Population Dynamics of Male and Female American Horseshoe Crabs (Limulus polyphemus) During Breeding and Non-breeding Seasons in Great Bay and Little Bay

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Fisheries Resources


Winsor Watson UNH - Department of Biological Sciences Co-Principal Investigator
Chris Chabot Plymouth State University Principal Investigator


The American horseshoe crab,Limulus polyphemus, plays important roles in medicine, commerce and the environment. Ecologically, numerous species of marine fishes and marine turtles feed on either the eggs, larvae or adult stages (Shuster, 1982), including the endangered loggerhead turtle (Tanacredi, 2001) and a variety of migratory birds. Limulus are also considered keystone benthic organisms as they greatly affect the community of the seafloor when they plow through the substratum while feeding on bivalve molluscs and polychaete worms (Shuster, 1982) to a depth of 18 cm (Kraeuter and Fegley, 1994). Limulus have also been important parts of commercial catches over the past two hundred years and have been used historically as fertilizer and bait (Shuster, 1982); over 2 million were harvested for use as eel and conch bait in 2000 alone (Sargent, 2002). Biomedical firms also make use of large numbers of horseshoe crabs. The blood of Limulus is used as a very sensitive, and FDA required (Berkson and Shuster, 1999), assay for the presence of gram-negative bacteria and their toxins in products such as dialyzers, antibiotics, and intravenous solutions (Levin et al., 2003).

Currently, there is serious concern that populations of Limulus may be declining due to over-fishing and other factors. Population declines have been cited on Cape Cod (Widener and Barlow, 1999), Florida (Provencha et al., 1992), and Delaware Bay (Michels.1996; Hall and Shuster, 2001). In Delaware Bay, the harvesting of Limulus for bait has been specifically blamed for the decline (Botton et al., 1994). The fact that gravid females are much preferred as eel bait (because of increased attractiveness to eels) and by biotech firms for their blood (because of their increased blood volumes; Sargent, 2002) is of particular concern.

Appropriate management and protection of any species requires a sound scientific understanding of the species of interest. Yet, despite the economic, ecological, and medical importance of this species, very little is known about the population dynamics of this species or any potential differences between the sexes. "Limulus" in Great Bay/Little Bay are a unique resource in that they represent a population that has not been fished to any large extent. As cited above, this is contrary to other better-studied populations such as those in Delaware Bay and Cape Cod.

The work outlined in this proposal will provide insight into some aspects of the population dynamics of Limulus and may, in turn, help managers and other scientists develop management practices that will prevent further declines in their populations. Of equal importance, the data provided may help us to develop accurate methods for monitoring the abundance of the species--it is difficult to monitor a species' health if one knows little about >90% of its lifespan.


Our overall objective is to determine the broad, global patterns of movements in a population of Limulus. There are at least two compelling reasons to look at this question in Limulus in Great Bay/Little Bay:

1) Unlike other populations of Limulus (e.g., Delaware Bay), this population has not been historically fished. This provides a unique scientific opportunity for studying a relatively untouched fishery.

2) The structure of Great Bay/Little Bay is very much unlike Delaware Bay where it is thought most animals leave the Bay to over-winter on the continental shelf. This is in contrast to tag/recapture studies conducted in Maine indicating that animals may stay resident (S. Shaller, in prep.). Thus, the patterns of movement we document in this study may be different than those found south of Cape Cod and thus may be more pertinent to the many populations of Limulus that reside north of Cape Cod.

Specific Objectives

Objective 1: To determine the seasonal movement patterns of Limulus caught in Great Bay/Little Bay. We will additionally focus on postulated sex differences in movement patterns.

Objective 2: To determine the abundance of Limulus in Great Bay and Little Bay during non- breeding seasons with an additional focus on sex ratio.

Objective 3: To collect enough data relative to Objectives 1 and 2 to write three new research proposals for submission in 2007. Specifically:

a) A new NSF proposal to the Division of Biological Oceanography (Marine Ecology) examining the relationship between geophysical factors and Limulus abundance and movement patterns

b) A new Northeast Consortium proposal in collaboration with local lobster fisherman to effect a tag/release/recapture program to examine the movement patterns and estimate the size of the population in different areas along the coast of NH and Maine

c) A new NH Sea Grant proposal to examine possible genetic (mtDNA) differences among populations in the Gulf of Maine (with possible joint funding with Maine Sea Grant). Animals used will be retrieved from both isolated bays as well as via offshore dredging (possibly from the National Coastal Assessment program).


