Detailed Research Information

Detailed project information for
Study Plan Number 02065

Branch : Aquatic Ecology Branch

Study Plan Number : 02065

Study Title : Status, trends, and genetic population structure of Horseshoe crabs (Limulus polyphemus) in Delaware Bay

Starting Date : 04/01/1999

Completion Date : 07/31/2004

Principal Investigator(s) : Smith, Dave R. & King, Timothy L.

Primary PI : Smith, Dave R.

Telephone Number : (304) 724-4467

Email Address : david_r_smith@usgs.gov

SIS Number : 5003571

Primary Program Element : Fisheries and Aquatic Resources

Second Program Element : Fish and Aquatic Habitats

Status : Completed

Abstract :
Because horseshoe crabs play a keystone role in the Delaware Bay ecosystem (ASMFC 1998), we are seeking to understand factors that determine the distribution of their eggs across the beach profile and the availability of their eggs to shorebirds. Knowledge of horseshoe crab egg distribution is important for designing an effective sampling strategy to estimate and monitor egg density. Understanding factors that determine horseshoe crab egg availability to shorebirds could contribute to effective habitat management and future conservation of migrating shorebirds.

Egg distribution across the beach profile

We hypothesize that horseshoe crab egg distribution across the beach profile is shaped by the interaction of beach morphology and density-dependent factors. We predict that, all else being equal, eggs would be spread widely across the foreshore of wide, low-sloped beaches, but would be tightly distributed on narrow, steep beaches. The suspected mechanism for this distributional effect involves the spatial pattern of spawning and the geometric effect of beach slope on location of high tides. Distance between the highest and lowest high tide is shorter on narrow, steep beaches than on wide, low-sloped beaches. Thus, because horseshoe crabs spawn at high tide, spawning and egg deposition would tend to occur in a more constricted pattern on narrow, steep beaches and in a more widespread pattern on wide, low-sloped beaches. However, we predict that the effect of beach morphology shrinks as high numbers of spawning horseshoe crabs are forced by crowding to deposit their eggs over a wider swath of beach regardless of width and slope. Thus, the hypothesized interaction is as follows – beach width and slope determines egg distribution on beaches with moderate to low levels of spawning, but not on beaches with high levels of spawning.

Our contention about egg distribution was supported empirically by an association between beach width and egg density within a 3 m strip oriented parallel to the waterline on beaches, which had received moderate to low levels of spawning, i.e., < 1 spawning female per m2 at the high tide line (Smith et al. unpubl. data). A possible explanation for this observation is that the 3 m strip captured a higher proportion of egg abundance (and thus higher density) on narrow beaches because of the constricted distribution. Also, consistent with the above-hypothesized interaction, beach width was not a factor when spawning activity was high, i.e., > 1 spawning female per m2 at the high tide line.

Knowledge of egg distribution is critical to sampling and monitoring horseshoe crab eggs because we cannot effectively sample eggs and estimate egg abundance if we don’t know where to find them on beaches of varied morphology. On the 16 Delaware Bay beaches surveyed in 1999 (Smith et al. unpubl. data), slopes ranged from 3 to 7 and widths ranged from 6 m to 24 m. (Slope and width tended to be inversely related: beaches were narrow and steep, wide and low-sloped, or intermediate width and slope.) The difference between semidiurnal tides is maximal at new and full moons when the majority of horseshoe crabs spawn (Barlow et al. 1986). And that difference covers a wider area on low-sloped beaches. For example, a tidal fluctuation of 0.3 m translates to a coverage of 5.7 m on a beach with a 3 slope and 2.5 m on a beach with a 7 slope (distance covered = tidal fluctuation/sin[slope]). Given this variation, the area over which eggs are sampled becomes critical to reliable comparison of egg density/abundance among beaches.

Egg availability to shorebirds

Horseshoe crabs deposit their eggs 10 – 15 cm below the substrate surface and out of reach of shorebirds (Loveland et al. 1996, Botton et al. 1992). It has been hypothesized that eggs are brought to the surface and made available to shorebirds by the burrowing action of spawning females (Myers 1986, Loveland et al. 1996, Berkson and Shuster 1999). Kraeuter and Fegley (1994) found that horseshoe crab burrowing disturbed sediment to an average depth of 11 cm and observed localized disturbance to greater sediment depth. However, Smith et al. (unpubl. data) failed to observe a relationship between surface egg density (0 – 5 cm deep) and spawning activity on 16 Delaware Bay beaches surveyed in 1999, which would be expected if the process of egg mixing was largely density-dependent. A weak association was found between surface egg density and beach width; narrow beaches tended to have higher surface egg densities.

