In the advancement of vertebrate life, the African and South American lungfish

represents a landmark in the shift from life in water to life on land (Fishman et al, 1992). They

are tropical fish that have survived with little evolutionary change for the last 350 million years.

The key to this survival is the ability for the lungfish to breathe air by using lungs as well as

water by using gills. Both the African and South American lungfish have a unique way of

dealing with their environments which has helped them survive for such a long period of time

(Fishman et al, 1992).

The African lungfish (Protopterus aethiopicus) are elongated, eel-like fishes, with thread-

like pectoral and pelvic fins. They have soft scales, and the dorsal and tail fins are fused into a

single structure. They can either swim like eels, or crawl along the bottom, using their pectoral

and pelvic fins (Rainer, 2009). The African lungfish is an example of how the evolutionary

transition from breathing water to breathing air can happen. Lungfish are exposed to water with

low oxygen content or situations into which their aquatic environment dries up. Their adaptation

for dealing with these conditions is an out pocketing of the gut, related to the swim bladder of

other fishes that serves as a lung (Rainer, 2009). The lung contains many thin-walled blood

vessels, so blood flowing through those vessels can pick up oxygen from air gulped into the lung.

The African lungfishes are obligate air breathers, with reduced gills in the adults (Bruton, 1998).

They have two anterior gill arches that retain gills, though they are too small to function as the

sole respiratory apparatus (Rainer, 2009). The lungfish heart has adaptations that partially

separate the flow of blood into its pulmonary and systemic circuits. The atrium is partially

divided, to that the left side receives oxygenated blood and the right side receives deoxygenated

blood from the other tissues (Rainer, 2009). These two blood streams remain mostly separate as

they flow through the ventricle leading to the gill arches. As a result oxygenated blood mostly

goes to the anterior gill arches and the deoxygenated blood mostly goes to the posterior arches.

African lungfishes generally inhabit shallow waters such as swamps and marshes, however they

are also found in larger lakes such as Lake Victoria (Kees, 2002). They can live out of water for

many months in burrows of hardened mud beneath a dried-up stream bed.

The South American lungfish (Lepidosiren paradoxa) is the single species of lungfish

found in swamps and slow-moving waters of the Amazon, Paraguay, and lower Paraná River

basins in South America (Shubin, 2008). Notable as an obligate air-breather, it is the sole

member of its family Lepidosirenidae. This species has an elongate, almost eel-like body. It may

reach a length of 125 centimeters. The pectoral fins are thin and threadlike, while the pelvic fins

are somewhat larger, and set far back. The fins are connected to the shoulder by a single bone,

which is a marked difference from most fish, whose fins usually have at least four bones at their

base; and a marked similarity with nearly all land-dwelling vertebrates (Shubin, 2008). The gills

are greatly reduced and essentially non-functional in the adults. Juvenile lungfish feed on insect

larvae and snails, while adults are omnivorous, adding algae and shrimp to their diet, crushing

them with their heavily mineralized tooth-plates (Shubin, 2008). The fishes' usual habitats

disappear during the dry season, so they burrow into the mud and make a chamber about 30-50

cm down, leaving a couple of holes to the surface for air. This capacity has made it possible for

the lungfish to withstand seasonal bouts of intense tropical heat which turns the swamps and

marshes they live in to hard dry crusted mud (Fishman et al, 1992). When this occurs the African

and South American Lungfish begin the process of aestivation in order to survive these harsh

conditions.

Aestivation is described as as a state of reduced metabolic activity in which certain

animals become quiescent (Randall et al, 2002). It is a resting interval associated with warm, dry

periods in areas that have alternating wet and dry seasons. Animals are induced to aestivate when

drought and heat interfere with their activities. Biologists also describe aestivation as a state of

torpidity induced in animals by excessive dry heat (Randall et al, 2002). Aestivation is seen

chiefly in the tropics during the long, hot, dry season.

