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= Sea Butterfly Swimming = Sea butterflies, also called pteropods or Thecosome with scientific name ,are a small pelagic swimming sea snail. Sea butterflies are holoplanktonic gastropod mollusks that spend their entire life in water column without touching any solid surfaces. They include some of the world's most abundant gastropod species. Sea butterflies have a calcified shell which is sensitive to ocean acidification. Sea butterfly shells can vary widely in shape such as coiled, needle-like, triangular, globules and bottle shaped. Because of the rock-like heavy shells sea butterflies are negatively buoyant in nature. They are rather difficult to observe, since the shell is mostly colorless, very fragile and usually less than 1 cm in length. So, in order to sustain their pelagic life style, they must flap their wing like appendages to swim actively. Sea butterflies have a pair of wings like parapodia used for swimming by flapping like insects. Sea butterflies have a very complex wing stroke kinematics and fluid dynamics around their wings and body .

Vertical Distribution and Diel vertical migration
Sea butterflies could be found oceans around the world. Sea butterflies are distributing up to few hundred depth in ocean, but highest abundances occurred consistently in 0-25 m layer in both night and day and abundance decreased with depth in general. Day and night distribution were significant, but difference in depth intervals were not significant. The local community structure of the pteropods were determined largely by vertical migrations. Diel vertical migration (DVM), also known as diurnal vertical migration, is a pattern of movement used by some organisms, such as copepods, living in the ocean and in lakes. Sea Butterflies also do DVM, they swim up close to the sea surface at night to feed on the Phytoplankton and sink to much deeper and darker area to avoid from their visual predators. DVM length is up to few hundred meters which is thousands of body length distance of sea butterflies when considering sea butterflies have body length of sub-centimeter. Every day, they migrate vertically in the water column, following their planktonic prey. At night they hunt at the surface and return to deeper water in the morning. Thus swimming is vital for their survival in the ocean.

Swimming Mechanism
Although sea butterflies and insect are very different, the physics underlying the fluid dynamics force generations are very similar . Sea butterflies have much less wing beat frequency than insects, besides sea butterfly flow medium (water) are big advantages in terms of study the fluid dynamics and also explore the physics of the unsteady fluid dynamics in general by using high speed video cameras and particle image velocimetry techniques. Most interests in research were shown to insects in literature maybe because the animals are easy to access than pteropods. Most of the unsteady fluid mechanics principles were studied either insects or models that mimics insects. The similarity between insect aerodynamics and sea butterfly hydrodynamics naturally links ones attention and look for evidences that already well studied so far, thus to borrow knowledge from insects to study more about the swimming mechanism of the sea butterfly.

Kiørboe at. al. (2014) studied zooplankton swimming. They found that across taxa and sizes dedicated swimming produces flow fields with a much smaller spatial extension and a faster spatial attenuation than those produced by the plankton for which feeding, and swimming are integrated. For planktons that swim to relocate, propulsion has been optimized to minimize the fluid disturbance. For plankton in which swimming is constrained by a simultaneous need to feed, the fluid disturbance generated is many folds higher with a consequently higher risk of being detected by a predator. Predation is a strong selective agent in shaping the motility and propulsion strategy of zooplankton.

Role of Leading Edge Vortex
Leading edge vortex (LEV) plays great role in lift generation on insect flight. LEV plays larger effects and gives enough force to the insects fly. Force was underestimated based on the inviscid theory. Since the flight in nature is always 3D, spanwise vortex create tip vortex loop that help to stabilize the LEV, thus enhance overall lift generation further. Thus, insect can generate more force than its weight. Study on swimming behavior of Clione limacina, which is a negatively buoyant sea snail, shows that Their wing tips nearly touch or slightly overlap in the sagittal plane at the end of each half-cycle of the wing beat. C. limacia wing tip trajectory forms a horizontal figure-of-eight and wing movements produce lift in both upstroke and downstroke by actively adjusting wing orientation in both strokes. However, in Clione wing flapping, there is no ‘clap-and-fling’ of two wing surfaces like insects do, instead the wings do a version of ‘clap-and-fling’ mechanism with the animal body. The anterior part of the wings moves away from the body before the posterior part and presumably a leading-edge vortex is formed, ensuring immediate lift generation and stabilizing the wings against the stall that would otherwise be likely at the high angles of attack seen in Clione. It seems that the LEV created during the 'fling' is shed off from the wing upper surface and forced posteriorly, completing the downward motion of a complete vortex ring around the animal to generate hydrodynamic force.

However, not all researchers agree with the LEV based lift generation. Borrell et. al. (2005) claimed that Clione antarctica generates thrust primarily drag based propulsion (by rowing).

There are also some researchers believe marine snails use both lift based and drag based propulsion to swim. Szymik and Satterlie (2011) performed experimental study on Clione limacine which is a shell-less negatively buoyant marine snail. C. limacine wing kinematics are highly consistent form wingbeat to wingbeat, but it changes a lot between slow and fast speed. A significant increase in wing velocity and angle of attack when the animal changes from slow swimming speed to fast swimming speed were observed. C. limacine may use unsteady fluid mechanics features such as leading-edge vortex and wake recapture to generate fluid dynamic forces because of their use of high angle of attacks. However, based on the wing orientation Szymik and Satterlie (2011) conclude that C. limacine may also use drag-based propulsion with a combination of lift-based propulsion.

Clap-and-Fling
clap and fling mechanism or also called Weis-Fogh mechanism of lift generation by Torkel Weis-Fogh

Chang and Yen (2012) conclude that Limacina helicina ascend along a sawtooth trajectory in mostly linear and sometimes helical paths. The repertoire of movements indicates that elements of both rowing and flying are incorporated in the swimming of L. helicina with the added element of rotation. Clap-and-fling mechanism is applied during the stroke cycle. Limacina helicina has inverse relationship between wingbeat frequency and body size. Murphy at. al. (2016) studied Limacina helicina swimming by using 3D particle image velocimetry system. They conclude that L. helicina stroke its wings in a characteristic figure-of-eight pattern by extreme rotation of its body to produce lift. Generate extra lift by fling its wings (the well-known Weis-Fogh ‘clap-and-fling’ mechanism). Adhikari at. al. (2016) studied 	Limacina helicina antarctica which is negatively buoyant shelled snail, swimming showed the saw tooth like trajectory. They found that during power stroke attached vortex ring connecting the leading- and trailing edge vortex of the wing. During the recovery stroke leading edge vortex only formed. These vortices are crucial for pteropod force generation and enhancement. The ocean acidification may negatively effects on shelled pteropod swimming. Karakas at. al. (2018) studied 	Atlanta selvagensis which is a shelled heteropod. They found that heteropod flaps its shell as a second swimming appendage in synchrony with its fin to propel the animal (generate forces). Unique swimming mechanism by two dissimilar appendages.

Lower End Intermediate Reynolds Number Flow Regime
At low Reynolds number flow, both the inertia force and viscous force acting on the moving object become important and cannot be neglected like in high Reynolds number moving objects, i.e. airplane, large birds and large swimmers. Santhanakrishnan at. al. (2018) studied flow structure around rotating wing at low Reynolds number regime. They showed that there are drastic differences in the aerodynamics of flight at the scale of the smallest flying animals. At Reynolds number less than 100 regime, viscous force dominates LEV stabilization over spanwise flow.

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