Biology ︱ Professor Melanie Stiassny Fishes in the lower An extreme case of species divergence and convergent evolution

In its lower reaches the Congo he Nile might be Earth’s longest Professor Stiassny and her New York River is the world’s fastest and river, and the Amazon might collaborator Liz Alter believe that the contain the most water, but participation of Congolese partners and deepest river, and this short T Many fish species have adapted to the stretch hosts great biological neither are as deep nor as fast-flowing students, and the building of in-country extreme hydrological conditions of the diversity. Many fishes inhabiting as are places along the Congo. This is ichthyological (the branch of zoology lower Congo, through biological processes that are the focus of Professor Melanie the lower Congo River display particularly evident in the river’s lower devoted to the study of fishes) capacity Stiassny’s research. adaptations to life in such an reaches, extending for two-hundred is central to its success. Referred to as extreme environment, and often miles upstream from the Atlantic Ocean, the Congo Project by its collaborators, similar adaptations have arisen where huge rapids and deep underwater it aims to document the different fish adaptation and diversification of species in fishes distant from each other canyons create a highly turbulent region species inhabiting the lower Congo River, inhabiting extreme environments. Despite the harsh conditions of the central on the tree of life. Professor that may appear uninhabitable at first which are currently poorly understood. stream of the lower Congo, this stretch Melanie Stiassny, along with glance. Indeed, the intense surface rapids Some exciting discoveries have been HYDROLOGICAL AND her partners at the American largely prevent the movement of motile made by the Congo Project so far; BIOLOGICAL DIVERSITY of the river boasts an exceptionally high Museum of Natural History, , however, many fish species have physically and genetically distinct species Despite the harsh conditions of the diversity of fish species. CUNY Graduate Center and adapted to these extreme hydrological have evolved similar traits in a process central stream of the lower Congo, York College, the University conditions through biological processes known as ‘convergent evolution’, where this stretch of the river boasts an banks entirely separated by the high- where populations of a single species of Kinshasa, and the University that are the focus of Professor Melanie separate populations undergoing similar exceptionally high diversity of fish velocity central stream. The intense flow that have been separated over time of Marien Ngouabi, have been Stiassny’s research in this hyper-diverse selective pressures evolve to exhibit species. The lower Congo occupies between the low flow velocity regions on evolve into genetically distinct lineages. studying the fish of this region region, a collaborative and long-term similar morphological traits. This research just 2% of the area of the total Congo either side of the river channel hinders for over ten years, and their work has produced many research program between the American is important as it documents one of the basin, but contains nearly 30% of all the movement of fishes both across EVOLUTION IN ISOLATION interesting findings. Museum of Natural History, CUNY few cases where this type of evolution species, and 30% of those are endemic and up/down the river, and thus the The concept of speciation has served Graduate Center and York College, has occurred among distantly related to the lower Congo, meaning that they fishes inhabiting the separated, low flow as a cornerstone of evolutionary theory the University of Kinshasa (Democratic species over a relatively short timeframe are only found in this region. Trying to velocity areas are physically separated since the 19th Century. Understanding Republic of Congo; DRC), and the and at extremely small spatial scale. The answer the question of why so many by the water itself and rarely or never the process by which one species can University of Marien Ngouabi (Republic lower Congo system holds the potential species have arisen within the lower encounter one another. These conditions evolve into two or more genetically of Congo; RC). to provide a greater understanding of the Congo, Stiassny’s group has focused on are ideal for the occurrence of speciation, distinct species is vital for understanding the unique physical properties of the river itself. The annual discharge of the Congo is extremely high, second only to the Amazon, and combining this with the varying width and steep incline of the lower Congo generates complex flow regimes that have affected the development of life in the river. Below lethops the surface of the river, beneath its raging rapids, the team has documented Embolism an extraordinarily complex hydrological system that has created different and distinct ‘pockets’ throughout the river that potentially serve as habitat refuges for different species and populations. Surveys of the river have recorded Catastrophic Decompression Syndrome regions of low water velocity near the (CDS)

