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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 Délégation à l’information et à la communication Tél. : +33 (0)4 91 99 94 90 - fax : +33 (0)4 91 99 92 28 - [email protected] © GF Turner PRESS OFFICE : © GF Turner Scientific bulletin n°315 - April 2009 For further information