Sea angels (

clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...

Gymnosomata) are a large group of small free-swimming

sea slug Sea slug is a common name for some Marine biology, marine invertebrates with varying levels of resemblance to terrestrial Slug, slugs. Most creatures known as sea slugs are gastropods, i.e. they are Sea snail, sea snails (marine gastropod moll ...

s, classified into six separate families. They are

pelagic The pelagic zone consists of the water column of the open ocean and can be further divided into regions by depth. The word ''pelagic'' is derived . The pelagic zone can be thought of as an imaginary cylinder or water column between the sur ...

opisthobranch Opisthobranchs () is a now informal name for a large and diverse group of specialized complex gastropods which used to be united in the subclass Opisthobranchia. That taxon is no longer considered to represent a monophyletic grouping. Euopistho ...

s in the clade Gymnosomata within the larger

mollusc Mollusca is a phylum of protostome, protostomic invertebrate animals, whose members are known as molluscs or mollusks (). Around 76,000 extant taxon, extant species of molluscs are recognized, making it the second-largest animal phylum ...

clade

Heterobranchia Heterobranchia, the heterobranchs (meaning "different gill"), is a taxonomic clade of snails and slugs, which includes marine, aquatic, and terrestrial gastropod molluscs. Heterobranchia is one of the main clades of gastropods. Currently Het ...

. 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 The Thecosomata (collective/plural: ''thecosomes'', meaning "case/shell-body"), or sea butterflies, are a taxonomic suborder of small, pelagic, free-swimming sea snails known as holoplanktonic opisthobranch gastropod mollusks, in the order Pt ...

. 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

These organisms have a wide geographic range, from polar regions, under sea ice, to equatorial (tropic) seas. From spring to autumn, sea angels live at a depth of 200 m in the Sea of Okhotsk. In winter, they migrate to the coast of north Hokkaido with drift ice.

Description

In this

, the foot of the gastropod has developed into wing-like flapping appendages (parapodia) and larval sea angels discard their embryonic shells a few days after hatching. Both adaptations suit their free-swimming oceanic lives. The adaptations also explain the common name sea angel and the scientific name of the order: From

Greek Greek may refer to: Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group *Greek language, a branch of the Indo-European language family **Proto-Greek language, the assumed last common ancestor of all kno ...

''gymnos'' meaning "naked" and ''soma'' meaning "body". The lack of a heavy shell for gymnosomes allows them to hover on the upper water surfaces for longer periods, along with a streamlined body that reduces the drag coefficient. Also, gymnosomes have a low aspect ratio which aids them to maneuver and accelerate quickly. 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 Thecosomata (collective/plural: ''thecosomes'', meaning "case/shell-body"), or sea butterflies, are a taxonomic suborder of small, pelagic, free-swimming sea snails known as holoplanktonic opisthobranch gastropod mollusks, in the order ...

; the angels have terminal mouths with the

radula The radula (; : radulae or radulas) is an anatomical structure used by mollusks for feeding, sometimes compared to a tongue. It is a minutely toothed, chitinous ribbon, which is typically used for scraping or cutting food before the food enters ...

common to mollusks, and tentacles to grasp their prey, sometimes with suckers similar to

cephalopod A cephalopod is any member of the molluscan Taxonomic rank, class Cephalopoda (Greek language, Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral symm ...

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.SATTERLIE, R. A., LABARBERA, M., & SPENCER, A. N. (1985). Swimming in the Pteropod Mollusc, Clione Umacina: I. Behaviour and Morphology. Journal of experimental biology, 116(1), 189-204.

Behavior

Gymnosomata are carnivorous, feeding only on their fellow pteropods, the

. Their lifestyles have coevolved with those of their prey, with their feeding strategy adapting to the morphology and consistency of the thecosome shell. By rowing their "wings" back and forth at 1–3 Hz, the sea angels swim at speeds up to . This is about twice as fast as their prey, the sea butterfly. Tracing the wingtip showed a ‘figure-of-eight’ trajectory and stroke plane orthogonal to the caudal-rostral axis, this pattern was observed in some aerial insects, small birds, and

such as hummingbirds and ''Limacina helicina''. It is not yet clear whether the sea angel uses its swimming appendages as 'rowing paddles' or as 'wings'. 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 Amphipoda () is an order of malacostracan crustaceans with no carapace and generally with laterally compressed bodies. Amphipods () range in size from and are mostly detritivores or scavengers. There are more than 10,700 amphipod species cur ...

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. Even the size of the gymnosomes is correlated to the size of their prey, which they recognize by means of touch and grab using their sometimes-suckered buccal cones. A combination of hooks and a toothed radula are employed to scour the flesh from the thecosomes' shells. '' C. limacina'' stimulates the neuromuscular system to chase the prey, this notably increases the motion of the wings, minimizes the gap between wingtips at the end of each stroke, lessening the aspect ratio, and increases the deflection of the wings. Therefore, reducing the drag forces and overcoming stall. Also, the increment in the angle of attack along with low Reynold number for sea angels resembles the fluid dynamic forces acting on insects such as ''

Drosophila ''Drosophila'' (), from Ancient Greek δρόσος (''drósos''), meaning "dew", and φίλος (''phílos''), meaning "loving", is a genus of fly, belonging to the family Drosophilidae, whose members are often called "small fruit flies" or p ...

''. Gymnosomes slowly beat their wing-like parapodia in a rowing motion to propel their "perfectly streamlined" bodies through the upper 20 m of the water column. Although usually slow-moving, beating their wings once or twice per second, they are capable of bursts of speed when they need to pursue their prey, calling a separate suite of muscles into action to obtain the higher beat frequency.

Reproduction and development

Like many gastropods, sea angels are simultaneous

hermaphrodite A hermaphrodite () is a sexually reproducing organism that produces both male and female gametes. Animal species in which individuals are either male or female are gonochoric, which is the opposite of hermaphroditic. The individuals of many ...

s with internal fertilization. A fertilized animal later releases a gelatinous egg mass, and the eggs float freely until hatching. Their embryonic shells are lost within the first few days after hatching. The gymnosomes, like other shell-less opisthobranchs, discard their shells at metamorphosis, with the retractor muscles being severed and the shell lost. 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,

, is superficially similar to sea angels, but are not closely related; some authorities include both

and

Gymnosomata 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 pre ...

as separate branches of the

order Order, ORDER or Orders may refer to: * A socio-political or established or existing order, e.g. World order, Ancien Regime, Pax Britannica * Categorization, the process in which ideas and objects are recognized, differentiated, and understood ...

Pteropoda, whereas others list them as distinct orders within the subclass

. They have larger, broader parapodia, and most of that species retain a shell; they are commonly known as

. In the new taxonomy of Bouchet & Rocroi (2005), the

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

External links

* * list of ''Clione'' entries in the Sea Slug Forum

* 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 ArchiveReconstruction of the phylogeny of Opisthobranchia; Journal of Molluscan studies
{{Authority control Euopisthobranchia