Swima tawitawiensis, Osborn & Haddock & Rouse, 2011

Osborn, Karen J., Haddock, Steven H. D. & Rouse, Greg W., 2011, Swima (Annelida, Acrocirridae), holopelagic worms from the deep Pacific, Zoological Journal of the Linnean Society 163 (3), pp. 663-678 : 675-677

SWIMA TAWITAWIENSIS SP. NOV.

Common name: Orange bomber

Type material: Holotype, and only specimen, collected from the Celebes Sea off Tawi-Tawi, Philippines (4°58.00′N, 120°14.16′E), x.2007 at 2836 m by ‘ Exploring the Inner Space of the Celebes Sea 2007 Expedition’ using ROV Max Rover Global Explorer operated from R / V Hydrographer Presbitero. Body severed into two pieces when recovered from ROV (anterior fragment, head to chaetiger 5, posterior fragment, 27 chaetigers and pygidium; Fig. 6A). The specimen is deposited at the National Museum of the Philippines ( NMA 0437 , Table 1).

Diagnosis: Swima with nuchal organs forming oblique lines, each with medial end curved into U-shape. Three equally long, subulate branchiae just posterior to nuchal organs, one medial, with smooth sides, two lateral with 90° projection one-third to one-half distance from base. Notochaetae broad and flattened with a fine, spinous tip. Compound neurochaetae with short (barely as long as the medial element is wide), straight, round in cross section, spinous distal element and broad flattened medial element.

Etymology: Named after the location where it was collected, Tawi-Tawi, Philippines. The locality name is simplified and the adjectival ending - ensis is added.

Holotype description: Body transparent ( Fig. 6A). Chaetigers with distinct parapodial lobes and numerous long chaetae, posterior body smoothly tapered ( Fig. 6A), with gelatinous sheath through which narrow, short, clavate papillae extend. Gelatinous sheath mostly peeled from body, visible as shreds around bases of chaetae. Total preserved body length greater than 45 mm (anterior fragment 15 mm, posterior fragment 30 mm) and 6 mm widest width. Pygidium unadorned.

Head consists of prostomium, peristomium, at least three achaetous segments possessing two forms of branchiae, not retractable ( Fig. 6F). Prostomium is tissue posterior to palps supporting nuchal organs. Nuchal organs raised ciliated ridges forming oblique lines with medial ends curved into U-shape ( Fig. 6F, H). No eyes. Peristomium surrounds prostomium completely. Grooved frontal palps transparent to orange in life, tapered, coiling at tips, reaching to first chaetiger ( Fig. 6E). Buccal organ anteroventrally located, unarmed, bilobed, forming eversible lateral lips, inner lobe transparent to black ( Fig. 6G). Slight ridge found posterior to nuchal organs and anterior to row subulate branchiae, indicating segment margin ( Fig. 6F). Two branchial forms: (1) single row of three equally long (to second segment), subulate, transparent to white, single medial with smooth sides, lateral pair with 90° projection one-third to one-half distance from base ( Fig. 6E–H), and (2) elliptical branchiae. Photograph taken immediately after collection shows an elliptical branchia attached to the right nephridiopore papillus ( Fig. 6A), no elliptical branchiae retained with preserved specimen. Branchial scars lateral, first slightly ventral of lateral midline just posterior to row subulate branchiae, second on medial half of nephridiopore, third and fourth on following achaetous segments on saclike projections of body wall ( Fig. 6E). Branchial scars found in the same locations and identical to those in S. bombiviridis and S. fulgida sp. nov. except the fourth, which is located on an additional achaetous segment instead of on first chaetiger. Elliptical branchiae yellow-green in life. Large projection supporting fourth elliptical branchia scar on left ( Fig. 6E top arrow).

Chaetigers undifferentiated into thoracic and abdominal. Noto- and neuropodial lobes form single, nearly smooth projection ( Fig. 6C). One to four clavate papillae lollipop-shaped, found between noto- and neuropodial lobes ( Fig. 6C). Notochaetae simple with smooth body, no articulations, broad and flat, paddle-like, narrow abruptly to fine point at distal tip ( Fig. 6B). Neurochaetae compound, with broad, flat medial element, cylindrical distal element ( Fig. 6D). High magnification reveals fine whorls of spines making distal tips noto- and neurochaetae appear segmented ( Fig. 6B, D), bases appear striated under high magnification. Chaetigers 4–5 with developing gonads, gonopore low, broad papilla just posterior to right fifth neuropodium.

