Sea angel

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Gymnosomata (sea angel)
Temporal range: Frasnian-present [1]
Sea angel.jpg
Clione limacina
Scientific classification OOjs UI icon edit-ltr.svg
Domain: Eukaryota
Kingdom: Animalia
Phylum: Mollusca
Class: Gastropoda
Subclass: Heterobranchia
Clade: Euopisthobranchia
Order: Pteropoda
Suborder: Gymnosomata
Families

Sea angels (clade 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 opisthobranchs in the clade Gymnosomata within the larger mollusc clade Heterobranchia. Sea angels were previously referred to as a type of pteropod.

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. [2] Fossils of the group go back to the Middle Frasnian stage of the Late Devonian period. [ citation needed ]

Distribution

Sea angels in Australian waters Sea angels .jpg
Sea angels in Australian waters

These organisms have a wide geographic range, from polar regions, under sea ice, to equatorial (tropic) seas. [3] 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 clade, 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".

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. [4]

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. [5]

Behavior

Gymnosomata are carnivorous, feeding only on their fellow pteropods, the Thecosomata. [3] Their lifestyles have coevolved with those of their prey, with their feeding strategy adapting to the morphology and consistency of the thecosome shell. [3]

By rowing their "wings" back and forth at 1–3 Hz, the sea angels swim at speeds up to 100 mm/s (0.22 mph). 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 sea butterflies such as hummingbirds and Limacina helicina. [6] [5] It is not yet clear whether the sea angel uses its swimming appendages as 'rowing paddles' or as 'wings'. [7]

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. [8]

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. [3] Even the size of the gymnosomes is correlated to the size of their prey, [3] which they recognize by means of touch and grab using their sometimes-suckered buccal cones. [3] A combination of hooks and a toothed radula are employed to scour the flesh from the thecosomes' shells. [3]

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 . [9]

Gymnosomes slowly beat their wing-like parapodia [3] in a rowing motion [10] to propel their "perfectly streamlined" [10] 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. [3]

Reproduction and development

Like many gastropods, sea angels are simultaneous hermaphrodites 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, [lower-alpha 1] discard their shells at metamorphosis, with the retractor muscles being severed and the shell lost. [11] 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). [12]

Taxonomy

Cladogram of sea angels according
to Peijnenburg, Janssen, et al. (2022). [13]


   Pteropoda   
  Gymnosomata  
     

  Clione antarctica

     
  (sea angels)  
   Thecosomata   

 (19 species in 13 genera) 

 (  sea butterflies  ) 

Note that the use of Pteropoda as the containing clade is traditional, but contested. Others would omit Pteropoda and use its containing clade Euopisthobranchia or subclass Heterobranchia.

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:

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

Footnotes

  1. ↑ Opisthobranchs with no shells are the gymnosomes (sea angels), the sacoglossa, and nudibranchs.

Related Research Articles

<span class="mw-page-title-main">Sea butterfly</span> Suborder of molluscs

The Thecosomata, or sea butterflies, are a taxonomic suborder of small, pelagic, free-swimming sea snails known as holoplanktonic opisthobranch gastropod mollusks, in the order Pteropoda. Most pteropods have some form of calcified shell, although it is often very light, even translucent.

<span class="mw-page-title-main">Sea slug</span> Group of marine gastropods

Sea slug is a common name for some marine invertebrates with varying levels of resemblance to terrestrial slugs. Most creatures known as sea slugs are gastropods, i.e. they are sea snails that, over evolutionary time, have either entirely lost their shells or have seemingly lost their shells due to having a significantly reduced or internal shell. The name "sea slug" is often applied to nudibranchs and a paraphyletic set of other marine gastropods without apparent shells.

<span class="mw-page-title-main">Opisthobranchia</span> Informal group of gastropods

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.

<i>Limacina</i> Genus of gastropods

Limacina is a genus of swimming predatory sea snails commonly known as sea butterflies in the family Limacinidae. This genus contains some of the world's most abundant gastropod species.

<span class="mw-page-title-main">Clionidae</span> Family of gastropods

The Clionidae are a family of sea angels, which are a group of pelagic marine gastropods.

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.

The Notobranchaeidae, or "naked sea butterflies", are a taxonomic family of floating sea slugs, specifically under the subclass Opistobranchia, also called "sea angels".

<i>Clione</i> Genus of gastropods

Clione is a genus of small, floating sea slugs, pelagic marine gastropod mollusks in the family Clionidae, the sea angels.

Paedoclione doliiformis is a species of sea angel, a small floating sea slug, a pelagic marine gastropod mollusk in the family Clionidae.

<span class="mw-page-title-main">Pteropoda</span> Order of molluscs

Pteropoda are specialized free-swimming pelagic sea snails and sea slugs, marine opisthobranch gastropods. Most live in the top 10 m of the ocean and are less than 1 cm long. The monophyly of Pteropoda is the subject of a lengthy debate; they have even been considered as paraphyletic with respect to cephalopods. Current consensus, guided by molecular studies, leans towards interpreting the group as monophyletic.

<span class="mw-page-title-main">Pterotracheoidea</span> Superfamily of molluscs

The Pterotracheoidea is, according to the Taxonomy of the Gastropoda, a taxonomic superfamily of sea snails or sea slugs, marine gastropod molluscs in the clade Littorinimorpha. They are commonly called heteropods or sea elephants.

<span class="mw-page-title-main">Cymbulioidea</span> Superfamily of gastropods

Cymbulioidea is a taxonomic superfamily of pelagic "sea butterflies", one group of swimming sea snails. They are holoplanktonic opisthobranch gastropod molluscs in the clade Thecosomata.

