Konrad Wothe / Small Striped Swordtail (Graphium policenes) and Nymphalid Butterfly (Nymphalidae) feeding on flower nectar, Mahale Mountains National Park, Tanzania

Konrad Wothe / Small Striped Swordtail (Graphium policenes) butterflies sipping minerals on shore, Lake Tanganyika, Mahale Mountains National Park, Tanzania

Chien Lee / Fivebar Swordtail (Graphium antiphates) and Common Jay (Graphium doson) butterflies feeding on minerals on riverbank, Gunung Mulu National Park, Malaysia

Luiz Claudio Marigo / Periander metalmark / Periander swordtail butterfly (Rhetus periander) sucking water with mineral salts from the ground on the margin of Cristalino River, Alta Floresta, Mato Grosso State, Brazil.

Dave Bevan / Green swordtail (Xiphophorus helleri) male, captive

Jane Burton / Swordtail {Xiphophorus helleri} male Tuxedo lyre-tail displaying to female Red lyre-tail in foreground, captive, from Central America

Jane Burton / Green swordtail {Xiphophorus helleri} and Bronze catfish {Corydorus aeneus} coexist in some parts of their range, captive, from central america

Jane Burton / Red swordtail {Xiphophorus helleri} males sparring, swimming backwards at each other with bodies arched and tails displayed, females in the foreground, captive, from Central America

Jane Burton / Red swordtail {Xiphophorus helleri} male chasing female, captive, from Central America

Jane Burton / Green swordtail {Xiphophorus helleri} female with young fry sinking away below her to avoid being eaten, captive, from Central America, sequence 3/3

Jane Burton / Close up of gonopodium of male Red swordtail {Xiphophorus helleri} captive

Jane Burton / Green swordtail {Xiphophorus helleri} female jerking away shortly after giving birth to baby fry, captive, from Central America, sequence 2/3

Jane Burton / Green swordtail {Xiphophorus helleri} male with young females in the background, this male started life as a female and, after giving birth to several broods, underwent a sex change to become a fully...

Jane Burton / Swordtail {Xiphophorus helleri} 'Sunset' male with mosquito larvae on which this species feeds, captive, from Central America

Jane Burton / Swordtail {Xiphophorus helleri} one-day-old young fry, captive, from Central America

Jane Burton / Green swordtail {Xiphophorus helleri} female giving birth to live young, captive, from Central America, sequence 1/3

Jane Burton / Green swordtail fish {Xiphophorus helleri} in Central American coastal stream tropical fish tank, captive

Jane Burton / Swordtail {Xiphophorus helleri} male Tuxedo lyre-tail (below) and female Red lyre-tail (above), captive, from Central America

Jane Burton / Swordtail {Xiphophorus helleri} Green 'Sunset' female feeding on mosquito larvae with 'Sunset' male below, captive, from Central America

Jane Burton / Green Swordtail {Xiphophorus helleri} male and female (with swollen belly), captive, from Central America

Jane Burton / Red Swordtail fish (Xiphophorus helleri) male chasing a female, captive, UK

Thomas Marent / Dyson's Swordtail (Rhetus dysonii) butterfly, Manu National Park, Peru

PREMAPHOTOS / Periander swordtail / variable beautymark butterfly (Rhetus periander) in rainforest, Brazil

Jane Burton / Swordtail (Xiphophorus helleri), captive.

Jane Burton / Swordtail (Xiphophorus helleri) female giving birth to live young. Captive

Claus Meyer / Swordtail Butterfly (Opsiphanes bassus) portrait, Caatinga ecosystem, Brazil

Claus Meyer / Swordtail Butterfly (Opsiphanes bassus), Caatinga ecosystem, Brazil

Norbert Wu / Swordfish (Xiphias gladius) worldwide, can tolerate temperatures of five degrees Celsius and dive to 650 meters, uses sword to kill prey such as squid, Sardinia, Italy, Mediterranean Sea