www.researchoutreach.org www.researchoutreach.org the process of evolution itself. There are several different forms of speciation, but here, we focus on allopatric speciation, Behind the Research which occurs when populations of one species become physically isolated by events such as continental drift or the Professor Melanie Stiassny formation of mountain ranges and evolve into separate, distinct species when they E: [email protected] T: +1 212 769 5796 face different conditions and pressures W: http://www.amnh.org/our-research/staff-directory/melanie-stiassny to which they must adapt. References Different species may develop similar Research Objectives traits to adapt to similar ecological Professor Stiassny’s research focuses on the systematics http://vimeo.com/channels/nsf#24733777 an introduction niches in different parts of the world, and evolutionary morphology of fish faunas of the Old to Professor Stiassny’s NSF project. which is known as convergent evolution World tropics. - the process by which species evolve http://www.youtube.com/watch?v=AdU1PgLb03g independently and yet display similar Detail TEDxBroadStreetNY: Melanie Stiassny - Learning from or the same traits. For example, Old Old World and New World vultures are not Ancient Fish. phylogenetically related, but have evolved and New World vultures are not independently to feed on carrion. Department of Ichthyology, phylogenetically related, yet both are American Museum of Natural History, https://www.youtube.com/watch?v=tObYa9KQb8w Science adapted to feeding on carrion, sharing 79th Street at Central Park West, Bulletins: Evolution in Action—Isolation and Speciation in the their featherless heads and soaring or dying washed up on the shore. It a species of elephantfish, the spiny eels New York, Lower Congo Region. behaviours when searching for food. was later determined that these fish Mastacembelus brichardi and crassus, NY 10024 However, these vultures live and have dwell in the depths of the river and Notoglanidium pallidum, a species of https://www.careergirls.org/role-model/ichthyologist/#my- evolved independently on different occasionally become caught in strong African big-eye catfish, andGymnallabes Bio story- career advice from Melanie Stiassny. continents. The process of evolution upwelling currents, bringing them to nops, a species of walking catfish. This is a Melanie Stiassny is the Axelrod Research Curator of is slow, taking tens of thousands to the surface. Adapted to extremely high clear example of phenotypic convergence Ichthyology at the American Museum of Natural History, millions of years, as the geological factors pressures, deep-ocean fish are unable to – the “reduced eyes, lack of pigmentation, and Professor in the museum’s Richard Gilder Graduate separating populations are similarly survive should they rise above a certain elongated body” phenotype has evolved School. She is Lead Curator for the museum’s iconic Hall Personal Response slow. However, the rapid currents and of Ocean Life, and serves as scientific advisor to various deep canyons of the lower Congo River conservation organisations. She has authored numerous have sped up this process. Numerous …the “reduced eyes, lack of scientific papers, books, and articles and is a popular What is it that makes the case of phylogenetic observations of populations divided lecturer in Museum educational outreach programs. convergence in the lower Congo River so unique? by river currents, rather than mountain pigmentation, elongated body” The lower Congo River is one of the most extreme ranges, were made in the lower Congo, Funding freshwater habitats on Earth. Vast volumes of water plunge and this case is unique in that such phenotype has evolved separately National Science Foundation, down a narrow channel to the ocean, forming some of the largest whitewater rapids and the deepest underwater evolution has occurred over such a small AMNH Axelrod Curatorship, in species from different families. canyons on Earth. The river is home to an extraordinary spatial scale. Therefore, the lower Congo Carnegie Corporation of New York, array of fishes, many of which are bizarre in appearance River is an ideal place for studying the depth as the change in pressure causes separately in species from distantly related National Geographic Society and exhibit numerous adaptations to life in turbulent, diversification and evolution of species the formation of gas bubbles in their families. The current phase of the Congo high-energy waters. Among these are striking examples as it occurs rapidly at such a small tissues. This phenomenon is known as Project aims to continue to investigate this Collaborators of morphological convergence, strongly suggesting spatial scale. catastrophic decompression syndrome, extreme phenotypic convergence and • S. Elizabeth Alter (CUNY Graduate Center and York a shared signature of selection. The fishes of the lower and the same appears to be true for the establish how quickly these phenotypes College) Raoul Monsembula and Justin Mbimbi Congo present an unparalleled natural experiment to investigate the variety of mechanisms by which convergent DIVERSITY IN THE LOWER CONGO bottom-dwelling fish of the lower Congo. emerged, how similar the morphologies (University of Kinshasa) phenotypes have repeatedly evolved across deep The Congo Project commenced as a of species in the same and different • Victor Mamonekene and Armel Ibala phylogenetic time. collaborative effort between the American Upon the retrieval of a live individual of L. classes exhibiting these phenotypes (University of Marien Ngouabi). Museum of Natural History, the University lethops, gas bubbles appeared to rapidly are and determine whether these traits of Kinshasa, DRC, and the University of form within its tissues as it died, strongly evolved during a similar timeframe. Marien Ngouabi, RC in 2006. The aims suggestive of its adaptation to a high- were to record fish diversity in the lower pressure environment. Compared with The research conducted by the Congo Congo, understand the distributions of other individuals of its genus residing in Project is significant as it has identified different species, and elucidate how these the Congo River, L. lethops is the only the lower Congo River as a region species evolved and their populations species apparently adapted to withstand where convergent evolution can be formed in such a diverse environment. the conditions of the river’s bottom, with observed and recorded, which is vital The first signs of the lower Congo’s bizarre its depigmented skin, reduced eyes, and for understanding the mechanisms fish populations were discovered when elongated body. Yet, L. lethops is not the behind such evolutionary patterns. The the local fishing community noticed only species exhibiting these traits. Such project continues to describe the diverse that the ‘blind ’, Lamprologus a morphology has also been discovered array of fish species found in the lower lethops (which lacks pigment and has in a series of phylogenetically distant Congo region, which had been previously skin-covered eyes) was only found dead species, including Stomatorhinus microps, poorly understood.

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