Thermiphione rapanui, McCowin, Marina F. & Rouse, Greg W., 2018

McCowin, Marina F. & Rouse, Greg W., 2018, Phylogeny of hydrothermal vent Iphionidae, with the description of a new species (Aphroditiformia, Annelida), ZooKeys 779, pp. 89-107 : 94-96

Thermiphione rapanui sp. n. Figures 6, 7, 8, 9

Type-locality.

German Flats, hydrothermal vents of Pacific Antarctic Ridge, 110°55'W, 37°48'S.

Material Examined.Type specimens. Holotype (SIO-BIC A8557) from German Flats, hydrothermal vents of Pacific Antarctic Ridge, (type locality above), HOV Alvin Dive 4088, 2216m depth, 22 March 2005; fixed in 10% SW formalin, preserved in 50% ethanol. The holotype was not sequenced directly to avoid damage but was morphologically identical to sequenced specimens from the same locality. Post-preservation, holotype 10 mm long, 8.5 mm wide including parapodia, 31 segments. Paratypes: 1 specimen (SIO-BIC A7969) fixed and preserved in 95% ethanol, same location as holotype, post-preservation 9 mm long, 7 mm wide, 29 segments; 1 specimen (SIO-BIC A7970) from same location as holotype: anterior of specimen (approximately 14 segments) fixed in 10% SW formalin and preserved in 50% ethanol and posterior (approximately 14 segments) fixed and preserved in 95% ethanol; 2 specimens (SIO-BIC A7971, juvenile; SIO-BIC A7972) from the western flank of the Easter Microplate, East Pacific Rise, 115°34'W, 23°32'S, HOV Alvin Dive 4096, 2595m depth, 6 April 2005. SIO-BIC A7971 fixed and preserved in 95% ethanol, post-preservation 7 mm long, 4 mm wide, 19 segments; SIO-BIC A7972: anterior of specimen (approximately 20 segments) fixed in 10% SW formalin and preserved in 50% ethanol and posterior (approximately 9 segments) fixed and preserved in 95% ethanol.

Diagnosis.

Ventrally flattened, oval-shaped body. Between 29 and 31 segments, with 13 pairs of elytra on segments covering dorsum. Elytra covered completely by polygonal areas enclosing areolae, with marginal papillae covering edges. Prostomium bilobed and slightly rounded. Eyes absent. Lateral and median antennae absent. Segment 1 with pair of smooth palps and pair of tentaculophores plus enlarged anterior cirri (tentacular cirri). Mouth anterior with eversible pharynx. Segment 2 with buccal cirri. Segment 3 with dorsal tubercles. Dorsal cirri long with short styles. Ventral cirri short. Anus dorsal. Parapodia biramous with dense bundles of feathered notochaetae and less dense hooked neurochaetae.

Description.

In life, elytra pale brown with yellow tinge, becoming slightly paler after preservation. Body ventrally flattened, slightly tapered at anterior and posterior ends (Figure 6 A–D). Holotype with 31 segments, 13 pairs of elytra, bacterial filaments on elytra (Figure 6A, B). One mature paratype SIO-BIC A7969, 29 segments, 13 pairs of elytra (Figure 6C, D). One juvenile paratype (SIO-BIC A7971), 19 segments, eight pairs of elytra (identified by scars; elytra lost in sampling).

