Sea angel
Overview
Sea angels previously known as one kind of pteropod, are a large group of small swimming sea slugs in six different families. These are pelagic marine
Marine (ocean)
Marine is an umbrella term. As an adjective it is usually applicable to things relating to the sea or ocean, such as marine biology, marine ecology and marine geology...

 opisthobranch gastropod molluscs in the clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...

 Gymnosomata within the larger clade Heterobranchia
Heterobranchia
Heterobranchia, the heterobranchs or Euthyneura, is a taxonomic clade of snails and slugs, which includes species from the sea, the land and freshwater; marine, aquatic and terrestrial gastropod mollusks....

.

Sea angels are also sometimes known as "cliones", but this is potentially misleading because the family Clionidae
Clionidae
Clionidae is a family of sea angels, which are a group of pelagic marine gastropods.They are shaped a little like angels, and have flapping "wings", hence their common name. They are gelatinous, mostly transparent pteropods, and they only have shells in their embryonic stage...

 is just one of the families within this clade.

Recent molecular data suggests that the gymnosomata form a sister group to the Thecosomata, other planktonic, weakly or non-mineralized gastropods, although this long-standing hypothesis has had some recent detractors.
In this clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...

, the foot of the gastropod has developed into wing-like flapping appendages (parapodia) and their shells have been lost.
Unanswered Questions
Encyclopedia
Sea angels previously known as one kind of pteropod, are a large group of small swimming sea slugs in six different families. These are pelagic marine
Marine (ocean)
Marine is an umbrella term. As an adjective it is usually applicable to things relating to the sea or ocean, such as marine biology, marine ecology and marine geology...

 opisthobranch gastropod molluscs in the clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...

 Gymnosomata within the larger clade Heterobranchia
Heterobranchia
Heterobranchia, the heterobranchs or Euthyneura, is a taxonomic clade of snails and slugs, which includes species from the sea, the land and freshwater; marine, aquatic and terrestrial gastropod mollusks....

.

Sea angels are also sometimes known as "cliones", but this is potentially misleading because the family Clionidae
Clionidae
Clionidae is a family of sea angels, which are a group of pelagic marine gastropods.They are shaped a little like angels, and have flapping "wings", hence their common name. They are gelatinous, mostly transparent pteropods, and they only have shells in their embryonic stage...

 is just one of the families within this clade.

Recent molecular data suggests that the gymnosomata form a sister group to the Thecosomata, other planktonic, weakly or non-mineralized gastropods, although this long-standing hypothesis has had some recent detractors.

Description

In this clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...

, the foot of the gastropod has developed into wing-like flapping appendages (parapodia) and their shells have been lost. These are both adaptations which 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 language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...

 gymnos meaning "naked" and soma meaning "body."

The other suborder of pteropods, Thecosomata, are superficially similar to sea angels but are not closely related. They have larger, broader parapodia, and most species retain a shell; they are commonly known as sea butterflies.

Sea angels are gelatinous, mostly transparent and very small, with the largest species (Clione limacina) reaching 5 cm. Clione limacina is a polar species; those found in warmer waters are far smaller. Some species of sea angel feed exclusively on sea butterflies; the angels have terminal mouths with the radula
Radula
The radula is an anatomical structure that is used by molluscs for feeding, sometimes compared rather inaccurately to a tongue. It is a minutely toothed, chitinous ribbon, which is typically used for scraping or cutting food before the food enters the esophagus...

 common to mollusks, and tentacles to grasp their prey, sometimes with suckers similar to cephalopod
Cephalopod
A cephalopod is any member of the molluscan class Cephalopoda . These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, and a set of arms or tentacles modified from the primitive molluscan foot...

s. Their "wings" allow sea angels to swim much faster than the larger (usually fused) wings of sea butterflies.

Another large polar species of sea angel, Clione antarctica
Clione antarctica
Clione antarctica is a species of a pelagic marine gastropod mollusks in the family Clionidae.- Distribution :Distribution of Clione antarctica include Southern Hemisphere in polar waters of Antarctica.- Ecology :...

, defends itself from predators by synthesizing a previously unknown molecule, named pteroenone. Because of this secretion, predators will not eat the sea angel and a species of amphipod has taken advantage of this fact: 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.

The animals are simultaneous hermaphrodite
Hermaphrodite
In biology, a hermaphrodite is an organism that has reproductive organs normally associated with both male and female sexes.Many taxonomic groups of animals do not have separate sexes. In these groups, hermaphroditism is a normal condition, enabling a form of sexual reproduction in which both...

s, and fertilization occurs internally. A gelatinous egg mass is released during spawning, and the eggs float freely until hatching. Their embryonic shells are lost within the first few days after hatching.

