| Sea angel | |
|---|---|
| Clione limacina | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Mollusca |
| Class: | Gastropoda |
| (unranked): | clade Heterobranchia clade Euthyneura clade Euopisthobranchia clade Gymnosomata |
| Families | |
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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 opisthobranch gastropod molluscs in the clade Gymnosomata within the larger clade Heterobranchia.
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 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.[1]
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In this clade, 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 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 common to mollusks, and tentacles to grasp their prey, sometimes with suckers similar to cephalopods. 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, 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 hermaphrodites, 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.
The gymnosomes, like other shell-less opisthobranchs,[note 1] discard their shell at metamorphosis, with the retractor muscles being severed and the shell lost.[2] 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).[3]
The organisms have a wide geographic range, from sub-sea ice polar regions to equatorial seas.[4]
Gymnosomata are carnivorous, feeding only on their fellow pteropods, the thecosomata.[4] Their lifestyle has co-evolved with that of their prey, with their feeding strategy adapting to the morphology and consistency of the thecosome shell.[4]
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.[4] Even the size of the gymnosomes is correlated to the size of their prey,[4] which they recognize by means of touch and grab using their sometimes-suckered buccal cones.[4] A combination of hooks and a toothed radula are employed to scour the flesh from the thecosomes' shells.[4]
Gymnosomes slowly beating their wing-like parapodia[4] in a rowing motion[5] to propel their "perfectly streamlined"[5] 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.[4]
The IPCC 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.[6]
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.