ArticlePDF Available

Influence of Environmental Factors on Spawning of the American Horseshoe Crab (Limulus polyphemus) in the Great Bay Estuary, New Hampshire, USA

  • New York Sea Grant / Science and Resilience Institute at Jamaica Bay

Abstract and Figures

The Great Bay Estuary, New Hampshire, USA is near the northern distribution limit of the American horseshoe crab (Limulus polyphemus). This estuary has few ideal beaches for spawning, yet it supports a modest population of horseshoe crabs. There is no organized monitoring program in the Great Bay Estuary, so it is unclear when and where spawning occurs. In this 2-year study (May through June, 2012 and 2013), >5,000 adult horseshoe crabs were counted at four sites in the estuary. The greatest densities of horseshoe crabs were observed at Great Bay sites in the upper, warmer reaches of the estuary. Peaks of spawning activity were not strongly correlated with the times of the new or full moons, and similar numbers of horseshoe crabs were observed mating during daytime and nighttime high tides. While many environmental factors are likely to influence the temporal and spatial patterns of spawning in this estuary, temperature appears to have the most profound impact.
Content may be subject to copyright.
237© Springer International Publishing Switzerland 2015
R.H. Carmichael et al. (eds.), Changing Global Perspectives on Horseshoe Crab
Biology, Conservation and Management, DOI 10.1007/978-3-319-19542-1_13
Chapter 13
The Life History Cycle of Limulus polyphemus
in the Great Bay Estuary, New Hampshire
Helen Cheng , Christopher C. Chabot , and Winsor H. Watson III
Abstract The overall goal of this chapter is to provide an overview of the life
history cycle of the American horseshoe crabs ( Limulus polyphemus ) that reside in
the Great Bay Estuary, New Hampshire, U.S.A. Great Bay horseshoe crabs generally
spawn during high tides in the spring. Based on recent work and studies, spawning
appeared to be triggered by increases in water temperature, and animals seemed to
prefer to spawn in the warmest sections of the estuary. However, in contrast to horse-
shoe crabs in some other areas of the U.S.A., peaks of spawning activity did not
necessarily correspond with the new and full moons, or with the highest tides, and
similar numbers of animals were observed spawning during day and night high tides.
Once the eggs hatch, it is hypothesized that their planktonic larvae are likely trans-
ported to the upper regions of the estuary where they settle on the expansive mudfl ats
that characterize most of the Great Bay Estuary. At ~9 years (about the 17th instar
stage), males appear to reach sexual maturity, while it appears that females molt one
more time before reaching sexual maturity. This difference, along with a tendency
for males to approach mating beaches more often than females, may contribute to a
sex ratio that is skewed towards males at most spawning beaches in the estuary.
Keywords Limulus polyphemus Great Bay Estuary Spawning Juveniles
Mating Thermal preferences Estuary Sex ratios Tidal rhythms
13.1 Introduction
In this overview, our goal is to bring together the results of published and more
recent unpublished studies to provide an overview of the life history cycle of
the horseshoe crabs that reside in the Great Bay Estuary, New Hampshire, U.S.A.
H. Cheng W. H. Watson III (*)
Department of Biological Sciences , University of New Hampshire , Durham , NH 03824 , USA
C. C. Chabot
Department of Biological Sciences , Plymouth State University , Plymouth , NH 03264 , USA
This estuary is located at the northern end of the range of horseshoe crabs and,
perhaps as a result, there are both similarities and differences in the behavior and
life history of this subpopulation of Limulus compared to other populations that
have been extensively studied in other areas. We will begin this overview in the
early spring, when overwintering adults begin to migrate to intertidal areas to
spawn, and we will end with the juvenile horseshoe crabs that grow up in the estuary
and reach sexual maturity after ~17 molts (Sekiguchi et al. 1988 ).
13.2 The Great Bay Estuary of New Hampshire, U.S.A.
The Great Bay Estuary is a complex, semi-enclosed, embayment near the New
Hampshire-Maine border. It is separated into three different areas: the Piscataqua
River, Little Bay, and Great Bay (Fig. 13.1 ). The rivers that fl ow into the Great Bay
Estuary drain a watershed that extends more than 1,600 km
2 , and this convergence
of land and water shapes the features and uses of the ecosystem (Short 1992 ). There
are fi ve dominant aquatic habitats: eelgrass meadows, mudfl ats, salt marsh, channel
bottom, and rocky intertidal. Additionally, the large quantities of water that move in
and out of the estuary create some of the strongest tidal currents in North America
Fig. 13.1 Map of the Great Bay Estuary, New Hampshire, U.S.A. The Great Bay Estuary is 16 km
inland from the Gulf of Maine and the Atlantic Ocean
H. Cheng et al.
(Short 1992 ). This tidal exchange structures the Great Bay Estuary ecosystem by
affecting water quality, habitat extent, and species distributions.
The Great Bay Estuary appears to support a large population of horseshoe crabs
(Watson et al. 2009 ; National Marine Fisheries Service 2010 ; Schaller et al. 2010 ;
Watson and Chabot 2010 ). Using tag-recapture methods and the simplifi ed Lincoln-
Peterson Index calculation (Krebs 1998 ), preliminary population estimates in 2012
and 2013 indicated that there were approximately 122,430 horseshoe crabs that
spawned along the shorelines of the Great Bay Estuary in the spring (Cheng, unpub-
lished data).
13.3 Spawning Activity
Horseshoe crab spawning in the estuary generally occurs during the months of May
and June, when both mature males and females approach spawning beaches. More
than 95 % of the females approaching spawning beaches have a male attached
(amplexed) to them (Cheng 2014 ), and interestingly, since we often observe male:
female pairs while diving throughout the year, pair formation is not unique to the
mating season. Beach approaches by single males are also common; however,
amplexed pairs outnumber single males by ~3:1 (2.86 pairs to each single male).
13.3.1 Large-Scale Movements to Spawning Beaches
In the Great Bay Estuary, horseshoe crabs tend to overwinter in deep areas that are
2–5 km from where they spawn (Schaller et al. 2010 ; Watson and Chabot 2010 ). In
contrast to reports from the mid-Atlantic States such as New York, New Jersey,
Maryland, Delaware and Virginia (Shuster and Botton 1985 ; Botton and Ropes
1987 ; Walls et al. 2002 ), horseshoe crabs in northern bays and estuaries, such as the
Great Bay Estuary, Taunton Bay, Maine and Pleasant Bay, Massachusetts, do not
appear to move into coastal waters or offshore at any time of the year (Moore and
Perrin 2007 ; James-Pirri 2010 ; Schaller et al. 2010 ). Based on high resolution track-
ing of animals equipped with ultrasonic transmitters, horseshoe crabs are very sed-
entary during the coldest months of the year (Schaller et al. 2010 ; Watson and
Chabot 2010 ). Then, in the spring, as estuarine waters warm and exceed ~8–10 °C,
they become active and migrate further up into the estuary where it tends to be
warmer and the salinity is often lower. These data are consistent with similar telem-
etry studies in Taunton Bay and Pleasant Bay (Moore and Perrin 2007 ; James-Pirri
2010 ) and with laboratory investigations showing the close relationship between
activity rhythms and water temperature (Watson et al. 2009 ; Chabot and Watson
2010 ; Chabot et al. 2011 ).
While this pattern of moving up estuary, or into bays, appears to be consistent at
different locations, it is not clear what cues these animals use to fi nd their spawning
13 The Life History Cycle of Limulus polyphemus in the Great Bay Estuary, New
beaches. While chemical, tactile, and visual cues likely aid their ability to locate
each other (Barlow 1983 ; Barlow et al. 1984 , 2001 ; Powers and Barlow 1985 ;
Barlow and Powers 2003 ; Schwab and Brockmann 2007 ; Saunders et al. 2010 ) and
suitable beaches for laying their eggs (Botton et al. 1988 ), these same cues are prob-
ably not useful for guiding them long distances to the general area where they spawn
because odors would be rapidly diluted and dispersed in most locations; tactile cues
only are effective in their immediate vicinity; and visual cues are only useful for
distances of a few meters, especially in an estuary. Therefore, it is more likely that
horseshoe crabs are detecting physical cues and gradients of things such as salinity
and/or temperature. James-Pirri ( 2010 ) found that female horseshoe crabs were
located in the shallow upper-regions of Pleasant Bay in the spring, moved towards
the deeper portions of the bay in the fall, and then moved up to the upper regions
again the following spring. Schaller et al. ( 2010 ) also found a similar trend in
seasonal movements in the Great Bay Estuary and further reported that when tem-
peratures exceeded 10–11 °C in the spring, horseshoe crabs started to become active
and moved up-estuary. Then, when water temperatures became colder in the fall, the
horseshoe crabs moved to deeper areas with more stable conditions where they
spent the winter. Moreover, we fi nd many spawning horseshoe crabs in the warmest
parts of the estuary and up into the rivers that empty into the estuary, and our
preliminary laboratory studies, described later in this overview, also suggest that
horseshoe crabs might seek the warmest regions of bays and estuaries to spawn.
