High Cryptic Diversity across the Global Range of the Migratory Planktonic Copepods Pleuromamma piseki and P. gracilis

Abstract

Although holoplankton are ocean drifters and exhibit high dispersal potential, a number of studies on single species are finding highly divergent genetic clades. These cryptic species complexes are important to discover and describe, as identification of common marine species is fundamental to understanding ecosystem dynamics. Here we investigate the global diversity within Pleuromamma piseki and P. gracilis, two dominant members of the migratory zooplankton assemblage in subtropical and tropical waters worldwide. Using DNA sequence data from the mitochondrial gene cytochrome c oxidase subunit II (mtCOII) from 522 specimens collected across the Pacific, Atlantic and Indian Oceans, we discover twelve well-resolved genetically distinct clades in this species complex (Bayesian posterior probabilities >0.7; 6.3–17% genetic divergence between clades). The morphologically described species P. piseki and P. gracilis did not form monophyletic groups, rather they were distributed throughout the phylogeny and sometimes co-occurred within well-resolved clades: this result suggests that morphological characters currently used for taxonomic identification of P. gracilis and P. piseki may be inaccurate as indicators of species’ boundaries. Cryptic clades within the species complex ranged from being common to rare, and from cosmopolitan to highly restricted in distribution across the global ocean. These novel lineages appear to be ecologically divergent, with distinct biogeographic distributions across varied pelagic habitats. We hypothesize that these mtDNA lineages are distinct species and suggest that resolving their systematic status is important, given the ecological significance of the genus Pleuromamma in subtropical-tropical waters worldwide.

Eukaryotic plankton diversity in the sunlit ocean

“…2B), but typically, three to eight times more rDNA OTUs were found than described morphospecies in the best-known lineages within these categories. This is within the range of the number of cryptic species typically detected in globally-distributed pelagic taxa using molecular data (28,29). The general congruency between genetic and morphological data in the cataloged compartment of eukaryotic plankton suggests that the protocols used, from plankton sampling to DNA sequencing, recovered the known eukaryotic biodiversity without major qualitative or quantitative biases.…”

Species are hypotheses: avoid connectivity assessments based on pillars of sand

“…Consequently, for organisms for which no recent taxonomic revision is available, the sampling scheme should both follow the requirements of population biology and phylogenetic systematics. Only a widespread sampling of a supposedly cosmopolitan species may reveal several diverging lineages restricted to isolated areas or distinct habitats as exemplified by the study of the supposedly ubiquitous and widespread amphipod Eurythenes gryllus (Lichtenstein in Mandt, 1822) (Havermans et al 2013) or the copepods Pleuromamma piseki Farran, 1929, and Pleuromamma gracilis Claus, 1863 (Halbert et al 2013). Collecting in undersampled or unexplored areas may also reveal new divergent lineages.…”

Global phylogeography of Oithona similis s.l. (Crustacea, Copepoda, Oithonidae) – A cosmopolitan plankton species or a complex of cryptic lineages?

“…Thus, it has 36 been assumed that allopatric speciation may be less important in pelagic species than 37 sympatric or parapatric speciation (Norris, 2000). During the past years molecular studies 38 have revealed that many supposedly widespread species are mosaics of several cryptic or 39 pseudocryptic species (Andrews et al, 2014; Burridge et al, 2015; Cornils and Held, 2014; 40 Darling et al, 2007;Halbert et al, 2013; Hunt et al, 2010;Miyamoto et al, 2012). The mode …”