[ENVIRONMENT ★★ – THE MOST EXTREME FISH ON EARTH]
(P1) There are male fish, and female fish, and these fish mate to create little fish, which inherit traits from both parents.
(P2) Over a few years, the little fish grow and grow, until they become old fish.
(P3) Fish breathe using their gills.
(P4) Fish also live in water. We take that one for granted – hence the phrase “A FISH OUT OF WATER,” meaning something weird or odd, something that shouldn’t exist.
(P5) Fish can’t live out of water.
(P6) And to get about, they swim, ideally in huge expanses of water, such as rivers, lakes, seas and oceans.
(P7) However, there is one group of fish that does not follow any of these rules.
(P8) Known as killifish, these small INNOCUOUS-looking creatures measure just a few inches long.
(P9) Many species are FLAMBOYANT and kept as pets in aquaria. But others look ordinary, and in their own way are among the oddest fish on the planet. A few can claim to be the world’s most extreme fish.
(P10) That’s because some killifish don’t REPRODUCE like other fish. In fact, one species doesn’t reproduce in the same way as any other vertebrate.
(P11) Killifish don’t grow old. They don’t mature like other fish, and many killifish species will live and die with a year. One species survives for less than three months, making it among the shortest-lived of all known vertebrates.
(P12) They have other ways of breathing, apart from using their gills.
(P13) And swimming isn’t really their thing. They prefer to live puddles and pools, and have evolved their whole life strategy to cope without water, rather than with it.
(P14) Some species can survive out of water for more than two months. They will leave it to go for a walk and they even hunt on land.
(P15) Some have even been found living in trees.
(P16) There are a number of killifish species living across Africa and South America. Most are adapted to living in places where water is scarce, or where water levels change DRASTICALLY from season to season.
(P17) Some do live in larger bodies of water, but many survive in ISOLATED pools that can dry up.
(P18) That creates a FUNDAMENTAL challenge – how do the fish survive and reproduce when the water eventually runs out?
(P19) One major adaption, which different killifish species have independently evolved, is to live their whole lives within a single year, using a number of unique tricks.
(P20) During the wet season, when pools are full, the fish hatch. They quickly grow to maturity and start SPAWNING, which they keep doing until the pools start to dry. At that point all the fish perish.
(P21) This cycle of life can be so quick that one species, the turquoise killifish (Nothobranchius furzeri) of equatorial Africa, lives for an average of just 10 weeks.
(P22) But they leave behind eggs in the soil, and these eggs represent the entire surviving populations of killifish.
(P23) These eggs survive the dry season in a DORMANT state, buried in the soil until the following rainy season. The return of the rains causes eggs to hatch and the cycle begins anew.
(P24) To do this, different killifish are able to stopping growing as embryos, and then start again when conditions are right. And they can stop at different times during their development, delaying the growth of major organs, such their skulls, hearts and circulatory systems. They can then delay hatching for days, weeks or months.
(P25) Other killifish have evolved other extreme lifestyles.
(P26) The mangrove killifish (Kryptolebias marmoratus), for example, is a hermaphrodite, possessing both male and female sex organs. Other animals can do that.
(P27) But the mangrove killifish is the only known vertebrate hermaphrodite that can FERTILISE itself, producing genetically identical clones.
(P28) Most fish breathe with their gills, passing water over them to extract oxygen. The mangrove killifish, however, can also breathe through its skin.
(P29) That’s critical for the fish’s survival as mangrove killifish live in the most extreme habitat, within the mangrove forests of the West Atlantic, a tangle of thick tree roots and deep mud.
(P30) Decaying leaves from red mangrove trees sink into the mud, and are broken down by bacteria. They spew out NOXIOUS hydrogen sulphide gas, and the water contains little oxygen.
(P31) Other fish species live in mangrove swamps, but they usually move in and out as water conditions vary. Mangrove killifish spend their whole lives here. This water then disappears during the dry season, leaving the fish high and dry.
(P32) Some survive in diminishing pools, while others are left clinging to the mud or tree branches. Essentially, the fish have moved onto land as the water RECEDES, and survive by breathing air through their skin. They even excrete waste through their skin and tests have shown that mangrove killifish can survive out of water for up to 66 days.
(P33) The fish are so well adapted to life on land that they will lay their eggs on damp mud and occasionally FORAGE across the mud above the tide line.
(P34) They get around by PERCHING upright on their bellies, and then purposely WADDLE their bodies to explore their habitat.
(P35) They even use their tails to flip into the air.
(P36) Moving through the air in this way is particularly difficult for fish, as the environment is so ALIEN to them. Fish have bodies designed to move in more DENSE water that supports their body weight.
(P37) Killifish do it by first lying either in a PRONE position or on their sides. Then they lift their head, curling the body. Then they straighten their back, forcing their tail down and into the ground. They do it with such force that they turn their body into a missile, propelling it many body lengths into the air, before landing.
(P38) They can use this technique to hunt and pounce on other animals. In the lab they have been shown capable of hunting and capturing crickets, and can pluck insects from leaves high above themselves, before taking them back into the water to eat.
(P39) Extreme indeed.
If you found the passage difficult to read or had problems understanding specific words or idiomatic expressions, please discuss them with your tutor. The following discussion questions should be answered in your own words and with your own arguments.
- Briefly summarize the content of the article in your own words.
- Why do you think that biologists are interested in unusual species like the killifish?
- People always think of fish and water together. How do killifish use water differently than other fish?
- How do mangrove killifish have babies?
- Is it possible to keep killifish as pets?
EXPRESSIONS TO PRACTICE:
What do the following expressions mean? Practice using each expression in a sentence; extra points if you can use it in conversation.
- A fish out of water