Aphelochaeta tanyperistomia, Blake, 2019

Blake, James A., 2019, New species of Cirratulidae (Annelida, Polychaeta) from abyssal depths of the Clarion-Clipperton Fracture Zone, North Equatorial Pacific Ocean, Zootaxa 4629 (2), pp. 151-187 : 161-164

Aphelochaeta tanyperistomia new species

Figures 7–8

urn:lsid:zoobank.org:act:4554FCD3-1A1D-4788-893B-831AF59011D9

Tharyx sp. A: Wilson & Hessler 1987: Appendix E (in part).

Tharyx sp. C: Wilson & Hessler 1987: Appendix E (in part).

Material examined. North Equatorial Pacific Ocean, abyssal plain, Clarion-Clipperton Fracture Zone, NOAA BIE Project site, coll. D.D. Trueblood, Sandia box corer, Sta. DDT-01-94, veg. 6, 5– 10 cm fraction, 25 Jul 1994, 12°55,788′N, 128°35.843′W, 4851 m, 1 paratype ( USNM 1557535 ) ; Sta. DDT-4-94, veg. 22, 5– 10 cm fraction, 27 Jul 1994, 12°55.967′N, 128°35.758′W, 4851 m, holotype ( USNM 1557536 ) ; Sta. DDT-7-93, veg. 4, 2– 5 cm fraction, 02 Sep 1993, 12°56.303′N, 128°35.311′W, 4844 m, 1 juvenile ( USNM 1557537 ) ; veg. 7, 5– 10 cm 1 juvenile ( USNM 1557538 ) ; Sta. DDT-9-94, 29 Jul 1994, 12°54.980′N, 128°35.440′W, 4877 m, 1 juvenile ( USNM 1557539 ).— ECHO I , DOMES Site C, R/ V Melville cruise, coll. R. Hessler, 0.25 m 2 Sandia box core, Sta. 350, 1– 5 cm fraction, 14 Jun 1983, 14°38.1226′N, 125°26.8208′W 4506 m, 1 specimen ( LACM-AHF Poly 11263) ; Sta. H 351, 5– 10 cm fraction, 14 Jun 1983, 14°37.6334ʹN, 125°26.3840ʹW, 4516 m, 1 specimen ( LACM-AHF Poly 11264) ; Sta. 356, 1– 5 cm fraction, 21 Jun 1983, 14°42.4541′N, 125°24.2664′W, 4518 m, 1 specimen ( LACM-AHF Poly 11256) ; Sta. H 358, 1– 5 cm frac- tion, 23 June 1983, 14°42.1930′N, 125°24.2556′W, 4516 m, 1 specimen ( LACM-AHF Poly 11266) ; Sta. H 362, 1– 5 cm fraction, 18 Jun 1983, 14.7013°N, 125.4309°W, 4480 m, 1 specimen ( LACM-AHF Poly 11267) .

Description. A small species, holotype complete, in three parts, about 8 mm long, 0.3 mm wide across narrow peristomium, 0.70 mm wide across expanded anterior segments, and 0.33 mm wide across posterior segments with about 40 setigerous segments total. Paratype smaller, incomplete, 2.45 mm long, with 27 setigerous segments. Body of holotype with unusually elongate, narrow pre-setiger region ( Fig. 7A ) and first 3–4 setigers followed by 10–12 expanded anterior segments, then narrowing again in posterior segments. Body generally cylindrical throughout with anterior expanded segments and dorsum elevated over parapodia; no ventral or dorsal grooves evident. Color in alcohol opaque white with no pigmentation.

Pre-setiger region elongate, narrow, as long as first eight setigers in holotype ( Fig. 7A ) and first 12 setigers in paratype ( Fig. 7B ). Prostomium relatively short, rounded on anterior margin, paratype and juvenile with distinct palpode on tip ( Figs. 7B , 8 A–C); eyespots absent; nuchal organs not observed on holotype; present as narrow grooves on paratype at posterior border with peristomium ( Fig. 7B ). Peristomium two to three times longer than wide; surface appearing wrinkled, with no annular rings evident ( Fig. 7 A–B); dorsal surface forming low crest. Dorsal tentacles arising mid-dorsally near posterior margin of peristomium ( Fig. 7 A–B). First pair of branchiae arising on setiger 1 dorsal to notosetae; branchiae arising from similar location on subsequent setigers ( Fig. 7A ); branchiae or stubs observed over first half of body.

Parapodia moderately developed as expanded lobes or shoulders best observed in anterior expanded segments; posterior segments with small podial lobes from which setae arise. Setae all capillaries with 5–7 relatively long capillaries in anterior notopodia with 1–2 additional very long, natatory-like capillaries ( Fig. 8 A–B, D); capillaries reduced in number to 4–5 per setiger posteriorly and not as long; neurosetae also long, with up to 8–10 capillaries in anterior setigers, reduced to 4–6 posteriorly.

Body of holotype terminating in pygidium with flattened ventral lobe.