13.3.2 Timing of Mating Activity
While there is some evidence from certain study locations that horseshoe crabs tend
to mate during the days that coincide with the new and/or full moons (Rudloe 1980 ;
Cohen and Brockmann 1983 ; Barlow et al. 1986 ; Smith et al. 2002 ; Brockmann
2003 ; Table 13.1 ), in our study location there was no clear bias towards increased
Table 13.1 Comparison of major factors found to infl uence spawning activity among locations,
including the Great Bay Estuary (Cheng
2014 )
Delaware Bay
(New Jersey,
c Mashnee Dike
Great Bay
Estuary (New
cycle More
during full
moon than
new moon
during full
moon than
new moon
Only rst half of
season, more
animals during
full and new
throughout May
and fi rst week of
June, regardless
of moon phase
throughout May
and June,
regardless of
moon phase
More spawning
animals during
full and new
e (continued)
H. Cheng et al.
Table 13.1 (continued)
Delaware Bay
(New Jersey,
c Mashnee Dike
Great Bay
Estuary (New
occurred on
spring tides
occurred on
spring tides;
during high
tides of full
moon than
high tides
of new
Poor indicator
alone of
More spawning
animals observed
during spring
tides, though
signifi cant
numbers spawn
on neap tides
(lowest high
No relationship
of spawning
animals to
highest high
tides (spring
tides); spawning
occurred on
spring tides and
neap tides
(lowest high
f [Mean high
tide height
during full
and new
~1.1 m; ~
23.2 % from
neap high
f [Mean high
tide height
during full
and new
~1.1 m; ~
20.3 %
from neap
high tides
f [Mean high tide
height during
full and new
moon ~1.6 m;
~10.4 % from
neap high tides
(quarter moons)]
f [Mean high tide
height during full
and new moon
~1.2 m;
~7.28 % from
neap high tides
(quarter moons)]
f [Mean high tide
height during
full and new
moon ~2.1 m;
~ 6.80 % from
neap high tides
(quarter moons)]
observed at
observed at
day than at
Spawning only
observed at night
4 In 2012, more
observed at day;
in 2013, no
between day and
[Day tides
higher than
[Day tides
higher than
night by ~
0.2 m]
observed day and
night (dependent
on highest tide of
a given day)
[Night tides
higher than day
by ~0.5 m during
spring tides,
~0.1 m during
neap tides]
[Daily tide
depends on lunar
phase, varies
during spring
tides by ~1 m]
a Rudloe ( 1980 )
b Cohen and Brockmann ( 1983 )
c Smith et al. ( 2002 )
d Cavanaugh ( 1975 )
e Barlow et al. ( 1986 )
f Tide heights were gathered from May 2014 NOAA Tide Predictions
13 The Life History Cycle of Limulus polyphemus in the Great Bay Estuary, New
spawning at these times or during the highest high tides (Fig. 13.2 ; Table 13.1 ;
Cheng 2014 ). There was no signifi cant difference in the densities of spawning
horseshoe crabs during the days around the new moon, full moon, and equivalent
time periods outside of the moon cycles for both 2012 and 2013 combined ( p = 0.430;
Kruskal-Wallis test). This could be due to the characteristics of the tidal system and
spawning beaches in the Great Bay Estuary. For example, certain well-studied
Florida beaches (such as north and west of Apalachee Key and Seahorse Key;
Table 13.1 ) have minimal tides throughout most of the month, except during the full
and new moons. Therefore, if horseshoe crabs use water depth as a cue for the
occurrence of high tides (and this appears to be the case; Watson et al.
2008 ), and
thus for aggregating on spawning beaches, these might be the only times of the
month when that cue might be above their detection threshold. In contrast,
tide heights outside the full and new moons (neap tides) in the Great Bay Estuary
are always large enough to entrain horseshoe crabs to the tidal rhythm (Chabot and
2010 ), and the small increases in tide height that occur around the new and
full moons (6.8 % increase from neap tide to spring tides; Table 13.1 ) might not be
signifi cant enough to cause a change in their behavior.
Another difference between the spawning beaches surrounding the Great Bay
Estuary and those in some other areas along the East Coast is that the Great Bay
beaches are generally shallow and not very long. Therefore, during a large high tide,
Fig. 13.2 The relationship between spawning horseshoe crab density (number of animals per square
meter) and mean temperature per 5-day interval for May ( M ) and June ( J ) of 2012 and 2013. Five-day
intervals when the full moon ( open circle ) and new moon ( darkened circle ) occurred are indicated
H. Cheng et al.
the water can completely submerge a spawning beach, leaving little room for
mating where the water meets the beach. This might actually lead to fewer animals
mating during these higher tides and thus might, in part, explain why we did not fi nd
a close relationship between tide height, or a lunar cycle, and the number of mating
horseshoe crabs (Fig. 13.2 ; Cheng 2014 ).
In contrast to a number of previous reports from studies conducted in other parts
of the geographic range of Limulus , there was no clear tendency for horseshoe crabs
to spawn during the night high tides in comparison to daytime high tides (Table 13.1 ;
Fig. 13.3 ; Cheng 2014 ). In fact, in 2012 there were signifi cantly more crabs spawn-
ing during the day ( p = 0.016, Wilcoxon matched-pairs signed-ranks test, n = 12 sur-
vey dates; Fig. 13.3 ). Powers et al. ( 1991 ), Chabot et al. ( 2007 ), and Watson et al.
( 2008 ) proposed that horseshoe crabs evolved the ability to increase their visual
sensitivity at night so they could effectively mate during either, or both, day and
night high tides, which they appear to do in many locations. However, even though
horseshoe crabs might be able to see well at night, due to the turbidity of many
estuaries and the low amount of contrast and color difference between horseshoe
crabs and the substrate, it might be somewhat more advantageous for them to mate
during the day when they can probably see each other and other objects better
(Barlow 1983 ; Barlow et al. 1984 , 2001 ; Krutky et al. 2000 ).
13.3.3 Temperature
In the Great Bay Estuary both the timing and location of horseshoe crab mating are
closely correlated with water temperature. For example, while spawning typically
commences each year in May, in 2012, as the result of a warm winter and an unusual
Fig. 13.3 Mean spawning horseshoe crab densities during day and night high tides in 2012 ( n = 12
survey dates) and 2013 ( n = 39 survey dates). Bars represent standard error of the means
13 The Life History Cycle of Limulus polyphemus in the Great Bay Estuary, New
warm spell in March and April, horseshoe crabs moved towards spawning areas
sooner and spawned about 2 weeks earlier than usual (Fig. 13.2 ; Cheng 2014 ). In
contrast, in 2013 spawning activity started at the more typical time in mid-May,
when water temperatures began to warm and surpassed 11–13 °C (Fig. 13.2 ), and
then continued into June. This is consistent with reports that the greatest spawning
activity generally occurs at this time at other locations within their geographic range
(Shuster 1979 , 1982 ; Shuster and Botton 1985 ; Barlow et al. 1986 ).
The rst large surge in spawning activity in the Great Bay Estuary is usually
followed by two to three more peaks, which are separated by 2–8 days, and these
peaks are generally associated with rising water temperatures. Thus, spawning may
be modulated by the rate of increase in water temperature, rather than just higher
temperatures. An additional factor causing the periodic peaks in spawning activity
might be a physiological refractory period during which female horseshoe crabs
need to recharge their ability to produce and extrude mature eggs. Thus, even in the
absence of higher tides associated with the full and new moons, or peaks in water
temperature, there will be oscillations in the number of females spawning through-
out the season.
