Sea butterfly

The Thecosomata (collective/plural: thecosomes, or sea butterflies, are a taxonomic suborder of small, pelagic, free-swimming sea snails known as holoplanktonic opisthobranch gastropod mollusks, in the order Pteropoda (also included within the informal group Opisthobranchia). Most pteropods have some form of calcified shell, although it is often very light, even translucent. as their large numbers are an essential part of the ocean food chain, they are a significant contributor to the oceanic carbon cycle.

Their shells are bilaterally symmetric and can vary widely in shape, ranging from coiled or needle-like to triangular or globular.

The shell is present in all life cycle stages of the Cavolinioidea (euthecosomata). In the Cymbulioidea (pseudothecosomata), adult Peraclididae also bear shells; the Cymbuliidae shed their larval shells and develop a cartilaginous pseudoconch in adulthood. Only the Desmopteridae lack any rigid covering when mature.

Behavior and distribution

Swimming kinematics

Molluscan pteropods develop their feet into a pair of wing-like parapodia in the growing phase. These 'wings' are highly flexible, as the orientation of the muscles is different, and they have a hydrostatic skeleton filled with a pressurized fluid. Thus, the high bending-angle supports the parapodia to diminish the drag forces generated by the classic "clap-and-fling" maneuver; additionally, it aids in carrying the extra weight of a shell and ascending 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; it then rotates its 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 overall 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; the extreme shell rotation also assists in raising the wingtips at the end of each stroke to create a figure-eight pattern, common for flying insects. In contrast, flying insects and shell-less pteropods encounter higher resistance forces that limit the body rotation.

Sea butterflies range from the tropics

Occasionally, thecosomans swarm in large numbers, and can sometimes be found washed ashore in flotsam, especially along the coastline 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. Measuring up to 5 cm wide—many times larger than their own bodies. If disturbed, they simply abandon the web and flap slowly away.

Each day, they embark on a regular diel vertical migration through the water column in their pursuit of planktonic prey. At night, they forage at the surface and return to deeper waters by the morning.

Fossil record

Geologically-speaking, Thecosomata is a rather young group, being known from the Late Paleocene of the Cenozoic Era.

thumb|Sea butterfly pseudoconch

The group is known within 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. 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.

Although most Thecosomata have some form of calcified shell, mature Gymnosomata have none.

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.

Footnotes

References