Juvenile Morphology. Two complete juveniles, with 11 and 17 setigers, respectively, were encountered in a single box core from Sta. DDT-7-93.

An 11-setiger juvenile (USNM 1557538, veg 4), 2.7 mm long, 0.1 mm wide across peristomium, 0.2 mm wide across expanded anterior section, and 0.09 across posterior segments. Pre-setiger region narrow and elongate as in adults, with distinct palpode on anterior end of prostomium ( Fig. 8 A–C). Peristomium relatively smooth, but with irregular lobes or wrinkles evident; dorsal tentacles and branchiae not developed. Expanded anterior segments with only four setigers ( Fig. 8 A–B), rapidly narrowing to long middle and posterior section, terminating in narrow pygidial segment with rounded lobe ( Fig. 8A ).

Second juvenile (USNM 1557539, veg. 7), with 17 setigerous segments, 2.2 mm long, 0.18 mm wide across peristomium, 0.40 mm across expanded anterior section, and 0.18 mm across far posterior segments. Specimen with ten setigers in expanded anterior area ( Fig. 8D ), and seven setigers in posterior section. Palpode on anterior margin of prostomium appears partially retracted. Rudiment of one dorsal tentacle observed on posterior margin of peristomium and clear spots evident on several anterior segments suggesting sites of branchial development. With pair of fragile anal cirri evident arising laterally from pygidial lobe ( Fig. 8E ).

Methyl Green stain. No pattern.

Etymology. The epithet is from the Greek, tany, for long or stretched and peristome, for the region around the mouth, and refers to the unusual elongate peristomial morphology of this species.

Remarks. Aphelochaeta tanyperistomia n. sp. is unusual among bitentaculate cirratulids in the nature of the elongate pre-setiger region. The wrinkled epidermis of the peristomium on both the holotype and paratype suggest that the entire pre-setiger region is subject to considerable expansion and contraction: the pre-setiger region and first four setigers of the paratype are contracted whereas the same pre-setiger and anterior setigers of the holotype appear stretched. The long stretched-out region of the holotype followed by an expanded segmental region suggests that the species is an active burrower using hydrostatic pressure to produce cracks in the sediment into which the worm moves. The mechanics of this kind of cirratulid burrowing were described by Che & Dorgan (2010) and Dorgan et al. (2011) for Cirriformia moorei Blake, 1996 , a common intertidal species in California. The topic was also reviewed by Blake & Magalhães (2019). The holotype, paratype, and one juvenile of A. tanyperistomia n. sp. were taken from the lower 5–10 cm fraction of 10 cm subcores of the box core; the smaller 11-setiger juvenile was taken from the middle 2–5 cm fraction. These results suggest that these worms are deep burrowers.

Other species of Aphelochaeta with long pre-setiger regions are: A. antelonga Dean & Blake, 2016 from off Costa Rica to Chile, A. striata Dean & Blake, 2016 also from Costa Rica, and A. elongata Blake, 1996 from northern California ( Blake 1996, 2018; Dean & Blake 2016). All three of these species have a pre-setiger region that is 2–3 times as long as wide, but unlike A. tanyperistomia n. sp. their pre-setiger region is relatively smooth or with shallow peristomial transverse grooves instead of a strongly wrinkled surface with no evidence of transverse grooves.

The presence of a distinct palpode on the tip of the prostomium of the paratype and two juveniles of A. tanyperistomia n. sp. is reminiscent of similar structures in some species of Paraonidae and Opheliidae and appears to be unique within the Cirratulidae . The palpode is likely retracted in the holotype. In paraonids, Strelzov (1973, 1979) suggested that the prostomial palpode was tactile in function and he noted that it was capable of retracting by means of a muscle innervated directly from the brain.

The juveniles exhibit features that not only provide important developmental information on this species, but for bitentaculate cirratulids in general. The developmental morphology revealed in the 11- and 17-setiger juveniles clearly suggests a pattern where the elongate, narrow pre-setiger region develops early and the initial segmentation produces elongate narrow segments that are subsequently enlarged and modified in the anterior half of the body, resulting in the characteristic expanded anterior or thoracic region followed by the narrow posterior or abdominal region. The anterior expanded region consists of only four segments in the 11-setiger juvenile, ten segments in the 17-setiger juvenile and up to 15 setigers in the larger holotype and paratype. The dorsal tentacles and branchiae appear later in development, which has been documented for other cirratulids ( Blake 1975).

Distribution. Abyssal Pacific Ocean, 4506–4877 m.

Figures

FIGURE 7. Aphelochaeta tanyperistomia n. sp. Holotype (USNM 1557536): A, anterior end, left lateral view. Paratype (USNM 1557535): B, anterior end, left lateral view.

FIGURE 8. Aphelochaeta tanyperistomia n. sp. 11-setiger juvenile (USNM 1557538):A. entire worm, dorsal view; B, anterior end, dorsal view; C, detail of prostomium palpode. 17-setiger juvenile (USNM 1557539): D, anterior end middle segments, dorsal view; E, posterior end and pygidium, dorsal view. All stained with Shirlastain A.