Internal anatomy visible through transparent body wall and gelatinous sheath. Ventrally located double nerve cord with two pairs fused ganglia per segment, diverge just posterior to peristomium to surround buccal organ, fused again just posterior to palp attachment. Single pair of large anterior, semitransparent nephridia reaching back to fifth chaetiger, ventrally orientated from lateral origin, folding back anteriorly, then narrowing to lead to lateral nephridiopores ( Fig. 6F). Gut running from buccal organ straight for one-third body length, coiled in midregion, straight in posterior region. Anterior gut black, expandable to near body width ( Fig. 6E). Heart body and dorsal blood vessel extend through anterior one third of body. Gonads ventral in chaetigers 4–5 ( Fig. 6G), chaetiger 6 severely damaged so unknown if contained gonads as well.

Remarks: This species is most similar to S. fulgida sp. nov. and S. bombiviridis and is sister to the former based on DNA sequence data ( Figs 1 , 3 ). Swima tawitawiensis sp. nov. possesses a pair of lateral subulate branchiae, which is absent in both other species ( Fig. 6F). We suggest that the medial subulate and the lateral projections are branchiae because of the large afferent and efferent blood vessels that run their length and originate from just anterior to the heart body. These structures possess no obvious sensillae. All three projections appear to originate from a single achaetous segment because a slight ridge of tissue connects them ( Fig. 6F). Horned and Tiburon bombers, Chauvinelia spp. and Helmetophorus rankini have similar subulate structures, but possess multiple rows or segments of them, each row possessing various numbers of projections. Each of those species also possesses convoluted nuchal organs that wind along the bases of the subulate branchiae, further distinguishing them from S. tawitawiensis sp. nov.

The broad, flattened notochaetae and compound neurochaetae further differentiate S. tawitawiensis sp. nov. from S. fulgida sp. nov. and S. bombiviridis and the other members of the swimming clade, as do the less pronounced interramal lollipop-shaped papillae and U-shaped medial end of the nuchal organs. Swima tawitawiensis sp. nov. and S. fulgida sp. nov. share a darkly pigmented foregut. The posterior-most pair of elliptical branchiae scars of S. tawitawiensis sp. nov. is not on the first chaetiger as found in the other two Swima species , but instead they lie on an additional achaetous segment. The body-wall projections supporting the small ring-like scars where the elliptical branchiae were attached also differentiate S. tawitawiensis sp. nov. from the other two Swima species.

There was variation from one side of the specimen to the other with respect to the form of the posteriormost elliptical branchia scars; this appears to be an abnormal enlargement of the tissue because nothing of the sort was found on the other side of the animal or on any specimens of the closely related species S. fulgida sp. nov. and S. bombiviridis . No digitiform branchiae were found on the holotype but they might be expected to occur on additional specimens of this species considering the variation found in the other Swima species.

In all Swima species , the broadened and coiled region of the midgut is always the first portion of the body to deteriorate upon collection (possibly because of release of digestive enzymes when damaged or dying). The gut of S. tawitawiensis sp. nov. is presumably coiled similar to that of S. fulgida sp. nov. and S. bombiviridis , but this could not be determined from the specimen because the body wall was severely damaged, with the gut spilled out.

Ecology: Found in the Celebes Sea at 2836 m depth within 30 m of the seafloor. Animals were not observed on the seafloor. The specimen was dead upon recovery and not checked for bioluminescence.

Figures

Figure 1. Ninety-five per cent majority rule consensus tree from Bayesian analyses of five concatenated genes [18S, 28S, 16S, cytochrome oxidase I (COI) and cytochrome b (cytb)] from cirratuliform annelids, showing Swima as part of Acrocirridae (Osborn & Rouse, 2010). Support indicated as posterior probabilities, bootstraps from the maximum likelihood analysis and from the parsimony analysis. Asterisks indicate 1.0 or 100% support.

Figure 3. Ninety per cent majority rule consensus cytochrome oxidase I (COI) gene tree from Bayesian analyses of cirratuliform annelids, showing Swima as part of Acrocirridae and support for Swima monophyly. Unsupported branches were collapsed. Support indicated as posterior probabilities. Asterisks indicate 1.0 or 100% support.