<span class="mw-page-title-main">Clionoidea</span> Superfamily of gastropods

Clionoidea is a taxonomic superfamily of sea slugs, specifically naked pteropods, marine opisthobranch gastropod mollusks in the clade Gymnosomata. They are sometimes called "sea angels" or "naked sea butterflies" along with the other superfamily in the Gymnosomata. They can be found anywhere from the surface to a depth of 350 meters. They are transparent and small, with the largest of the species being up to 5 cm.

Clione limacina, known as the naked sea butterfly, sea angel, and common clione, is a sea angel found from the surface to greater than 500 m (1,600 ft) depth. It lives in the Arctic Ocean and cold regions of the North Atlantic Ocean. It was first described by Friderich Martens in 1676 and became the first gymnosomatous "pteropod" to be described.

<i>Limacina helicina</i> Species of gastropod

Limacina helicina is a species of small swimming planktonic sea snail in the family Limacinidae, which belong to the group commonly known as sea butterflies (Thecosomata).

<i>Limacina rangii</i> Species of gastropod

Limacina rangii is a species of swimming sea snail in the family Limacinidae, which belong to the group commonly known as sea butterflies (Thecosomata).

Clione antarctica is a species of "sea angel", a sea slug, a pelagic marine gastropod mollusk in the family Clionidae, the "sea angels".

Limacina retroversa is a distinct species of swimming planktonic gastropods, belonging to a group of predatory sea snails known as sea butterflies (Thecosomata). The name Limacina retroversa describes the unique morphology of this sea snail, including its slug-like body and coiled, backwards-turning shell. They are typically found in the epipelagic zone of cold, polar waters, but can be found worldwide, in any ocean. L. retroversa are currently under threat, as their numbers are decreasing due to rising global carbon levels and other human-caused climate threats.

Clione okhotensis is a species of sea angel, a pelagic marine gastropod in the family Clionidae.

Clione elegantissima is a species of sea angel in the family Clionidae.

References

  1. ↑ "Order Gymnosomata Blainville 1824 (sea angel)". PBDB. Archived from the original on 2020-06-18. Retrieved 2020-06-10.
  2. ↑ Klussmann-Kolb, A.; Dinapoli, A. (2006). "Systematic position of the pelagic Thecosomata and Gymnosomata within Opisthobranchia (Mollusca, Gastropoda) - revival of the Pteropoda". Journal of Zoological Systematics and Evolutionary Research . 44 (2): 118. doi: 10.1111/j.1439-0469.2006.00351.x .
  3. 1 2 3 4 5 6 7 8 9 Seibel, B. A.; Dymowska, A.; Rosenthal, J. (2007). "Metabolic temperature compensation and coevolution of locomotory performance in pteropod molluscs". Integrative and Comparative Biology . 47 (6): 880–891. doi: 10.1093/icb/icm089 . PMID   21669767.
  4. ↑ Karakas, F., Wingate, J., Blanco-Bercial, L., Maas, A. E., & Murphy, D. W. (2020). Swimming and sinking behavior of warm water pelagic snails. Frontiers in Marine Science, 749.
  5. 1 2 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.
  6. ↑ Murphy, D. W., Adhikari, D., Webster, D. R., & Yen, J. (2016). Underwater flight by the planktonic sea butterfly. Journal of Experimental Biology, 219(4), 535-543.
  7. ↑ Murphy, D.; Adhikari, D.; Webster, D.; Yen, J. (2016). "Underwater flight by the planktonic sea butterfly". Journal of Experimental Biology . 219 (4): 535–543. doi: 10.1242/jeb.129205 . PMID   26889002.
  8. ↑ Borrell, B. J., Goldbogen, J. A., & Dudley, R. (2005). Aquatic wing flapping at low Reynolds numbers: swimming kinematics of the Antarctic pteropod, Clione antarctica. Journal of Experimental Biology, 208(15), 2939-2949.
  9. ↑ Szymik, B. G., & Satterlie, R. A. (2011). Changes in wingstroke kinematics associated with a change in swimming speed in a pteropod mollusk, Clione limacina. Journal of Experimental Biology, 214(23), 3935-3947.
  10. 1 2 Morton, J. E. (2009). "Observations on the Gymnosomatous Pteropod Clione Limacina (Phipps)" (PDF). Journal of the Marine Biological Association of the United Kingdom . 37 (2): 287–297. doi:10.1017/S0025315400023687. S2CID   86798952. Archived (PDF) from the original on 2017-09-22. Retrieved 2019-07-12.
  11. ↑ Gibson, Glenys D. (2003). "Larval Development and Metamorphosis in Pleurobranchaea maculata, with a Review of Development in the Notaspidea (Opisthobranchia)". Biological Bulletin . 205 (2): 121–132. doi:10.2307/1543233. JSTOR   1543233. PMID   14583510. S2CID   24629524. Archived from the original on 2011-06-16. Retrieved 2010-06-04.
  12. ↑ Janssen, A.W. (2003). "Notes on the systematics, morphology and biostratigraphy of fossil holoplanktonic Mollusca, 13. Considerations on a subdivision of Thecosomata, with the emphasis on genus group classification of Limacinidae". Cainozoic Research. 2 (1–2): 163–170.
  13. ↑ Peijnenburg, Katja T.C.A.; Janssen, Arie W.; Wall-Palmer, Deborah; Goetze, Erica; Maas, Amy E.; Todd, Jonathan A.; Marlétaz, Ferdinand (2020-09-24). "The origin and diversification of pteropods precede past perturbations in the Earth's carbon cycle". Proceedings of the National Academy of Sciences . 117 (41): 25609–25617. Bibcode:2020PNAS..11725609P. doi: 10.1073/pnas.1920918117 . ISSN   0027-8424. PMC   7568333 . PMID   32973093.

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