We hypothesize, as with egg distribution, that egg availability is determined by the interaction of beach morphology, wave energy, and density-dependent factors. Above a threshold of spawning activity, eggs are brought to the surface by repeated burrowing. However, below that threshold beach geometry and wave/wind action contribute to egg availability through sediment erosion and deposition. Jackson and Nordstrom (1993) confirmed that depths of sediment activation are greater for steeper beaches. Thus, a large proportion of deposited eggs can surface even where spawning activity is moderate or low particularly on steeper beaches. Knowledge of beach characteristics that contribute to egg availability can contribute to identifying, managing, and conserving shorebird-feeding habitat. For example, if a high proportion of eggs surface on narrow, steep beaches then that information, along with other information on habitat requirements, would be useful in habitat protection and management.

OBJECTIVES: 1) Determine the effect of beach morphology, wave energy, and level of spawning activity on egg distribution and availability to shorebirds. 2) Use information on variation in egg distribution to make recommendations on survey protocol to estimate and monitor egg density. 3) Identify beach characteristics associated with egg availability to assist habitat protection efforts (such as, land purchase, horseshoe crab/shorebird sanctuary designation, landowner easement acquisition).

HYPOTHESIS TO BE TESTED: 1) Horseshoe crab eggs are distributed over less of the beach foreshore on narrow, steep beaches than on wide, low-sloped beaches except on beaches with spawning activity > 1 nesting female per m2 at the high tide line in which case egg distribution is independent of beach width and slope. 2) A greater proportion of horseshoe crab eggs surface into the top 5 cm of sediment on narrow, steep beaches and on beaches with greater wave energy except on beaches with spawning activity >1 nesting female per m2 at the high tide line in which case the burrowing action of spawning females outweighs effects of beach morphology and wave energy.

