Horseshoe crab (Limulus polyphemus) larvae abundance and distribution:
patterns in a small estuary
Jaymie
1
Frederick ,
Ken
2
Able , Rosemarie
2
Petrecca
1Maine
Maritime Academy, Corning School of Ocean Studies, Pleasant St. Castine, ME 04420
2Rutgers University Marine Field Station, Institute of Marine and Coastal Sciences, 800 c/o 132 Great Bay Blvd. Tuckerton, NJ 08087
55
45
45
Larvae per 1000m3
55
35
25
experiment.
ACKNOWLEDGEMENTS
Special thanks to Daniel Gibson (Worchester Polytechnic Institute) for sharing his
knowledge and enthusiasm of horseshoe crabs and to Jackie Toth (RUMFS) for calculating
horseshoe crab densities for the plankton net data. I would also like to thank the NSF and
RIOS for funding this project and supplying the materials and resourced needed to
complete the project.
Results
LAB REARING
Individuals in the lab were raised as far as
the second instar stage (Fig. 4). Larvae
ranged from 2.8 to 3.7mm; 2nd instars
ranged from 4.9 to 5.9mm. Aside from size,
the 2nd instars have longer telsons and
overall shape is comparable to that of an
Figure 4. Trilobite larvae
Figure 5. 2nd instars adult (Fig. 5). On average it took 10-14
(right) and 2nd instar (left)
days for larvae to reach the 2nd instar stage.
SWIMMING BEHAVIOR
Larvae were in the water column more than the 2nd instars. On average the larvae
were in the water column 19% of the time during the day, and 0.3% of the time at
night. This finding is interesting as the literature states that the larvae are more
active at night (Botton et al., 2003, and Rudloe 1981). The 2nd instars landed on the
bottom and there was no swimming behavior from any of the individuals in either the
day or night trials.
Larvae were more present in the water column during low flow than the 2nd instars.
The 2nd instars were more influenced by the higher flow rate.
PLANKTON SAMPLING
SEASONAL AND ANNUAL VARIATION
Larval horseshoe abundance data has been collected for the last six years from
Little Sheepshead Creek showing that horseshoe crabs have annually been
spawning in this small estuarine system and that peak abundance in the water
column is in mid-July (Fig. 6). There is large annual variation in density of larvae
(Fig. 6). Some possible explanations include variation in number of spawning adults
that enter the estuary system, environmental conditions, and larval availability to
sampling gear.
SPATIAL PLANKTON SAMPLING
In the June spatial plankton samples, no larvae were caught at any of the sites. In
July, larvae were found in samples from all three sites. While limited data was
collected, it suggests that there could be a spatial relationship as todensity of larvae;
Little Sheepshead Creek having the highest density and Jimmy’s Creek having the
lowest (Fig. 7).
25
15
15
5
5
-5
6/14/2005
65
7/14/2005
5.c 55
8/14/2005
Date
9/14/2005
10/14/2005
5.d
-5
6/14/2006
45
8/14/2006
Date
9/14/2006
10/14/2006
1000m3
350
Larvae per
35
25
15
250
150
50
5
-5
6/14/2007
5.e
7/14/2006
450
7/14/2007
65
8/14/2007
Date
9/14/2007
-50
6/14/2008
10/14/2007
5.f
7/14/2008
8/14/2008
Date
9/14/2008
10/14/2008
450
55
1000m3
35
Larvae per
Larvae per 1000m3
350
45
25
250
150
15
50
5
-5
6/14/2009
LAB REARING
Eggs were collected from the beach next to Little Sheepshead Creek Bridge
and brought back to the Rutgers University Marine Station (RUMFS)
laboratory. In the laboratory eggs were kept in aerated containers (20cm
diameter 7cm deep). After hatching out larvae (1st instars) were put inflow
through containers which were placed in the RUMFS flow through laboratory.
Containers were marked with the egg collection date and the hatching date
so that length of stage could be monitored. Individuals were measured and
classified using the methods of Sekiguchi et al. (1988). The 2nd instars, or
individuals that have molted once since hatching, were fed brine shrimp.
SWIMMING BEHAVIOR
Swimming experiments were preformed with both larvae
(1st instars)and 2nd instars. Fifteen trials were run for each stage
during the day and another 15 for each stage at night. One
individual was dropped into a 200ml graduated (Fig. 1)cylinder
filled to 100ml, and watched their behavior for a period of
15 minutes.
Experiments to see what influence flow rate had on behavior in
the water column were performed in a 10 gallon aquarium. Three
trials were run for each larvae and 2nd instars during the day, and
another three trials for each were done at night. Twenty
Figure 1.
Swimming
individuals were used for each trial. Each trial consisted of two
behavior
flow rates; “slow” between 5-10cm/sec and “fast” 5-20cm/sec.
