Online Resource 1 Comparison of major factors found to influence spawning activity in different locations (from literature cited), including the Great Bay Estuary, New Hampshire. Florida locations: 1) Rudloe 1980; 2) Brockmann and Johnson, 2011. Delaware locations: 3) Smith et al. 2002a; 4) Smith et al. 2010. Massachusetts locations: 5) Cavanaugh 1975; 6) Barlow et al. 1986. Information on tide height from locations (from NOAA Tides and Currents 2014) are provided. ↑ indicates percentage of increase of tide heights between spring and neap tides.
Apalachee Bay (Florida)1 / Seahorse Key (Florida)2 / Delaware Bay(New Jersey, Delaware)3,4 / Mashnee Dike (Massachusetts)5,6 / Great Bay Estuary
(New Hampshire)
Lunar Cycle / More spawning horseshoe crabs were present during the full moon than new moon / No difference in spawning horseshoe crabs between new and full moon high tides; more pairs and unpaired males were present on new moon / More spawning horseshoe crabs were present during full and new moon of the first half of spawning season only 3 / · Spawning occurred throughout May and first week of June, regardless of moon phase 5
· More spawning horseshoe crabs were present during full and new moon 6 / Spawning occurred throughout May and June, regardless of moon phase
Highest high tides
/ Spawning only occurred during spring tides
[Mean high tide height during full and new moon ~1.1 m; ↑~23.2% from neap high tides (quarter moons)] / Spawning strongly correlated with actual maximum high tide height; more horseshoe crabs were present during tides over 125 cm
[Mean high tide height during full and new moon ~1.1 m;
↑~20.3% from neap high tides (quarter moons)] / Poor indicator alone of spawning activity 3
[Mean high tide height during full and new moon ~1.6 m; ↑~10.4% from neap high tides (quarter moons)] / More spawning horseshoe crabs were observed during spring tides, though significant numbers spawned during neap tides (lowest high tides) 6
[Mean high tide height during full and new moon ~1.2 m; ↑~7.28% from neap high tides (quarter moons)] / No relationship between spawning horseshoe crabs and the highest high tides (spring tides); spawning occurred during spring tides and neap tides (lowest high tides)
[Mean high tide height during full and new moon ~2.1 m; ↑~6.80% from neap high tides (quarter moons)]
Day versus night / More spawning horseshoe crabs were observed at night
[Day tides height higher than night] / No difference in horseshoe crab pairs between day and night high tides; more unpaired males were present at night / More horseshoe crabs were observed during nighttime tides 4
[Night tides higher than day tides] 4 / · Spawning was only observed at night 5
· Spawning was observed day and night (dependent on highest tide of a given day) 6
[Daily tide difference depends on lunar phase, varies during spring tides by ~1 m] 6 / In 2012, more spawning horseshoe crabs were observed during the day day; in 2013, there was no difference between day and night
[Night tides higher than day by ~0.5 m during spring tides, ~0.1 m during neap tides]
Temperature / Horseshoe crabs were present at 11–33°C; presence was more likely when above 20°C / Initiation of spawning correlated to water temperature 4 / Appears to have the strongest influence in spawning activity; most spawning activity occurred between ~15 to ~20°C
Wave height / Wave height correlated with the number of horseshoe crabs / Wave height did not correlate with spawning density, except when water depth was high due to high waves (20–30 cm). More horseshoe crabs pairs were present when waves were high and the number of unpaired females was strongly associated with higher waves / During lower wave heights (≤0.3 m), there were more spawning horseshoe crabs 3 / More horseshoe crabs were present during intermediate waves than when there were no waves and >0.3 m waves