The greatest numbers of spawning animals were always observed at the beaches
that were located furthest up into the estuary, where the water temperature was also
signifi cantly warmer (Fig. 13.4 ; these beaches do not appear to differ in any obvious
way from other beaches in the estuary; Cheng 2014 ). This observation is consistent
with recent behavioral assay laboratory studies demonstrating that horseshoe crabs
signifi cantly spend more time on the warmer side than the colder side of a Y-maze
( p = 0.023; unpaired t -test, Fig. 13.5 ; Cheng 2014 ). Thus, at least in the Great Bay
Estuary, seasonal and daily changes in water temperature appear to be the primary
drivers of mating activity and strongly infl uence both when and where animals mate.
Fig. 13.4 Graduated bubble map of spawning horseshoe crab density (horseshoe crabs/m 2 ) at each
survey site, along with the mean temperatures at each site in 2012 ( a ) and 2013 ( b )
H. Cheng et al.
Fig. 13.5 The mean percent time spent by horseshoe crabs in the experimental zone (warm water)
during control and experiment phases of Y-maze behavioral studies. Each experiment included
a 1-h control period and a 1-h experimental period. In the fi rst experiment ( a ), animals were given
a choice between ambient water and warmer water ( n = 6), while in the second experiment
( b ), animals were given a choice between ambient and ambient (Control), and then cold and warm
water (Experiment; n = 7). Bars represent the standard error of the means
13 The Life History Cycle of Limulus polyphemus in the Great Bay Estuary, New
13.4 Growing Up in the Great Bay Estuary
13.4.1 Juvenile Horseshoe Crabs and the Location of Nursery
The extensive mudfl ats of the Great Bay Estuary appear to be important horseshoe
crab feeding habitats for adults and juveniles (Lee 2010 ). These areas are generally
characterized by an abundance of meiofaunal and infaunal prey and the absence of
predators, which are usually restricted to deeper, subtidal habitats (Gunderson et al.
1990 ; Morrison et al. 2002 ; Holsman et al. 2006 ). During SCUBA surveys carried
out at times of high tides on mudfl ats throughout the Great Bay Estuary, we found
the most juvenile horseshoe crabs in the upper regions of the estuary, in Great Bay
proper, and in areas that were not exposed to air at low tide. Juveniles were found
both adjacent to adult horseshoe crab spawning beaches, as well as in areas that
were at least 0.5–2.5 km from a known breeding beach (Fig. 13.6 ; Cheng 2014 ).
Interestingly, there were no juvenile horseshoe crabs found during any of the sur-
veys in Little Bay, despite surveying adjacent to documented spawning beaches.
This may be due to the way larvae are carried by tidal currents, and perhaps the way
that larvae move up and down in the water column (Rudloe 1979 ; Shuster 1982 ;
Sekiguchi 1988 ; Botton and Loveland 2003 ; Ehlinger and Tankersley 2006 ).
13.4.2 Interactions Between Larval Behavior and Tidal
Many estuarine, benthic invertebrates produce planktonic larvae that have evolved
strategies to increase their chances of either being retained in the estuary or carried
out of the estuary. This is accomplished using tidal-infl uenced vertical migration or
selective tidal-stream transport (Forward and Tankersley 2001 ; Forward et al. 2003 ).
After hatching, horseshoe crabs become planktonic (Rudloe 1979 ; Shuster 1982 ;
Sekiguchi 1988 ; Botton and Loveland 2003 ) and often migrate vertically within the
water column (Ehlinger and Tankersley 2006 ). However, it is unclear how long they
stay planktonic, or if they time their excursions into the water column in a manner
that would infl uence their transport within the estuary.
Tidal currents vary throughout the Great Bay Estuary, with fast moving currents
ranging from 0.75 to 1.5 m/s in some areas of Little Bay to currents less than 0.5 m/s
in the vast expanses of Great Bay (Short 1992 ). Drifter studies, using devices
designed to mimic the passive drift of larvae, have been conducted in the Great Bay
Estuary to investigate American lobster ( Homarus americanus ) larval transport
(Goldstein 2012 ) and in all cases the drifters were retained within the estuary,
regardless of whether they were released on a fl ood or ebb tide. Moreover, those
released in Little Bay ended up in Great Bay (“upstream”). Recently, we repeated
some of these drifter studies, releasing them adjacent to selected mating beaches in
Little Bay and Great Bay, and most of them, as predicted from Goldstein’s ( 2012 )
H. Cheng et al.
work, were transported up into Great Bay (Fig. 13.6 ; Watson et al. unpublished
data). Thus, it would appear that planktonic larvae are rarely exported out of the
system and may, in fact, use these currents to increase their retention in the estuary.
Specifi cally, horseshoe crab larvae that are hatched in spawning grounds in Little
Bay are likely transported to areas in Great Bay where the current speeds are reduced
in magnitude, and then they settle on appropriate soft bottom sediments.
13.4.3 Growth and Maturation
The majority of the juvenile horseshoe crabs found SCUBA diving were 45–85 mm
in prosomal width (Fig. 13.7 ). Smaller juveniles were either in different areas or not
captured because they were hard to see due to their size and cryptic coloration. It is
Fig. 13.6 Graduated bubble map of juvenile horseshoe crab densities (horseshoe crabs/m
2 ) in
Great Bay and Little Bay, based on dive surveys in 2012 and 2013 combined. Adult spawning
survey beaches (Cheng
2014 ) are indicated by Δ’s (also, refer to Fig. 13.4). X’s indicate that no
juveniles were found during surveys in these areas. The gray lines show the paths taken by two
drifters that were released in Little Bay ( 1 ) and Great Bay ( 2 ) in 2014. Note that there is a tendency
for the drifters to move up into Great Bay
13 The Life History Cycle of Limulus polyphemus in the Great Bay Estuary, New
also likely that many could have been buried in the sediment at the time the surveys
were conducted. Based on size frequency plots, it appears as if juveniles in the Great
Bay Estuary increase in size by about 30 % with each molt (Fig. 13.7 ), which is
consistent with previous studies (Carmichael et al. 2003 ; Burton et al. 2009 ).
Carmichael et al. ( 2003 ) measured the growth of juvenile horseshoe crabs in
Pleasant Bay, Massachusetts and also found that horseshoe crabs grew ~25–35 %
larger with each molt. Based on the sizes of juveniles collected in the Great Bay
Estuary, these horseshoe crabs were likely 1–8 years old (according to Table I from
Sekiguchi et al. 1988 and Figure VI-44 from Sekiguchi 1988 ).
During shoreline surveys on nearby spawning beaches, the most abundant small
males were 120 mm in prosomal width, and the most abundant small females were
150 mm (Fig. 13.8 ; Cheng 2014 ). Thus, we rarely saw horseshoe crabs that were
100–115 mm in either our dive surveys or spawning surveys. It is likely that this
“missing” cohort of juveniles were beginning to move into areas that were deeper
than where the smaller juveniles were found and yet were not sexually mature, so
they were also not observed on the mating beaches.
After undergoing approximately 17 molts, in the span of 7–11 years, horseshoe
crabs reach their terminal molt and stop growing (Shuster 1950 ; Walls et al. 2002 ).
Given the sizes of the smallest males and females observed spawning in the Great
Bay Estuary (Fig. 13.8 ), it appears as if males reach sexual maturity one molt sooner
than females. This was also been observed in laboratory studies (Sekiguchi 1988 ;
Sekiguchi et al. 1988 ). They suggest that males reach sexual maturity at year 9, at
Fig. 13.7 Size-frequency distribution of live juveniles ( n = 116) and molts ( n = 101) collected
diving and on beaches. Estimated ages of horseshoe crabs (According to Sekiguchi
1988 ) are
indicated for the most abundant size cohorts
H. Cheng et al.
the 17th instar stage, and females reach sexual maturity at year 10, at the 18th instar
stage. If males reach sexual maturity one molt sooner than females, this might
explain both their size differences and, in part, the male-skewed sex ratios observed
on mating beaches, both in New Hampshire (1.4 males to 1 female; Cheng 2014 )
and elsewhere (Rudloe 1980 ; Cohen and Brockmann 1983 ; Smith et al. 2002 ;
Carmichael et al. 2003 ; James-Pirri et al. 2005 ). For example, in Florida, Rudloe
( 1980 ) and Cohen and Brockmann ( 1983 ) observed sex ratios of four males to one
female (4:1), and in Delaware Bay, sex ratios have been reported to reach up to 5:1
(Shuster and Botton 1985 ; Smith et al. 2002 ). In some areas the sex ratio is even
more skewed, but this might be the result of harvesting for the larger females by the
biomedical and bait industries (James-Pirri et al. 2005 ). Data gathered from Pleasant
Bay, a location where horseshoe crabs historically are harvested in large numbers
for biomedical purposes (Rutecki et al. 2004 ; James-Pirri et al. 2005 ), showed nine
males for every one female (James-Pirri et al. 2005 ).
