Swima, OSBORN ET AL., 2009

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 : 669

SWIMA OSBORN ET AL., 2009

Type species: Swima bombiviridis Osborn et al., 2009

Diagnosis (emended): Swimming acrocirrids with thick gelatinous sheath penetrated throughout by long, clavate papillae. Body transparent. One or more lollipop-shaped, inter-ramal papillae projecting well beyond gelatinous sheath. With more than 30 long (more than body width) chaetae per parapodium. Eyes absent. Head not retractable. Nuchal organs just posterior to palps as simple, slightly raised ciliated ridges making no more than single 180° bend, not curving around bases of subulate branchiae. Possessing single, medial subulate branchia either individually or as part of a single row of subulate branchiae immediately posterior to palps and anterior to segmental branchiae, not easily lost. Sometimes with single row of more than 30 digitiform branchiae just posterior to subulate branchiae. Nephridiopores as papillae on second achaetous segment. Four pairs of segmental branchiae modified as ellipsoid, bioluminescent structures, the second of which is attached to basal portion of nephridiopore papillae. Segmental branchiae small (largest less than half width widest body), easily lost, leaving obvious circular scars.

Remarks: Photos for the type species ( S. bombiviridis , Fig. 2) are reproduced from Osborn et al. (2009: supplement), with permission, to represent the characters of the genus and for comparison to the species described here. Swima shares the following features with other Acrocirridae : several achaetous anterior segments ( Figs 2A, C, 4E, 5B , 6E), shape of prostomium, presence of nephridiopores near second branchiae ( Figs 2C, 5B , 6F), gonads in three or fewer anterior segments ( Figs 2C, 4E, 5A , 6G), four pairs or fewer of branchiae that are easily lost, and simple, spinous notochaetae ( Figs 2E–H, 4G–H, 5F–H , 6B). Swima differs from Macrochaeta Grube, 1850 , Acrocirrus Grube, 1873 , Flabelligella Hartman, 1965 , and Flabelligena Gillet, 2001 in general body form ( Fig. 2A), the absence of eyes, and presence of more than 30 chaetae per parapodium ( Figs 2E, 4A, 6C). Swima differs from Flabelliseta incrusta Hartman, 1978 in the shape of the notopodial papillae, by possessing notochaetae, and by not adhering sediment particles to their gelatinous sheath. Swima is similar to Helmetophorus rankini Hartman, 1978 and Chauvinelia (consisting of Chauvinelia biscayensis Laubier, 1974 and Chauvinelia arctica Averincev, 1980 ), sharing with them the nature of their buccal organ and possibly the ability to swim, although the latter is unconfirmed in Chauvinelia and Helmetophorus . Members of Swima differ from members of Helmetophorus and Chauvinelia by lacking a cephalic hood and elongate achaetous anterior segments, possessing a medial subulate branchia, having simple, not convoluted nuchal organs, and in general body size. Swima and Chauvinelia further differ from Helmetophorus by possessing lollipop-shaped interramal papillae ( Figs 2D, 4C) and more than 30 chaetae per parapodium. Although the papillae drawn by Glasby & Fauchald (1991) from the types suggest Helmetophorus ’s interramal papillae are lollipop-shaped, examination of the type material indicates that they are not because they lack the extremely bulbous, granular appearing, solid distal tips. Instead, Helmetophorus has flaccid, nongranular, moderately bulbous distal tips on their clavate interramal papillae.

Analysis of molecular sequences ( Figs 1 , 3 ) shows that Swima forms a well-supported clade distinct from all previously known acrocirrids and flabelligerids available for these analyses. Helmetophorus , Chauvinelia , Flabelliseta , and Flabelligella were unavailable for genetic analyses, but as detailed above, they are distinguishable from Swima based on morphology. Helmetophorus and Chauvinelia are the most likely of these missing taxa to form a clade with Swima , but their morphology clearly distinguishes them from Swima as outlined above.

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.

Figure 5. Swima fulgida sp. nov. paratype 1 (A–C, F–H) and paratype 2 (D–E), images A, B, D taken from live animals. A, ventral view, whole animal. Dark pigmented foregut and coils of orange midgut are visible through the transparent body wall. B, dorsal view, anterior. C, illustration showing location of features visible in B. Grey arrows indicate left elliptical branchiae scars; black arrow indicates right nuchal organ. The bases of the palps, the medial subulate branchia (sb), nuchal organs, and nephridiopore papillus (np) are visible. D, ventral view, anterior. E, illustration showing location of features visible in C. Developing gonads indicated by arrows; posterior fold of the left nephridia (n), and dorsal blood vessel (bv) visible. F, light micrograph of notochaetae, stitched compilation of four images. G, light micrograph of shaft of chaeta. H, scanning electron micrograph of shaft of chaeta. Scale bars: A = 5 mm, D = 1 mm, F = 0.4 mm, H = 10 Mm.