DOI: 10.5061/dryad.63xsj3v02
title: Data from: Biogeography of shell morphology in over-exploited shellfish reveals adaptive tradeoffs on human-inhabited islands and incipient selectively driven lineage bifurcation

description: Aim: To identify potentially human-mediated biogeographic patterns in
 selection and adaptive tradeoffs affecting the evolution an over-exploited
 shellfish. Location: Hawaiian Archipelago Taxon: Mollusca, Gastropoda,
 Patellagastropoda, Nacellidae, Cellana exarata, ‘Opihi makaiauli Methods:
 We surveyed phenotypic characters associated with temperature and
 predation avoidance across the entire species range and tested for
 differences in the relationship between these characters and latitude, on
 islands with and without humans. Results: Among all limpets surveyed,
 there was a bimodal distribution in shell color (light, dark) and a
 parapatric pattern of shell coloration across the archipelago with lighter
 shells being prevalent on the uninhabited islands and darker, more
 camouflaged shells being prevalent on the inhabited islands.  On
 the cooler, uninhabited islands, all morphometric characters associated
 with thermal avoidance (surface area, height, and doming) increased with
 decreasing latitude. On the hotter, inhabited islands, however, shells
 were flatter, less variable, and less adapted for avoiding thermal stress
 than predation.  Main Conclusions: The biogeographic patterns in
 shell phenotype and previous genetic studies suggest that the population
 is beginning to bifurcate in response to disruptive and directional
 selection as well as geographic isolation between the islands with and
 without humans. Decreased phenotypic and genetic diversity on the
 inhabited islands despite much larger populations of ‘opihi suggests a
 prominent historical bottleneck. The prevalence of maladaptive dark, flat
 phenotypes for thermal avoidance on the inhabited islands suggests that
 predation is a stronger selective force, driving adaptive tradeoffs in
 shape and color.  We propose that this is likely a case of
 fisheries-induced evolution and a millennium of harvesting is the most
 likely selective pressure driving the observed biogeographic patterns in
 shell morphology.  The flatter, darker shells will allow body
 temperatures to rise higher in direct sunlight, therefore we hypothesize
 that the thermal niche of ‘opihi is narrower on inhabited islands and will
 continue to narrow as Earth warms.  This study highlights the
 utility of using intraspecific biogeographic patterns in phenotype to
 identify adaptive tradeoffs in response to varying selective pressures and
 identify nascent ecologically driven lineage splitting.
description: Living and presumably healthy<i> Cellana exarata</i>
 were collected from eight Hawaiian Islands between 2012 and 2016: ‘Ōnū
 (ON, Gardner Pinnacles, Puha honu), Lalo (LA, La Perouse Pinnacles,
 Mokupāpapa), Mokumanamana (MM, Necker), Nihoa (NI), Kaua‘i (KA), O‘ahu
 (OA), Maui (MA), and Hawai‘i (HI, Big Island; Table 1; Kikiloi et al.,
 2017). The NWHI were accessed during intertidal monitoring cruises
 sponsored by Papahānaumokuākea Marine National Monument, and eight ‘opihi
 were collected from each of six, 1 cm size classes. In the MHI, shells of
 legal harvesting size (&gt; 3 cm) were haphazardly collected along a 7
 km section of coastline on Maui, two locations on Kaua‘i (Miloli‘i and
 Kekaha) and O‘ahu (Kaka‘ako, ‘Ᾱina Moana), and one location on each of the
 remaining islands. All sites were composed of natural basalt formations
 with the exceptions of the O‘ahu and Hawai‘i sites (basalt riprap) and
 Kekaha (emergent coral reef). Site selection was largely haphazard because
 shell morphology (at the level investigated here) was observed to be more
 homogenous within than between the NWHI and MHI. Images were taken of the
 lateral face of each individual with other measurements and measurements
 from images in csv files which are processed and analyzed using R code
 found https://github.com/jdselwyn/Opihi_Morphology.