Fig. 13.8 A comparison of the size frequency distribution of ( a ) molts and live juvenile horseshoe
crabs ( n = 225), and ( b ) adult spawning horseshoe crabs ( n = 668)
13 The Life History Cycle of Limulus polyphemus in the Great Bay Estuary, New
13.5 Final Comments
The results from these collective studies on the American horseshoe crabs that reside
in the Great Bay Estuary shed new light on this northern population. The Great Bay
Estuary is characterized by large seasonal fl uctuations in temperature and salinity,
typical of high latitudinal temperate estuaries (Watson et al. 2009 ). Maximum tem-
peratures occur during mid-summer through the fall. In addition, tidal cycles cause
temperature and salinity fl uctuations, with ebb tide water temperatures being warmer
than fl ood tides, and fl ood tides bringing more saline waters into the estuary (Short
1992 ). All of these environmental factors play a role in the activities of horseshoe
crabs in this location, and thus the behaviors and distributions observed in this study
may be a result of local adaptations to the Great Bay Estuary.
The overall numbers of adult horseshoe crabs that return to spawn year after year
along the shorelines, embayments, and estuaries of the U.S. Atlantic Coast are
decreasing, or have decreased, in a number of regions. Though there are popula-
tions, such as those in Delaware Bay, that have shown annual fl uctuations (Smith
and Michels 2006 ) and are reported to be stable or increasing in recent years, popu-
lations in New York and Massachusetts show continuing declines and have not
recovered (ASMFC 2009 , 2010 , 2013 ). Additional pressure from coastal develop-
ment is reducing the quality of spawning beaches and possible nursery habitats
where juvenile horseshoe crabs can develop. The Great Bay Estuary could serve as
an important baseline for comparisons of horseshoe crab population health across
the entire U.S. Atlantic coast since there has been little to no reported commercial
harvest for horseshoe crabs in recent years (J. Carloni 2013 personal communica-
tion) and no history of biomedical company activity (State of New Hampshire 2001 ,
2002 ). Continued monitoring and assessments of the Great Bay horseshoe crab
population might be useful in determining whether shifts in sex ratios, distribution,
abundance, growth rates and other characteristics are the result of climate change
or harvesting.
Acknowledgements The authors thank the many research assistants, volunteers, and talented
SCUBA divers, who have assisted with fi eld surveys and behavioral studies and experiments.
Thanks also to David Shay for his daily maintenance of the facilities at Jackson Estuarine
Laboratory. This work was supported by grants from the University of New Hampshire School of
Marine Science and Ocean Engineering, New Hampshire Sea Grant, and NSF (NSF-IOS 0920342)
to CCC and WHW III.
ASMFC (Atlantic States Marine Fisheries Commission) (2009) Terms of reference and advisory
report to the horseshoe crab stock assessment peer review. Stock Assessment Report No. 09-02.
Atlantic States Marine Fisheries Commission, Washington, DC
ASMFC (Atlantic States Marine Fisheries Commission) (2010) Stock assessment overview:
horseshoe crab. Atlantic States Marine Fisheries Commission, Washington, DC
H. Cheng et al.
ASMFC (Atlantic States Marine Fisheries Commission) (2013) Horseshoe crab stock assessment
update. Atlantic States Marine Fisheries Commission, Washington, DC
Barlow RB Jr (1983) Circadian rhythms in the Limulus visual system. J Neurosci 3:856–870
Barlow RB Jr, Powers MK (2003) Seeing at night and fi nding mates: the role of vision. In: Shuster
CN Jr, Barlow RB Jr, Brockmann HJ (eds) The American horseshoe crab. Harvard University
Press, Cambridge, MA, pp 83–102
Barlow RB Jr, Powers MK, Kass L et al (1984) Vision in Limulus mating behavior during the day
and at night. Biol Bull 167:522
Barlow RB Jr, Powers MK, Howard H et al (1986) Migration of Limulus for mating: relation to
lunar phase, tide height, and sunlight. Biol Bull 171:310–329
Barlow RB Jr, Hitt JM, Dodge FA (2001) Limulus vision in the marine environment. Biol Bull
Botton ML, Loveland RE (2003) Abundance and dispersal potential of horseshoe crab ( Limulus
polyphemus ) larvae in the Delaware Estuary. Estuaries 26(6):1472–1479
Botton ML, Ropes JW (1987) Populations of horseshoe crabs, Limulus polyphemus , on the north-
western Atlantic continental shelf. Fish Bull 85(4):805–812
Botton ML, Loveland RE, Jacobsen TR (1988) Beach erosion and geochemical factors: infl uence
on spawning success of horseshoe crabs ( Limulus polyphemus ) in Delaware Bay. Mar Biol
Brockmann HJ (2003) Nesting behavior, a shoreline phenomenon. In: Shuster CN Jr, Barlow RB
Jr, Brockmann HJ (eds) The American horseshoe crab. Harvard University Press, Cambridge,
MA, pp 33–49
Burton WH, Kelley FS, Franks EA (2009) Distribution of juvenile horseshoe crabs in subtidal
habitats of Delaware Bay using a suction-dredge sampling device. In: Tanacredi JT, Botton
ML, Smith DR (eds) Biology and conservation of horseshoe crabs. Springer, New York,
pp 285–293
Carmichael RH, Rutecki D, Valiela I (2003) Abundance and population structure of the Atlantic
horseshoe crab Limulus polyphemus in Pleasant Bay, Cape Cod. Mar Ecol Prog Ser
Cavanaugh CM (1975) Observations on mating behavior in Limulus polyphemus . Biol Bull
Chabot CC, Watson WH III (2010) Circatidal rhythms of locomotion in the American horseshoe
crab, Limulus polyphemus : underlying mechanisms and cues that infl uence them. Curr Zool
Chabot CC, Betournay SH, Braley NR et al (2007) Endogenous rhythms of locomotion in the
American horseshoe crab, Limulus polyphemus . J Exp Mar Biol Ecol 345(2):79–89
Chabot CC, Yelle JF, O’Donnell CB et al (2011) The effects of water pressure, temperature, and
current cycles on circatidal rhythms expressed by the American horseshoe crab, Limulus
polyphemus . Mar Freshwater Behav Physiol 44(1):43–60
Cheng H (2014) The environmental infl uences on American horseshoe crab ( Limulus polyphemus )
behavior and distribution in the Great Bay Estuary, New Hampshire, U.S.A. M.S. thesis,
University of New Hampshire
Cohen JA, Brockmann HJ (1983) Breeding activity and mate selection in the horseshoe crab,
Limulus polyphemus . Bull Mar Sci 33:274–281
Ehlinger GS, Tankersley RA (2006) Endogenous rhythms and entrainment cues of larval activity
in the horseshoe crab, Limulus polyphemus . J Exp Mar Biol Ecol 337(2):205–214
Forward RB Jr, Tankersley RA (2001) Selective tidal-stream transport of marine animals. Oceanogr
Mar Biol 39:305–353
Forward RB Jr, Tankersley RA, Welch JM (2003) Selective tidal-stream transport of the blue crab
Callinectes sapidus , an overview. Bull Mar Sci 72:347–365
Goldstein JS (2012) The impact of seasonal movements by ovigerous American lobsters ( Homarus
americanus ) on egg development and larval release. Dissertation, University of New Hampshire
Gunderson DR, Armstrong DA, Shi YB et al (1990) Patterns of estuarine use by juvenile English
sole (
Parophrys vetulus ) and Dungeness crab ( Cancer magister ). Estuaries 13(1):59–71
13 The Life History Cycle of Limulus polyphemus in the Great Bay Estuary, New
Holsman KK, McDonald P, Armstrong DA (2006) Intertidal migration and habitat use by subadult
Dungeness crab Cancer magister in a NE Pacifi c estuary. Mar Ecol Prog Ser 308:183–195
James-Pirri MJ (2010) Seasonal movement of the American horseshoe crab Limulus polyphemus
in a semi-enclosed bay on Cape Cod, Massachusetts (USA) as determined by acoustic telem-
etry. Curr Zool 56(5):575–586
James-Pirri MJ, Tuxbury K, Marino S et al (2005) Spawning densities, egg densities, size struc-
ture, and movement patterns of spawning horseshoe crabs, Limulus polyphemus , within four
coastal embayments on Cape Cod, Massachusetts. Estuaries 28(2):296–313
Krebs CJ (1998) Estimating abundance: mark-recapture. In: Krebs CJ (ed) Ecological methodol-
ogy, 2nd edn. Benjamin Cummings, Menlo Park, pp 19–69
Krutky MA, Atherton JL, Smith S et al (2000) Do the properties of underwater lighting infl uence
the visually guided behavior of Limulus ? Biol Bull 199:178–179
Lee WJ (2010) Intensive use of intertidal mudfl ats by foraging adult American horseshoe crabs
Limulus polyphemus in the Great Bay Estuary, New Hampshire. Curr Zool 56:611–617
Moore S, Perrin S (2007) Seasonal movement and resource-use patterns of resident horseshoe crab
( Limulus polyphemus ) populations in a Maine, USA Estuary. Estuar Coast 30(6):1016–1026
Morrison M, Francis M, Hartill B et al (2002) Diurnal and tidal abundance changes in the fi sh
fauna of a temperate tidal mudfl at. Estuar Coast Shelf Sci 54:793–807
National Marine Fisheries Service Management Division (2010) Programs improving manage-
ment of ASMFC managed species in New Hampshire: annual report. New Hampshire Fish and
Game Department, Durham
Powers MK, Barlow RB Jr (1985) Behavioral correlates of circadian rhythms in the Limulus visual
system. Biol Bull 169(3):578–591
Powers MK, Barlow RB Jr, Kass L (1991) Visual performance of horseshoe crabs day and night.
