Swima bombiviridis is the type species of the genus Swima, which was only described in 2009. Swima bombiviridis and its close relatives are pelagic marine polychaete worms spending most or all of their life living in the deep sea at depths greater than 1800 meters. Some, like S. bombiviridis, live well above the bottom and presumably never land and are exceptional swimmers, hence the generic name. The common name for this species is the ‘green bomber’ owing to a series of green spheres attached just behind the head that are easily shed by the animal when it is disturbed. These are also the basis for the specific epithet, bombiviridis, which is Latin for green bombs. Once detached, the bombs will glow with green light for up to a minute. Swima bombiviridis belongs to the polychaete family Acrocirridae. Nearly all other acrocirrids are benthic, living in sediments or under rocks from shallow water down to the deep sea. An evolutionary analysis of acrocirrids has shown that Swima bombiviridis and its pelagic relatives evolved from benthic acrocirrid ancestors and share many morphological features with other acrocirrids, although some features appear to have evolved to suit their pelagic lifestyle. Like its benthic relatives, Swima bombiviridis has a pair of grooved palps that it presumably uses for feeding. However, while other acrocirrids usually have four pairs of elongate branchiae (gills) behind the head that are easily detachable, in S. bombiviridis and some of its other pelagic relatives these gills appear to have been evolutionarily transformed to become detachable bioluminescent orbs, the ‘bombs’. These may well not function as branchiae any more and instead these animals have evolved new structures that appear to be branchiae. Swima and its swimming relatives also have elongate paddle-like chitinous chaetae in bundles along their bodies that are quite different from those of their benthic relatives and are almost certainly for use in swimming. (Osborn et al. 2009; Osborn et al. 2011; Osborn and Rouse 2011)

Swima with transparent gut. Possessing a thick, transparent gelatinous sheath penetrated throughout by narrow clavate papillae, simple noto- and neurochaetae, and three achaetous anterior segments supporting ellipsoid, bioluminescent, derived branchiae that are less than 1.5 mm in length.

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