Horseshoe crabs play an important ecological role in the food web for migrating shorebirds, finfish, and Atlantic loggerhead turtles (Caretta caretta), a federally-listed threatened species that uses the Chesapeake Bay as a summer nursery area (Keinath et al. 1987). For more detailed information on the horseshoe crab and its ecological niche, please see Horseshoe Crabs & Their Neighbors.
The principle shorebirds observed include ruddy turnstone (Arenaria interpres), red knot (Calidris canutus), semipalmated sandpiper (Calidris pusilla), sanderling (Calidris alba), dowitcher (Limnodromus spp.), and dunlin (Calidris alpina) (Dunne et al., 1982). Other shorebirds frequenting sandy beaches include western sandpiper (Calidris mauri), the black-bellied plover (Pluvialis squatarola), semipalmated plover (Charadrius semipalmatus), the threatened piping plover (Charadrius melodus), and willet (Catoptrophorus semipalmatus) (Burger, et al., 1977). The dominant species of shorebirds that use the Delaware Bay for staging are the red knot, ruddy turnstone, semipalmated sandpiper, and sanderling, representing approximately 88 percent of all shorebirds within the Delaware Bay (Gelvin-Innvaer, 1996). The Delaware Bay staging area is unique and of particular importance to shorebirds for the following reasons: shorebirds use few major stopovers during the spring migration; shorebirds arrive at stopover sites with little or no fat reserves; shorebirds demonstrate fidelity to staging areas (Wander and Dunne, 1982). An estimated 80 percent and 30 percent of the hemispheric population of red knots and sanderlings, respectively, use the Delaware Bay as a staging area (American Bird Conservancy, 1997).
Despite high shorebird abundance within the Delaware Bay, counts of sanderlings and semipalmated sandpipers declined significantly over a seven-year period from 1985 to 1992 (Clark et al., 1993). The decline in shorebirds in the Delaware Bay between 1986 and 1997 is statistically significant (p<0.05) (Clark and Niles, unpublished data, 1997). The Delaware Division of Fish and Wildlife also reports a 45 percent decline in peak counts of shorebirds from 1990-1996 compared to data from 1986-1989. The International Shorebird Survey also indicated a decline in sanderlings between 1975 and 1983. Declines in shorebird numbers may be the result of several threats, including the potential overharvest of horseshoe crabs.
During the two- to three-week staging period, shorebirds undergo weight gains of 40 percent or more (e.g., increasing body weight from 54 to 79 grams over 3 weeks) (Myers, 1986). Much of this weight gain results from feeding on horseshoe crab eggs. In particular, sanderlings are estimated to consume as much as 30.9 grams of eggs per day per bird (approximately 8,300 eggs). However, the estimated overall metabolic efficiency is low (39 percent) and is among the lowest recorded value of a vertebrate feeding on food of animal origin, based on experiments on captive birds (Castro et al., 1989). Low metabolic efficiency is attributable to the high percentage of eggs that pass through the bird's digestive tract unbroken. Metabolic efficiency of broken horseshoe crabs eggs is much higher (69 percent) than the metabolic efficiency of unbroken horseshoe crab eggs (Castro et al., 1989). Tsipoura and Burger (1998) indicate that under natural conditions, assimilation efficiency of horseshoe crab eggs may be higher than suggested by Castro et al. (1989) because sand in the diet may assist in breaking and grinding down horseshoe crab eggs.
Shorebirds require an enormous energy input every day due to their high basal metabolic rates. In addition, shorebirds typically have high daily energy expenditures and are among the longest-distance migrant animals in the world (Kersten and Piersma, 1987; Myers et al., 1985). Castro et al. (1989) concluded that sanderlings, and possibly other shorebirds, compensate for low metabolizable energy of horseshoe crab eggs by consuming large quantities of eggs. This gluttonous behavior is an absolute necessity, given the short amount of time they have to refuel, and it is facilitated by the sheer abundance of eggs, the ease in obtaining them, and the rapidity with which they pass through the birds digestive tract.