Vis Neurosci 7:179–189
Rudloe A (1979) Locomotor and responses of larvae of the horseshoe crab, Limulus polyphemus .
Biol Bull 157:494–505
Rudloe A (1980) The breeding behavior and patterns of movement of horseshoe crabs Limulus
polyphemus in the vicinity of breeding beaches in Apalachee Bay, Florida. Estuaries
Rutecki D, Carmichael RH, Valiela I (2004) Magnitude of harvest of Atlantic horseshoe crabs,
Limulus polyphemus , in Pleasant Bay, Massachusetts. Estuaries 27(2):179–187
Saunders KM, Brockmann HJ, Watson WH III et al (2010) Male horseshoe crabs Limulus polyphe-
mus use multiple sensory cues to locate mates. Curr Zool 56:611–617
Schaller SY, Chabot CC, Watson WH III (2010) Seasonal movements of American horseshoe crabs
Limulus polyphemus in the Great Bay Estuary, New Hampshire (USA). Curr Zool
Schwab RL, Brockmann HJ (2007) The role of visual and chemical cues in the mating decisions
of satellite male horseshoe crabs, Limulus polyphemus . Anim Behav 74:837–846
Sekiguchi K (1988) Biology of horseshoe crabs. Science House, Tokyo
Sekiguchi K, Seshimo H, Sugita H (1988) Post-embryonic development of the horseshoe crab.
Biol Bull 174:337–345
Short FT (ed) (1992) The ecology of the Great Bay Estuary, New Hampshire and Maine: an estua-
rine profi le and bibliography. NOAA – Coastal Ocean Program Publication, Durham
Shuster CN Jr (1950) Observation on the natural history of the American horseshoe crab, Limulus
polyphemus . Woods Hole Oceanogr Inst Contrib 564:18–23
Shuster CN Jr (1979) Distribution of the American horseshoe crab, Limulus polyphemus . In:
Cohen E (ed) Biomedical applications of the horseshoe crab (Limulidae). Alan R. Liss,
New York, pp 3–26
Shuster CN Jr (1982) A pictorial review of the natural history and ecology of the horseshoe crab
Limulus polyphemus , with reference to other Limulidae. In: Bonaventura J, Bonaventura C,
Tesh S (eds) Physiology and biology of horseshoe crabs. Alan R. Liss, New York, pp 1–52
Shuster CN Jr, Botton ML (1985) A contribution to the population biology of horseshoe crabs,
Limulus polyphemus (L.), in Delaware Bay. Estuaries 8:363–372
H. Cheng et al.
Smith DR, Michels SF (2006) Seeing the elephant: importance of spatial and temporal coverage in
a large-scale volunteer-based program to monitor horseshoe crabs. Fisheries 31(10):485–491
Smith DR, Pooler PS, Swan BL et al (2002) Spatial and temporal distribution of horseshoe crab
( Limulus polyphemus ) spawning in Delaware Bay: implications for monitoring. Estuaries
State of New Hampshire (2001) Horseshoe crab fi sheries and management program annual report
for 2000. New Hampshire Fish and Game, Concord
State of New Hampshire (2002) Horseshoe crab fi sheries and management program annual report
for 2001. New Hampshire Fish and Game, Concord
Walls EA, Berkson J, Smith SA (2002) The horseshoe crab, Limulus polyphemus : 200 million
years of existence, 100 years of study. Rev Fish Sci 10:39–73
Watson WH III, Chabot CC (2010) High resolution tracking of adult horseshoe crabs ( Limulus
polyphemus ) in a New Hampshire estuary using fi xed array ultrasonic telemetry. Curr Zool
Watson WH III, Bedford L, Chabot CC (2008) Dissociation between circadian rhythms of visual
sensitivity and circatidal rhythms of locomotion in the horseshoe crab Limulus polyphemus .
Biol Bull 215:46–56
Watson WH III, Schaller SY, Chabot CC (2009) The relationship between small- and large-scale
movements of horseshoe crabs in the Great Bay Estuary and Limulus behavior in the labora-
tory. In: Tanacredi JT, Botton ML, Smith DR (eds) Biology and conservation of horseshoe
crabs. Springer, New York, pp 131–148
13 The Life History Cycle of Limulus polyphemus in the Great Bay Estuary, New

Supplementary resources (2)

... The physical and chemical cues they use to locate suitable beaches for spawning have been previously documented. For example, physical characteristics of a beach, such as wave energy, sediment type, beach morphology, and high tide inundation, influence where horseshoe crabs spawn and lay eggs for developmental success (Rudloe and Herrnkind 1976, 1980, Botton et al. 1988, Smith et al. 2002a, b, Swan, 2005, Vaquez et al. 2015a, Cheng et al. 2016, as well as chemical features such as the amount of peat, sediment redox potential, ambient oxygen, and pore water hydrogen sulfide (Botton et al. 1988, Saunders et al. 2010, Vasquez et al. 2015a, b, 2017. Horseshoe crabs have likely evolved to prefer beaches with the right combination of these characteristics in order to optimize egg development and larval survival. ...
... This was most obvious in 2012 when horseshoe crabs initiated their migration much earlier than usual because of an unusually early, warm spring . A parallel study showed that, as a result, horseshoe crabs were observed on spawning beaches 2-3 weeks earlier than reported in other years (Cheng et al. 2016). These observations, in part, led to the hypothesis that horseshoe crabs can sense thermal gradients and they use them to help guide their migrations up into the estuary where optimal beaches for spawning are located. ...
... Previously, Reynolds and Casterlin (1979a) demonstrated in that horseshoe crabs will avoid extreme cold or hot water, choosing to occupy water at temperatures from 15 to 40 °C, when given a potential range of 0-50 °C. However, in the Great Bay Estuary, during the time period when horseshoe crabs typically migrate from where they overwinter to the beaches where they spawn, the water temperature is typically between 12 and 20 °C, with increasing mean surface water temperature from the open coast to the inner estuary (Short 1992;Cheng et al. 2016). Therefore, in this study, we aimed to determine if they could detect, and exhibit preferences for smaller changes in temperature than previously demonstrated by Reynolds and Casterlin (1979a). ...
