Molecular approaches uncover cryptic diversity in intertidal Ligia isopods (Crustacea, Isopoda, Ligiidae) across the southern Africa coastline.

Taylor M Greenan,Carlos A Santamaria,Charles L Griffiths

doi:10.7717/peerj.4658

Abstract

Recent phylogeographic studies along the coastline of southern Africa have uncovered cryptic diversity in several coastal invertebrates, including direct developing crustaceans in the superorder Peracarida. These findings indicating the possible existence of additional cryptic diversity in other yet to be studied peracarids, particularly those known to harbor said cryptic diversity in other regions of the world. Isopods in the genus Ligia are one such taxon. They inhabit patchy rocky beaches, are direct developers, avoid the open water, and exhibit other biological traits that severely constrain their dispersal potential (e.g., poor desiccation resistance). These traits are thought to have led to long-term isolation of populations, and allopatric diversification in Ligia species around the world; however, Ligia species in southern Africa, where three endemic Ligia species of uncertain validity are known to exist, remain unstudied to date. In this study, we used mitochondrial and nuclear markers to characterize Ligia collected in 18 localities from Namibia to the KwaZulu-Natal region of South Africa. We report the presence of cryptic lineages within Ligia species in the region that suggest the need for taxonomic reevaluation of these isopod species.

Highlights

We successfully amplified 658-bp of the Cytochrome Oxidase I (COI) mtDNA gene for 111 Ligia individuals from 18 localities across southern Africa (Fig. 1): 10 that were identified as L. glabrata, 28 as L. dilatata, and 73 as L. natalensis
The ‘‘Eastern’’ cluster we observe a deep split between Clade C and all other lineages in the cluster, with divergences amongst these lineages matching and/or exceeding those seen for pairwise comparisons between L. glabrata and L. dilatata (8.5–10.7% COI Kimura 2-Parameter distances (K2P), Table 2), as well as those previously reported for other Ligia species pairs, such as L. perkinsi/L. hawaiensis (11.9–16.7% COI K2P; Santamaria et al, 2013)
By using morphological identifications as well as nuclear and mitochondrial markers to characterize 18 Ligia populations from southern Africa, we report patterns that suggest the biodiversity of these isopods is under-reported in this region

Summary

Introduction

Recent phylogeographic work on coastal invertebrate species has led to the discovery of cryptic diversity in poorly dispersing species around the world (e.g., Chan, Tsang & Chu, 2007; Hurtado, Lee & Mateos, 2013; Radulovici, Sainte-Marie & Dufresne, 2009; Santamaria et al, 2017; Santamaria et al, 2016; Santamaria, Mateos & Hurtado, 2014; Santamaria et al, 2013; Varela & Haye, 2012). Adult Ligia isopods avoid open water and quickly attempt to regain the shore when submerged (Barnes, 1932; Barnes, 1935), exhibit low desiccation and submergence resistance (Barnes, 1936; Barnes, 1938; Todd, 1963; Tsai, Dai & Chen, 1997; Tsai, Dai & Chen, 1998; Zhang et al, 2016) and poor locomotion on non-rocky substrates These traits limit both their overland and overwater dispersal potential, which may lead to severely restricted gene flow between populations, long term isolation, and in turn allopatric and potentially cryptic diversification, as reported for L. hawaiensis (Santamaria et al, 2013; Taiti et al, 2003), L. exotica and L. cinerascens (Hurtado et al, 2018; Yin et al, 2013), L. occidentalis (Hurtado, Mateos & Santamaria, 2010), L. baudiniana (Santamaria, Mateos & Hurtado, 2014), L. oceanica (Raupach et al, 2014), as well L. vitiensis and L. dentipes (Santamaria et al, 2017). Molecular characterization of yet to be studied Ligia species may uncover evidence suggestive of cryptic diversification

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Discussion

Conclusion