Element levels in muscle and eggs of a keystone species, the Atlantic horseshoe crab ( Limulus polyphemus) : risk to consumers that eat crabs depends upon tissue and route of exposure

Atlantic horseshoe crabs (Limulus polyphemus) are keystone species of estuarine food webs located from Maine to Florida. Crab eggs are critical for migrant red knot (Calidris canutus rufa, federally threatened), other birds, invertebrates, fish, and turtles. This study compared levels of arsenic (As), cadmium (Cd), chromium (Cr), lead (Pb), mercury (Hg), and selenium (Se) in horseshoe crab eggs and muscle from 2019 to 2021 obtained from Delaware Bay (New Jersey). Except for Cr, levels in muscle did not vary by year, but As, Cd and Hg varied significantly by year. Mercury in eggs increased from a mean of 11 ng/g in 2019 to 244 ng/g in 2021 and averaged 121 ng/g (2019) and 166 ng/g (2021) in leg muscle. Levels of Hg were just below the range known to adversely affect sensitive birds and mammals (adverse sublethal effects risk level [ASRL] = 200 ng/g). Mean As (4,330 ng/g to 8690 ng/g) were above the invertebrate minimum adverse effects risk level (ASRL, 2000 ng/g). Pb levels in leg muscle were 453 ng/g (2019) and 387 ng/g (2021), above ASRL levels ( > 200 ng/g) known to adversely affect many different animal tissues. Levels in eggs represent recent exposure, while levels in muscle reflect longer term sequestration. Crab tissues may currently pose a health risk to horseshoe crabs themselves and to sensitive biota because some species feed on them almost exclusively for a month during the crab spawning season. Hg levels from 2019/2021 were higher than in the 1990s. Risk to crabs and the other species that consume them depends upon date, tissue, and route of exposure.