similar in shape and length to antennae (Fig. 9C). Segment one dorsally fused to prostomium, following segments clearly demarcated (Fig. 9A, C, E). Pair of elongated dorsal cirri [tentacular cirri] on each of segments 1 (~ 0.25 mm long), 2 (~ 0.28 mm long) (Fig. 9A, C). All elongated dorsal cirri cirriform, tapering distally. Pair of elongated ventral cirri on segment 2 (~ 0.2 mm long) (Fig. 9A, C). Conical, tapering regular ventral cirri (~ 0.08 mm long) begin on segment 3 continuing posteriorly (Fig. 9C). Bulbous, rounded dorsal cirri (~ 0.08 mm long) begin on segment 4 continuing posteriorly (Fig. 9A). Dorsal cirri absent on segment 3 (Fig. 9A). Parapodia uniramous, notopodial chaetae absent; neuropodium with central fascicle containing ~5–8 compound chaetae; one simple emergent acicula (Figs 8F, H, 10B, D, E). Compound chaetal shaft cylindrical; thin, flattened pointed blade extended from curved joint (Figs 9D, 10B, D, E). Pygidium with one pair of cirriform pygidial cirri tapering distally (~ 0.2 mm long) (Fig. 9B). Proboscis protrudes ¼ body length, smooth until ½ distance distal, then lined with papillae (Fig. 9E).

Variation. There was a very slight variation in animal length and number of segments in comparison to the original description (Blake 1985). The specimens studied here reached ~100 segments and a total length of 22 mm (Fig. 8E) as opposed to ~90 segments and a total length of 20 mm described by Blake (1985). All other characters match the original description.

Remarks. The G. aristata samples used in our DNA analyses were collected along the EPR, over 3,000 km from the type locality (= Galapagos Rift). However, the specimens matched Blake’s description and the examined paratype closely. Previous authors have recorded G. aristata specimens from the EPR, though these were not taxonomic studies (Jenkins et al. 2002; Govenar et al. 2004; 2005). Here we extend the range of G. aristata to the EPR, though final confirmation requires DNA sequencing of G. aristata from the Galapagos Rift vents. Unique features of G. aristata shared between specimens used in this study and the paratypes include the hook-like joint of the compound chaetae (Figs 9D, 10D), fusion of segment one dorsally but not ventrally to the prostomium (Figs 9A, C, 10A, C), the prostomium size and shape (Figs 9A, C, 10A, C).

Galapagomystides aristata is morphologically most like G. kathyae n. sp. and G. bobpearsoni n. sp. in that all have segment 1 dorsally fused to the prostomium. However, in the phylogenetic analyses, G. aristata was found to be the sister group to a clade comprising G. kathyae n. sp. and G. verenae. Galapagomystides aristata has a smooth proboscis as mentioned in previous descriptions (Blake 1985; Pleijel 1991), though there are papillae located at the distal third of the proboscis (Fig. 9E). Pleijel’s (1991) diagnosis of Galapagomystides claimed nuchal organs present as dorso-lateral ciliated pits between the prostomium and segment 1, however nuchal organs were not located in this study.