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

Abstract

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.

Full text

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
U.S.A.
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
e-mail: win@unh.edu
C. C. Chabot
Department of Biological Sciences , Plymouth State University , Plymouth , NH 03264 , USA

238
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.

239
(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…

240
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 )
Apalachee
key
(Florida)
a
Seahorse
key
(Florida)
b
Delaware Bay
(New Jersey,
Delaware)
c Mashnee Dike
(Massachusetts)
d,e
Great Bay
Estuary (New
Hampshire)
Lunar
cycle More
spawning
animals
during full
moon than
new moon
More
spawning
animals
during full
moon than
new moon
Only fi rst half of
spawning
season, more
spawning
animals during
full and new
moon
Spawning
occurred
throughout May
and fi rst week of
June, regardless
of moon phase
d
Spawning
occurred
throughout May
and June,
regardless of
moon phase
More spawning
animals during
full and new
moon
e (continued)
H. Cheng et al.

241
Table 13.1 (continued)
Apalachee
key
(Florida)
a
Seahorse
key
(Florida)
b
Delaware Bay
(New Jersey,
Delaware)
c Mashnee Dike
(Massachusetts)
d,e
Great Bay
Estuary (New
Hampshire)
Highest
high
tides
Spawning
only
occurred on
spring tides
Spawning
only
occurred on
spring tides;
more
observed
during high
tides of full
moon than
high tides
of new
moon
Poor indicator
alone of
spawning
activity
More spawning
animals observed
during spring
tides, though
signifi cant
numbers spawn
on neap tides
(lowest high
tides)
e
No relationship
of spawning
animals to
highest high
tides (spring
tides); spawning
occurred on
spring tides and
neap tides
(lowest high
tides)
f [Mean high
tide height
during full
and new
moon
~1.1 m; ↑ ~
23.2 % from
neap high
tides
(quarter
moons)]
f [Mean high
tide height
during full
and new
moon
~1.1 m; ↑ ~
20.3 %
from neap
high tides
(quarter
moons)]
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)]
Day
versus
night
More
spawning
animals
observed at
night
More
spawning
animals
observed at
day than at
night
Spawning only
observed at night
4 In 2012, more
spawning
animals
observed at day;
in 2013, no
difference
between day and
night
[Day tides
height
higher than
night]
[Day tides
higher than
night by ~
≤0.2 m]
Spawning
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
difference
depends on lunar
phase, varies
during spring
tides by ~1 m]
e
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…

242
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
Watson
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.

243
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…

244
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 fi 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.

245
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…

246
13.4 Growing Up in the Great Bay Estuary
13.4.1 Juvenile Horseshoe Crabs and the Location of Nursery
Habitats
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
Currents
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.

247
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…

248
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.

249
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…

250
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.
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