DOI: 10.5061/dryad.d51c5b0c1
title: Evolution in changing seas: The loss of plasticity under predator invasion and warming oceans

description: The impact of invasive predators during the early stages of invasion is
 often variable in space and time. Such variation is expected to initially
 favor plasticity in prey defenses but fixed defenses as invaders become
 established. Coincident with the range expansion of an invasive predatory
 crab in the Gulf of Maine we document rapid changes in shell thickness – a
 key defense against shell crushing predators – of an intertidal snail.
 Field experiments, conducted 20 years apart, revealed that temporal shifts
 in shell thickness were driven by the evolution of increased trait means
 and erosion of thickness plasticity. The virtual elimination of the
 trade-off in tissue mass that often accompanies thicker shells is
 consistent with the evolution of fixed defenses under increasingly certain
 predation risk.
description: <em>Crab Surveys</em> Crab surveys
 were conducted at eight wave-protected sites to characterize regional
 variation of green crab abundance in the Gulf of Maine. Four sites were in
 the northern Gulf (Quoddy Region of Maine) while the remaining four were
 in the southern Gulf (Nahant to Cape Ann, Massachusetts). Surveys lasted
 for one hour, began and concluded within two hours of low tide, and were
 conducted monthly from July to October in 2017-2018 and 2021. Historical
 crab density data for 1973 and 2003 were obtained from previously
 published surveys. <em>Clinal Variation in L.
 obtusata Shell Thickness and Tissue Mass</em>
 Between 1995 and 1997, our previous work
 (<em>29</em>) collected samples of <em>Littorina
 obtusata</em> from 25 populations spanning GOM rocky intertidal
 habitats. Between 2017 and 2018, we collected samples from 22 of the same
 populations (snails were not present at 3 of the sites sampled in
 1995-1997) to examine how phenotypic clines may have changed following
 increased ocean temperatures and increased green crab density in the
 northern GOM. For each population, snails (n = 50 per population) were
 haphazardly collected while attempting to maximize size range and returned
 to the laboratory for measurement of shell length and shell thickness with
 digital calipers. Mean shell thickness (hereafter, shell thickness) was
 calculated by taking the mean of whorl thickness and opposite whorl
 thickness. We then used a C-clamp to crack the shell of each snail and
 shell fragments were separated from soft tissue. Shell fragments and soft
 tissue were placed in separate aluminum trays and dried at 60°C for 48h
 before weighing on an analytical balance.
 <em>Reciprocal Transplant Experiment in the
 Field</em> To examine how shell thickness and its
 plasticity have changed after 20 years (1998-2018), in 2018 we repeated
 the reciprocal transplant experiment that was conducted in 1998. To allow
 a robust comparison of the two experiments, we carefully replicated all
 aspects of the 1998 experiment including using the same experimental
 chambers that were deployed into the field 20 years earlier. In early May
 2018, we collected juvenile <em>L. obtusata </em>(5-6 mm in
 shell length) from a northern (Quoddy Head, Lubec, ME) and southern
 (Lobster Cove, Manchester, MA) site in the Gulf. All snails were
 individually tagged with a color-coded dot of permanent ink that was then
 sealed with cyanoacrylate glue. We measured initial shell length and shell
 thickness with digital calipers (± 0.01 mm) and shell mass and tissue mass
 (± 0.001 g) were estimated using a non-destructive weighing
 technique. After completing initial measurements, we
 transported snails from both populations to the northern or southern site
 in mid-May. At each site, we placed six snails (hereafter, response
 snails) from a single population and approximately 60 g of brown algae
 (<em>Ascophyllum nodosum</em>) as food into 24 separate,
 replicate cylindrical containers (5-cm height x 10-cm diameter) that had
 mesh windows (mesh size = 3mm) to allow water flow. Hence, at each site 12
 replicate containers housed snails from either the northern or southern
 population and 6 replicates for each population exposed snails to either
 the presence (Crab) or absence (No Crab) of predation risk. To create
 these risk treatments, each container stocked with response snails was
 secured beneath a similar container that was perforated on all sides and
 housed either (a) a mature male green crab (Crab) and 30 conspecific
 snails (hereafter, stimulus snails) or (b) just 30 stimulus snails (No
 Crab) to serve as a control. Each pair of stimulus-response containers was
 placed inside a large, replicate cylindrical chamber (11-cm height x 28-cm
 diameter) that had mesh windows (mesh size = 3mm) to permit water flow.
 These large chambers were anchored haphazardly in the mid-intertidal zone
 (~1.5 m MLW). Ocean temperature was monitored at 5-minute intervals during
 the experiment with dataloggers (Tidbits, model UTBI-001, Onset Computer
 Corp.) that were placed inside 3 replicate chambers at each site. Every 14
 days we replaced stimulus snails in both the Crab and No Crab containers.