Each spring and early summer horseshoe crabs (Limulus polyphemus) approach beaches at high tide and spawn. While we have some understanding of the factors that influence which beaches they choose and how they find their mates, the environmental cues they use to guide their large-scale movements to the vicinity of these spawning areas remain to be elucidated. In the Great Bay Estuary, New Hampshire, when water temperatures exceed ~11 °C in the spring, horseshoe crabs move into the upper regions of the estuary that are generally characterized by lower salinities and warmer water. Therefore, these abiotic factors could serve as cues to guide them to their preferred spawning beaches. The first goal of this study was to determine if they would prefer certain temperatures or salinities in a Y-maze. When given a choice between ambient temperature water and water that was 2–3 °C warmer, horseshoe crabs spent significantly more time on the warm water side of the Y-maze. However, when given a choice between ambient water and water that had a salinity that was 2–3 psu lower, they did not express a preference. The second goal of the study was to determine if horseshoe crabs could detect small changes in water temperature, using a cardiac assay. When 17 horseshoe crabs were exposed to water that was at least 2.6 °C warmer than ambient water, they all expressed a “startle response”, characterized by a brief slowing of their heart rate followed by a prolonged increase of their heart rate. These results suggest that water temperature, but not salinity, is likely to be one of the most important cues used by horseshoe crabs to guide their large-scale movements and migrations to the vicinity of known spawning beaches each spring.Keywords Limulus polyphemus Behavior Y-maze Cardiac assay Temperature Salinity Migrations
... This is supported by abundant accumulations of their molts (e.g., in the Bertie Waterlime), the occurrence of juveniles and their trackways in marginal settings (as described here), their reproductory and respiratory paleophysiology, and comparisons with living horseshoe crabs and semiterrestrial crustaceans (Braddy, 2001a;Vrazo and Braddy, 2011). Many amphidromous animals cluster together to maximize reproductive potential, e.g., the xiphosuran Limulus Müller, 1785, a living relative of the eurypterids, which undertake seasonal mass migrations onto beaches along the eastern coast of the USA (Cheng et al., 2016). ...
Palmichnium gallowayi (Sharpe, 1932) new combination from the Middle Ordovician Martinsburg Formation (proximal deltaic facies) of Rondout, near Kingston, New York State, is redescribed. It consists of opposing series of five tracks, the outer two large and pear-shaped, the inner three smaller and elliptical, arranged in a chevron converging in the direction of travel, on either side of a wide medial impression. It is attributed to a medium-sized stylonurid eurypterid using a decapodous gait, crawling onto the shoreline, traversing the intertidal zone, a behavior interpreted as part of its reproductive life cycle. This provides the earliest ichnological evidence for the ‘mass-molt-mate’ hypothesis, which proposes that eurypterids migrated en masse into nearshore environments to molt and mate.
... Differences in the timing of spawning activity at individual sites could also have influenced the composition of egg stages presented here. While spawning activity can be synchronous around the timing of the full moon and the new moon in some areas of the horseshoe crab's geographic range (Barlow et al., 1986), many studies have shown that other local environmental factors can more strongly influence spawning (Rudloe, 1985;Cheng et al., 2016;Sasson et al., 2020). Anecdotal information from suppliers of horseshoe crabs to the biomedical industry indicates that in the early spring, horseshoe crabs may spawn on sandy beaches two to three days before spawning in marsh habitats; but variation in spawning locations appears to be driven primarily by local weather conditions and does not systematically differ between habitats as the season progresses. ...
Full-text available
For animal embryos that develop externally, the physio-chemical environment can substantially affect offspring viability. In the case of the American horseshoe crab (Limulus polyphemus), sediment conditions along estuarine shorelines influence development rates and embryonic viability. Sandy beach habitats are considered to have optimal conditions for horseshoe crab embryonic development; however, spawning is often observed outside of these optimal habitats, in areas such as salt marshes, where reduced oxygen availability is thought to decrease the viability of eggs laid in these sediments. We excavated horseshoe crab eggs, embryos, and trilo-bites laid naturally in marsh and beach sediments in South Carolina to compare their development and viability between habitats. We found all developmental stages in both marsh and beach habitats. For two of three sampling areas, trilobites were more likely to be found at beaches than at marshes. Mul-tivariate analyses demonstrate that the prevalence of early and middle developmental stages was similar between habitats but that beaches had a greater proportion of late-stage trilo-bites than marshes. The lower likelihood of finding trilobites at some marshes may reflect differences in spawning phenol-ogy between habitats or reduced rates of embryonic development in marshes compared to beaches, leading to potentially different developmental timelines rather than a true reduction in viability. Nevertheless, the substantial proportions of eggs laid in salt marshes that survive to the trilobite stage indicate that spawning in this habitat could represent a previously un-derappreciated source of recruitment for horseshoe crab populations that may need to be incorporated into population assessments.
... Spawning survey dates are designated around the May-early June full and new moons (spring tides), when spawning activity in the middle Atlantic tends to peak. When the density of spawning adults is low (Basudev et al., 2013;Sclafani, McKown & Udelson, 2013;Nelson et al., 2015;Pati, Biswal & Dash, 2015) or if spawning in a particular embayment is not tightly coupled to the lunar phases (Ehlinger, Tankersley & Bush, 2003;Cheng, Chabot & Watson, 2016), larger sampling units and/or more frequent shoreline surveys may be required. Spawning intensity can also be inferred by sampling the density of eggs (Shuster & Botton, 1985;Botton, Loveland & Jacobsen, 1994;Smith et al., 2002a;Pooler et al., 2003;James-Pirri et al., 2005;Botton et al., 2006;Karpanty et al., 2006;Weber & Carter, 2009;Beekey, Mattei & Pierce, 2013;Botton et al., 2018), or by enumerating the number of 'nests', which are shallow depressions left in the sand by the females as they deposit their eggs (Nelson et al., 2015;Fairuz-Fozi et al., 2018). ...
• Horseshoe crab population sizes and trends have been previously studied using surveys of spawning adults and counts of eggs from surface (top 5 cm) and deep (20 cm) sediment samples. The correlations between surface and deep eggs were studied at two locations, Delaware Bay and Jamaica Bay, USA, and the correlations between egg densities and spawning counts were examined in Jamaica Bay. • There were significantly higher densities of eggs in deep sediments than in surface sediments. Only about 10% of the variability in surface egg density was explained by deep egg density. The numerical patterns between surface and deep eggs were similar between Delaware Bay and Jamaica Bay and across sampling dates. • Nearly 20% of the deep samples in the combined data from Delaware Bay and Jamaica Bay with an egg density of ≥100,000 m⁻² had zero surface eggs. Therefore, the use of surface eggs as an indicator of habitat suitability and spawning intensity may seriously underestimate the importance of a beach for spawning horseshoe crabs. • When paired with nearest survey date, Jamaica Bay spawning indices did not predict deep or surface egg densities. This may be related to a temporal mismatch between survey methods, the extreme overdispersion (patchiness) of the eggs, and/or the dynamics of egg distribution after exhumation. • Both egg density and spawning surveys can provide useful data on habitat suitability for horseshoe crabs and can offer excellent opportunities for student and citizen scientist engagement. More labour is required for egg surveys than spawning surveys because of the time required to sample, sort, and enumerate the eggs.
... Horseshoe crabs have specific spawning and nursery requirements related to a combination of beach topography and physico-chemical parameters such as dissolved oxygen, chlorophyll a content, total sulfide content, and sediment grain size (Hsieh and Chen, 2009;Vasquez et al., 2015;Cheng et al., 2016;Xie et al., 2020). Therefore, the identification of optimal nursery habitats for releases is the first step. ...