For More Information : http://www.lsc.usgs.gov/aeb/2065/

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Branch :	Aquatic Ecology Branch
Study Plan Number :	02065
Study Title :	Status, trends, and genetic population structure of Horseshoe crabs (Limulus polyphemus) in Delaware Bay
Starting Date :	04/01/1999
Completion Date :	07/31/2004
Principal Investigator(s) :	Smith, Dave R. & King, Timothy L.
Primary PI :	Smith, Dave R.
Telephone Number :	(304) 724-4467
Email Address :	david_r_smith@usgs.gov
SIS Number :	5003571
Primary Program Element :	Fisheries and Aquatic Resources
Second Program Element :	Fish and Aquatic Habitats
Status :	Completed
Abstract :	Because horseshoe crabs play a keystone role in the Delaware Bay ecosystem (ASMFC 1998), we are seeking to understand factors that determine the distribution of their eggs across the beach profile and the availability of their eggs to shorebirds. Knowledge of horseshoe crab egg distribution is important for designing an effective sampling strategy to estimate and monitor egg density. Understanding factors that determine horseshoe crab egg availability to shorebirds could contribute to effective habitat management and future conservation of migrating shorebirds. Egg distribution across the beach profile We hypothesize that horseshoe crab egg distribution across the beach profile is shaped by the interaction of beach morphology and density-dependent factors. We predict that, all else being equal, eggs would be spread widely across the foreshore of wide, low-sloped beaches, but would be tightly distributed on narrow, steep beaches. The suspected mechanism for this distributional effect involves the spatial pattern of spawning and the geometric effect of beach slope on location of high tides. Distance between the highest and lowest high tide is shorter on narrow, steep beaches than on wide, low-sloped beaches. Thus, because horseshoe crabs spawn at high tide, spawning and egg deposition would tend to occur in a more constricted pattern on narrow, steep beaches and in a more widespread pattern on wide, low-sloped beaches. However, we predict that the effect of beach morphology shrinks as high numbers of spawning horseshoe crabs are forced by crowding to deposit their eggs over a wider swath of beach regardless of width and slope. Thus, the hypothesized interaction is as follows – beach width and slope determines egg distribution on beaches with moderate to low levels of spawning, but not on beaches with high levels of spawning. Our contention about egg distribution was supported empirically by an association between beach width and egg density within a 3 m strip oriented parallel to the waterline on beaches, which had received moderate to low levels of spawning, i.e., < 1 spawning female per m2 at the high tide line (Smith et al. unpubl. data). A possible explanation for this observation is that the 3 m strip captured a higher proportion of egg abundance (and thus higher density) on narrow beaches because of the constricted distribution. Also, consistent with the above-hypothesized interaction, beach width was not a factor when spawning activity was high, i.e., > 1 spawning female per m2 at the high tide line. Knowledge of egg distribution is critical to sampling and monitoring horseshoe crab eggs because we cannot effectively sample eggs and estimate egg abundance if we don’t know where to find them on beaches of varied morphology. On the 16 Delaware Bay beaches surveyed in 1999 (Smith et al. unpubl. data), slopes ranged from 3 to 7 and widths ranged from 6 m to 24 m. (Slope and width tended to be inversely related: beaches were narrow and steep, wide and low-sloped, or intermediate width and slope.) The difference between semidiurnal tides is maximal at new and full moons when the majority of horseshoe crabs spawn (Barlow et al. 1986). And that difference covers a wider area on low-sloped beaches. For example, a tidal fluctuation of 0.3 m translates to a coverage of 5.7 m on a beach with a 3 slope and 2.5 m on a beach with a 7 slope (distance covered = tidal fluctuation/sin[slope]). Given this variation, the area over which eggs are sampled becomes critical to reliable comparison of egg density/abundance among beaches. Egg availability to shorebirds Horseshoe crabs deposit their eggs 10 – 15 cm below the substrate surface and out of reach of shorebirds (Loveland et al. 1996, Botton et al. 1992). It has been hypothesized that eggs are brought to the surface and made available to shorebirds by the burrowing action of spawning females (Myers 1986, Loveland et al. 1996, Berkson and Shuster 1999). Kraeuter and Fegley (1994) found that horseshoe crab burrowing disturbed sediment to an average depth of 11 cm and observed localized disturbance to greater sediment depth. However, Smith et al. (unpubl. data) failed to observe a relationship between surface egg density (0 – 5 cm deep) and spawning activity on 16 Delaware Bay beaches surveyed in 1999, which would be expected if the process of egg mixing was largely density-dependent. A weak association was found between surface egg density and beach width; narrow beaches tended to have higher surface egg densities. We hypothesize, as with egg distribution, that egg availability is determined by the interaction of beach morphology, wave energy, and density-dependent factors. Above a threshold of spawning activity, eggs are brought to the surface by repeated burrowing. However, below that threshold beach geometry and wave/wind action contribute to egg availability through sediment erosion and deposition. Jackson and Nordstrom (1993) confirmed that depths of sediment activation are greater for steeper beaches. Thus, a large proportion of deposited eggs can surface even where spawning activity is moderate or low particularly on steeper beaches. Knowledge of beach characteristics that contribute to egg availability can contribute to identifying, managing, and conserving shorebird-feeding habitat. For example, if a high proportion of eggs surface on narrow, steep beaches then that information, along with other information on habitat requirements, would be useful in habitat protection and management. OBJECTIVES: 1) Determine the effect of beach morphology, wave energy, and level of spawning activity on egg distribution and availability to shorebirds. 2) Use information on variation in egg distribution to make recommendations on survey protocol to estimate and monitor egg density. 3) Identify beach characteristics associated with egg availability to assist habitat protection efforts (such as, land purchase, horseshoe crab/shorebird sanctuary designation, landowner easement acquisition). HYPOTHESIS TO BE TESTED: 1) Horseshoe crab eggs are distributed over less of the beach foreshore on narrow, steep beaches than on wide, low-sloped beaches except on beaches with spawning activity > 1 nesting female per m2 at the high tide line in which case egg distribution is independent of beach width and slope. 2) A greater proportion of horseshoe crab eggs surface into the top 5 cm of sediment on narrow, steep beaches and on beaches with greater wave energy except on beaches with spawning activity >1 nesting female per m2 at the high tide line in which case the burrowing action of spawning females outweighs effects of beach morphology and wave energy.
For More Information :	http://www.lsc.usgs.gov/aeb/2065/
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U.S. Department of the Interior \|\| U.S. Geological Survey 11700 Leetown Road, Kearneysville, WV 25430, USA URL: http://www.lsc.usgs.gov Maintainer: lsc_webmaster@usgs.gov Last Modified: October 21, 2002 dwn Privacy Policy and Disclaimers \|\| FOIA \|\| Accessibility