Rather than probe below the surface of the substrate, shorebirds typically forage for horseshoe crab eggs as the eggs are uncovered by successive waves of nesting crabs and erosion from localized storms (Botton et al., 1994). It is important to note that the majority of shorebirds feed on eggs from nests that have been disrupted, and that these eggs are no longer viable. Shorebirds, therefore, do not have an adverse effect on the breeding success of the horseshoe crab. Horseshoe crab eggs are the most abundant food item on Delaware Bay beaches during the migratory staging of shorebirds. Botton et al. (1994) found few other available macroinvertebrates and concluded that shorebirds are feeding primarily on horseshoe crab eggs because of their abundance. However, it is likely that shorebirds supplement their diet with ingestion of other food items during the stopover period (Botton, 1984b).
Macroinvertebrate densities on the Delaware Bay beaches rarely exceed 200 per square meter during horseshoe crab spawning season and are several orders of magnitude less than horseshoe crab egg densities. As a result, shorebirds showed a preference for beaches with higher number of horseshoe crab eggs (Botton et al., 1994). Tidal cycle, human disturbance, and competition among shorebirds and gulls may limit access to horseshoe crab eggs by shorebirds. Burger et al. (1996) concluded that a mosaic of habitat types ranging from mudflats to high marshes is essential to sustain the high population of shorebirds using the Delaware Bay during spring migration. In addition, Burger et al. (1996) documented the importance of marshes for foraging in several species of shorebirds. Shorebirds do abandon beaches at night to roost in isolated marshes, possibly to reduce the risk of predation by nocturnal wildlife (Bryant and Pennock, 1991). Clark et al. (1993) estimated that only 15-20 percent of the migrating semipalmated sandpipers and no more than 30 percent of the migrating dunlin were observed in salt marshes, feeding on prey other than horseshoe crab eggs.
Forage data (stomach contents) collected from sanderlings, ruddy turnstones, least sandpipers, semipalmated sandpipers, dunlin, and red knots on Delaware Bay beaches along the New Jersey coast (N=70) indicate that horseshoe crab eggs represent the majority of food items taken by shorebirds. Horseshoe crab eggs comprised, on average, 57.3 percent of the total stomach contents (Tsipoura and Burger, 1998). As such, horseshoe crab eggs were not taken to the exclusion of other items, such as polychaete worms and arthropods. Based on fat-free weights, red knot, ruddy turnstone, sanderling, and semipalmated sandpiper increased body mass up to 70 to 80 percent while staging on the Delaware Bay (Tsipoura and Burger, 1998). This rate of weight gain is the highest recorded for any stopover site in the world and is considered to be the result of feeding on horseshoe crab eggs. Additionally, Tsipoura and Burger (1998) reported that the mass movement of shorebirds (from the New Jersey side to the Delaware side of the Delaware Bay) is correlated with availability of horseshoe crab eggs. The ruddy turnstone provides one possible exception to the interaction between horseshoe crab egg availability and bird distribution. These birds use their bill to dig into the sand and make holes several inches deep, thereby reaching the eggs that are buried deeper in the substrate. Tsipoura and Burger (1998) found high concentrations of egg membranes in gut samples of ruddy turnstones that were captured on Thompson's Beach, New Jersey and hypothesized that the decline in abundance of surface eggs may not have been a deterrent to the foraging success of this species, provided sufficient numbers of eggs were still available in the lower strata.
Despite significant shorebird predation on horseshoe crab eggs, such activity probably has little impact on the horseshoe crab population (Botton et al., 1994). Horseshoe crabs place egg clusters at depths greater than 10 centimeters, which is deeper than most short-billed shorebirds can reach. The horseshoe crab eggs that are available for shorebird consumption are brought to the surface by wave action and by the burrowing activity of spawning horseshoe crabs. These exposed eggs would probably not survive to hatching due to heat stress or desiccation (Botton et al., 1994). Additionally, horseshoe crabs continue to spawn at least one month after the departure of most of the shorebirds. Horseshoe crab larval densities have been observed regularly exceeding 100,000 per square meter in July and August (Botton et al., 1992). For these reasons, it is unlikely that shorebird predation has a substantial adverse impact on the reproductive success of horseshoe crabs in the Delaware Bay.
