Shuster found that horseshoe crab reproductive success is greatest under the following conditions: - The egg clusters are moistened by water with salinity of at least 8 parts per thousand. In examining the moisture requirement, Penn and Brockmann found that in Delaware, horseshoe crabs tended to place their nests in sand that was about three percent saturated.
Eggs that were buried above this zone were more likely to desiccate, and the saturated sediments of the lower beaches contained insufficient interstitial oxygen concentrations for egg development to occur. The moisture content of the sediment is determined largely by the size of the grains in the sediment. The grain size of the beaches in Delaware that had the greatest horseshoe crab spawning concentrations, as reported by Shuster and Botton , had grain sizes ranging from 0.
Beaches used by spawning horseshoe crabs in South Carolina and Florida have much smaller grain sizes. In South Carolina, grain sizes on study beaches used by horseshoe crabs are between 0. The mechanism by which horseshoe crabs locate preferred spawning habitat is not completely understood.
While horseshoe crabs spawn in greater numbers and with greater fecundity along sandy beaches, horseshoe crabs can tolerate a wide range of physical and chemical environmental conditions, and they will spawn in less suitable habitats if ideal conditions are not encountered. Therefore, the presence of large numbers of horseshoe crabs on a beach is not necessarily an indicator of habitat suitability Shuster, Interestingly, it appears that horseshoe crabs can detect hydrogen sulfide, which is produced in the anaerobic conditions of peat substrates.
These anaerobic conditions reduce egg survivability, and horseshoe crabs avoid peat substrates Botton et al. Jacobsen pers. Beach slope is also thought to play an important role in determining the suitability of beaches for horseshoe crab spawning Shuster, pers.
Horseshoe crabs generally travel downslope after spawning and appear to become disoriented on flat areas Jacobsen, pers. Field experiments by Botton and Loveland determined that beach slope is more significant than vision in orientation behavior and identified poor orientation performance on flat beaches.
Horseshoe crabs show rapid seaward orientation on beaches with slopes of approximately six degrees Botton and Loveland, Although the optimal beach slope is unknown, beaches commonly used by horseshoe crabs in New Jersey have slopes of three to seven degrees seaward U. Fish and Wildlife Service, However, Thompson concluded that while parameters controlling site selection for spawning would normally favor beaches with an optimal slope i.
Erosion is also an important component in spawning success. Erosion of the substrate in which eggs are deposited would increase egg and larval mortality. Thompson suggested that short-term, seasonal erosion characteristics may be more important than long-term conditions.
In addition to the intertidal zone used for spawning, horseshoe crabs also use shallow water areas less than 12 feet deep such as intertidal flats and shoal water as nursery habitat in their juvenile life stages. The presence of offshore intertidal flats may also influence the use of certain beaches by spawning horseshoe crabs. Horseshoe crabs may congregate on intertidal flats to wait for full moon high tides because these flats provide protection from wave energy.
Thompson identified that preferentially selected spawning sites were located adjacent to large intertidal sand flat areas. In addition to providing protection from wave energy, sand flats typically provide an abundance of available food for juvenile horseshoe crabs. Since several tidal cycles may be required to complete spawning, offshore intertidal flats may provide safe areas to rest between tide cycles. Growth and Diet Horseshoe crab eggs typically hatch 14 to 30 days after fertilization Sekiguchi, et al.
The optimum temperature for egg development has been estimated to be between 30C and 35C Jegla and Costlow, The larval stage begins when the eggs hatch and the larvae emerge. The larvae swim and feed for a period of approximately six days.
Although this free-swimming period provides the possibility of wide dispersion, when it is over, most larvae settle in shallow, intertidal areas near the beaches where they were spawned to complete their first molt Shuster, This molt into the first juvenile instar occurs approximately 20 days after emergence Jegla and Costlow, Some larvae, while still in the egg capsule, delay emergence, overwinter within beach sediments, and hatch the following spring Botton et al.
This phenomenon was observed during the winters of to , and densities of 1, to10, live trilobites per square meter were observed in sediment depths greater than 15 centimeters Botton et al.
Upon hatching, these larvae follow the same cycle described above. Juvenile horseshoe crabs generally spend their first and second summer on the intertidal flats, usually near breeding beaches Rudloe, ; Shuster, Thompson found significant use of sand flats by juvenile horseshoe crabs in South Carolina. Older crabs move out of intertidal areas and are found a few miles offshore except during breeding migrations Botton and Ropes, After the larvae and young juveniles leave the beach environment, they do not return to the beach until they are sexually active adults Rudloe, Horseshoe crabs swim or crawl as their primary means of locomotion.
Both larvae and juveniles are more active at night than during the day Rudloe, ; Shuster, Thompson, Juveniles typically feed prior to the daytime low tide, then burrow into the sand, remaining inactive for the remainder of the day Rudloe, ; Thompson, Because horseshoe crabs lack jaws, they crush and pulverize their food with the spiny bases of their legs and then place the food in the mouth.
