Kirkegaardia fragilis Blake, 2016, new species

Kirkegaardia fragilis new species

Figure 25

Material examined. North equatorial Pacific Ocean, abyssal plain, Clarion-Clipperton Fracture Zone, NOAA BIE Sta. DDT-07-94, 12°55.023′N, 128°35.451′W, 0.25 m 2 box core, 4879 m, 29 July 1994, coll. D.T. Trueblood, holotype and 2 paratypes (USNM 1407165–6); Sta. 02-93, 12°56.166′N, 128°35.520′W, 4869 m, coll. 10 August 1993, fragments and tessellated tubes (JAB); Sta. 06-93, 12°55.780′N, 128°35.881′W, 4858 m, coll. 0 1 September 1993, 1 posterior fragment (JAB).

Description. A small fragile species with elongate, narrow body, cylindrical throughout; without dorsal or ventral grooves. Holotype in two parts, collectively measuring 4.2 mm long, 0.17 mm wide across thoracic segments, with 40 setigerous segments. One paratype a larger anterior fragment, 0.3 mm wide, 2.1 mm long, with 12 setigerous segments. Color in alcohol opaque white; largest paratype with black pigment spots on ventrolateral sides of peristomium and anterior margin of setiger 1.

Pre-setigerous region of holotype elongate, 1.7x as long as wide and as long as first four setigerous segments; prostomium conical, tapering to narrow, rounded apex (Fig. 25 A); eyes absent; nuchal organs not observed. Peristomium relatively smooth, with thin lateral lines suggesting 2–3 annulations, but these not distinct; no achaetous segments evident (Fig. 25 A). Setiger 1 distinct from peristomium. Dorsal tentacles arise near posterior end of peristomium, with one intact tentacle present on holotype (Fig. 25 A); first pair of branchiae on setiger 1, dorsal to notosetae (Fig. 25 A); holotype with one branchia retained on setiger 5; other branchiae identified by scars when stained with Shirlastain A (Fig. 25 A). Branchiae located dorsal to notosetae throughout.

Thoracic region of holotype with five segments, each about 3.5x as wide as long (Fig. 25 A); paratypes with 5– 7 thoracic segments of similar proportions. Transition to long narrow abdominal region abrupt, with first segments as long as wide, becoming beadlike in middle abdominal segments, about 1.3x as long as wide (Fig. 25 B); posterior abdominal segments less bead-like, transitioning to short far posterior region with about eight segments becoming weakly expanded, flattened ventrally, terminating in narrow, elongate, conical, unsegmented pygidial lobe (Fig. 25 C).

Parapodia low mounds, shifted somewhat dorsal in thoracic segments and first few abdominal segments, thereafter lateral (Fig. 25 B). Setae all capillaries, consisting of narrow setae with smooth blades in thoracic segments and shorter setae with broad base tapering to narrow capillary tip, with fine denticles along one edge of expanded base in middle and posterior segments, denticles not evident along narrow apex; denticles present on both noto- and neurosetae from about setiger 20, or middle of body; denticles not visible on capillaries of last 4–5 setigers. Denticles weakly visible at 400x, best observed at 1000x; individual denticles numerous, with sharp points directed basally (Fig. 25 D); notosetae with denticles directed ventrally and denticles of neurosetae directed dorsally, vis-à-vis. Setae numbering 6–8 in both noto- and neuropodia in thoracic segments, 5–7 in middle body segments; and 4–5 in posterior segments.

Methyl Green stain. Prostomium staining light green, rest of body not retaining stain after differentiation.

Etymology. The species name fragilis, is from Latin, and refers to the narrow fragile nature of this species.

Remarks. Kirkegaardia fragilis belongs to a group of species in which the parapodia are not elevated over the dorsum of the thoracic region and no dorsal groove is produced; further, the first pair of branchiae occur on setiger 1 and, apart from some retention of stain on the prostomium, there is effectively no distinctive MG staining pattern for this species.

This species has no close congeners except perhaps K. carolina n. sp., which is another small, threadlike deepwater species that occurs in upper slope depths off North Carolina and that also has tessellated tubes and branchiae from setiger 1. K. fragilis n. sp. differs from K. carolina n. sp. in having instead of lacking denticulate notosetae, in lacking instead of having annular rings on the peristomium, and by having a more expanded posterior end.

Biology. The type specimens were isolated from the 0–2 mm profile of 10-cm subcores from the box core, suggesting that this species inhabits sediments near the surface. Thin membranous tessellated tubes were found with non-type specimens in a sample from Sta. 2-93 suggesting that this species extends its branchiae into the overlying water from within the tube. The holotype is a male with sperm packets in the coelom that contain numerous sperm with small rounded nuclei.

Distribution. Known only from the Pacific Ocean in abyssal depths 4858–4879 m.