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:''Not to be confused with angel sharks Sea angels (clade Gymnosomata) are a large group of small, swimming sea slugs classified into six different families. They are pelagic opisthobranchs in the clade Gymnosomata within the larger 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. ==Description== In this clade, the foot of the gastropod has developed into wing-like flapping appendages (parapodia) and their shells have been lost. 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 other suborder of pteropods, Thecosomata, is 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. ''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. 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, pteroenone. Because of this secretion, predators will not eat the sea angel, and a species of amphipod has taken 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. 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Sea angel」の詳細全文を読む スポンサード リンク
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