Unraveling the secret life of cryptic algal species: evolution, phylogeography and eco-physiology of the red alga, Bostrychia intricata

Molecular techniques have enhanced our ability to unravel the evolutionary history and hidden diversity of species, and to explain how historical events have helped to shape the demography and dispersal of populations. Cryptic species are typically defined as two or more genetically distinct species that are morphologically indistinguishable. The discovery of cryptic diversity has become a challenge for biologists in understanding the species concepts and biodiversity patterns. Many current studies have revealed the existence of cryptic species, but few studies have focused on their ecological and biological aspects.  Bostrychia, a filamentous red alga, has long been used as a model system for studies of evolutionary process and biogeographic history. In the Southern Hemisphere, there are four endemic species: B. arbuscula, B. gracilis, B. intricata and B. vaga. Bostrychia intricata is widely distributed in the Southern Hemisphere, whereas the other three species have more restricted distributions. The aim of this study was to reveal the evolutionary history, phylogeographic pattern and eco-physiological trait within B. intricata.  Phylogenetic analysis based on combined data (mitochondrial COI, chloroplast rbcL and nuclear 28S) strongly supported the monophyly of the four Southern Hemisphere Bostrychia species, with B. vaga as a sister species to the other three. Multigene phylogeny and COI-based species delimitation revealed cryptic species diversity within B. intricata and B. vaga. Additionally, a COI-based phylogeographic study indicated the existence of three cryptic B. intricata species (N2, N4 and N5) in New Zealand. Population analyses demonstrated that cryptic species N2 populations recently expanded, possibly after the Last Glacial Maximum (LGM), while N4 was more diverse, showing a stable population, which possibly persisted during the LGM. The results suggested that the contrasting pattern in population structure and demographic histories between cryptic species was probably due to the difference in the evolutionary history and survival ability.  Growth experiments clearly showed that cryptic species N4 had a significantly higher specific growth rate than other two species, N2 and N5, in different salinities and temperature, suggesting physiologically differentiation between these cryptic species. Additionally, the small-scale distribution of B. intricata at Moa Point, Wellington of three cryptic species showed that N4 was found at the higher tidal position than species N2 and N5. Cryptic species N2 occurred in more wave-exposed areas than other two species. These results suggest spatial niche differences between cryptic species, possibly allowing them to sympatrically co-exist. This study highlights the fact that cryptic species are distinctly different in many biological characteristics, while maintaining \identical morphologies.