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.

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 We also located an unpublished manuscript that added information from Delaware Bay and a Smithsonian Institution website provided sizes for animals from the Indian River Lagoon in Florida,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_female = 73, n_male = 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 < 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