The food supply provided by horseshoe crab eggs in Delaware has been estimated at 290 metric tons (Delaware Department of Natural Resources and Environmental Control, 1987). Based on an estimate of the shorebirds total energy requirements, Castro and Myers (1993) calculated that 539 metric tons of horseshoe crab eggs would be needed to sustain the spring migration of shorebirds through the Delaware Bay (assuming the shorebirds ate only horseshoe crab eggs). From this calculation, Castro and Myers (1993) estimated that the total number of females needed to lay the eggs consumed by shorebirds would be approximately 1,820,000. Assuming a sex ratio of 1:1, approximately 3,640,000 horseshoe crabs are required to sustain the shorebird migration stopover in Delaware Bay. However, these calculations assume that shorebirds feed exclusively on horseshoe crab eggs. Tsipoura and Burger (1998) indicated that although horseshoe crab eggs are a significant part of the shorebirds diet, the birds supplement their diet with other food resources. Botton et al. (1994) estimated that an average of 44,000 eggs per square meter would be needed to sustain the entire shorebird population in the Delaware Bay. Their data indicate these densities currently occur within most Delaware Bay beaches. A significant decrease in the number of horseshoe crabs could leave a large portion of migrating shorebirds without either the necessary food resources to complete their trip to the Arctic breeding grounds or the required fat reserves upon arrival to initiate egg laying and incubation.
The horseshoe crab or soldier crab as it is sometimes called, is arguably the most interesting creature on our coast. Although most people would not put it high on their list of graceful and beautiful animals, it generally leaves a lasting first impression on those who encounter it.
Spaniards exploring Florida's West Coast were impressed; naming Cockroach Bay after a creature that is neither an insect nor a true crab, but more closely related to spiders and scorpions. Understandably, those who were first to note Limulus polyphemus probably had little interest in taxonomy, but were more concerned with the practical value of their discoveries.
The French explorers were also impressed by the "king crab," and it is worth noting that Samuel de Champlain's map from his 1604 voyage to "New France" has on its margin, sketches of a few New World creatures that obviously impressed him. One of those animals is a horseshoe crab with the intriguing, presumably Native American word "Sijuenoc" scratched in next to it. The map was widely distributed in its day and I've seen a copy on display at the Acadia National Park.
Much has been written on the horseshoe crab's remarkable life history. In fact, it has been called the most intensively studied marine invertebrate in the world. However, little mention is made of the great variety of creatures that benefit from the crab or live in association with this common denizen of the shallow waters off our shore.
Over the years I have been compiling an ever growing list of creatures that somehow rely on horseshoe crabs, and have decided that if I had only one animal to choose as a teaching tool about life in the sea, this venerable arthropod would be on the top of my list. Examining a mature crab is like perusing a text book on invertebrate zoology, and almost any specimen you might pickup has at least two or three other species in tow.
Most of the myriad creatures that are found clinging or growing attached to the horseshoe crab are probably opportunists, but a few residents of the moving menagerie depend on the crab for survival. Looking over a well inhabited crab, I'm often reminded of Jonathan Swift's jingle:
"Big fleas have little fleas
In an evolutionary sense, the horseshoe crab is a conservative fellow, changing little since it first left trails between Paleozoic tide pools hundreds of millions of years ago, enduring great changes through the earth's history.
It is about as close as nature gets to a permanent fixture in this dynamic environment, and like any firm substrate in the sea, its surface is quickly covered by "fouling" organisms that require a safe haven from siltation, as well as access to the water currents that deliver food and oxygen. Because it is large, long lived and mobile, the horseshoe crab is a magnet for a variety of the invertebrates in the sea as it migrates from the continental shelf to our estuaries to spawn each spring.
A horseshoe crab that has recently shed, especially a rapidly growing youngster, has a smooth and beautiful olive colored shell that is free of scratches and marine growth. From observations in the aquarium and field, it appears that one way juveniles keep their shells squeaky clean, intentionally or incidentally, is by regularly plowing through the sand and spending extended periods of time completely buried while at rest.
For the aquarist worried about rotting fish in their tank, this disappearing act, which might last for over a week, makes them unnerving pets; but its all part of the crabs repertoire in the wild. While diving in rivers in the summer, I've observed them burrowing into the sediments to avoid being pushed around by tidal currents. While treading up clams from the marsh in winter, I've dug up young crabs in the sandy creeks, apparently hibernating. This may be where juvenile horseshoe crabs "mysteriously" disappear to during their first few winters, surviving the cold and keeping free of fouling organisms at the same time.
