Heterospio bidentata, 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 : 37-38

Heterospio bidentata new species

Figures 16–17

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Material examined. Continental slope off Queensland, Eastern Australia , Coral Sea Marine Park, R / V Investigator, Sta. 134, coll. 14 June 2017, Brenke sledge, start 23.750°S, 154.572°E, 2093 m to end 28.774°S, 154.546°E, 2156 m, holotype ( AM W.52715) .

Description. A moderately sized species, holotype (AM W.52715) only available specimen, incomplete, with 14 setigers, 10 mm long, 0.20 mm wide across thoracic setigers. Body long, narrow, threadlike, divided into thoracic region with crowded segments and abdominal region with elongate cylindrical segments; posterior segments missing. Thoracic parapodia weakly swollen, elevated over dorsal and ventral surfaces ( Fig. 16A–C ); abdominal parapodia reduced; cylindrical, dorsal and ventral surfaces smooth, lacking grooves or ridges. A thin membrane observed on some abdominal setigers suggesting remnants of an adhering tube. Color in alcohol light tan; pigment absent.

Pre-setiger region narrow, about as long as first 2.5 setigers ( Fig. 16A–C ). Prostomium elongate, weakly diamond-shaped, tapering anteriorly to rounded tip, posteriorly extending over peristomium to anterior margin of setiger 1 as a dorsal crest ( Fig. 16A–B ); eyes absent, nuchal organs not observed. Peristomium with two lateral grooves, producing two rings apparent laterally, but not crossing dorsal and ventral surfaces ( Fig. 16A–C ); dorsally interrupted by dorsal crest; dorsal tentacles absent, scars not apparent. Ventrally, mouth a semi-circular opening on first peristomial ring, with seven short lobes on anterior lip, none on posterior lip ( Fig. 16C ). Branchiae present on setigers 2–5 as short stubs or scars dorsal to notosetae ( Fig. 16A–B ).

Thoracic region consisting of six short setigers, each about as wide as long and a seventh setiger about 1.5 times longer than setiger 6 ( Fig. 16A–B ); setigers 1–7 slightly flattened dorsally with parapodia weakly inflated and elevated over dorsum; similarly inflated ventrally; setiger 8 greatly elongate, as long as first six thoracic segments, setiger 9 equally as long as setiger 8, setigers 8–14 comprising available elongate middle body or abdominal setigers, each about 15 times longer than short thoracic setigers, cylindrical with parapodia reduced, not conspicuous.

All parapodia biramous with setal fascicles arising from anterior edge of segments. All thoracic notopodia with 8–12 capillaries; thoracic neuropodia of setiger 1 with five curved bidentate spines ( Fig. 16D–E ) and 3–4 capillaries; setigers 2–7 with 6–8 capillary noto- and neurosetae. Elongate setiger 8 with capillaries, transitioning to acicular spines in both noto- and neuropodia on setiger 9, setae arranged in short fan-shaped fascicles consisting of two rows of setae with acicular spines in anterior row and capillaries in posterior row; parapodia of setigers 9–13 lateral with noto-and neuropodia close together leaving broad dorsal and ventral gaps, not producing encircling cinctures. Thoracic neuropodial bidentate spines of setiger 1 strongly curved, with thick blunt-tipped main fang on concave side of shaft surmounted by thin apical tooth on convex side ( Fig. 16D–E ). Abdominal acicular spines with thick, weakly curved shaft tapering to narrow pointed tip ( Fig. 16F–H ); aristate setae and subuluncini not observed.

Far posterior segments and pygidium unknown.

Methyl Green staining. Methyl Green imparts a distinctive pattern in which the pre-setiger region and dorsal and ventral surfaces of the thoracic setigers are heavily stained ( Fig. 17 ). The stain is concentrated in numerous large glandular cells that occur in those areas.

Remarks. Heterospio bidentata n. sp. is only the fourth species of the genus reported to have curved neuropodial spines in setiger 1 and the second with bidentate hooks. Two species reported with unidentate hooks are H. catalinensis ( Hartman, 1944) from southern California in shelf depths ( Hartman 1944, 1957) and Heterospio sp. A reported by Uebelacker (1984) from the Gulf of Mexico. Heterospio bidentata n. sp. and H. alata n. sp., both described in the present study, are the first having bidentate hooks in the neuropodia of setiger 1.

In H. bidentata n. sp. the apical tooth is a distinct secondary tooth arising directly from the shaft; whereas in H. alata n. sp. the apical “tooth” is an extension of a flange or crest that occurs along the convex side of the shaft and extends over the curved tip or main tooth of the hook providing the bidentate appearance. See earlier remarks for H. alata n. sp.

Heterospio bidentata n. sp. is further unusual among species of the genus in having the transition from defined tight setal fascicles to paired rows of setae with spines occurring on setiger 9. In most species, this transition occurs on setiger 10; in the other species with bidentate hooks, H. alata n. sp., the transition occurs at setiger 11. In addition, the rows of spines and capillaries on setigers 9–13 of H. bidentata n. sp. are short and limited to the lateral sides of individual segments, leaving wide dorsal and ventral gaps instead of producing the encircling cinctures that characterize most species of the genus.

Etymology. The epithet is from bidens, Latin for two-toothed, in reference to the bidentate neuropodial hooks present on setiger 1 of this species.

Distribution. Abyssal Plain off Queensland, Eastern Australia, Coral Sea, 2093–2156 m.

Figures

FIGURE 16. Heterospio bidentata n. sp. Holotype (AM W.52715): A, anterior end (setiger 1–8), dorsal view; B, anterior end (setiger 1–3), dorsal view; C, anterior end (setiger 1–3), ventral view; D–E, neuropodial acicular bidentate hooks from setiger 1; F–H, acicular spines from setiger 12. Arrows denote location of neuropodial hooks on setiger 1.

FIGURE 17. Heterospio bidentata n. sp. Holotype (AM W.52715):Anterior end (setiger 1–6) with Methyl Green stain.Arrows denote concentrations of MG.