Download Darwin`s dream : the evolution of African cichlids

Scientific bulletin n°315 - April 2009
© Aquarium de la Porte Dorée-Paris
The cichlids are tropical freshwater fish well
known to aquarists for
their display of magnificent colours and the
unparalleled diversity
of species. Moreover,
those of the East African
Great Lakes (Malawi,
Tanganyika and Victoria)
are renowned among
evolution researchers :
they give an exceptional
illustration of adaptive
radiation, the rapid appearance of a multitude
of species starting from
a common ancestor.
Most cichlids adopted
an original reproductive strategy : mouthbrooding1. This practice
enabled the cichlids of
the three extensive lakes
to branch out and colonize, from their original habitats on the lacustrine
floor, to rocky shore and
pelagic2 environments.
IRD researchers and
their partners3 compared
the reproduction strategies of East African
cichlid species. They
demonstrated a separate
but parallel evolution
in the different environments and in each of the
lakes.
To survive in their new
habitats, the fish took to
the extreme the reproduction strategy of their
benthic2 ancestors. They
reduced the number of
eggs produced per batch
but increased their size.
Incubation time was prolonged as well.
Darwin’s dream :
the evolution of African cichlids
The cichlids of the East African Great Lakes display some magnificent colours.
Natural selection helped shape cichlids
life-history evolution. Fish biologists
from the IRD and their partners have studied the evolution of reproduction strategies of such fish in Lakes Malawi, Victoria
and Tanganyika. They retraced the evolutionary history of these fish from their original benthic habitats2, as they branched
out to rocky coasts and the pelagic2 zone.
As they colonized these two new environments, the East African cichlids adapted
their reproduction strategy in similar ways
but completely independently in the three
different lakes. The adaptations moreover
appeared in parallel in each type of habitat. This discovery suggests that, in each
lake, the rocky and pelagic habitats were
colonized independently of each other by
benthic ancestors. Therefore these fish
did not undergo any evolutionary transition between the rocky shore and pelagic
habitats.
The research team showed that in all three
of the great lakes the cichlids’ most recent
common ancestor was a benthic species:
it lived near the sandy-mud bottoms at the
lake edges. The lacustrine cichlids would
probably originally have come from rivers,
where rocky habitats are generally limited
to reaches with strong currents and no pelagic zones.
The cichlids take the mouthbrooding
strategy to its height. The requirements
for survival in the face of competition
pressure from other fish in their new pelagic2 environment pushed the cichlids of
the East African Great Lakes to develop
their benthic ancestors’ reproduction strategy, the practice of mouthbrooding, to its
limits. The fish that adopted this original
mode of reproduction lay a small number
of large eggs per batch and take care of
their offspring over long periods after hatching. The research revealed that the further they colonized the pelagic habitats,
and to a lesser extent the rocky areas, the
more the cichlids reduced their fecundity,
by lowering the number of eggs produced
by each female at laying time. At the same
time the egg size increased. Moreover,
the incubation time was extended: the fry
can stay in the parent’s mouth for several
weeks, until they grow to 3 cm. In some
species living close to rocks, even freeswimming juveniles can return to shelter
in the mother’s mouth in case of danger.
Institut de recherche pour le développement - 44, boulevard de Dunkerque, CS 90009
F-13572 Marseille Cedex 02 - France - www.ird.fr
You can find the IRD photos concerning this bulletin, copyright free for the press, on www.ird.fr/indigo
CONTACT :
Fabrice DUPONCHELLE
Unité de recherche
Caractérisation et valorisation de la diversité ichtyologique pour une aquaculture
raisonnée (CAVIAR)
Address :
IRD
Castilla 18
1209 -Lima 18
Pérou
Tel : (+511) 445 68 98
[email protected]
REFERENCES :
Duponchelle F., Paradis E.,
Ribbink A. J, Turner G. F.
Parallel life history evolution in mouthbrooding
cichlids from the African
Great Lakes, PNAS, 105
(40) : 15475- 15480, 2008
Doi: 10.1073_
pnas.0802343105
KEY WORDS :
CICHLIDAE, REPRODUCTION,
MOUTHBROODING,
EVOLUTION
The researchers demonstrated that in
different sites, diverse species exposed
to similar environments develop analogous characters, proof of evolution
driven by natural selection. Most cases
of evolutionary convergence observed up
to now concern morphological characteristics, like shape or colour. Now this work
has yielded one of the rare documented
examples of adaptive evolution in parallel
with a reproduction strategy.
This strategy is diametrically opposed to
that of other surface4 water fish which lay
thousands of tiny eggs dispersed in all directions and left to their fate, with a low
chance of survival.
The cichlids thus optimized the survival of
their offspring in rocky and pelagic environments, where food is scarcer and predation
more intense. The fact that they develop
from larger-sized eggs would render the
juveniles more able to find their food and
escape from their predators. They would
also better withstand prolonged periods
without food. No other pelagic bony4 fish
in the world, whether marine or freshwater,
uses this unusual reproduction system.
Comparison was made of the quantity
and size of eggs produced by the mouthbrooding cichlids living in pelagic, benthic
and rocky habitats of the three East African
Great Lakes. Fecundity of the fish and egg
diameter differ significantly both between
the different environments and from one
lake to another. In the same lake, pelagic
cichlids can lay on average 10 times fewer
eggs than benthic cichlids. And their eggs
are 2 to 3 times larger : their diameter can
reach 7 mm in some species. The cichlids
from the rocky shore environments show
intermediate characteristics. This substantial difference in egg size and fecundity
between habitats is most marked in Lake
Tanganyika, the most ancient of the three
lakes. It is less distinct in the most recent
of the three lakes, Lake Victoria.
This study has brought better understanding of the processes of evolutionary
adaptation.
Gaëlle Courcoux - DIC
Translation - Nicholas flay
1. The eggs the larvae develop inside the mouth
(buccal cavity), usually in the mother, although
some species are paternal mouthbrooders, the
father ensuring mouthbrooding, or biparental
ones. The eggs are most often laid in a nest,
then fertilized before being taken into the parent’s mouth. However, in some surface-water
cichlids, laying is performed in the open water.
In this case, the mother immediately retrieves
the eggs in her mouth and goes to pick up the
sperm from the male’s genital papillae (mouth
fertilization).
2. The pelagic zone corresponds to the surface
waters. The space near the floor of lakes and
rivers is called the benthic zone.
3. These research projects were conducted
jointly with scientists from South African Institute for Aquatic Biodiversity, Grahamstown, and
Bangor University in Wales, UK, in the context
of the valorization of results from the SADC/
GEF Programme’s Lake Malawi Biodiversity
Conservation Project
4. The cichlids here are compared with other
pelagic bony fish. Some pelagic cartilaginous
fish, such as sharks or rays, also display a reproduction strategy favouring the production of
a small number of large-size descendants.
Vincent Coronini
+33 (0)4 91 99 94 87
[email protected]
INDIGO,
IRD PHOTO LIBRARY :
Daina Rechner
+33 (0)4 91 99 94 81
[email protected]
www.ird.fr/indigo
When the cichlids of the East African Great Lakes passed from the benthic environment towards the rocky and
pelagic habitats, they adopted similar reproduction strategies, in a parallel way but independently in each lake.
Gaëlle Courcoux, coordinator
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Tél. : +33 (0)4 91 99 94 90 - fax : +33 (0)4 91 99 92 28 - [email protected]
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Scientific bulletin n°315 - April 2009
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