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Boston University Marine Program, Marine Biological Laboratory, Woods Hole, Massachusetts 02543
1 University of Virginia, Charlottesville, VA.
Horseshoe crabs, Limulus polyphemus, are important predators in shallow environments (1). The feeding habits of adult horseshoe crabs have been examined (2), but the natural diet of juveniles has not. Juvenile crabs grow through 16 to 17 instars (3), and the increases in size during development suggest that nutritional sources may change (4). We identified likely components of juvenile diets using 
13C and 15N stable isotope signatures of juvenile horseshoe crabs and their potential prey species.
To obtain samples for isotope analyses, we collected juvenile crabs from intertidal sand flats at Nauset Beach, Massachusetts, during June and July 2002. Instars of crabs were determined based on data of Carmichael et al. (unpublished) on size of juveniles at each instar. To determine the size of juveniles, we measured the widest portion of the crab prosoma to the nearest 0.1 mm using vernier calipers. To assess position of juveniles within food webs, we collected potential prey items by hand and by sieving sediment from 0.3 m x 0.3 m x 0.1 m grabs from intertidal areas.
All specimens were dried at 60 °C, ground, and sent to the University of California—Davis Stable Isotope Facility to measure 13C and 15N signatures of crabs and potential prey species by mass spectrometry. To estimate maximum possible contributions of different taxa of prey items to the diet, we applied a linear mixing model (5) to the isotope data.
15N and 13C of horseshoe crabs changed as crabs grew (Fig. 1a). Signatures of the first instar juveniles are likely inherited from the parent, since first instars live off yolk made by the parent (6). The signatures of first instars were enriched in 15N by approximately 0.32
and depleted in 13C by approximately 2.8 relative to the average adult signature. These results extend, to an invertebrate, the range of results from studies on birds, which show yolk to be enriched in 15N and depleted in 13C relative to the parent signature (7).
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15N signatures became heavier, up to instar 11. The shift in 15N after instar 3 confirms the usual finding that, as consumers increase in size, they move up trophic steps because they seek larger prey; and larger prey characteristically show heavier signatures (4). The 15N signatures of adults remain similar to instar 11, and their offspring then start the isotopic cycle again. As the horseshoe crab juveniles grew, they evidently made use of mixed diets. To roughly quantify the proportion of different items contributing to the mixed diet, we calculated possible maximal contributions to diets of the different instars (Fig. 1c). For instars 2–3, benthic and suspended matter made up the diet. As crabs went through instars 5–11 (Fig. 1b and 1c), the contribution of polychaetes increased markedly; and molluscs contributed less than a quarter of the diet, considerably less than reported for adults (2).
The change in 15N values according to life cycle of horseshoe crabs depends on the size of the crab and their prey. The changes in 13C, in contrast, tell us that as horseshoe crabs grow, they make remarkable shifts in food webs. Judging by the 13C signature, the instar 2 juveniles probably assimilated food from organisms that derived their nutrition in part from phytoplankton and macroalgal food webs, which have 13C values of about -21 and -17, respectively (9), and in part from a food web based on Spartina alterniflora, the salt marsh cordgrass, which has a 13C of -13 (10) (vertical gray lines in Fig. 1b). By instar 3, the juveniles largely shifted to prey that depended primarily on Spartina alone. This dependency on the salt marsh-supported food web continued until instar 11. Adult signatures markedly changed from those of juveniles, again turning to the phytoplankton- and macroalgae-based parts of the food web. This pattern likely exists because juveniles largely eat small benthic polychaetes, amphipods, and isopods, whereas adults consume more bivalves (2), which are suspension feeders assimilating phytoplankton.
The isotopic data showed remarkable shifts in the diet and food web position of juvenile horseshoe crabs as they grow. Juvenile diets changed on the basis of prey size, as well as shifting from a food web based on phytoplankton to one supported by salt marsh producers. The crabs then returned to the phytoplankton-based food web as adults. These shifts in position in the food web reflect the changing array of prey consumed by horseshoe crabs of different instars and demonstrate that this species depends on widely different food webs. Conservation of horseshoe crab populations, therefore, depends on suitable management that assures both a phytoplankton and macroalgal source, as well as continued accessibility of salt marsh habitats.
This work was supported by a grant from the Friends of Pleasant Bay and a grant from the National Science Foundation REU (OCE-0097498). Many thanks to Dianne Suggs for field assistance and to the Cape Cod National Seashore for providing access to Nauset Beach. Thanks also to Paulina Martinetto, Mirta Teichberg, and Brad Williams for lab assistance.
Literature Cited
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