However, the most important feature of their lives that keeps the shell clean is rapid growth, and they shed most frequently when they are juveniles up to five times a year. This, more than any other factor prevents larvae of other creatures that are continuously settling out of the plankton community from becoming permanently established on their shells.
For horseshoe crabs, it is, quite literally, a drag getting old. As a crab ages and its growth rate slows, it sheds less frequently and begins to display a striking variety of hitchhikers. The assortment, size and growth rate of this zoological 5 o'clock shadow, gives us an idea of how recently the crab has shed and becomes a gauge of its growth rate; information that is otherwise difficult to obtain from animals that lose their entire shell, and any markers that are placed on them, as they grow.
A number of these creatures associate with horseshoe crabs because they are a source of food. Others are permanent residents, apparently living intimately with the horseshoe and nowhere else. Others may be attracted to settle on the crab because of the presence of members of their own species. Most of the other hitchhikers may settle out of their plankton stage randomly, and with luck, end up living on Limulus.
Many animals use the horseshoe crab for food, although the adult crab is so large that few things bother it. The shell of an adult crab is often riddled with scars from its few enemies. People are the worst culprits when the crab is inshore, poking and stabbing them, often because they look monstrous in their faunal overcoats. Some specimens have evenly spaced, but healed gashes across the shell, testimony to the crab's great recuperative capabilities. These are often propeller scars, however, I like to embellish things a bit for curious youngsters and add, "Although it could be the bite of a shark or loggerhead turtle!" perhaps the only two creatures that regularly try to tackle an adult crab.
Even before birth and in death, the horseshoe crab is exploited. Shorebirds are noted for their dependence on the crab's eggs, but I've seen many other birds, even ducks, treading the sand for them at the water's edge. At the high tide mark, I've found tiny nematode worms wriggling among the egg clumps in nests, presumably feeding on the eggs. Gulls pounce on overturned crabs and tear them apart to eat the gill and muscle tissue, and flies and sand fleas are soon attracted to the dead crab and utilize what remains of its flesh. The crab exacts some revenge though; it harbors an encysted flatworm in its gut, which matures into a parasite in the gull's intestine.
As on most other submerged surfaces in the sea, there develops a film of bacteria feeding on organic materials adhering to the crab's shell. These pioneers set the stage for the rest of the fouling community of invertebrates, and may ultimately help cause the demise of individual crabs by damaging the shell. On adult crabs in their terminal molt, the microorganisms begin to take a toll, as they do with most things; and as time goes on their shell becomes darker and shows more pockmarking from the presence of bacteria that utilize the chitin.
What I really find interesting are those creatures that are dwelling with crabs. The most unique confederate of the crab is the Limulus leech, an inconspicuous, but regular "ectocommensal" found on the underside of the crab. The "leech" is a flatworm, one of the Platyhelminthes; an interesting phylum because most of its members are parasitic - like tapeworms in humans; or commensal apparently harmless or even beneficial to their host. The Limulus leech is not a true "bloodsucking" parasite in its behavior or classification, although you'd think so, judging from the name bdelloura that taxonomist borrowed from the Greeks. (A hint to the word's pronunciation and origins is the bdellometer - pronounced "delometer" a 19th century medical gadget developed as a substitute for leeches.)
Bdelloura is found around the book gills and leg joints of crabs, especially on older females that have not shed for a long time. For those of us who are squeamish about wiggly things in our hands, they are often present in disconcerting abundance, and are said to be toxic if eaten just in case you're the adventurous type.
We tend to button hole animals as being either aquatic or terrestrial, over-looking the third lifestyle that is so im-portant and prevalent, that of the distant cousin who appears at the door looking for a place to live. Symbiotic relationships can benefit only one partner (Com-mensal), both (Mutualism), or be detrimental to one (Parasitism). The Limulus leech exhibits features of all three "isms", at least in the literature, and biologist don't all seem to agree on its exact relationship to the crab.
