Heterospio dibranchiata, Blake & Maciolek, 2023

Blake, James A. & Maciolek, Nancy J., 2023, New species and records of Heterospio (Annelida, Longosomatidae) from continental shelf, slope and abyssal depths of the Atlantic Ocean, Pacific Ocean, Indian Ocean and adjacent seas, Zootaxa 5260 (1), pp. 1-74 : 23-27

Heterospio dibranchiata new species

Figures 8–9

urn:lsid:zoobank.org:act:1FB493ED-53AC-4828-BE77-784002C767DD

Material examined. (40 specimens) Gulf of Mexico, off Louisiana. Matterhorn Platform Survey , coll. J. A. Blake, Chief Scientist. Sta. M-5S, 10 Nov 2008, BC, 28.724°N, 88.826°W, 955 m, holotype ( MCZ 163727 ) , 9 paratypes ( MCZ 163728 ) ; Sta. M-1S, 10 Nov 2008, BC, 28.739°N, 88.826°W, 875 m (2, MCZ 163729 ) ; Sta. M-2S, 10 Nov 2008, BC, 28.738.738°N, 88.826°W, 878 m, 1 paratype ( MCZ 163730 ) ; Sta. M-3S, 10 Nov 2008, BC, 28.737°N, 88.826°W, 884 m, 2 paratypes ( MCZ 163731 ) ; Sta. M-4S, 10 Nov 2008, BC, 28.733°N, 88.826°W, 908 m, 1 paratype ( MCZ 167732 ) ; Sta. M-1E, 10 Nov 2008, BC, 28.742°N, 88.822°W, 867 m, 1 paratype ( MCZ 163733 ) ; Sta. M-2E, 10 Nov 2008, BC, 28.742°N, 88.821°W, 867 m, 1 paratype ( MCZ 163734 ) ; Sta. M-3E, 10 Nov 2008, BC, 28.742°N, 88.82°W, 868 m, 2 paratypes ( MCZ 163735 ) ; Sta. M-4E, 10 Nov 2008, BC, 28.742°N, 88.815W 891 m (1, MCZ 163736 ) ; Sta. M-5E, 09 Nov 2008, BC, 28.742°N, 88.805°W, 921 m, 3 paratypes ( MCZ 163737 ) ; Sta. M-2N, 10 Nov 2008, BC, 28.746°N, 88.826°W, 825 m (1, MCZ 163738 ) ; Sta. M-4N, 10 Nov 2008, BC, 28.752°N, 88.826°W, 821 m, 1 paratype ( MCZ 163739 ) ; Sta. M-3W, 10 Nov 2008, BC, 28.742N, 88.831°W, 850 m, 2 paratypes ( MCZ 163740 ) ; Sta. M-4W, 10 Nov 2008, BC, 28.742°N, 88.837° W, 839 m, 1 paratype ( MCZ 163741 ) .— Virgo Platform Survey , coll. J.A. Blake, Chief Scientist. Sta. V-1E, 11 Nov 2008, BC, 29.182°N, 88.164°W, 340 m (1, MCZ 163742 ) ; Sta. V-1W, 11 Nov 2008, BC, 29.185°N, 88.172°W, 340 m (1 post end MCZ 167743 ) ; Sta. V-1N, 11 Nov 2008, BC, 29.185°N, 88.171°W, 335 m (1, MCZ 163744 ) .— Matterhorn Gas Pipeline Survey , coll. P.A. Neubert, Chief Scientist. Sta. 2C, 05 Jun 2009, BC, 28.872°N, 88.88°W, 533 m (3, MCZ 163745 ) ; Sta. 6C, 04 Jun 2009, BC, 28.872°N, 88.941°W, 282 m (1, MCZ 163746 ) ; Sta. 8C, 04 Jun 2009, BC, 28.863°N, 88.984°W, 241 m (1, MCZ 163747 ) ; Sta. 9C, 04 Jun 2009, BC, 28.897°N, 88.946°W, 229 m (2, MCZ 163748 ) .— Matterhorn Oil Export Pipeline Survey , coll. J.A. Blake, Chief Scientist, Sta. 4, 10 Jul 2008, 28.821°N, 88.985W, 364 m (1, MCZ 154127 ) .

