Sea butterfly

Sea butterfly
Sea butterflies, scientific name Thecosomata (thecosomes, "case / shell-body"), are a taxonomic suborder of small pelagic swimming sea snails. They are holoplanktonic opisthobranch gastropod mollusks. Most Thecosomata have some form of calcified shell, although it is often very light and / or transparent.
The sea butterflies include some of the world's most abundant gastropod species,
and because of their large numbers are an essential part of the food chain, and a significant contributor to the oceanic carbon cycle.
The sea butterflies are included in the Pteropoda order, and are also included in the informal group Opisthobranchia.
Morphology
Sea butterflies float and swim freely in the water, and are carried along with the currents. This has led to a number of adaptations in their bodies. The shell and the gill have disappeared in several families. Their gastropodal foot has taken the form of two wing-like lobes, or parapodia, which propel the animal through the sea by slow flapping movements.
Most Thecosomata have some form of calcified shell, although often very light.
They are rather difficult to see, since their shell (if present) is mostly transparent, very fragile, and usually tiny: Less than 1 cm in length. Although their shell may be so fine as to be transparent, it is nevertheless calcareous, and an important part of the ocean calcite cycle.
Their shells are bilaterally symmetric and can vary widely in shape: coiled, needle-like, triangular, or globular.
The shell is present in all stages of the Cavolinioidea (euthecosomata) life cycle. In the Cymbulioidea (pseudothecosomata), adult Peraclididae also bear shells; the Cymbuliidae shed their larval shells and develop a cartilaginous pseudoconch in adulthood; only the Desmopteridaen lack any rigid covering as adults.
Behavior and distribution
Swimming Kinematics
Molluscan pteropods develop their feet into a pair of wing-like (parapodia) in the growing phase. The wings are highly flexible because the orientation of the muscles is different and have a hydrostatic skeleton filled with pressurized fluid. Therefore, the high bending angle supports the parapodia to diminish the drag forces generated by the classic clap-and-fling maneuver; it also, helps to carry the extra weight of the shell and ascend in the water column for the diel vertical migration.
The power stroke for L. helicina starts with a sharp rotation of it is body accompanied by an increase in swimming speed, then L. helicina rotates the shell in the opposite direction to initiate the recovery stroke and swims upward with a speed less than the power phase. There is a drop in swimming speed between power and recovery strokes which develop a sawtooth trajectory in the sagittal plane. The hyper-pitch of the round shell of L. helicina diminishes the rotational drag and the moment of inertia; also, the extreme shell rotation assists in raising the wingtips at the end of each stroke to create a figure-of-eight pattern, which is common for aerial insects. In contrast, flying insects and shell-less pteropods encounter higher resistance forces that limit the body rotation.
Sea butterflies range from the tropics to the poles.
They are "holoplanktonic": That is, they spend their whole lives floating among the plankton, rather than just being planktonic during their larval stage. As such, the thecosomata are the most common (in terms of diversity, species richness, and abundance) in the top of the ocean, and become rarer the deeper one samples.
Sometimes, they swarm in large numbers and can be found washed up in flotsam, especially along the coast of eastern Australia.
Diurnal vertical migration
Thecosomata beat their wing-like parapodia to "fly" through the water.
When descending to deeper water, they hold their wings up.
They migrate vertically from day to night, so the community structure changes on a 24 hour cycle; during the day many organisms take refuge at water depths in excess of 100 m.
Feeding
Little is known about the behaviour of sea butterflies, but they are known to have a peculiar way of feeding.
They are generally herbivorous, mostly passive plankton feeders, just floating along with the currents, ventral-side up, although some may become active feeders at times.
They catch planktonic food by entangling it in a mucous web that can be up to 5 cm wide – many times larger than themselves. If disturbed, they abandon the net and flap slowly away.
Every day, they migrate vertically in the water column, following their planktonic prey. At night they graze at the ocean surface and return to deeper water in the morning.
Fossil record
This is, geologically speaking, a rather young group, having evolved from the Late Paleocene in the Cenozoic Era.
The group is represented in the fossil record from shells of those groups within the clade that mineralized.
These carbonate shells are a major contributor to the oceanic carbon cycle, making up as much as 12% of global carbonate flux. However the low stability of their aragonitic shells means that few end up being preserved in sediments as fossils, mostly being deposited in shallow waters of tropical seas.
Importance in the food chain
These creatures, which range from lentil- to orange-sized, are eaten by various marine species, including a wide variety of fish that are, in turn, consumed by penguins and polar bears. The sea butterflies form the sole food source of their relatives, the Gymnosomata. They are also consumed by sea birds, whales, and commercially important fish. However, if sea butterflies are consumed in large quantities fish can get "black gut", which makes them unsellable.
Taxonomy
Along with its sister group, the sea angels (Gymnosomata), the sea butterflies (Thecosomata) are included in the order Pteropoda.
The validity of the pteropod order is not universally accepted; it fell out of favour for a number of years, but recent molecular evidence suggests that the taxon should be revived.
Although most Thecosomata have some form of calcified shell, mature Gymnosomata have none.
Ponder & Lindberg
Order Thecosomata de Blainville, 1824
Infraorder Euthecosomata
Superfamily Limacinoidea
Family Limacinidae de Blainville, 1823
Superfamily Cavolinioidea
Family Cavoliniidae H. and A. Adams, 1854
Family Cliidae
Family Creseidae
Family Cuvierinidae
Family Praecuvierinidae
Infraorder Pseudothecosomata
Superfamily Peraclidoidea
Family Peraclidae Tesch, 1913
Superfamily Cymbulioidea
Family Cymbuliidae Gray, 1840
Family Desmopteridae Dall, 1921
Bouchet & Rocroi
In the new taxonomy of Bouchet & Rocroi (2005) Thecosomata is treated differently :
Clade Thecosomata :
Superfamily Cavolinioidea Gray, 1850 (= Euthecosomata)
Family Cavoliniidae Gray, 1850 (1815)
Subfamily Cavoliinae Gray, 1850 (1815) (formerly Hyalaeidae Rafinesque, 1815 )
Subfamily Clioinae Jeffreys, 1869 (formerly Cleodoridae Gray, 1840 - nomen oblitum)
Subfamily Cuvierininae van der Spoel, 1967 (formerly : Cuvieriidae Gray, 1840 (nom. inv.); Tripteridae Gray, 1850 )
Subfamily Creseinae Curry, 1982
Family Limacinidae Gray, 1840 (formerly : Spirialidae Chenu, 1859 ; Spiratellidae Dall, 1921 )
† Family Sphaerocinidae A. Janssen & Maxwell, 1995
Superfamily Cymbulioidea Gray, 1840 (= Pseudothecosomata)
Family Cymbuliidae Gray, 1840
Subfamily Cymbuliinae Gray, 1840
Subfamily Glebinae van der Spoel, 1976
Family Desmopteridae Chun, 1889
Family Peraclidae Tesch, 1913 (formerly Procymbuliidae Tesch, 1913
Bouchet & Rocroi (2005) move the family Limacinidae into the superfamily Cavolinioidea, making redundant the superfamily Limacinoidea erected for it in Ponder & Lindberg's taxonomy. The families Creseidae and Cuvierinidae are demoted to subfamilies of Cavoliniidae (Creseinae and Cuvierininae). The infraorder Pseudothecosomata becomes the superfamily Cymbulioidea. The family Peraclididae is included in the superfamily Cymbulioidea as the family Peraclidae, making the superfamily Peraclidoidea redundant.
See also
Ocean acidification
Clione antarctica
Footnotes
References
Sources
Euopisthobranchia
Extant Thanetian first appearances