A. Experiment 1: Assessing population dynamics using tagged individuals

i) Rationale

We recently received an NSF grant to look at short-term, daily and tidal patterns of movement during the summer in Limulus. My collaborator on this project, Win Watson, also received Sea Grant development funds to track a few of these animals outside of the mesocosm for a short period of time during the breeding season last year. The data that we have collected has helped us to realize that animals captured and tagged near Jackson Estuarine Laboratory (JEL) seem to stay within a few kilometers of JEL at least over the short-term. A very important part of this funding request will allow us to track the seasonal movements of these animals using these tagged animals. It is important to stress that the current NSF grant funds studies of animal movement in a mesocosm only (with very limited funds for boat time--$500 over three years). Here we are requesting funds to track these animals primarily during the non-breeding season (which comprises over 90% of their lives). Further, we propose to specifically look at possible Male/Female differences in movement patterns. Recent mitochondrial DNA (mtDNA) evidence suggests that males tend to wander much more than females. Importantly, along with helping us to understand the population dynamics of this species, these data will help to provide the evidence needed to develop three research grants cited above (Objective 3b).

ii) Methods

We currently have 25 animals (10 male and 15 female) that have been fitted with sonar tags. We propose to log their movement throughout the estuary using both hand-held sonar and automatic data-logging devices (VR2s). The VR2s log the approach (100m) of a tagged animal and will be deployed at strategic sites around Great and Little Bay (Fox Point, Green Can in upper Great Bay, channel draining area near Portsmouth Country Club, and two bracketing JEL). The data from these loggers will be downloaded bi-weekly and will help us to log their movements and to determine the area in which to search for animals using the hand-held VR100. The VR100 will be used to localize the animals to a 50-meter range on a bi-weekly basis.

iii) Expected Outcomes

Overall, the data will help us to understand the seasonal range of movement of Limulus in Great Bay/Little Bay. We predict that the range will be an order of magnitude less than that suggested for animals that breed in Delaware Bay (<5 km versus 50+ km). These differences, along with the fact that they represent an unfished, more "naturalistic" population, will help us to make the point in future funding proposals that our northern Limulus are substantially different than the southern populations and should be studied.

B. Assessing population dynamics with modified lobster traps

i) Rationale

While beach observations of most (southern) populations of breeding Limulus have indicated a highly male-biased sex ratio of 4:1 (Cohen and Brockmann, 1983), there are no studies that we are aware of that have helped us to determine if these ratios are a population phenomenon or are just based on behavioral differences (i.e., males simply visit breeding beaches five times more than females). Thus, the use of different population sampling methods is critical to help to distinguish between these possibilities.

Traditional lobster traps also catch some horseshoe crabs. Watson and colleagues have recently developed a modified lobster trap that is more selective for Limulus than current traps (the heads, the openings between the kitchen and parlor, and escape vents are all modified). This makes it possible to estimate the distribution and abundance of horseshoe crabs throughout the estuary even during months when mating is not taking place. By fishing these traps both during and after breeding season we will also be able to determine: 1) the migratory patterns of Limulus in the estuary; 2) the times of year when they are molting; 3) where they aggregate in the winter, and; 4) the sex ratio of the population. Interestingly, beach observations on another unfished northern population (Taunton Bay, ME) have indicated a less male-biased sex ratio of approximately 2:1 (S. Shaller, in prep.). It is tempting to speculate that these ratios represent a more normal (unfished) state, but it still does not differentiate between beach approaches and the actual sex ratio of the population.

ii) Methods

Twenty modified lobster traps will be deployed from Dover Point through Little Bay/Great Bay up to the mouth of the Squamscott and the drainage from Portsmouth Country Club. Ten of these will be modified to allow for best capture of females and ten will be modified for best capture of males (which are approximately 5 cm smaller than females on average). These traps will be fished biweekly while deployed.

iii) Expected Outcomes

The data collected will help us to determine if the observed beach sex ratio (2:1 male-biased) is reflected by the population as a whole or if the observations may be better attributed to a male behavioral bias for increased beach visits during the breeding season. We will compare the data collected with (ongoing) surveys of the mating beaches to determine if horseshoe crabs tend to stay near breeding beaches during non-breeding seasons. Importantly, data collected from this proposal will also be used to write new grants to more fully address this issue using these and additional methods (genetic analysis, dredging, beach counts, etc.). Additionally, funding of this proposal will help us to demonstrate the uniqueness of the population compared to more southern populations of Limulus. This finding will also help us to procure additional funding.