Neotrygon Maskrays, a Complex Chondrichthyan Radiation in the Cenozoic: How and When!

Neotrygon maskrays are distinguished by a dark mask-like band over the eyes and represent an ideal group for understanding how evolutionary forces, geography and environmental dynamics may have shaped marine populations in the Indo-Australian archipelago. This is because, they are confined to the Indo-West Pacific and four of the five nominal species exist as narrow-ranging endemics along the interface between the Australian and Eurasian tectonic plates. Further, they inhabit shallow coastal and continental shelf waters. This habitat makes them easy targets to population fragmentation due to environmental pressure, such as cycles of glaciations responsible for the formation of phylogeographic breaks and/or contact zones.
In this paper, Melody assesses the partitioning of the genetic diversity within the genus, the taxonomic validity of some morphs and the timing of species divergence. The results are interpreted in the light of large scale paleo-climatic and geological events. Finally, conclusions are discussed given the formation of biogeographic barriers, responsible for vicariance and population contraction/expansion or secondary contact mechanisms across the Indo-Pacific.

 

Abstract

Maskrays of the genus Neotrygon (Dasyatidae) have dispersed widely in the Indo-West Pacific being represented largely by an assemblage of narrow-ranging coastal endemics. Phylogenetic reconstruction methods reproduced nearly identical and statistically robust topologies supporting the monophyly of the genus Neotrygon within the family Dasyatidae, the genus Taeniura being consistently basal to Neotrygon, and Dasyatis being polyphyletic to the genera Taeniurops and Pteroplatytrygon. The Neotrygon kuhlii complex, once considered to be an assemblage of color variants of the same biological species, is the most derived and widely dispersed subgroup of the genus. Mitochondrial (COI, 16S) and nuclear (RAG1) phylogenies used in synergy with molecular dating identified paleoclimatic fluctuations responsible for periods of vicariance and dispersal promoting population fragmentation and speciation in Neotrygon. Signatures of population differentiation exist in N. ningalooensis and N. annotata, yet a largescale geological event, such as the collision between the Australian and Eurasian Plates, coupled with subsequent sea-level falls, appears to have separated a once homogeneous population of the ancestral form of N. kuhlii into southern Indian Ocean and northern Pacific taxa some 4–16 million years ago. Repeated climatic oscillations, and the subsequent establishment of land and shallow sea connections within and between Australia and parts of the Indo-Malay Archipelago, have both promoted speciation and established zones of secondary contact within the Indian and Pacific Ocean basins.

Relevant literature

• M Puckridge, PR Last, WT White and N Andreakis (2012) Phylogeography of the Indo-West Pacific maskrays (Dasyatidae, Neotrygon): a complex example of chondrichthyan radiation in the Cenozoic. Ecology and Evolution. DOI: 10.1002/ece3.448.