Heterospio aruba Blake & Maciolek 2023, new species

Heterospio aruba new species

Figure 4

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Material examined. (3 specimens) Caribbean Sea, off Island of Aruba, R / V Alpha Helix, Cruise CARIB I, Sta. ND-16, 26 Jun 1977, dredge, 12.506°N, 70.045°W, 30 m, holotype (USNM 1673074), 2 paratypes (USNM 1673075).

Description. All three specimens incomplete. Body elongate, narrow; fragments with 11–13 setigers, nine anterior thoracic setigers with first eight crowded, each about four times wider than long, followed by an elongated setiger 9 about 3.5 times longer than each of first eight setigers (Fig. 4A–B). Following abdominal segments each about as long as entire thoracic region. Thoracic segments 1–9 with typical dorso-lateral setal fascicles confined to localized parapodia; these transitioning from setiger 10 to abdominal segments where setae arise from long transverse parapodia resulting in broad cincture-like fascicles or rows mostly surrounding anterior margin of each segment, with dorsal and lateral parapodial gaps. Dorsal surface relatively smooth along entire body; mid-ventral surface of setigers 1–8 with low ridge extending posteriorly from mouth over peristomium to about setiger 8 (Fig. 4B); ventral surface of setiger 9 and abdominal segments smooth, similar to dorsal surface.

Holotype (USNM 1673074) largest fragment, with 13 setigers, 5.8 mm long, 0.4 mm wide across thorax and 0.45 mm wide across abdominal segments. Larger paratype with 11 setigers, 3.93 mm long, 0.29 mm wide across thorax and abdomen; smaller paratype long, thin, threadlike, with 12 setigers, 4.36 mm long, 0.12 mm wide across thorax and 0.07 mm wide across abdominal segments. Color in alcohol light tan.

Pre-setiger region triangular in shape, about as long as first three thoracic setigers (Fig. 4A–B). Prostomium triangular, wide at base, tapering to narrow rounded tip; eyespots absent, nuchal organs narrow slits along posterior margin (Fig. 4A). Peristomium with two dorsolateral rings dorsally separated by groove; no dorsal tentacles present, but scars evident in medial notch in groove (Fig. 4A). Peristomial lobes not extending onto smooth ventral surface; mouth arising at level between the two peristomial lobes, consisting of simple narrow transverse slit (Fig. 4B), proboscis not emergent on any specimen.

Branchiae on setigers 2–8 on holotype (Fig. 4A), setigers 2–6 on larger paratype, and 2–3 on small paratype; branchiae or their stubs or scars clearly evident when present, not observed on setiger 9. Individual complete branchiae, when present, thin, rounded in cross section, tapering to rounded tip; 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. Neuropodia of holotype with prominent digitate postsetal lobes on all nine thoracic setigers, shortest on setigers 1–3, then longer on setigers 4–8, then short again on setiger 9 (Fig. 4B); these short, rounded lobes on larger paratype, rudimentary on smaller paratype. Abdominal setigers round in cross section and bearing setae in transverse rows encircling anterior border of each segment. Abdominal parapodia from setiger 10 as narrow elongate lobes, becoming thicker and more prominent on setigers 11–13; noto- and neuropodia with distinct dorsal and ventral gaps between setal fascicles; lateral gaps between noto- and neuropodia narrower.

All thoracic notopodia of setigers 1–9 with 10–25 long capillaries in spreading fascicles; capillaries of neuropodia more numerous with up to 25–30 arranged in tight, dense fascicle. Noto- and neuropodia of setiger 10 with capillaries arranged in two transverse rows with distinct dorsal and lateral gaps; a few aristate spines also present in anterior row of neuropodia. Setigers 11–12 with noto- and neuropodia with aristate-tipped acicular spines in anterior row (Fig. 4D) and capillaries in posterior row; setiger 13 similar but with acicular spines (Fig. 4C) in neuropodia in first row and thin capillaries in second row, notopodia with aristate spines and few acicular spines in anterior row and capillaries in posterior row.

Posterior region not present among fragments.

Methyl Green staining. Body stains uniformly and de-stains with no pattern.

Remarks. Heterospio aruba n. sp. is similar to the original concept of H. longissima by Ehlers (1874, 1875) in having eight short thoracic segments and a longer ninth setiger, followed by even longer segments. However, H. aruba n. sp. has acicular and aristate spines among the abdominal setae from setiger 10 instead of only capillaries. It is possible that some acicular spines with rounded tips may simply be ones that have lost the fragile arista. Heterospio aruba n. sp. is unusual in having prominent digitate neuropodial postsetal lamellae on setigers 1–9. Short neuropodial lamellae were illustrated, but not described or discussed for H. indica from the Indian Ocean in about 20 m (Parapar et al. 2016: Fig. 2B). Heterospio indica is reported to have thickened capillary setae and a few subuluncini in abdominal parapodia, but no acicular or aristate spines (but see H. indica below); in contrast H. aruba n. sp. has both aristate and acicular spines.

Etymology. The epithet for Heterospio aruba n. sp. is based on its discovery offshore the Island of Aruba, in the Caribbean Sea.

Distribution. Caribbean Sea, off Island of Aruba, 30 m.