Sea Angels
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Sea angels (
clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...
Gymnosomata) are a large group of small free-swimming sea slugs, not to be confused with Cnidarians (Jellyfish and other similar creatures), classified into six different families. They are pelagic
opisthobranch Opisthobranchs () is now an 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. Euopisth ...
s in the clade Gymnosomata within the larger
mollusc Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000  extant species of molluscs are recognized. The number of fossil species is esti ...
clade Heterobranchia. Sea angels were previously referred to as a type of pteropod. Sea angels are also sometimes known as "cliones" but this is potentially misleading because the family Clionidae is just one of the families within this clade. 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. Fossils of the group go back to the Middle Frasnian stage of the
Late Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, wher ...
period.


Distribution

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


Description

In this
clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...
, 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 ''gymnos'' meaning "naked" and ''soma'' meaning "body". Sea angels are gelatinous, mostly transparent, and very small, with the largest species (''
Clione limacina ''Clione limacina'', known as the naked sea butterfly, sea angel, and common clione, is a sea angel (pelagic sea slug) found from the surface to greater than depth.Gofas, S. (2012). ''Clione limacina''. Accessed through: World Register of Marine ...
'') 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 The radula (, ; plural radulae or radulas) is an anatomical structure used by molluscs 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 ...
common to mollusks, and tentacles to grasp their prey, sometimes with suckers similar to
cephalopod A cephalopod is any member of the molluscan class Cephalopoda (Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head ...
s. 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. It is not yet clear whether the sea angel uses its swimming appendages as 'rowing paddles' or as 'wings'. Another large polar species of sea angel, ''
Clione antarctica ''Clione antarctica'' is a species of "sea angel", a sea slug, a pelagic marine gastropod mollusk in the family Clionidae, the "sea angels". Distribution The distribution of ''Clione antarctica'' is within the Southern Hemisphere, in the p ...
'', defends itself from predators by synthesizing a previously unknown 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 9,900 amphipod species so far descr ...
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.


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. 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. 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 In reproductive biology, a hermaphrodite () is an organism that has both kinds of reproductive organs and can produce both gametes associated with male and female sexes. Many Taxonomy (biology), taxonomic groups of animals (mostly invertebrate ...
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, 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 Order, ORDER or Orders may refer to: * Categorization, the process in which ideas and objects are recognized, differentiated, and understood * Heterarchy, a system of organization wherein the elements have the potential to be ranked a number of d ...
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 A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...
Gymnosomata is arranged as follows: *Superfamily
Clionoidea Clionoidea is a taxonomic superfamily of sea slugs, specifically naked (i.e. unshelled) pteropods, marine opisthobranch gastropod mollusks in the clade Gymnosomata. They are sometimes called "sea angels" or "naked sea butterflies" along with t ...
: **family Clionidae **family
Cliopsidae The Cliopsidae, common name sea angels, are a taxonomic family of small, free-swimming sea slugs, pelagic marine opisthobranch gastropod mollusks in the order Opisthobranchia. As is the case in all gymnosome pteropods, these sea angels ...
**family
Notobranchaeidae The Notobranchaeidae, or "naked sea butterflies", are a taxonomic family of floating sea slugs, specifically under the subclass Opistobranchia, also called "sea angels". Similar to other Pteropods, these pelagic marine heterobranch gastropod mo ...
**family
Pneumodermatidae The Pneumodermatidae are a family of sea angels, or small floating predatory sea snails or sea slugs. They are pelagic marine heterobranch opisthobranch gastropod mollusks in the clade Gymnosomata. These small pelagic snails lack shells (excep ...
*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) -


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
{{Taxonbar, from=Q1315611 Euopisthobranchia