Traditionally the feeling was that the leech was not a parasite and didn't harm the crab, but merely took advantage of the minute bits of organic material that drifted around while the crab was eating or perhaps grazed the film of fouling growth on the shell surface. More recently, biologists have started to reconsider Bdelloura as a parasite that may weaken the crab enough to contribute to its eventual demise.
The leech lays its eggs in the "pages" of the crab's book gills and these are visible as little dark spots. It may also use the cuticle of the gills as a substrate for chemical activity. In time, these actions weaken the gill surface and allow leakage of seawater and bacteria into the crab's body. This may account for the crab's hypersensitivity to bacteria in its blood, which makes it of interest to the medical field. Eventually the crab begins to suffer from the onslaught.
The largest of the three Bdelloura species is easily seen and measures up to a half inch in length. Like its two smaller and less conspicuous cousins it is known only from the horseshoe crab, but is easy to find if you know where to look. Although the flatworms seem to be more abundant on females, all you need to do is flip over any large horseshoe crab to find a few.
The flatworms appear as small, pale pieces of fleshy tissue that move when you touch them. Under the magnifying glass, they are fascinating to watch as they glide across your finger, no doubt trying to escape this hot, alien world onto which they have suddenly been transplanted. How they manage to stay with the crab during the molting process is a bit of a mystery. I've never found them on young crabs and suspect it takes a considerable amount of time or good luck for a community of them to get established. Do they spread between crabs during mating? With the death of their host, how do they deal with such a disaster in their otherwise secure existence?
There are many other more conspicuous residents found on horseshoe crabs. Poring over the Phylogenetic tree that forms my ever growing list, I see nine major phyla of invertebrates regularly represented. Some are probably no better off on a crab than they would be on a rock, but others seem to thrive as hitchhikers and consistently can be found attached at specific spots on the horseshoe crab.
Sponges are filter feeders and are usually restricted to locations where there is enough water movement to bring plankton to them and to prevent burial by siltation. Brightly colored red beard sponges and other fouling Porifera occasionally become established on the posterior of horseshoe crabs; probably when the water is cool and the crab is half buried in a dormant stage. They never seem to get a chance to grow very large; at least I've never seen any more than an inch or so long. These slow growers are better suited for cosmopolitan lives on an immovable rock in a current swept channel where they won't be buried.
Coelenterates are some of the pioneering animals of the fouling community in our waters and are represented on horseshoe crabs by anemones and hydroids. The ghost anemone, a common, nondescript intertidal species, and the colorful striped anemone, an immigrant from Japanese waters, can sometimes be found if the crab's dorsal side is closely inspected. They can only fully be appreciated underwater since they close up when the crab is lifted out of the water. Like the lovely pink sea strawberries, hydroids that are also found on Limulus, they must be observed underwater to be fully appreciated. Their exquisite shape is lost when they are not supported by the water.
Snail fur, a stout, bristly cousin of sea strawberries, is more durable out of the water, but is not as attractive or easy to see without a magnifying glass. However, it is easy to feel and a colony's velcrolike texture contrasts greatly with the smooth glistening shell of the horseshoe crab.
Although the flatworms are well represented by Bdelloura, other worms are less common on horseshoe crabs. The Annelids or segmented worms are present, but never in such great numbers. Free living annelids like the scale worm (Lepidonotus) sand and bloodworms (Nereis and Glycera) may be temporary residents, as they would be on a rock or among seaweeds. They may also be potential food items that the crab has dug out of the sediments. It is difficult to decide which, but it is not unusual to pull a crab out of a net and find a worm or two gliding among its fouling growth community or trapped by the surface tension of the film of water on the crab's shell.
Tube building worms are common on the backs of horseshoe crabs. Older crabs oftentimes have a filigree of tunnels intertwined on the highest portion of the shell where it is rarely buried. Some worm species, like Sabellaria, glue sand grains together to form a protective tube. Obviously the crab must have spent time on the sandy bottom for the sand castle worm to collect construction materials, so this helps us trace its movements.
Other worms extract calcium from the water to make their home and the bottom type where the crab lives is not a factor. The loosely coiled Hydroids is fairly common on our crabs in the Middle Atlantic, and I've been told the tightlycoiled Spirorbis worm is found on crabs in the cool waters at the crabs northern limit in Frenchman's Bay, Maine, where Champlain may have encountered them.
