Horseshoe Crab Habitat Model
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Draft Date:
June 2001

Species:
Horseshoe crab, Limulus polyphemus

Use of Study Area Resources:
All life stages. On the North American Atlantic coast this species is found from mid-coast Maine to the Gulf of Mexico.

Horseshoe crabs are invertebrates of estuarine and marine environments. Habitat selection and use in the study area has not been thoroughly studied, requiring us to rely on source information from other areas. Reproductive, juvenile and adult non-reproductive habitat are all found in the study area, with known spawning sites as far north in Maine as Hog Bay in Franklin, and Salt Pond in Blue Hill (Born 1977). Reproductive habitat may be the most limited in availability.

Adults breed on beaches, retire to shallow embayments through the summer, and then move to deeper waters offshore for the winter (Shuster 1982). Eggs develop and hatch within 2 weeks or more of spawning (Shuster 1990). Rates of development show some variation depending on salinity, temperature, and dissolved oxygen (Jegla and Costlow 1982, Penn and Brockman 1994). Maturation is believed to be slower in colder waters (Born 1977) and eggs may overwinter and hatch in the spring (Botton et al. 1992). Upon hatching from the nest, the tiny "trilobite" larvae are free swimming in the water column- mostly swimming up and then drifting down (Shuster 1990). This larval stage lasts approximately 6 days, at the end of which the larvae molt to a 'first-tailed' instar, resembling a miniature adult, and settle to the bottom (Shuster 1990). Juveniles molt up to five times the first year, two or three times the second year, "a couple of times the third year, and thereafter, once a year" (Shuster 1990). Maturity requires 15 or more molts over approximately 9 or 10 years, respectively, for males or females (Shuster 1990). Both juveniles and adults feed on infauna (worms, clams, mussels), and will burrow into sediments where they can rest or feed and presumably be protected from predation.

Habitat Requirements:
Cover.Suitable substrate for breeding, juvenile, and adult stages consists of sand, muddy-sand, gravel or crushed shell, without reducing (anoxic) conditions (Botton et al. 1988, Shuster 1990). Beaches used for spawning should have low to moderate wave energy; spawning crabs cannot withstand buffeting from waves a foot or more high, or they are washed off the beach (Shuster 1955 in Thompson m.s.). Moreover, storms prevent adults from spawning and may wash out nests and eggs (Shuster 2000).

Salinity. Egg development requires 10-15 ppt (parts per thousand); optimal levels for development are 20-30 ppt (Jegla and Costlow 1982, Sugita 1988). Shuster (1982) regards a salinity of 8 ppt as minimal for survival of eggs, larvae or adults.

Water Depth. In Maine spawning has been documented on protected beaches at the high tide line (Shuster 1953 unpublished data, C. Hodgson, personal communication). There is also reason to believe that in rare instances individuals spawn on sub-tidal flats (Rudloe 1980). Juveniles use shoal waters and flats up to -12' deep as nursery areas through the first summer and winter (Brady and Schrading 1997). After gaining in size the second season, sub-adults move to progressively deeper waters to winter (Shuster 1982). Trawling surveys collected 74% of horseshoe crabs in depths down to -65', and 92% to -98'; they have been collected as deep as -1097 meters (Botton and Ropes 1987).

Model:
Habitat was mapped for each of the three stages (breeding, juvenile, and general adult use), and these maps were combined (see below). The breeding habitat model was based on suitability of substrate (see Cover Suitabilities table, below), salinity, exposure to waves, and water depth. Substrate, salinity, and depth were considered in the adult and juvenile habitat models; the latter also depended on proximity to suitable breeding habitats.

Cover Suitabilities (all stages):
NWI Designations
(wetlands only)
Cover Types Cover Suitability
(0 - 1 scale)
Upland deciduous forest
Upland coniferous forest
Upland mixed forest
Grassland
Upland scrub/shrub
Cultivated
Developed
Bare ground
PEM, L2EM Lake/pond, emergent vegetation
PFOcon Palustrine forest, conifer
PFOdec Palustrine forest, deciduous
PSSdec Palustrine scrub shrub, deciduous
PSScon Palustrine scrub shrub, conifer
PAB, L2AB Lake/pond, aquatic vegetation
L1UB, PUB Lake/pond, unconsolidated bottom
L2US Lake, unconsolidated shore
L2RS Lake, rocky shore
R1UB Riverine subtidal unconsolidated
Rper Riverine perennial
E1AB Estuarine subtidal vegetated
E1UB Estuarine subtidal unconsolidated bottom

1.0

E2AB Estuarine intertidal algae
E2EM Estuarine intertidal emergent
E2RS, R1RS Estuarine, tidal river rocky shore
E2SS Estuarine intertidal shrub
E2US, R1US Estuarine, riverine intertidal unconsolidated shore

1.0*

M1AB Marine subtidal vegetated
M1UB Marine subtidal unconsolidated bottom

1.0

M2AB Marine intertidal algae
M2RS Marine intertidal rocky shore
M2US Marine intertidal unconsolidated shore

1.0*

NOTES * cobble and peat substrates, identified from NWI, regarded as unsuitable

Depth Suitability: Only intertidal depths were considered suitable for breeding; intertidal to -12' (referenced to mean low water, mlw) was regarded as suitable for juveniles. Only waters from intertidal to -99' mlw were scored as suitable adult non-breeding habitat, due to the proportionately higher levels of use by horseshoe crabs in that range.