Description. An elongate, threadlike species. Holotype (MCZ 163727) complete, with 27 setigers (9 thoracic, 15 abdominal, 3 on posterior section), 14.9 mm long, 0.1 mm wide across narrow thoracic setigers, 0.26 mm wide across middle abdominal segments, and 0.22 wide across bulbous posterior section ( Fig. 8A ). A smaller complete specimen (MCZ 163744) with 28 setigers (9 thoracic, 15 abdominal, 4 on posterior section), 7.27 mm long, 0.08 mm wide across thoracic setigers, 0.12 mm wide across abdominal segments, and 0.07 mm across posterior section. One small complete juvenile also present (see below). Paratypes and other specimens incomplete. Large paratype (MCZ 163735) with 13 setigers, 23.3 mm long, 0.21 mm wide across thoracic setigers, and 0.71 mm across abdominal segments; small paratype (MCZ 163728) with 13 setigers, 5.71 mm long, 0.15 across thoracic setigers, 0.12 across abdominal segments. Body divided into thoracic region with nine setigers, setigers 1–6 short, slightly wider than long; setigers 7–9 becoming progressively longer with setiger 7 about 2 times longer than setiger 6, setiger 8 about 3.5 times longer than setiger 6; setiger 9 greatly elongated, about 9 times longer than setiger 6. Color in alcohol opaque white to light tan; pigment entirely absent.

Pre-setiger region short, as long as first two thoracic setigers ( Fig. 9A–C ). Prostomium narrow, triangular, tapering to narrow apex ( Fig. 9A–C ); eyespots absent; nuchal organs distinct slits on posterior lateral margins ( Fig. 9A–B ). Dorsal surface with narrow ridge or crest extending from prostomium mid-dorsally over peristomium ( Fig. 9B ). Peristomium a single ring, interrupted dorsally by dorsal ridge ( Fig. 9A–B ). Mouth narrow vertical opening bordered anteriorly by three short narrow lobes, laterally by two large protruding lobulated lobes that could be mistaken for protruding proboscis, and posteriorly by 7–9 lobes connecting to elongate narrow ridge extending posteriorly over peristomium and setigers 1–2 ( Fig. 9C ). Dorsal tentacles observed on one specimen, arising in groove or notch lateral to dorsal crest; tentacles thickened filaments.

Branchiae present on setigers 2–3 ( Figs. 8A–B , 9A–B ) on all specimens examined except smallest juveniles, where present only on setiger 2; branchiae long, thin ( Fig. 8B ), rounded in cross section, tapering to rounded tip; each with narrow ciliated groove; internal blood vessel extends along entire length.

All parapodia biramous with setal fascicles arising from near anterior edge of segment. All thoracic setigers slightly flattened dorsally with parapodia weakly inflated and elevated over dorsum and bearing setal fascicles in tight bundles. Abdominal setigers 10 and following round in cross section and bearing setae in transverse rows encircling each anterior end of segment. Abdominal parapodia from setiger 10 narrow elongate lobes, becoming thicker and more prominent from setigers 11–12; noto- and neuropodia with distinct dorsal and ventral gaps between setal fascicles; lateral gaps between noto- and neuropodia narrower.

Thoracic notopodia of setigers 1–9 with 10–15 long capillaries in spreading fascicles; capillaries of neuropodia more numerous, with up to 20–25 setae arranged in tight, dense fascicle. Noto- and neuropodia of setiger 10 with capillaries arranged in two elongate transverse rows with distinct dorsal and lateral gaps. Setiger 11 with notosetae of both rows all capillaries; neuropodia with anterior row of acicular spines and posterior row with capillaries; setiger 12 with both noto- and neuropodia with anterior row of acicular spines and posterior row of capillaries. Setiger 13 and all subsequent abdominal setigers (to setiger 24 in holotype) with both noto- and neuropodia with anterior row of acicular spines and posterior row of narrower spines with long aristate tip. Acicular spines thick, tapering to narrow pointed tip ( Figs. 8D–E , 9D ); aristate spines narrower than acicular spines, with long, tapering tip ( Figs. 8D–E , 9E–G ). Far posterior bulbous section of holotype oval-shaped ( Fig. 8C ), with terminal anal opening; with three parapodia, each bearing 1–2 hooked spines in each ramus; each spine short, with curved pointed tip ( Fig. 8F ).