Full-text available
As a well-known example of “living fossil”, horseshoe crabs are ecologically significant macroinvertebrates in coastal and estuarine ecosystems. The tri-spine horseshoe crab, Tachypleus tridentatus, has been widely utilized for Tachypleus amebocyte lysate production and food consumption since the 1980s, which led to considerable population declines along the west coast of the Pacific Ocean. The declining horseshoe crab population is expected to have ecological and social impacts. Stock enhancement through captive rearing of juveniles is cited as an important alternative to repopulate the native T. tridentatus, which in turn supports sustainable resource utilization and research activities. The hatchery production techniques for this species have gradually developed following the mass culture efforts in Japan since the late 1980s. However, the previous studies have primarily concerned the feed types and husbandry conditions to maximize the growth and survival of the juveniles. Little is known about the practicability and effectiveness of releasing large numbers of hatchery-bred individuals through releasing programs. In this review, we (1) summarize the available captive breeding and rearing techniques, (2) discuss the release strategies that could potentially improve the survival of released juveniles, and (3) identify the future opportunities and challenges in establishing technical frameworks to support responsible stock enhancement programs for T. tridentatus. The information should benefit future horseshoe crab fisheries management efforts in the attempt to restore the severely depleted populations.
... Limited tidal ranges also affect horseshoe crab movement and habitat use, which may explain why horseshoe crabs are not found west of Louisiana in the northwestern GOM [14]. Other variables that may contribute to local horseshoe crab habitat suitability include wave height, wind speed, wind direction, salinity, dissolved oxygen, water quality, beach slope, near shore bathymetry, sediment grain size, and drainage capacity of sediments [2][3][15][16][17][18]. In the northcentral GOM, freshwater outflow from the Mobile Bay and Mississippi River watersheds, which are among the largest in the U.S., discharge east and west of the barrier island system that comprises the known habitat for horseshoe crabs. ...
Full-text available
This study provides regional-scale data on drivers of horseshoe crab ( Limulus polyphemus ) presence along the northcentral Gulf of Mexico coast and has implications for understanding habitat suitability for sparse horseshoe crab populations of conservation concern worldwide. To collect baseline data on the relationship between environmental factors and presence of horseshoe crabs, we surveyed four sites from the Fort Morgan peninsula of Mobile Bay, Alabama (AL) to Horn Island, Mississippi (MS). We documented number, size and sex of live animals, molts, and carcasses as metrics of horseshoe crab presence and demographics for two years. Data were compared to in situ and remotely sensed environmental attributes to assess environmental drivers of occurrence during the time of study. Overall, greater evidence of horseshoe crab presence was found at western sites (Petit Bois and Horn Islands) compared to eastern sites (Dauphin Island, Fort Morgan peninsula), mediated by a combination of distance from areas of high freshwater discharge and interannual variation in weather. Higher sex ratios also were found associated with higher occurrence, west of Mobile Bay. Land cover, particularly Bare Land and Estuarine Emergent Wetland classes that are common to western sites, was most predictive of live animal and to some extent carcass occurrence. Our findings suggest that small-scale variation in habitat quality can affect occurrence of horseshoe crabs in sparse populations where density is not a limiting factor. Data from molts and carcasses were informative to supplement live animal data and may be useful to enhance ecological assessment and support conservation and management in regions with sparse populations.
Full-text available
According to an International Union for Conservation of Nature (IUCN) Red List assessment (RLA), the American horseshoe crab ( Limulus polyphemus ), an iconic coastal species, is at risk of extirpation in some regions within its range where small and vulnerable populations occur. However, the RLA does not consider future status beyond viability and does not attempt to identify the conservation necessary to effectively mitigate threats and recover the species to full ecological functionality. To aid in conservation planning for vulnerable species, the IUCN developed the Green Status of Species assessment (GSA) process to complement the RLA. This paper describes the application of the GSA process to assess the recovery potential of the American horseshoe crab. First, specific Limulus populations within spatial units for conservation were delineated, and their statuses were defined based on viability and ecological functionality. Then conservation actions were identified that would promote recovery and affect their near‐ and long‐term population status under different conservation scenarios. Horseshoe crab conservation has relied on, and will continue to depend on, effective harvest regulation. However, as currently conceived, conservation is not expected to mitigate habitat loss at the scale required to restore range‐wide ecological functionality, primarily because habitat loss is widespread and affected by climate change. Thus, the GSA results, while indicating that there is potential for near‐term recovery gains, reveal that long‐term recovery is in doubt owing to expected loss of habitat. To conserve critical habitats for spawning and early life stages and achieve ecological functionality, it is imperative to identify and develop conservation plans at appropriate spatial scales. Unfortunately, such plans do not currently exist and need to be established. The GSA Green Score can then serve as a metric for monitoring recovery and gauging the effectiveness of conservation implementation.
Every year, more than 600000 horseshoe crabs are bled to produce Limulus amoebocyte lysate, which is used to detect Gram-negative bacteria in biomedical products. While numerous studies have shown that some horseshoe crabs die after being bled, less is known about what happens to those that are returned to their natural habitat. In this study, we used an array of VR2W acoustic receivers to track 10 bled and 10 control females during the mating season in the Great Bay Estuary, NH, USA. Animals were bled, or not, released where they had been initially captured, and tracked from 22 May to 26 June 2019. Bled and control females moved comparable distances at similar speeds during the weeks after they were released (controls: 90.3 m h ⁻¹ ; bled: 89.7 m h ⁻¹ ). The longer horseshoe crabs remained within the virtual positioning system array, the longer we were able to track them and the more beach approaches and mating attempts we were able to identify. When this relationship between the duration of time we were able to track a horseshoe crab and how many apparent mating attempts it expressed was taken into account, we found that bled females attempted to spawn half as often as control females, and this difference was significant. Overall, these data are consistent with previous findings indicating that females that are released back into their natural habitat after bleeding express similar levels of activity and seasonal movements but attempt to mate less than control animals, at least in the first few weeks after being bled.
For animals that develop externally, habitats where environmental conditions are optimal for embryonic development are sometimes assumed to represent the highest recruitment potential and thus support the majority of reproductive output for a species. However, organisms may spawn in areas considered sub-optimal for embryonic development. Thus, understanding spawning habitat selection decisions and their potential impacts on recruitment and ecological interactions is necessary for predicting population status and identifying critical habitats to inform sustainable conservation decisions and effective management approaches. The American horseshoe crab, Limulus polyphemus , is ecologically, economically, and biomedically important. Females come ashore to spawn in the sediment where eggs develop for 2 – 4 weeks. Horseshoe crabs have been thought to primarily use sandy beach habitat for spawning in part because this habitat has been shown to be optimal for embryonic development. Horseshoe crab eggs on sandy beaches are an essential part of the diet of many organisms, including shorebirds such as the rufa red knot which requires the eggs to fuel their migration to arctic spawning grounds. While horseshoe crabs have been observed spawning in alternative habitats such as salt marshes and peat beds, this behavior has been assumed to be rare and non-adaptive. In this study, we compare the use of beach and alternative habitats by horseshoe crabs for spawning. To do so, we conducted adult horseshoe crab spawning surveys and horseshoe crab egg surveys in beach and Spartina -dominated salt marsh alternative habitats in South Carolina, Connecticut, and New Hampshire, U.S.A. While spawning horseshoe crabs were more likely to be observed on beach habitats than in alternative habitats, potentially due to logistical constraints surveying alternative habitats, we found similar densities of spawning horseshoe crabs in both habitat types. We also tended to find more eggs in alternative habitats than on beaches. Taken together, these results suggest that alternative habitats likely represent a significant source of horseshoe crab spawning activity and recruitment that had not previously been quantified. We recommend this information be incorporated into horseshoe crab population assessments, habitat protections, and more directed research at understanding variability in habitat-specific horseshoe crab spawning and its relationship to migratory shorebirds.
Estuaries are among the most biologically productive and geomorphologically complex environments in the coastal zone. A review of research is presented, focusing on broad scale estuarine morphology and evolution and an examination of contemporary processes and forms in the intertidal zone. The chapter includes a discussion of current issues in estuarine research, including geomorphic-biotic interactions, human-modified estuaries, and restoration practices. The chapter concludes with a brief discussion of future areas of concern given current attention to climate variability and sea level rise.