35
7/14/2009
8/14/2009
Date
9/14/2009
10/14/2009
-50
6/14/2010
7/14/2010
8/14/2010
Date
9/14/2010
10/14/2010
Figure 6. Average density of larvae for each weekly larval sampling date since 2005. Graphs show
mean density + 1 s.d. to display the variation in density amongst the three tows taken each date. The
y axis scale because of differences in density. Note that data for 2010 has only been collected up to
90
July 20th.
Figure 7. Mean density of larvae for all three
tows at each sample site during the July spatial
sampling project; + 1 s.d. See Figure 3 for
location of sampling sites.
Larvae per 1000m3
Materials and Methods
Larvae per 1000m3
The Atlantic Horseshoe crab, Limulus polyphemus, is an ancient species that
is important to estuarine ecology, particularly for the food source its eggs are
to shore birds (Karpanty et al., 2006) and fish (Nemerson and Able, 2004),
as well as its economic importance; including bait and biomedical industries
(Walls et al., 2002). Negative human impacts on the horseshoe crab
population from bait fisheries and coastal development, have raised concern
over the management of this species (Odell et al., 2005). In order to protect
this species scientific studies need to address all aspects of its life cycle
including settlement. A majority of the work has been done studying
horseshoe crabs in large estuarine systems such as Delaware Bay (Botton
et al., 2003; and Karpantry et al., 2006), however, an understanding of the
role they play in small estuarine systems is relatively unstudied. The
objective of this study is to obtain an understanding of the reproductive
seasonality and distribution and abundance of larval horseshoe crabs in the
small estuarine system of Little Egg Inlet, Tuckerton, NJ.
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5.b
Larvae per 1000m3
Introduction & Objectives
LARVAL SAMPLING
On a weekly basis, a 1 meter (1mm mesh) circular
plankton net was used to collect larval horseshoe crabs
on the night flood tide. The net was deployed for three
replicates in the mid-water column (2 meters below the
surface) each for thirty minutes, off of the Little
Sheepshead Creek Bridge,
Tuckerton, NJ (Fig.2).
Figure 2. Plankton net sample
off of Little Sheepshead bridge.
Water flow was measured
for each replicate tow. Salinity and water temperature
were measured before the first tow and after the third
tow. To look at potential spatial distribution and
Great
Bay
Atlantic occurrence within the estuarine system, one sampling
Little
Ocean
date in June and July, 2010 additional samples were
Egg Inlet
collected from two other sites; Jimmy’s Creek, and
Thorofare Creek (Fig. 3). Data collected from these
Figure 3 Map of sampling sites; 2010 collections will be added to an existing data set of
L-Little Sheepshead, Creek Jlarval horseshoe crab abundance from Little
Jimmy’s Creek, and T- Thorofare Sheepshead Creek that originated in 2005. This data
Creek. RUMFS indicated by
will be used to analyze seasonal and annual patterns,
green dot.
as well as to look for potential environmental patterns.
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5.a
80
70
60
50
40
30
20
10
0
Little
Sheepshead
Thorofare
Jimmy's
Location
CONCLUSIONS
•Horseshoe crabs reproduce in Great Bay Little Egg Inlet Estuary.
•Larvae are available to sampling gear; this is supported by laboratory
observations.
•In the six years of data, the peak of larval density has been in the middle of
July.
•The spatial analysis started in 2010 suggests that larvae are distributed
through out the system and as such it is likely that they play a significant
ecological role.
•Overall, horseshoe crabs reproduce, survive hatching and early stages of
development, and as such may play an important role in small estuaries
such as the study site.
REFERENCES
Botton, M. L., R.E. Loveland, and A. Tiwari. Distribution, abundance, and survivorship of young-of-the-year in a
commercially exploited population of horseshoe crabs Limulus polyphemus. Mar. Ecol. Prog. Ser. 2003; 265: 175-184.
Karpanty, S. M., J. D. Fraser, J. Berkson, L. J. Niles, A. Dey, and E. P. Smith. Horseshoe crab eggs determine red knot
distribution in Deleware Bay. Journal of Wildlife Management. 2006; 70(6): 1704-1710.
Nemerson, D. M. and K. W. Able. 2004. Spatial patterns in diet and distribution of juveniles of four fish species in
Delaware Bay marsh creeks: factors influencing fish abundance. Marine Ecology Progress Series 276:249-262.
Odell, J., M. E. Mather, and R. M. Muth. A biosocial approach for analyzing environmental conflicts: A case study of
horseshoe crab allocation. BioScience. 2005; 55(9): 735- 747.
Rudloe, A. Aspects of the biology of juvenile horseshoe crabs, Limulus polyphemus. Bulletin of Mar. Sci. 1981; 31(1): 125133.
Sekiguchi, K., H. Seshimo, and H. Sugita, 1988. Post-embryonic development of the horseshoe crab. Biol. Bull. 174:
337-345.