The African Lungfish which inhabits the shallow waters of lakes and waters in Central

Africa, during the hot, dry summer when life in the evaporating waters and slime becomes

intolerable, they burrow into the mud forming a waterproof, subterranean cocoon that encase all

but the mouth (Kees, 2002). The cocoon opens to the surface by a narrow passage for breathing

air. The lungfish remains in the cocoon as an air breather until the waters return. While in the

cocoon the lungfish undergoes a series of extraordinary physiological changes that enable it to

survive without food or water until the annual drought is over (Kees, 2002). In an observation

done by Smith 1930, and Johansen in 1970, they found that the lungfish aestivating survived for

months to years without food or water (Fishman et al, 1992). They also observed a decline in

oxygen consumption within one day of life in the mud and then began to slowly stabilize at a

lower level. They observed that their body weights fell more slowly to stabilize at about eighty

five percent of their pre- aestivation levels (Fishman et al, 1992). The ways the African lungfish

deals with desiccation and starvation since neither food nor water is ingested during aestivation.

The prospect of desiccation is circumvented by the waterproofing cocoon and the arrest of urine

formation. The anuria, in turn, is handled by metabolic rearrangements that minimize the

consequences of fat and nitrogen metabolism which are the primary food sources that the

lungfish draws upon during aestivation (Fishman et al, 1992). As for starvation it is hard to

dissociate the physiologic effects but they say it’s correlated with changes in blood pressure,

heart rate and breathing pattern.

The South American lungfish, Lepidosiren paradoxa, inhabits swamps that dry out on a

seasonal basis within the Pantanal region (Mato Grosso, Brazil), which coincides with the winter

in the middle of the year. Information on aestivation in L. paradoxa is highly limited, except for

(Harder et al, 1999), who reported on cardiac frequencies during formation of a burrow. When a

lake dries out, L. paradoxa digs a hole into the mud and clay, which allows movements and

change of position. For the animals in water, they reported a cardiac frequency of 31 beats per

minute which became down-regulated by forty six percent during aestivation. As a major

difference, Protopterus secretes a protective mucous from the skin to form a hardened cocoon

(DeLaney et al, 1974). Much less is known about aestivation in L. paradoxa, in particular

concerning blood gases and osmolality.

In summary aestivation in the African and South American lungfish is a state of light

torpor to which the animal resorts when threatened by desiccation (Shubin, 2008). It is analogous

to the arousal state of hibernation when the “winter sleep” is ready to be interrupted. The

aestivating state is a remarkable phenomenon of which it is a state of suspended animation which

is rapidly reversible and as an illustration of how an organism can adapt to two different

environments without regard for homeostasis that characterizes mammalian organisms (Bruton,

1998).

References

Fishman,D. (1992). Aestivation in African Lungfish. American Philosophical Society,136(1),61

Rainer, J. (2009) Life. The Science of Biology, 7(2), 943

Bruton, M. (1998). Paxton, J.R. & Eschmeyer, W.N.. ed. Encyclopedia of Fishes. San Diego:

14(1), 70–72.

Kees, P. (2002) East African Wild Life Society, African Journal of Ecology, Decline of the

African lungfish (Protopterus aethiopicus) in Lake Victoria East Africa 40(6), 42-52

"Your Inner Fish" Neil Shubin, 2008,2009,Vintage, p.33

Randall, D. ( 2002) Eckert Animal Physiology: Mechanisms and Adaptations 19(5), 72-84

Harder, F. (1999) Biology of Tropical Fishes, The South American lungfish—adaptations to an

extreme habitat. 86(2), 99–110.

DeLaney, D. (1974) Journal of Experimental Physiology. Aestivation of the African lungfish

Protopterus aethiopicus: cardiovascular and pulmonary function 6(1), 111–128

Topic Number: 39

Title: Aestivation in Lungfish

Biology of Vertebrates

Submitted By: David Young 200641397

Lab Slot: 64