Why are there so many of species of coral reef fish? According to a new study, it\u2019s because about 50 million years ago, some fish figured out how to bite food from hard surfaces.\u00a0<\/p>\n
Evolution doesn\u2019t proceed at an even pace: Species evolve in jumps and spurts, followed by lulls. These periods of rapid diversification usually occur after a dramatic environmental change or upheaval, or when a lineage develops a new \u201cinnovation\u201d that allows them to use a previously inaccessible resource. For fish, the ability to feed from a hard surface was one such innovation.<\/p>\n
In a study published April 13 in Proceedings of the National Academy of Sciences<\/a>, University of California, Davis, researchers show that fish lineages that acquired the ability to scrape algae, snails and shellfish from hard surfaces evolved in more diverse ways than fish living higher in the water column, where there are no hard surfaces to feed from.\u00a0<\/p>\n \u201cThis is one of the reasons we have so many species of coral reef fish today,\u201d said Nick Peoples, graduate student in the Population Biology graduate group and lead author on the paper. \u201cWhen we account for the entire evolutionary history of fishes in different habitats, we find that this immense and unparalleled acceleration of diversification happened on coral reefs.\u201d<\/p>\n Why is biodiversity so unevenly distributed?<\/p>\n The study was spurred by Peoples\u2019 desire to understand why fish diversity is so unevenly distributed across different habitats and ecosystems.<\/p>\n \u201cCoral reefs have 6,000-7,000 species of fish, and there\u2019s a lot of diversity in freshwater environments in Africa, but there’s comparatively fewer species in the open ocean and deep sea,\u201d said Peoples. \u201cWe were interested in what is driving that pattern. The process of diversification is what structures the biodiversity we see today, so understanding that process can give us interesting insights into how biodiversity is generated and maintained.\u201d<\/p>\n Tracking evolution\u2019s pace<\/p>\n To investigate whether differences in the pace of evolution might have led to biodiversity differences between habitats, the researchers estimated the rate of fish evolution over the past 350 million years.<\/p>\n First, they classified 9,560 fish species into seven categories based on where they live: at the bottom of freshwater lakes or oceans; near \u2014 but not on \u2014 the bottom of freshwater lakes or oceans; higher in the water column; or on coral reefs. Then, they used a pre-existing evolutionary tree to estimate the rates of speciation (the emergence of new species) and extinction for each habitat during 1-million-year intervals.\u00a0<\/p>\n They showed that, around 50 million years ago, the rate of diversification increased by around 1.5 to 1.7x for fish species living in coral reefs or at the bottom of fresh or marine bodies of water. This acceleration was particularly pronounced for coral reef fish. In contrast, the rate of evolution remained relatively stable for fish that live in open water.\u00a0<\/p>\n This difference is due to the fact that open water habitats are less complex, the researchers said. With water on all sides, fish that live higher in the water column have fewer opportunities to develop new behaviors or anatomical features compared to fish that live in crowded coral reefs or lakebeds.<\/p>\n \u201cOur results show that the global biodiversity of fishes was shaped by the dynamic interplay between the traits of species and the features of the habitats they occupy,\u201d said Peoples.<\/p>\n Giving fish a clean slate\u00a0<\/p>\n The jump in fish evolution began shortly after the Paleocene-Eocene Thermal Maximum (PETM), a 200,000-year period of climate warming that occurred approximately 56 million years ago. During the PETM, global temperatures rose by 5\u25e6 to 8\u25e6C, which caused a rapid turnover in species, especially for plankton and corals living in shallow waters.<\/p>\n \u201cWe think this reorganization of what fishes could eat was almost like a blank slate for fishes to diversify on,\u201d said Peoples.\u00a0<\/p>\n When they compared the rate of evolution of corals during the same time period, the researchers didn\u2019t find any evidence for increased coral diversification after the PETM.<\/p>\n \u201cThis suggests that the acceleration in fish speciation after the PETM is due to the evolution of the fish themselves and how they’re interacting with the environment,\u201d said Peoples.\u00a0<\/p>\n By comparing diversification rates for different groups of fish, the researchers showed that only specific lineages underwent this pulse of evolution.<\/p>\n \u201cOn reefs, it’s the fish that eat parts of the reef that really took off and diversified,\u201d said Peter Wainwright<\/a>, professor in the Department of Evolution and Ecology and senior author on the paper. \u201cThe groups that really stand out in our analysis are parrotfish, butterflyfish, angelfish, surgeonfish, rabbitfish and triggerfish. These are all fish that feed by biting the reef, whereas we don’t see an increase in speciation in reef fish that feed in mid-water, like snappers, groupers and sea basses.\u201d<\/p>\n The study adds to previous research published by Wainwright\u2019s lab that also investigated the evolution of teeth<\/a> and biting in fish<\/a>.\u00a0<\/p>\n \u201cBiting is a unique mode of feeding that evolved recently in fish,\u201d said Wainwright. \u201cThis study shows at the macro level that these unique modes of feeding contributed to the incredible diversity of reef fishes that we see today.\u201d<\/p>\n The study was co-authored by Michalis Mihalitsis, who was then a postdoctoral fellow in Wainwright\u2019s lab and is now an assistant professor at the University of Guam.<\/p>\n The work was supported by the University of California, Davis. Peoples was supported by a University of California, Davis, Dissertation Year Fellowship and the Achievement Rewards for College Scientists (ARCS) Foundation.<\/p>\n","protected":false},"excerpt":{"rendered":"Why are there so many of species of coral reef fish? 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