Development

The gymnosomes, like other shell-less opisthobranchs,e.g. Ascoglossa & Nudibranch
Nudibranch
A nudibranch is a member of what is now a taxonomic clade, and what was previously a suborder, of soft-bodied, marine gastropod mollusks which shed their shell after their larval stage. They are noted for their often extraordinary colors and striking forms...

ia
discard their shell 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 it as yet outwith our grasp).

Distribution

The organisms have a wide geographic range, from sub-sea ice polar regions to equatorial seas.

Behaviour

Gymnosomata are carnivorous, feeding only on their fellow pteropods, the thecosomata. Their lifestyle has co-evolved with that 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 beating their wing-like parapodia in a rowing motion to propel their "perfectly streamlined" bodies through the upper 20 metres 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.

Survival threats

The IPCC
Intergovernmental Panel on Climate Change
The Intergovernmental Panel on Climate Change is a scientific intergovernmental body which provides comprehensive assessments of current scientific, technical and socio-economic information worldwide about the risk of climate change caused by human activity, its potential environmental and...

 reports that increasing atmospheric carbon dioxide is causing acidification of the oceans, which could eliminate pteropods from the Southern Ocean and cause serious repercussions throughout the food chain
Food chain
A food web depicts feeding connections in an ecological community. Ecologists can broadly lump all life forms into one of two categories called trophic levels: 1) the autotrophs, and 2) the heterotrophs...

.

Taxonomy

In the new taxonomy of Bouchet & Rocroi
Taxonomy of the Gastropoda (Bouchet & Rocroi, 2005)
The taxonomy of the Gastropoda as it was revised by Philippe Bouchet and Jean-Pierre Rocroi is currently the most up-to-date overall system for classifying gastropod mollusks...

 (2005), the clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...

 Gymnosomata is arranged as follows :
  • Superfamily Clionoidea
    Clionoidea
    Clionoidea is a taxonomic superfamily of sea slugs, specifically naked pteropods, marine opisthobranch gastropod mollusks in the clade Gymnosomata...

    :
    • family Clionidae
      Clionidae
      Clionidae is a family of sea angels, which are a group of pelagic marine gastropods.They are shaped a little like angels, and have flapping "wings", hence their common name. They are gelatinous, mostly transparent pteropods, and they only have shells in their embryonic stage...

    • family Cliopsidae
      Cliopsidae
      Cliopsidae, common name sea angels, are a taxonomic family of small free-swimming sea slugs, pelagic marine opisthobranch gastropod mollusks in the order Opisthobranchia....

    • family Notobranchaeidae
      Notobranchaeidae
      Notobranchaeidae is a taxonomic family of floating sea slugs or "sea angels", pelagic marine opisthobranch gastropod mollusks.The small pelagic snails of this family have no shell...

    • family Pneumodermatidae
      Pneumodermatidae
      Pneumodermatidae is 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....

  • Superfamily Hydromyloidea
    Hydromyloidea
    Hydromyloidea is a taxonomic superfamily of sea slugs, specifically sea angels, marine opisthobranch gastropod mollusks in the order Gymnosomata.-Taxonomy:There are two families within the superfamily Hydromyloidea:* Family Hydromylidae...

    :
    • family Hydromylidae
      Hydromylidae
      Hydromylidae is a taxonomic family of small sea slugs, marine opisthobranch gastropod mollusks in the suborder Gymnosomata, the sea angels.Hydromyles species are pelagic, carnivorous and viviparous.-Description:...

    • family Laginiopsidae
      Laginiopsidae
      Laginiopsidae are a taxonomic family of floating and swimming sea snails, sea slugs or sea angels, pelagic marine opisthobranch gastropod mollusks in the suborder Gymnosomata, the sea angels....



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

In popular culture

  • The Digimon
    Digimon
    , short for , is a Japanese media franchise encompassing digital toys, anime, manga and video games. The franchise's eponymous creatures are monsters of various forms living in a "Digital World", a parallel universe that originated from Earth's various communication networks.-Conception and...

    MarineAngemon is based on the Sea Angel in appearence and name.
  • Pokémon
    Pokémon
    is a media franchise published and owned by the video game company Nintendo and created by Satoshi Tajiri in 1996. Originally released as a pair of interlinkable Game Boy role-playing video games developed by Game Freak, Pokémon has since become the second most successful and lucrative video...

    water-types Phione and Manaphy are based on the sea angel.
  • The Skies of Arcadia
    Skies of Arcadia
    Skies of Arcadia, released in Japan as , is a console role-playing game developed by Overworks for the Dreamcast and published by Sega in 2000. Skies of Arcadia Legends, a port, was released for the GameCube in 2002. Legends was also in development for the PlayStation 2; however, the PS2 version...

    monsters Ghastling, Graver, Lurker, and Ghrost resemble sea angels.

Further reading

  • Mollusca - The Southern Synthesis Order Gymnosomata by L. Newman pages 985-989; Beesley, P.L., Ross, G.J.B. & Wells, A (eds) - ISBN 0-643-05756-0

External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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