Sea angels (clade Gymnosomata) are a large group of small free-swimming sea slugs, classified into six separate families. They are pelagic opisthobranchs in the clade Gymnosomata within the larger mollusc clade Heterobranchia. Sea angels were previously considered to be pteropods.

Sea angels are also sometimes known as "sea butterflies" but this is potentially misleading because the family Clionidae is just one of the families within this clade, and the term "sea butterfly" is also applied to the shelled Thecosomata.

Recent molecular data suggest the Gymnosomata form a sister group to the Thecosomata (other planktonic, weakly or nonmineralized gastropods), but this long-standing hypothesis has also had some recent detractors.

Distribution

right|thumb|250px|Sea angels in Australian waters

These organisms have a wide geographic range, from polar regions, under sea ice, to equatorial (tropic) seas.

Sea angels are gelatinous, mostly transparent, and very small, with the largest species (Clione limacina) reaching 5 cm. C. limacina is a polar species; those found in warmer waters are far smaller. Some species of sea angels feed exclusively on sea butterflies; the angels have terminal mouths with the radula common to mollusks, and tentacles to grasp their prey, sometimes with suckers similar to cephalopods.

Wings anatomy

The wing comprises seven groups of muscles, anterior oblique muscles for dorsal and ventral sides, posterior oblique muscles for dorsal and ventral sides, the longitudinal and transverse wing retractors muscles, and dorso-ventral muscles. The first four groups form a continuous sheet where the dorsal muscles are perpendicular to the ventral muscles. Transverse and longitudinal retractors muscles retract the wing along the span and the chord. The dorso-ventral muscles control the thickness of the wing by changing the haemocoelic pressure inside the wing, this also ejects the buccal apparatus from C. limacina’s head.

Behavior

Gymnosomata are carnivorous, feeding only on their fellow pteropods, the Thecosomata. Their lifestyles have coevolved with those of their prey, with their feeding strategy adapting to the morphology and consistency of the thecosome shell.

However, cold water sea angel C. antarctica likely accommodates drag-based force production for several reasons: (1) Low aspect ratio for C.antarctica causes deleterious lift-drag ratios. (2) High angle of attack typically ranges from 60 to 80 degrees. (3) The orthogonality between the surface of the wing and ascending trajectory of the sea angel.

Furthermore, Clione antarctica, defends itself from predators by synthesizing a previously unknown noxious molecule, pteroenone. Because of this secretion, predators will not eat the sea angel. A species of amphipod takes advantage of this trait: The amphipod will seize an individual of C. antarctica out of the water column, and carry it around for protection. Local population density of C. antarctica may reach extraordinary levels; up to 300 animals per cubic metre have been recorded.

Their hunting strategies are variable; some forms are ambush predators, sitting and waiting for their prey; whilst others actively pursue their prey; their metabolic rate is closely linked to that of their prey species.

Gymnosomes slowly beat their wing-like parapodia to propel their "perfectly streamlined"

The group does not truly, therefore, lack a shell. Few larval shells have been described (and consequently an understanding of their fossil record is as yet unknown).

Taxonomy

The other suborder of pteropods, Thecosomata, is superficially similar to sea angels, but are not closely related; some authorities include both Thecosomata and Gymnosomata as separate branches of the order Pteropoda, whereas others list them as distinct orders within the subclass Heterobranchia. They have larger, broader parapodia, and most of that species retain a shell; they are commonly known as sea butterflies.

In the new taxonomy of Bouchet & Rocroi (2005), the clade Gymnosomata is arranged as follows:

  • Superfamily Clionoidea:
  • family Clionidae
  • family Cliopsidae
  • family Notobranchaeidae
  • family Pneumodermatidae
  • Superfamily Hydromyloidea:
  • family Hydromylidae
  • family Laginiopsidae

The group that used to be the family Thliptodontidae is treated as Thliptodontinae, a subfamily of the family Clionidae.

Footnotes

References

Further reading

  • Mollusca - The Southern Synthesis Order Gymnosomata by L. Newman pages 985–989; Beesley, P.L., Ross, G.J.B. & Wells, A (eds) -
  • list of Clione entries in the Sea Slug Forum: [http://www.seaslugforum.net/search.cfm?searchstring=Clione]
  • Video of a sea angel in motion: https://web.archive.org/web/20040607043553/http://www.biol.sc.edu/~vogt/courses/neuro/neurobehavior.html#clione
  • Mikko's Phylogeny Archive
  • Reconstruction of the phylogeny of Opisthobranchia; Journal of Molluscan studies