Abstract
The clupeoid fishes are distributed worldwide, with marine, freshwater and euryhaline species living in either tropical or temperate environments. Regional endemism is important at the species and genus levels, and the highest species diversity is found in the tropical marine Indo-West Pacific region. The clupeoid distribution follows two general pattern of species richness, the longitudinal and latitudinal gradients. To test historical hypotheses explaining the formation of these two gradients, we have examined the early biogeography of the Clupeoidei in reconstructing the evolution of their habitat preferences along with their ancestral range distributions on a time-calibrated mitogenomic phylogeny. The phylogenetic results support the distinction of nine main lineages within the Clupeoidei, five of them new. We infer several independent transitions from a marine to freshwater environment and from a tropical to temperate environment that occurred after the initial diversification period of the Clupeoidei. These results combined with our ancestral range reconstruction hypothesis suggest that the probable region of origin and diversification of the Clupeoidei during the Cretaceous period was the tropical marine precursor to the present Indo-West Pacific region. Thus, our study favors the hypotheses of “Region of origin” and “Tropical conservatism” to explain the origins of the longitudinal and latitudinal gradients of clupeoid species richness, respectively. Additional geological and paleontological evidence further define the tropical marine paleo-region of origin as the eastern Tethys Sea region. The Cretaceous fossil record of the Clupeoidei is partially incongruent with the results here as it contains taxa found outside this region. We discuss three possible causes of conflict between our biogeographical hypothesis and the distributions of the Cretaceous clupeoid fossils: regional extinction, incomplete taxonomic sampling and incorrect timescale estimation.
Highlights
Species richness of worldwide-distributed groups of marine organisms is not uniform across regions and general gradients of diversity are apparent
The phylogenetic results are consistent with previous molecular studies in recovering the same major lineages [24,25,26,27,28,29]
Three of these lineages are the families Pristigasteridae, Chirocentridae and Engraulidae; each of these families is well defined by several morphological synapomorphies
Summary
Species richness of worldwide-distributed groups of marine organisms is not uniform across regions and general gradients of diversity are apparent. Unbalanced species richness amongst the tropical marine regions is a common pattern of longitudinal gradients of organism diversity [1] with the Indo-West Pacific (IWP) region comprising more species than any other tropical region [4,5]. These longitudinal and latitudinal gradients increased dramatically in strength through the Cenozoic era to reach their present maximum [1]. The ‘‘Region of origin’’ hypothesis postulates that a widely distributed group of organisms originated and diversified within the region that harbors the maximum species diversity [7]. The ‘‘Tropical conservatism’’ hypothesis holds that a latitudinally widely distributed group of organisms originated and diversified within tropical regions and have spread to temperate regions only more recently [11]
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