Juvenile morphology. A complete juvenile with 28 setigers (MCZ 163745) with nine thoracic, 16 abdominal, and three posterior setigers; 4.94 mm long and 0.06 mm across anterior setigers; body consistently narrow except for bulbous posterior end. A pair of long branchiae present on setiger 2 but absent on setiger 3. Setigers 7, 8, and 9 longer than individual setigers 1–6, but setiger 9 not as long proportionately as in adults; all abdominal segments of a relatively equal length.A second complete juvenile from Sta. 6C (MCZ 163746) with 28 setigers: nine thoracic, 15 abdominal, and four posterior, measuring 3.3 mm long, 0.73 mm across thoracic setigers, 1.22 mm across abdominal setigers, and 0.91 across the posterior section. This second juvenile with abdominal segments contracted, thus appearing shorter. Posterior bulbous section of juveniles similar in appearance to that of adults. Transverse setal fascicles of abdominal segments greatly reduced in juveniles, restricted to simple noto- and neuropodia with fewer setae (4–6 spines per fascicle), but have same arrangement of acicular spines and aristate spines in two rows as in adults from setiger 13. Short curved hooks in posterior region of same morphology, arrangement, and number as in adults.

These observations suggest that full segmental complement of 27–28 or more setigers of Heterospio dibranchiata n. sp. develop early in juveniles and that subsequent growth involves elongation, enlargement, and elaboration of individual segments and parapodia as well as an increase in number of setae. It is likely that this same growth pattern occurs in all species of Heterospio .

Methyl Green staining. Body stains uniformly; weak staining of pre-setigerous region not prominent.

Remarks. Heterospio dibranchiata n.sp. is unusual within the genus in having only two pairs of branchiae(setigers 2–3); these branchiae are remarkably long, and when complete are as long as one-third or more of the body length. Other unusual characters include: (1) a conical prostomium with a narrow anterior apex and a posterior extension or caruncle that extends over the peristomium as a dorsal crest merging with setiger 1, (2) a single peristomial ring instead of two where the curved nuchal organs can be seen immediately anterior to the peristomium on the posterior lateral margin of the prostomium, (3) a notch on the anterior border of the peristomium where the dorsal tentacles are attached, (4) an unusual oral apparatus where a narrow vertical oral opening is bordered laterally by two large protruding lobes resembling a protruding pharynx, and (5) only six short thoracic segments with setigers 7, 8, and 9 becoming increasingly longer, and setiger 9 is as long as setiger 10 of other species. Other characters are more typical of known species, such as the first nine setigers (thorax) having widely spaced and well-defined noto- and neuropodia with setal fascicles in setiger 10 that transition to setae arranged in transverse rows across each noto- and neuropodia, nearly encircling the body; when spines are present, these encircling rows of setae resemble the cinctures of spines found in cirratulids of the genus Chaetozone . The hooked spines of the posterior bulbous section are similar to those reported for H. hartmanae n. sp. and others in this study. Another species in this study found to have only two pairs of branchiae and a single peristomial ring is H. alata n. sp. from off Brunei, Island of Borneo (see below). However H. alata n. sp. is one of only four known species to have neuropodial hooks on setiger 1, whereas H. dibranchiata n. sp. has only capillaries in that position.

Biology. The presence of four complete specimens in these samples is unusual for a species of Heterospio and likely represents the careful manner in which they were collected; gentle handling minimized fragmentation and damage (see Materials and methods).

Heterospio dibranchiata n. sp. was widely distributed in the various survey samples at the platform and pipeline locations ranging from 241 – 955 m. About a third of the specimens (10) were collected from a single sample at Station M-5S at a depth of 955 m, the deepest location from which the species was collected. Sediment grain size at Sta. M-5S was sand (1.86%), silt (34.00%), and clay (64.14%), with TOC at 1.24%. Station M-5S (955 m) was the only location where H. dibranchiata n. sp. was among the top 10 species. The station was dominated by aplacophoran molluscs, polychaetes including cirratulids, paraonids, and spionids, and a thyasirid bivalve mollusc (Blake et al. unpublished data).

Etymology. The epithet dibranchiata is from the Greek, di -, for two or double and branchos for gill in reference to this species having only two pair of branchiae.

Distribution. U.S. northern Gulf of Mexico, off Louisiana, 241– 955 m.

Figures

FIGURE 8. Heterospio dibranchiata n. sp. Holotype (MCZ 163727): A, entire worm; B, anterior end, right lateral view; C, posterior end, right lateral view; D, subuluncini setiger 16; E, subuluncini and acicular spine setiger 13; F, posterior hook. A–C, stained with Shirlastain A.

FIGURE 9. Heterospio dibranchiata n. sp. A, anterior end, left lateral view; B, anterior end, dorsal view; C, anterior end, ventral view; D, acicular spines; E–G, subuluncini.A, C, paratype (MCZ 163728), B, paratype (163735), D–G, holotype (MCZ 163727).