This is a very small Class, comprising a few species of curious structure. They are all of diminutive size, and all swim in the open ocean, rarely approaching the shore, except when washed thither by accident. They are all characterised by having a membranous expansion, resembling a large fin, on each side of the head. By means of these organs, the little Pteropod rows itself about in the open sea perpetually; being unfurnished with any means of crawling, or of affixing itself to any solid body. Some of these animals, as the genera Hyalæa and Cleodora for example, have the body enclosed in a shell of elegant form, and of a texture resembling the thinnest glass, for delicacy and transparency. The Cleodora pyramidata, one of the species of the latter genus, is of extreme delicacy and beauty. The shell is glassy and colourless; very fragile; nearly in the form of a triangular pyramid; with an aperture at its base, from which proceeds a long and slender glassy spine; and a similar spine projects from each side of the middle of the shell. The hinder part of the animal is globular and pellucid, and in the dark vividly luminous, presenting a singularly striking appearance, as it shines through its perfectly transparent lantern. Both of these are found floating in great numbers on the surface of the tropical sea.
Others are entirely destitute of a shelly covering, as is that little species which occurs in enormous profusion in the Arctic Seas, and which we now proceed to describe.
Genus Clio("Whale-food.")
These little creatures have an oblong membranous body, without a mantle; a head formed of two rounded lobes, each of which is furnished with three long tentacles, capable of being withdrawn into a fold of skin, or protruded at pleasure. The mouth, which is terminal, has two small fleshy lips; and two eyes, of elaborate structure, are placed at the back of the neck.
The species best known is that which is commonly called by our northern voyagers, Whale-food (Clione limacina). Though not more than an inch in length, it occurs in such countless millions as to form the principal part of the nourishment required by the most gigantic of living creatures. The Clio bears some slight resemblance to a butterfly just emerged from the chrysalis, before the wings are expanded. Near the head there is on each side a large fin or wing, by the motions of which it changes its place.These motions are amusing; and as the little creatures are so abundant, they make the dreary sea quite alive with their gambols as they dance merrily along. In swimming, the Clio brings the tips of its fins almost into contact, first on one side, then on the other. In calm weather they rise to the surface in myriads, for the purpose of breathing; but scarcely have they reached it before they again descend into the deep. Mr. Scoresby kept several of them alive in a glass of sea-water for about a month, when they gradually wasted away and died. The head of one of these little creatures exhibits a most astonishing display of the wisdom of God in creation. Around the mouth are placed six tentacles, each of which is covered with about three thousand red specks, which are seen by the microscope to be transparent cylinders, each containing about twenty little suckers, capable of being thrust out, and adapted for seizing and holding their minute prey.
Thus, therefore, there will be three hundred and sixty thousand of these microscopic suckers upon the head of one Clio: an apparatus for prehension perhaps unequalled in the creation.
-Philip Henry Gosse
Pteropoda (common name pteropods, from the Greek meaning "wing-foot") 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.[1] Current consensus, guided by molecular studies, leans towards interpreting the group as monophyletic.[2]
Pteropoda encompasses the two clades Thecosomata, the sea butterflies, and Gymnosomata, the sea angels. The Thecosomata (lit. "case-body"[3]) have a shell, while the Gymnosomata ("naked body") do not. The two clades may or may not be sister taxa; if not, their similarity (in that they are both pelagic, small, and transparent, and both groups swim using wing-like flaps (parapodia) which protrude from their bodies) may reflect convergent adaptation to their particular lifestyle.
The group Pteropoda was established by Cuvier as "ptéropodes" in 1804.[4] François Péron and Charles Alexandre Lesueur thought the group to be larger, and so they also included the opisthobranch taxa (Phyllirhoë and Glaucus), the heteropoda taxa (Carinaria and Firola), and even the Ctenophora (Callianira). In 1810, these authors divided the whole group in two separate groups: Those with a shell and those without a shell.
In 1824, H.M.D. de Blainville named these two groups Gymnosomata and Thecosomata and named the combining order Aporobranchia instead of Pteropoda.[5] He rejected the additional genera, except Phyllirhoë which he upgraded to a third group that he called Psilosomata. Only much later was Phyllirhoë classified within the order Nudibranchia.
Other attempts were made to describe the Pteropoda: J.E. Gray divided the Pteropoda into Dactylobranchia (with just the genus Cavolinia) and Pterobranchia (including all the other genera).[6] Cuvier (and his followers) did not accept the classification by de Blainville; they preferred the original classification as described in Le Règne Animal.
Rang (1829) followed the Cuvierian classification but tried to include the character of having a distinct head or not.[7] The German naturalist L. Oken went one step further and, for the sake of symmetry, wanted each order to contain four families and each family to contain four genera.[8] P.A. 1829, divided the Pteropoda according to the size of their fins: "Macroptérygiens" (including only Pneumonoderma) and "Microptérygiens" (including all the others). W.B. Clark (1829) treated the Pteropoda as a family and emended the spelling to Pteropodidae (a name now re-used for a family of fruit bats)
Finally, all these attempts were abandoned and, as more and more species were described as a result of several scientific expeditions, the classification of the Pteropoda into Thecosomata and Gymnosomata was generally adopted.[a]
The relationship between these two clades is not unequivocally established, but it seems that they are sister taxa.[2]
Pteropods are estimated to have originated during the Early Cretaceous, around 133 million years ago, with the diversification into the major lineages occurring during the mid-late Cretaceous. The oldest known fossil pteropod is a member of Limacinidae from the early-middle Campanian deposits of the San Juan Islands.[9][10]
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A study was conducted on the West Coast of the United States to see ocean acidification's effects on pteropods.[12] Limacina helicina was used to test the sensitivity to decreasing pH.[12] This species of pteropod is potentially vulnerable to the corrosive waters associated with ocean acidification due to their calcium carbonate shell.[13] The shell of a pteropod was immersed in ocean water with the projected pH level that the water will reach by the year 2100. After a month and a half in the water, the shell had almost completely dissolved.[12]
Pteropods are found in all major oceans, usually 0–10 metres (0–33 ft) below the ocean surface and in all levels of latitude. Pteropods can be found lower than 10 meters, but in less amounts in terms of biomass, however, pteropod distribution is more spread out deeper based on findings. This can be explained as Pteropods from tropical areas become more common in deeper areas. They are not found commonly in the deep sea, in fact, few live lower than 500 meters below sea level. Continental shelves, areas containing many opportunities for nutrients, and productivity are locations in which Pteropods are usually populous, according to patterns in data. Springtime is a peak season for pteropoda, as they reach higher populations, though data shows that pteropoda south of the equator are less abundant seasonally. In addition, current data suggests that 93% of the world's pteropods are part of the Thecosomata family, while the 7% are Gymnosomata.[14]
Pteropoda (common name pteropods, from the Greek meaning "wing-foot") 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.
Pteropoda encompasses the two clades Thecosomata, the sea butterflies, and Gymnosomata, the sea angels. The Thecosomata (lit. "case-body") have a shell, while the Gymnosomata ("naked body") do not. The two clades may or may not be sister taxa; if not, their similarity (in that they are both pelagic, small, and transparent, and both groups swim using wing-like flaps (parapodia) which protrude from their bodies) may reflect convergent adaptation to their particular lifestyle.