 For appropriate replicates, we also confirmed that crabs were alive; any
 dead crabs were replaced immediately. Overall, we had to replace 7 crabs
 at the northern site and 7 crabs at the southern site. At day 45, we
 replaced the <em>Ascophyllum</em> that served as food for
 response snails in all replicates. After 90 days in the field, all
 response snails were returned to the Northeastern University Marine
 Science Center (Nahant, Massachusetts) for measurement of final snail
 shell length, shell thickness, and tissue mass.
description: # Evolution in changing seas: The loss of plasticity under predator
 invasion and warming oceans
 [https://doi.org/10.5061/dryad.d51c5b0c1](https://doi.org/10.5061/dryad.d51c5b0c1) ## Description of the data and file structure The dataset contains three Excel spreadsheets which encompass all of the data used in statistical analyses for this manuscript. The data include (1) the density of green crabs (*Carcinus maenas*) at multiple sites in the Gulf of Maine for 1973, 2003, 2017-2018, and 2021, (2) morphological measurements (shell length and thickness, shell and tissue mass) of snails (*Littorina obtusata*) that were reciprocally transplanted between a northern and southern site in the Gulf of Maine and raised in the presence and absence of green crab risk cues, and (3) morphological measurements (shell thickness and tissue mass) for multiple snail populations across the Gulf of Maine 1995-1997 and 2017-2018. All data are presented in tabular form. Spreadsheet descriptions and column definitions are provided below. ### Files and variables #### File: Trussell.Corbett.Updated.ReciprocalTransplantExperiment.xlsx **Description:** Contains the data used in analyses of initial and final shell thickness (mm), shell length (mm), shell mass (mg) and tissue mass (mg) of snails (*Littorina obtusata*) that were reciprocally transplanted between a northern and southern site in the Gulf of Maine and raised in the presence and absence of green crab risk cues. ##### Variables * **Year** – Denotes the year in which the experiment was conducted. * **Population** – Denotes the source population (North, South) of the snails used in the reciprocal transplant experiment. * **Location** – Denotes the site (North, South) at which snails were raised as part of the reciprocal transplant experiment. * **Risk Treatment** – Denotes whether snails were raised in the presence (Crab) or absence (No Crab) of green crab risk cues during the reciprocal transplant experiment. * **Replicate** - Denotes the specific unit that snails were raised in as part of the reciprocal transplant experiment. * **Initial Shell Thickness** – Represents the shell thickness (mm) of snails at the beginning of reciprocal transplant experiment. * **Initial Shell Length** – Represents the initial shell length (mm) of snails at the beginning of reciprocal transplant experiment. * **Initial Shell Mass** – Represents the initial shell mass (mg) of snails at the beginning of reciprocal transplant experiment. * **Initial Tissue Mass** – Represents the initial tissue mass (mg) of snails at the beginning of reciprocal transplant experiment. * **Final Shell Thickness** – Represents the shell thickness (mm) of snails at the end of reciprocal transplant experiment. * **Final Shell Length** – Represents the initial shell length (mm) of snails at the end of reciprocal transplant experiment. * **Final Shell Mass** – Represents the initial shell mass (mg) of snails at the end of reciprocal transplant experiment. * **Final Tissue Mass** – Represents the initial tissue mass (mg) of snails at the end of reciprocal transplant experiment. #### File: Trussell.Corbett.Updated.GreenCrabDensity.csv **Description:** Contains the data used in analyses of green crab (*Carcinus maenas*) density across multiple years and sites in the Gulf of Maine. ##### Variables * **Site Location Year** – Denotes the site name of the collection site and the year data were collected. * **Latitude Year** – Indicates the latitude of each sampling site. * **Green Crab Density Year** – Indicates the number of green crabs found at each site and year. #### File: Trussell.Corbett.Updated.ShellThickness.TissueMass.Cline.xlsx Description: Contains the data used in analyses of shell thickness (mm) and tissue mass (mg) for multiple snail (*Littorina obtusata*) populations in the Gulf of Maine for the years spanning 1995-1997 and 2017-2018. *Note that the cell for tissue mass in row 850 is empty because that sample was lost. ##### Variables * **Year** – Denotes the years in which the snail populations were sampled. * **Site Name** – Denotes the name of each sampling site. * **Latitude** – Indicates the latitude of each sampling site. * **Shell Length** – Indicates the shell length (mm) of each snail. * **Shell Thickness** – Indicates the shell thickness (mm) of each snail. * **Tissue Mass** – Indicates the tissue mass (mg) of each snail. * **Shell Mass** – Indicates the shell mass (mg) of each snail. ## Access information Data was derived from the following sources: * All data were collected by J.C. Corbett and G.C. Trussell. Green crab density data for 1973 (Welch et al. 1973) and 2003 (Edgell & Rochette 2008) were obtained from previously published work.