Abstract
The supralittoral zones of sandy beaches are particularly vulnerable to coastal development and other activities that cause localised habitat destruction. Supratidal species, such as peracarid crustaceans, which lack a pelagic larval phase and tend to avoid direct contact with the swash, are therefore expected to be distributed as isolated populations, with implications for their long-term survival. Genetic population structure of one such species, the pill bug Tylos capensis, was investigated based on mitochondrial cytochrome c oxidase subunit I (COI) haplotype sequences, to assess demographic history and regional population connectivity in the presence of potential dispersal barriers (an estuary, coastal cliffs) along the Indian Ocean coast of South Africa. Here, pill bugs demonstrated strong and significant genetic structure at the regional scale, with 3 distinct clades across the species’ geographic distribution. At a localised spatial scale, coastal cliffs intersecting the high shore appeared to be a strong barrier to gene flow between adjacent populations, while a permanently open estuary did not limit gene flow. Estimates of historic gene flow and patterns of COI differentiation coincided with greater habitat continuity during the Pleistocene glaciations at sea levels between -75 and -120 m, when the African south coast was probably dominated by sandy beaches. While gene flow among low-dispersing pill bug populations is unlikely to benefit from a network of closely spaced coastal protected areas, the isolated nature of this species, coupled with the cryptic diversity inherent in this taxon, emphasizes the need for their protection. The importance of protecting the intact littoral active zone of beaches is highlighted.
Highlights
Open coastlines are naturally patchy, heterogeneous habitats punctuated by rocky shores, mixed shores, wave-cut platforms, estuaries, and sea cliffs (e.g. Harris et al 2011), in addition to man-made interruptions
The supratidal is dominated by crustaceans including ocypodid crabs, talitrid amphipods, and oniscid isopods, as well as several insect taxa (Brown & McLachlan 2006)
A large proportion of these species is ovoviviparous (Defeo & Gómez 2005), exhibiting direct development or brooding. These species are important processors of beach wrack (e.g. Lowman et al 2019), and so perform a crucial ecosystem function (Coupland et al 2007). Despite their consistent ecological function, many species have a high degree of cryptic diversity and deep divergences among regional clades, e.g. oniscid ispods (Hurtado et al 2013, 2014, Niikura et al 2015) and talitrid amphipods (Pavesi & Ketmaier 2013, Baldanzi et al 2016)
Summary
Open coastlines are naturally patchy, heterogeneous habitats punctuated by rocky shores, mixed shores, wave-cut platforms, estuaries, and sea cliffs (e.g. Harris et al 2011), in addition to man-made interruptions. Beach habitats show further variability, as sediment properties, wave action, and tidal amplitude vary at local and regional spatial scales to form different beach morphodynamic types (McLachlan et al 2018). A large proportion of these species is ovoviviparous (Defeo & Gómez 2005), exhibiting direct development or brooding These species are important processors of beach wrack Lowman et al 2019), and so perform a crucial ecosystem function (Coupland et al 2007) Despite their consistent ecological function, many species have a high degree of cryptic diversity and deep divergences among regional clades, e.g. oniscid ispods (Hurtado et al 2013, 2014, Niikura et al 2015) and talitrid amphipods (Pavesi & Ketmaier 2013, Baldanzi et al 2016). The supratidal zone, being physically extreme, harbours unique and often cryptic faunal diversity of ecological importance
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