Full-text available
While several studies have documented the large-scale, seasonal movements of horseshoe crabs, little is known about their fine-scale, daily movement patterns. In this study we used a fixed array ultrasonic telemetry system to track the movements of 12 male and 16 female horseshoe crabs in the Great Bay estuary, New Hampshire. Data were obtained during the mating season, as well as during the remainder of the summer and fall, in the years 2005-2008. During the mating season animals were often, but not always, active during the high tides when they were approaching and leaving the spawning beaches. On average, both males and females approached mating beaches during 33% of the high tides they experienced and they most often made the transition from being inactive to active during the last two hours of an incoming tide. From April-October horseshoe crabs were significantly more active during high tide periods vs low tide periods, with no clear preference for diurnal vs nocturnal activity. After the mating season ended horseshoe crabs continued to move into shallower water at high tide and then return to deeper water at low tide. Observations by SCUBA divers suggest that during these excursions into the mudflats horseshoe crabs were digging pits in the sediment while foraging for food. Thus, the tidal rhythm of activity that has been so well documented during the mating season probably persists into the fall, and primarily involves foraging activities [Current Zoology 56 (5): 599-610, 2010].
Full-text available
The American horseshoe crab Limulus polyphemus expresses both tidal and daily rhythms of locomotion in the laboratory and the tidal rhythms can be entrained to artificial tides. The main purpose of this study was to determine the types of rhythms horseshoe crabs express when freely moving in their natural habitat where they are exposed to natural light:dark and tidal cycles. A secondary goal was to determine if their overall activity patterns and depth preferences changed during the year. In 2010 and 2011, 20 adult horseshoe crabs (11 males, 9 females) were fitted with ultrasonic tags and released in the Great Bay Estuary, NH, USA. The tags transmitted acceleration and depth data every 3 to 5 min from June until December during the year in which they were tagged, and from March to May of the following year. Acoustic transmissions from the tags were detected and logged by a series of VR2W receivers moored throughout the estuary. Accelerometer data were used to assess when animals were active and to determine (1) whether they were expressing tidal or daily rhythms and (2) their overall activity level each month. We discovered that horseshoe crabs were just as likely to express tidal rhythms as daily rhythms, despite being continuously exposed to natural tide cycles. In addition, there was a tendency to move into deeper water and become less active as water temperatures cooled in the fall, and then to move up into the estuary and become more active as water temperatures warmed in the spring.
Full-text available
While eye sensitivity in the American horseshoe crab Limulus polyphemus has long been known to be under the control of an endogenous circadian clock, only recently has horseshoe crab locomotion been shown to be controlled by a separate clock system. In the laboratory, this system drives clear activity rhythms throughout much of the year, not just during the mating season when horseshoe crabs express clear tidal rhythms in the field. Water temperature is a key factor influencing the expression of these rhythms: at 17°C tidal rhythms are expressed by most animals, while at 11°C expression of circatidal rhythms is rarely seen, and at 4°C rhythms are suppressed. Neither long (16:8 Light:Dark) nor short (8:16) photoperiods modify this behavior at any of these temperatures. Synchronization of these circatidal rhythms can be most readily effected by water pressure cycles both in situ and in the lab, while temperature and current cycles play lesser, but possibly contributory, roles. Interestingly, Light:Dark cycles appear to have synchronizing as well as "masking" effects in some individuals. Evidence that each of two daily bouts of activity are independent suggests that the Limulus circatidal rhythm of locomotion is driven by two (circalunidian) clocks, each with a period of 24.8h. While the anatomical locations of either the circadian clock, that drives fluctuations in visual sensitivity, or the circatidal clock, that controls tidal rhythms of locomotion, are currently unknown, preliminary molecular analyses have shown that a 71 kD protein that reacts with antibodies directed against the Drosophila PERIOD (PER) protein is found in both the protocerebrum and the subesophageal ganglion.
Full-text available
The 1977 peak population of spawning horsehoe crabs, Limulus polyphemus, in Delaware Bay, was comprised of about 222,000 males and 51,000 females. This estimate, based upon a shoreline survey of spawning intensity along Delaware and New Jersey beaches at the time of full moon tides in June, was corroborated by a quantification of egg clusters in a beach. Fecundity of gravid females was used, in conjunction with the egg cluster estimate, to approximate the number of females responsible for the observed quantity of eggs. The present spawning population of Delaware Bay is several fold larger than that which existed during the 1960's. From a longer historical perspective, however, the population is far from approaching the numbers and spawning intensity reported a century ago.
Full-text available
This overview combines our recent studies with existing information to develop more complete conceptual models of selective tidal-stream transport (STST) of ovigerous female and post-larvae of the blue crab Callinectes sapidus. During the first phase of the spawning migration, non-ovigerous females migrate seaward from brackish water to the mouths of estuaries following insemination. After oviposition, females with mature embryos undertake the second phase of the spawning migration, in which they undergo ebb-tide transport for movement seaward to release larvae and then migrate back into estuaries using flood-tide transport. Following larval development offshore, post-larvae or megalopae undergo flood-tide transport for up-estuary movement in which they ascend into the water column during flood tides at night and are on or near the bottom at all other times. This behavioral pattern is not due to a circatidal rhythm in activity since megalopae have a circadian rhythm. The timing of this endogenous rhythm is paradoxical because megalopae are active during the day phase and inactive at night. Neither exposure to a cycle in salinity change that simulates the natural tidal cycle nor step decreases in salinity alter this circadian rhythm. The behavior underlying flood-tide transport consists of behavioral responses to a sequence of cues. Megalopae ascend into the water column in response to the relative rate of increase in salinity during flood tide. Water turbulence due to flood-tide currents induces sustained swimming, and the decline in turbulence during slack water at end of flood tide induces settlement out of the water column. Environmental cues during ebb tide do not induce STST. Since light inhibits swimming, flood-tide transport does not occur during the day and is reduced when the time of slack water after flood tide occurs after sunrise. Future studies are needed to determine the behavioral basis of STST of females, and especially the reversal from ebb-tide to flood-tide transport.
While the, life history of the American horseshoe crab (Limulus polyphemus) has been well studied in other locations, little is known about the horseshoe crab population that resides in the Great Bay Estuary, New Hampshire. The goals of this thesis were to identify the factors that influence spawning activity of adult horseshoe crabs as well as determine the distribution of juvenile horseshoe crabs in the Great Bay Estuary. It was found that increases in horseshoe crab mating activity more strongly corresponded to increases in water temperature than the lunar cycle. The most spawning was observed in the warmest areas of Little Bay and Great Bay, and behavioral assays revealed that adult horseshoe crabs have a strong preference for warmer water. More juvenile horseshoe crabs were also found in the Great Bay area of the estuary, despite the presence of spawning beaches throughout the estuary. This is possibly due to tidal currents and the behavior of larval horseshoe crabs. Thus, a range of environmental conditions, such as water temperature, weather, and tidal currents influence the biology of horseshoe crabs that reside in the Great Bay Estuary throughout their life history cycle.
In terms of modelling population dynamics, the mark-recapture literature has in recent years been dominated by methods for estimating survival, as described in Chap. 7. In this chapter, we consider open-population mark-recapture methods for estimating abundance, survival and births. We first summarise conventional methods (Seber 1973, 1982).
Selective Tidal-Stream Transport (STST) is used by invertebrates and fishes for horizontal movement. in general, animals ascend from the bottom and are carried by tidal currents during one phase of the tide. During slack water, at the end of this tidal phase, they return to the bottom and remain there during the opposite tidal phase. Through this sequence, horizontal movement takes place in a series of saltatory steps. In coastal and estuarine areas, STST can be characterised as ebb- or flood-tide transport depending upon which phase of the tide is used for transport. Modelling studies indicate STST is a highly effective means of horizontal movement for life-history stages that have weak swimming abilities and for energy conservation by adults with strong swimming ability. Frequently, the direction of STST reverses within a species, especially at different physiological or life-cycle stages, For example, larvae of estuarine crabs undergo ebb-tide transport for migration out of estuaries for development offshore, whereas older post-larvae use flood-tide transport for movement up estuaries to nursery areas. The behaviour underlying STST is ascribed to (a) a tidal rhythm in activity or vertical migration or (b) behavioural responses to environmental cues associated with tides. If behavioural responses are involved, then recent studies suggest that no single cue is used for STST but that animals respond to a sequence of cues during transport. Collectively, STST is well documented in the field, but underlying behaviours need future study.