Salinity Suitability: Salinity coverages available for the study area were relatively coarse, having classes of 0-5, 5-35, and 35+ ppt. Accordingly, all areas > 5 ppt were regarded as suitable for all stages.

Proximity of juvenile to breeding habitats: Because of limited motility of larval and juvenile stages, juvenile habitat was mapped only within a half-mile radius of suitable breeding habitat.

Wave energy/wind fetch: Breeding beaches must be relatively protected from wave action. We approached this by examining wind fetch (the linear distance of open water along which wind may build surface waves) in relation to known spawning beaches.  We measured this distance in each of 8 compass headings around 11 known breeding sites. The maximum fetch values (measured in numbers of 30 m cells) ranged from 8 to 156 cells, or approximately 0.24 to 4.7 km. We reasoned that the lower value might reflect a minimum level of exposure needed to avoid relatively "stagnant" conditions, leading to deposition of mud or clay, and thus reducing conditions. The upper value may reflect exposure beyond which nests would be destroyed. Therefore, suitable conditions were identified as those having wind fetch within the range of 8 to 156 cells.

Breeding habitat = having suitable values for substrate, depth, salinity, and fetch.

Juvenile habitat = having suitable values for substrate, depth, salinity, proximity to breeding habitat.

Adult habitat = having suitable values for substrate, depth, salinity.

Combined habitat values: Habitat scores for the three stages were combined to yield a single habitat map. Areas having breeding habitat also were suitable for the other 2 stages, and thus were scored highest (1.0); other areas having juvenile habitat also were suitable for adults, and thus were scored as intermediate (0.7); areas with adult habitat only were scored 0.3; non-habitat was scored 0.

Sources:
Born, J.W.  1977.  Significant Breeding Sites of the Horseshoe Crab (Limulus polyphemus) in Maine and their relevance to the critical areas program of the State Planning Office, Maine State Planning Office, Planning Report 28, April, 1977; reissued 4/82.

Botton, M.L., R.E. Loveland and T.R. Jacobsen. 1988. Beach erosion and geochemical factors: influence on spawning success of horseshoe crabs (Limulus polyphemus) in Delaware Bay. Marine Biology 99:325-332.

Botton, M.L., R.E. Loveland and T.R. Jacobsen. 1992. Overwintering by trilobite larvae of the horseshoe crab, Limulus polyphemus, on a sandy beach of Delaware Bay (New Jersey, USA). Marine Ecology Progress Series 88:289-292.

Botton, M.L. and J.W. Ropes. 1987. Populations of horseshoe crabs, Limulus polyphemus, on the northwestern Atlantic and continental shelf. Fisheries Bulletin 85(4):805-812.

Brady, J.T. and E. Schrading. 1997. Habitat Suitability Index Models: Horseshoe Crab (Spawning Beaches) - Delaware Bay, New Jersey and Delaware, Unpublished, Eric Schrading: USFWS, Pleasantville, New Jersey Field Office. 10 pp.

Jegla, T.C. and J.D. Costlow. 1982. Temperature and salinity effects on development and early posthatch stages of Limulus, in Physiology and biology of horseshoe crabs: studies on normal and environmentally stressed animals, Alan R. Liss, Inc., NY:103-113.

Penn, D. and H.J. Brockman. 1994. Nest-site selection in the horseshoe crab Limulus polyphemus. Biological Bulletin 187:373-384.

Rudloe, A. 1980. The breeding behavior and patterns of movement of horseshoe crabs, Limulus polyphemus, in the vicinity of breeding beaches in Apalachee Bay, Florida. Estuaries 3(3):177-183.

Sekiguchi, K. 1988. Biology of horseshoe crabs. Science House Co., Ltd., Tokyo, Japan. 428 pp.

Schrading, E., T. O'Connell, S. Michels and P. Perra. 1998. Interstate Fishery Management Plan for Horseshoe Crab, Atlantic States Marine Fisheries Commission, Fishery Mgt Rpt 32. 59pp.

Shuster, C.N., Jr. 1950. Observations on the natural history of the American horseshoe crab, Limulus polyphemus in 3rd Report on Investigations of methods of improving the shellfish resources of Massachusetts, Woods Hole Oceanographic Institution:18-23.

Shuster, C.N., Jr. 1982. A pictorial review of the natural history and ecology of the horseshoe crab Limulus polyphemus, with reference to other Limulidae, In Physiology and biology of horseshoe crabs: studies on normal and environmentally stressed animals, Alan R. Liss, Inc., NY: 1-52.

Shuster, C.N., Jr. 1990. The American horseshoe crab, Limulus polyphemus. In R.B. Prior, (ed.), Clinical applications of the Limulus amoebocyte lysate test. CRC Press, Boston, MA: 15-25.

Shuster, C.N., Jr. and M.L. Botton. 1985. A contribution to the population biology of horseshoe crabs, Limulus polyphemus (L.), in Delaware bay. Estuaries 8(4):363-372.

Shuster, C.N., Jr. 2000. An introduction to horseshoe crabs and biodiversity. Paper given at NJT4B, Mad About Biodiversity Conf. Reed's Beech, NJ. May 6, 2000.

Sugita, K. 1988. Environmental adaptation of embryos. In Biology of horseshoe crabs. Science House Co., Ltd., Tokyo, Japan: 195-224.

Thompson, M. ms. Draft guidelines prepared for the ASMFC horseshoe crab management plan. Revision 3 Nov. 1999, SCDNR, Charleston, SC. 16 pp.