Pharynx everted anteriorly in holotype, with 9 pairs terminal papillae, and dorsal and ventral pairs of hook-shaped jaws (Figs 6 E–G, 7 A–C). Prostomium bilobed, slightly rounded; eyes lacking (Figure 6H). Dorsal small circular medial nodules on segments 4 (1), and 5-8 (2 per segment) (Figure 6H). Lateral and median antennae lacking (Figs 6 F–H, 7 A–C). Pair of smooth palps, longer than pair of tentaculophores plus enlarged anterior cirri (tentacular cirri) (Figs 6F, 7 A–B, D). Tentaculophores extending laterally to prostomium (Figs 6F, 7 A–B, D), each with single acicula and very thin, short capillary chaetae on inner side. Enlarged anterior cirri, dorsal cirri, and ventral cirri with papillae (Figure 7). Buccal cirri on segment 2, also papillate, appearing larger than remaining ventral cirri (Figure 7C, E). Thirteen pairs of elytra covering dorsum and oval in shape, on segments 2, 4, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 26 (Figure 8). First pair of elytra slightly compressed (Figure 8A); last pair much smaller in size and tapered at one end compared to other elytra (Figure 8 B–C). Elytra covered completely by polygonal (generally hexagonal) areas enclosing areolae (Figure 8 D–F). Thin, rounded marginal papillae covering lateral edges of elytra, sometimes sparsely extending towards posterior edges of elytra (Figure 8D-F). Remaining segments cirrigerous. Dorsal tubercles and dorsal cirri on segment 3, alternating on segments 6-29, with short, clavate papillae; anal cirri on segments 30, 31 (Figure 6B, D). Dorsal cirri long with short styles, usually extending to near tips of neurochaetae. Ventral cirri much shorter and smaller than dorsal cirri, present on segments 2-29 (Figure 7 B–C, E). Anus dorsal; short ventral anal cirri similar to posterior dorsal cirri. Parapodia biramous (Figure 9), with short, subconical notopodia anterodorsal to larger neuropodia (Figure 9). Dense bundles of slender feathered notochaetae, shorter than neurochaetae (Figure 9F, H, J, L). Longer, simple, or slightly hooked neurochaetae, less dense but more numerous than notochaetae (Figure 9G, I, K). Upper neurochaetae generally longer than lower neurochaetae, with length of neurochaetae gradually decreasing towards dorsal and ventral edges (Figure 9).

Variation. Paratypes vary in segment number from holotype and were observed with fewer bacterial filaments on elytra.

Genetic distance.

Paratype specimens from the 23°S sampling locality varied by two nucleotide bases from the holotype specimen, 37°S (Figure 4B). This genetic distance is so small that they are certainly all the same species. Unfortunately, our sampling was too limited for any analyses of connectivity.

Etymology.

Thermiphione rapanui sp. n. is named after the traditional Polynesian name for Easter Island (Rapa Nui), which lies near one of the paratype localities. Neither of the specimens from near Easter Island were chosen as the holotype as they were in poor condition.

Remarks.

Thermiphione rapanui sp. n. was collected from hydrothermal vents across 15 degrees of latitude, with the northernmost samples collected from the western flank of the Easter Microplate region at 23°S latitude, and the samples from further south collected on the East Pacific Rise at 37°S. The northernmost samples of Thermiphione rapanui sp. n. were collected from the same locality as samples of its sister taxon, T. tufari , which previously has only been recorded from slightly further north at 21°30'S ( Hartmann-Schröder 1992).

Thermiphione rapanui sp. n. differs from its sister taxon T. tufari in that it has 13 pairs of elytra instead of 14 pairs of elytra and the last pair of elytra are on segment 26 instead of segment 27 (compare dorsal photos of each in Figs 6A and 5C, respectively). Like T. tufari , the new species also has up to 31 segments ( Hartmann-Schröder 1992). Both T. tufari and T. fijiensis (Figure 5A) have 14 pairs of elytra and 30-31 segments (Pettibone, 1986), so elytral number may be convergent (Figure 3). Thermiphione was erected by Hartmann-Schröder (1992) and distinguished from other Iphionidae largely based on the presence of 14 pairs of elytra and 30-31 segments, but Iphionella risensis (Figure 5B), which nests within the Thermiphione (Figure 2), and Thermiphione rapanui sp. n. have 13 elytral pairs ( Pettibone 1986). However, the two latter species differ in that I. risensis has 28-29 segments ( Pettibone 1986) and T. rapanui sp. n. has 29-31 segments. T. rapanui sp. n. also differs from I. risensis in the presence of medial nodules on segments 6-8 in T. rapanui sp. n., which are absent on these segments in I. risensis ( Pettibone 1986).