Cryptic Kelp: Identifying and describing a new species in the southern bull-kelp genis Durvillaea

Abstract

Genetic analyses offer unprecedented insight into the evolutionary history of species and can reveal a wealth of previously cryptic biodiversity. Species showing no obvious morphological or ecological differences, despite genetic differentiation, can offer opportunities to understand the processes underlying divergence. The Australian endemic brown macroalgae, Durvillaea potatorum (Phaeophyceae), is an ideal organism to study how divergence can arise and eventually lead to speciation in highly-disturbed environments. This study builds on a previous genetic investigation of D. potatorum that revealed two genetically differentiated, reproductively isolated lineages, potentially representing ‘cryptic species’. The aim was to identify any differences that correspond with the genetic division within D. potatorum by using a combined genetic, morphological, ecological and histological analytical approach. A total of 228 individuals from seven sites around coastal Tasmania were collected and measured in situ for a range of morphological and ecological characteristics (e.g. depth below mean sea level or stipe length). Tissue samples were also collected for each individual to allow genetic analyses using mitochondrial (COI) and nuclear (28S) markers as well as histological examination of reproductive structures. Differences between the two lineages were examined by a combination of univariate and multivariate modelling analyses (including non-metric multidimensional scaling, analysis of similarity, Adonis and partial least squares). Genetic analyses confirmed the presence of two deeply divergent monophyletic clades, representing genetically distinct species: Durvillaea potatorum (sensu stricto) and Durvillaea lentus sp. nov. Despite highly variable phenotypes, morphological and ecological differences were found. Durvillaea lentus was typically found in deeper waters and had a relatively thinner, more slender stipe. Durvillaea lentus was also less responsive to ecological conditions but appeared to invest more heavily in repair structures such as stipitate lateral branches. Spatial distributions and patterns of genetic diversity in contemporary populations of both species support allopatric speciation coinciding with the onset of glaciation cycles in the late Pliocene. The current sympatric distribution around Tasmania’s east and west-coasts can be explained by a combination of ecological divergence and interglacial range expansion, facilitated by changes to land connectivity and ocean currents. This study reveals how both geography and ecology can drive divergence, and eventually result in speciation. These findings suggest that high levels of disturbance can promote cryptic diversity and highlight the importance of long-term changes and short-term fluctuations in ecological conditions as drivers of biodiversity. Show more