Examination of large exuviae
with mating scars:
Do female American horseshoe crabs, Limulus
polyphemus, molt after sexual maturity?
Elizabeth E. Hieb1
Ruth H. Carmichael1,2, Glenn Gauvry3, Carl N. Shuster, Jr.4
1Dauphin
Island Sea Lab, Dauphin Island, AL; 2University of South Alabama, Mobile, AL;
3Ecological Research & Development Group, Inc., Milton, DE; 4Virginia Institute of Marine
Science, Gloucester Point, VA
Introduction
Terminal molt = final molt at sexual maturity
after which animals begin mating and no
longer grow
Males
Females
Introduction
Discovery of female
molts with mating scars
Methods – Sample collection
• Targeted collection of
molts with mating
scars
• Classification of
exuviae
• Sex as female
Chelate pedipalps, genital
slits, low prosomal arch,
large size
Methods – mating scar patterns
• Regions of
scarring
• Axial ridge
• Projections
of opisthosoma
I
II
III
IV
Methods – mating scar patterns
• Area of scarring
• Straight length (anterior to posterior)
• Width (laterally)
• Intensity of
scarring
• 1 = least
• 2 = moderate
• 3 = most
Results – classification of exuviae
• 35 molts collected from 2001-2007
Female
• Demographics
•
91% female based on
prosomal width,
arch structure
•
25% female based on
pedipalps, genital Male
operculum
Results – classification of exuviae
Size range of live adult females vs. molts in this study
(Delaware Bay, USA)
N
PW (mm)
56
178-258
Shuster 1955, 1979
100
245-350
Shuster et al. 1961 (unpublished)
14
260-310
1979-1980
Shuster and Botton 1985
216
258 (0.07)a
1986-1989
Botton and Loveland 1992
9,749 220-330
2003-2005
Smith et al. 2009
35
2001-2007
This study (molts)
aData
176-257
Year
Source
reported as mean PW (coefficient of variation)
Visual age
• Majority
“young”
• Four
“middleaged”
Number of molts
Results – classification of exuviae
6
Middle aged
Young
5
4
3
2
1
0
180
196
212
228
244
Prosomal width (mm)
260
Results – mating scar patterns
Scarring across regions
Region
Mean area
(cm2)
Mean
intensity
I
11.06 ± 1.64 1.26 ± 0.09
II
2.15 ± 0.36
1.18 ± 0.12
III
IV
Mean
2.00 ± 0.25
0.63 ± 0.11
16 ± 2
(total)
1.97 ± 0.10
1.14 ± 0.11
1.4 ± 0.1
I
III
• Total area of
scarring or
• Mean intensity of
scarring
40
30
20
10
0
Mean intensity of
scarring
• No relationship
between molt
size (PW) &
Total area of
scarring (cm2)
Results – mating scar patterns
170
188
206
224
242
260
170
188
206
224
242
260
2.5
2.0
1.5
1.0
0.5
0.0
Prosomal width (mm)
Results – mating scar patterns
• Higher index of
scarring in larger
(230-257 mm)
molts
0.8
Index of scarring
• Index of scarring =
intensity per unit
area
0.6
0.4
0.2
0.0
170
188
206
224
242
260
Prosomal width (mm)
Discussion
• Sexual size dimorphism (Smith et al. 2009)
• Differential-maturity hypothesis
Males mature in fewer molts than females
• Differential-growth hypothesis
Juvenile females grow in greater increments than
juvenile males
• Indeterminate-molting hypothesis
Females continue to molt as adults
Discussion
• Advantages of an additional female molt
• Larger size allows for carrying more eggs
• Chance to repair & regenerate shell structure*
• Alternate explanation
• Disadvantageous
• Males are amplexing subadult females (excessive)
J. Bedell
Discussion
• If molts represent subadult females:
Lower intensity and area of scarring
Scarring on only molts that overlap with subadult
sizes
Discussion
• Smith et al. (2009) estimated females molting
after sexual maturity as < 1% of population
Delaware Bay = ~ 70,000 – 110,000 horseshoe
crabs
Possible increased fecundity & survival
• Quantitative approach to define mating scars
• Lab & field-based studies of scar pattern
formation are needed
Acknowledgements
• Funding
provided by the Dauphin Island Sea Lab
(DISL) & the Ecological Research and Development
Group (ERDG).
• Volunteers who searched for & collected molt
specimens included: Mary Van House, the crew of
the F/V Tony and Ann out of Ocean City, Maryland,
Capt. Jeffrey “Jeff” Eustler, Maury Estes, Noel
Wingers, Morgan Waites
• Benjie Lynn Swan (Limuli Laboratories)
provided a
base for field operations & specimens for study
Questions?