Abundance and population structure of the Atlantic horseshoe crab Limulus polyphemus in Pleasant Bay, Cape Cod

Abstract

Populations of horseshoe crabs Limulus polyphemus are widely distributed from Maine to the Gulf of Mexico, are commercially harvested, and are thought to have decreased in abundance in recent decades. To provide information needed to manage exploited populations of horseshoe crabs, we conducted comprehensive field sampling in a major shallow estuary containing preferred habitat for horseshoe crabs. The data we obtained are the first to comprehensively define abundance, determine sex ratios of horseshoe crabs across an entire estuary, and estimate cohort-specific growth and mortality of juvenile and adult horseshoe crabs. We found that juveniles were more abundant and suffered greater mortality than adults. Adults were largely found on the sediment surface, but 20% were buried shallowly in sediments. The male:female sex ratio in juveniles was 1.4:1, but the adult sex ratio was 2.3:1. Juveniles grew faster than adults, and adult crabs may plausibly molt as frequently as once per year rather than have a terminal molt. Spawning appeared to span late March to mid-July, and juveniles hatched at semilunar intervals during Year 0 and grew to 16.6 ± 0.9 mm prosomal width by the start of Year 1. The distinct semilunar cohorts of Year 0 coalesced into annual cohorts after Year 0. Females deposited large numbers of eggs, but only 0.001% survived to the end of Year 0, and approximately 78% of these juveniles reached adulthood. KEY WORDS: Population dynamics · Growth · Sex ratio · Mortality · Spawning · RecruitmentResale or republication not permitted without written consent of the publisher

Conservation status of the American horseshoe crab, (Limulus polyphemus): a regional assessment

“…The condition of the individual, which is probably age related, is also a factor in stranding-related mortality (Penn and Brockmann 1995;Smith et al 2010). Carmichael et al (2003) found that in Pleasant Bay, Massachusetts, adults had a lower estimated mortality rate than juveniles, and there was no significant difference in estimated mortality rate for adult males and females. In contrast, Butler (2012) found through analysis of mark-recapture data from Delaware Bay that adult male annual survival (77%, SE = 0.04) was greater than adult female survival (65%, SE = 0.09).…”

“…Growth is relatively rapid during the first several years, progressing through stages I-V in the first year, stages VI-VII the second year, stages VII-IX the third year, with a single molt per year after that until reaching maturity (Shuster 1982). The pattern of development appears to the same throughout the species' range despite large regional differences in environmental attributes (Estes et al 2015;Carmichael et al 2003). Shuster (1950) approximated that it took 9-12 years for horseshoe crabs to reach sexual maturity.…”

“…Unattached males return to the beach more frequently than females, creating male-biased OSR and male-male competition for mates (Rudloe 1980;Brockmann 1990;Smith et al 2002aSmith et al , 2010Brockmann and Smith 2009;Brockmann and Johnson 2011). The mean OSR in unharvested populations is generally 1.5-2.4 males to females (Rudloe 1980;Wenner and Thompson 2000;Schaller 2002;Brockmann and Johnson 2011), but in populations with female-biased harvest the sex ratio is elevated, i.e., 3 to 8 males/female (Smith et al 2002b(Smith et al , 2009aCarmichael et al 2003;James-Pirri et al 2005;Kreamer and Michels 2009). Mean OSR during the 2012-2013 spawning season in Chuburná, Yucatán, was 1.1 males to females (unpublished data, J. Gutiérrez and J. Zaldívar-Rae, Anáhuac Mayab University).…”

“…Harvest for the marine life aquaria trade or scientific and educational collection is small in comparison to other uses, but is significant in Florida where juveniles are collected in large numbers (Gerhart 2007). Substantial evidence suggests that overharvest can result in depleted populations and localized extirpations (Widener and Barlow 1999;Carmichael et al 2003;Rutecki et al 2004;Schaller et al 2005;Gerhart 2007;Smith et al 2009b;McGowan et al 2011b). …”

The Role of Horseshoe Crabs in the Biomedical Industry and Recent Trends Impacting Species Sustainability

“…With a characteristically high mortality rate the early stages of life for numerous marine species, approximately 0.001% of crabs survive through the first year (Carmichael et al, 2003;Sweka et al, 2007). Researchers have had some success from collecting horseshoe crab eggs, rearing them in a laboratory, and releasing the crabs as juveniles.…”