Aestivation in Lungfish
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Transcript of Aestivation in Lungfish
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