Generating thrust Foil shaped
fins generate
thrust when moved, the lift of the fin sets water or air in motion and pushes the fin in the opposite direction. Aquatic animals get significant
thrust by moving fins back and forth in water. Often the
tail fin is used, but some aquatic animals generate thrust from
pectoral fins. Cavitation damage can occur to the tail fins of powerful swimming marine animals, such as dolphins and tuna. Cavitation is more likely to occur near the surface of the ocean, where the ambient water pressure is relatively low. Even if they have the power to swim faster, dolphins may have to restrict their speed because collapsing cavitation bubbles on their tail are too painful. Cavitation also slows tuna, but for a different reason. Unlike dolphins, these fish do not feel the bubbles, because they have bony fins without nerve endings. Nevertheless, they cannot swim faster because the cavitation bubbles create a vapor film around their fins that limits their speed. Lesions have been found on tuna that are consistent with cavitation damage. Fish use multiple fins, so it is possible that a given fin can have a hydrodynamic interaction with another fin. In particular, the fins immediately upstream of the caudal (tail) fin may be proximate fins that can directly affect the flow dynamics at the caudal fin. In 2011, researchers using
volumetric imaging techniques were able to generate "the first instantaneous three-dimensional views of wake structures as they are produced by freely swimming fishes". They found that "continuous tail beats resulted in the formation of a linked chain of vortex rings" and that "the dorsal and anal fin wakes are rapidly entrained by the caudal fin wake, approximately within the timeframe of a subsequent tail beat".
Controlling motion Once motion has been established, the motion itself can be controlled with the use of other fins. The bodies of
reef fishes are often shaped differently from
open water fishes. Open water fishes are usually built for speed, streamlined like torpedoes to minimise friction as they move through the water. Reef fish operate in the relatively confined spaces and complex underwater landscapes of
coral reefs. For this manoeuvrability is more important than straight line speed, so coral reef fish have developed bodies which optimise their ability to dart and change direction. They outwit predators by dodging into fissures in the reef or playing hide and seek around coral heads. The pectoral and pelvic fins of many reef fish, such as
butterflyfish,
damselfish and
angelfish, have evolved so they can act as brakes and allow complex manoeuvres. Many reef fish, such as
butterflyfish,
damselfish and
angelfish, have evolved bodies which are deep and laterally compressed like a pancake, and will fit into fissures in rocks. Their pelvic and pectoral fins have evolved differently, so they act together with the flattened body to optimise manoeuvrability. When ready for mating, the gonopodium becomes erect and points forward towards the female. The male shortly inserts the organ into the sex opening of the female, with hook-like adaptations that allow the fish to grip onto the female to ensure impregnation. If a female remains stationary and her partner contacts her vent with his gonopodium, she is fertilised. The sperm is preserved in the female's oviduct. This allows females to fertilise themselves at any time without further assistance from males. In some species, the gonopodium may be half the total body length. Occasionally, the fin is too long to be used, as in the "lyretail" breeds of
Xiphophorus helleri. Hormone treated females may develop gonopodia. These are useless for breeding. Similar organs with similar characteristics are found in other fishes, for example the
andropodium in the
Hemirhamphodon or in the
Goodeidae or the
gonopodium in the
Middle Triassic Saurichthys, the oldest known example of
viviparity in a ray-finned fish.
Claspers are found on the males of
cartilaginous fishes. They are the posterior part of the pelvic fins that have also been modified to function as intromittent organs, and are used to channel semen into the female's
cloaca during copulation. The act of mating in sharks usually includes raising one of the claspers to allow water into a siphon through a specific
orifice. The clasper is then inserted into the cloaca, where it opens like an umbrella to anchor its position. The siphon then begins to contract expelling water and sperm.
Other functions Other uses of fins include walking and perching on the sea floor, gliding over water, cooling of body temperature, stunning of prey, display (scaring of predators, courtship), defence (venomous fin spines, locking between corals), luring of prey, and attachment structures. The
Indo-Pacific sailfish has a prominent dorsal fin. Like
scombroids and other
billfish, they streamline themselves by retracting their dorsal fins into a groove in their body when they swim. The
oriental flying gurnard has large pectoral fins which it normally holds against its body, and expands when threatened to scare predators. Despite its name, it is a
demersal fish, not a flying fish, and uses its pelvic fins to walk along the bottom of the ocean. Fins can have an adaptive significance as sexual ornaments. During courtship, the female
cichlid,
Pelvicachromis taeniatus, displays a large and visually arresting purple
pelvic fin. "The researchers found that males clearly preferred females with a larger pelvic fin and that pelvic fins grew in a more disproportionate way than other fins on female fish." ==Evolution==