Larvae feed on a variety of small polychaetes, nematodes, and nereis Shuster, Juvenile and adult horseshoe crabs feed mainly on molluscs, including razor clam Ensis spp. However, horseshoe crabs also prey on a wide variety of benthic organisms including arthropods, annelids, nemertean, and polychaete worms Botton, ; Botton and Haskin, In the Delaware Bay, horseshoe crabs prefer soft-shell clam Mya arenaria and small surf clam Mulinia lateralis over gem clam Gemma gemma despite the numerical dominance of the gem clam in the Delaware Bay Botton, The horseshoe crab is also an important predator of soft-shell clams in Massachusetts.
Shuster reported the consumption of sand worm Nereis spp. Botton found Botton also found vascular plant material in nearly 90 percent of all individuals. Botton and Ropes hypothesized that horseshoe crabs may control species diversity, richness, and abundance in areas where they prey upon small molluscs and polychaetes. No differences between diet and food preference are apparent between male and female horseshoe crabs.
Not surprisingly, Shuster identified that food for the horseshoe crab is abundant on the continental shelf in areas where horseshoe crabs abound. Adults The horseshoe crab must molt shed its chitinous exoskeleton to grow. Molting occurs several times during the first two to three years. As far as scientists know, adult Horseshoe Crabs, both males and females, do not molt.
Horseshoe Crabs are characteristic species of the estuary. The old shells are frequent reminders as they wash up on beaches, either as whole shells, or as disarticulated pieces, that the bay is not dead. It an underwater nursery area for young Horseshoe Crabs. If you are so inclined, feel free to take home a husked shell of a Horseshoe Crab for decoration.
Clean the empty shell by soaking it in freshwater for a day or two and then let it dry in the sun. A Horseshoe Crab's molt is a great reminder of the often unseen life that exists unexpectedly downstream from New York City.
If you do find what appear to be a long line of lifeless crabs washed up on the beach, contact state wildlife organizations as soon as possible. Loads of dead crabs on a beach could be an indicator of a serious water quality issue. The views expressed in this post are the author's own. Want to post on Patch? Register for a user account. Joe Reynolds , Neighbor. Posted Sun, Sep 22, at pm ET. Thank Reply Share. The rules of replying: Be respectful. This is a space for friendly local discussions.
No racist, discriminatory, vulgar or threatening language will be tolerated. Be transparent. Really, horseshoe crabs are just clumsy and they use their tail to flip themselves back over if they get overturned by a wave.
Though the horseshoe crab's shell is hard, it is very sensitive to the world around it. The crabs are especially sensitive to light. They have 10 eyes, a pair of compound eyes on the prosoma, and "photo receptors" in other areas, primarily along the tail.
Instead, gently pick it up by both sides of the prosoma using both hands. Horseshoe crabs are known to gather in large nesting aggregations, or groups, on beaches particularly in the mid-Atlantic states such as Delaware, New Jersey and Maryland in the spring and summer, where their populations are largest.
Horseshoe crabs can nest year-round in Florida, with peak spawning occurring in the spring and fall. The male fertilizes the eggs as the female lays them in a nest in the sand. Some males called satellite males do not attach to females but still have success in fertilizing the female's eggs by hanging around the attached pair. Most nesting activity takes place during high tides around the time of a new or full moon.
Horseshoe crab larvae emerge from their nests several weeks after the eggs are laid. Juvenile horseshoe crabs look a lot like adults except that their tails are smaller. The young and adult horseshoe crabs spend most of their time on the sandy bottoms of inter-tidal flats or zones above the low tide mark and feed on various invertebrates.
Horseshoe crabs are an important part of the ecology of coastal communities. Their eggs are the major food source for shorebirds migrating north, including the federally-threatened red knot. These shorebirds have evolved to time their migrations to coincide with peak horseshoe crab spawning activity, especially in the Delaware and Chesapeake Bay areas.
They use these horseshoe crab beaches as a gas station, to fuel up and continue their journey. Many fish species as well as birds feed on horseshoe crab eggs in Florida. Adult horseshoes serve as prey for sea turtles, alligators, horse conchs, and sharks.
Horseshoe crabs are also extremely important to the biomedical industry because their unique, copper-based blue blood contains a substance called "Limulus Amebocyte Lysate", or "LAL". This compound coagulates or clumps up in the presence of small amounts of bacterial toxins and is used to test for sterility of medical equipment and virtually all injectable drugs.
Anyone who has had an injection, vaccination, or surgery has benefited from horseshoe crabs! Additionally, research on the amazing and complex compound eyes of horseshoe crabs has led to a better understanding of human vision.
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