DOI: 10.5061/dryad.qjq2bvqmf
title: Horseshoe crab body size cline

description: Aim Adult body size often exhibits patterns across large-scale
 environmental gradients, creating ecogeographic clines. However, the form
 of body size clines varies across taxonomic groups, with linear and
 non-linear patterns in body size observed in nature. Non-linear body size
 clines have received less study, and questions remain about how
 environmental gradients interact to produce non-linear clines. We examined
 the body size of the American horseshoe crab (Limulus polyphemus), a
 widely distributed marine arthropod, and evaluated the hypothesis that
 temperature and active season length can interact multiplicatively to
 result in a dome-shaped distribution. Location Fourteen states in the
 United States of America and three Mexican states, representing the entire
 geographic range of the species. Methods We compiled environmental data
 and body size measurements from more than 49,000 individual horseshoe
 crabs. For each location, we extracted from the literature or calculated
 from raw data the mean male prosoma width and the mean female prosoma
 width. We applied a General Additive Modeling (GAM) approach to
 characterize the body size cline, test a hypothesis regarding temperature
 and season length, and explore evidence for the influence of additional
 environmental factors. Results Model results indicate temperature and
 season length could act multiplicatively to produce dome-shaped clines,
 and these findings align with and quantify previous anecdotal reports of a
 strong dome-shaped body size cline across latitude for horseshoe crabs.
 Main conclusions Active season length appears to become relatively more
 influential on horseshoe crab body size in the northern part of their
 range, while temperature effects per se appear to dominate in southern
 latitudes. For horseshoe crabs, the pattern of size variation is
 consistent with the predictions of Optimal Resource Allocation models, but
 more study is needed to elucidate mechanistic underpinnings. Considering
 climate change projections, results from our study suggest future shifts
 in horseshoe crab body sizes.
description: Data Collection Body Size Data
 We searched journal articles, books, and federal and state agency
 reports for published information on the body size of mature adult
 horseshoe crabs in North America. We also located an unpublished
 manuscript that added information from Delaware Bay and a Smithsonian
 Institution website provided sizes for animals from the Indian River
 Lagoon in Florida, USA. We supplemented these with original field data
 collected from Stony Brook, New York, USA (2007); Skidaway (2007) and
 Sapelo (2009) Islands, Georgia, USA; Seahorse Key, Florida, USA
 (1995–1997, 2007–2009); and the citizen science program Florida Horseshoe
 Crab Watch (2015–2022; Heres et al., 2021).  In total,
 we compiled measurements from 49,877 individuals from 77 different
 locations in North America (14 USA states and 3 Mexican states),
 representing the entire geographic range of the species. For each
 location, we extracted from the literature or calculated from raw data the
 mean male and female prosoma width (PW) in cm, resulting in 153
 observations of mean adult horseshoe crab body size. Before analysis, we
 filtered out means that were based on less than three individuals due to
 concerns that low sample size could result in biased estimates, resulting
 in a total of 144 estimates of mean body size
 (n<sub>female</sub> = 73, n<sub>male</sub> =
 71). Body size measurements were derived only from
 mature adults in their terminal molt. Animals were mostly collected while
 spawning on the beach, but some were collected offshore during trawl
 surveys. While most body size measurements were reported as PW, four sites
 used inter-ocular distance (IO) and these data were converted to PW before
 analysis.  Environmental Data For
 locations in the USA, we gathered values from the National Oceanic and
 Atmospheric Administration’s (NOAA) online databases on the climatological
 mean for 1) annual sea temperature (°C) at 0 m, 2) salinity (ppt) at 0 m,
 and 3) tidal range (m), calculated as the difference between mean higher
 high water (MHHW) and mean lower low water (MLLW). For sites in Mexico,
 data for tidal range (m) were retrieved from the National Autonomous
 University of Mexico: National Tidal Service. We used the 2018 NOAA World
 Ocean Atlas, a uniformly formatted, quality controlled, data set compiled
 from more than 20,000 separate archived datasets and standardized using
 quality flags and objective tests, to gather the objectively analyzed
 means of both temperature and salinity at a ¼° scale. We selected values
 for temperature and salinity at 0 m depth because horseshoe crabs are
 often found in relatively shallow waters (i.e., depth &lt; 10 m). We
 then averaged temperature and salinity to within a 1° scale over all
 available years (2005–2017) (Boyer et al., 2018). We calculated active
 season as the number of days per year above the minimum temperature at
 which horseshoe crabs are active using NOAA’s Center for Operational
 Oceanographic Products and Services data from the nearest harmonic tidal
 buoy for daily water temperature averages. Based on previous studies
 (detailed above), we used the following minimum autumn and spring
 temperatures in order to calculate active season for our different
 locations: (1) Maine to New Hampshire, USA: spring = 11°C, autumn = 12°C;
 (2) Massachusetts to Delaware, USA: spring = 13°C, autumn = 14°C; (3)
 Maryland to Georgia, USA: spring = 15°C, autumn = 16°C; and (4) Florida,
 USA to Yucatán, Mexico: spring = 17°C, autumn = 18°C. We used NOAA data to
 determine tidal range at all but the Mexico sites by subtracting the
 yearly mean of the average higher high tide and average lower low tide for
 the most recent available year (2022) as daily tidal highs and lows are
 only retained for one year (Parker, 2007). At the sites in Mexico, we took
 the average annual monthly high tides and low tides and subtracted from
 each other to obtain the average tidal range. 
 Boyer, T.P. Garcia, H. E.. Locarnini, R. A.. Zweng, M. M..
 Mishonov, A. V.. Reagan, J. R., … Smolyar, I. V. (2018). World Ocean Atlas
 2018. NOAA National Centers for Environmental Information. Dataset. https://www.ncei.noaa.gov/archive/accession/NCEI-WOA18. Accessed 22 November 2021. Heres, B., Crowley, C., Barry, S., & Brockmann, H., (2021). Using citizen science to track population trends in the American horseshoe crab (Limulus polyphemus) in Florida. Citizen Science: Theory and Practice, 6(1), 19, 1–12. https://doi.org/10.5334/cstp.385 Parker, B. B. (2007). Tidal Analysis and Prediction. Silver Spring, MD: Center for Operational Oceanographic Products and Services, National Ocean Service, National Oceanic and Atmospheric Administration. NOAA Special Publication NOS CO-OPS 3. https://doi.org/10.25607/OBP-191