Another ancient group, the Echinoderms, is sometimes represented on horseshoe crabs by the starfish. I've never found more than one or two tiny ones on a heavily encrusted crab, so they are probably only temporary residents and fall or move off in a short time. It's hard to imagine they could be any threat to the crab since their prey is bivalves, however, in captivity, aquarists have reported adult starfish and urchins grazing on the eyes of resting horseshoe crabs. Perhaps there is also some hazard in the wild for dormant crabs. Of the horseshoe crabs that arrive earliest in the spring at Sandy Hook, a surprising percentage have damaged eyes. Most seem disoriented and make landfall on the wrong side of their destination, ending up stranded by the rough surf on the ocean side and becoming food for gulls.
Several crustaceans are regular companions of the horseshoe crab. Mud crabs and sand shrimp are usually with the horseshoe crabs that we drag up with nets. The shrimp are no doubt incidentals in the catch and temporarily caught up with the crabs legs, although while diving and watching horseshoe crabs, I have seen shrimp and mud crabs hiding on, under, and around their shells as though the resting horseshoe crabs were algae covered rocks. Tiny juvenile spider and rock crabs also find a secure, prefabricated home in the crevices inside Limulus molts.
Give a crab a good shake in a bucket of clean water and after you remove it, you'll be surprised at what's swimming around. Scuds and marine sowbugs are likely to be found with horseshoe crabs, especially if they are heavily covered with growth. Both animals are quite common where the crabs pass through seaweed beds, and although they are strong and graceful swimmers, they are also good fish food so it is certainly to their advantage to stick close to seaweed, rocks, or horseshoe crabs that look like seaweed covered rocks.
Some years the bottom of the bay is carpeted with skeleton shrimp, and they too can be found on the fouling growth of the crab. The inchworm-like movements of these tiny crustaceans is often overlooked, especially when the substrate they are on is out of the water, but they are easy to spot if the crab is submerged.
There is no mistaking barnacles, and they regularly settle and attach permanently to the backs of horseshoe crabs. In fact, they are the crustacean you are most likely to find on Limulus. The crown they form on the cephalothorax is a mark of how deep the crab burrows into the substrate when it is at rest for long periods. Usually the circular scars of detached barnacles are also present, evidence that the crab has not shed for some time and burrowed deep enough at some point to smother the previous generation of barnacles.
The Mollusks are better represented on the horseshoe than any other phylum. Several species of bivalves become at-tached to the crabs, and a number of snails are also regularly found gliding around on them.
Oyster spat need to settle on a firm, silt free substrate, and crab backs occasionally fit the bill, especially in Delaware Bay farther south. Like bar-nacles, if they die after being smothered in the mud by the crab's burrowing habits, they leave behind scars on the lower shell.
Mussels also like to call the crab home and usually attach themselves near the hinge where water is circulated to the gills by the resting crab. Less frequently, they can be found on the underside of the crab around the legs and gills, and I've freed quite a few old souls whose move-ments were greatly impeded by clumps of sizable mussels.
The water of the Delaware Bay is sometimes too warm in the summer for edible blue mussels to thrive and grow large, but traditionally, residents in Fortesque (NJ) are said to have collected suitable mussels from crabs that move into the bay after wintering offshore. Up in Raritan Bay, we also find horse mussels on crabs in the spring as they arrive to spawn and small ribbed mussels on them after they've been up in the marshes during the summer.
Turn over a horseshoe crab in the shallows and you are likely to discover young soft and hard clams, and tiny gem shells hung up in the bristles surrounding its mouth. These are prey items of the crab; the dominant predator of the bay's benthic community and the reason why in the unenlightened and not too distant past, Massachusetts had a bounty on Limulus.
Sometimes you'll find the prey exacting a penalty too. As the crab treads the bottom for seed clams, it may inadvertently stick a claw into the open shell of a hard clam that is filtering water. A large clam can clamp down so tightly neither creature can free itself. If the waters were cleaner here on Sandy Hook Bay, on a lunchtime stroll in May I could easily collect enough quahogs from those crabs that are dragging around a living ball-and chain, to make myself a nice chowder. And without even getting wet above the ankles.
Asking around in New England, I've heard of one more bivalve that's been reported attached to Limulus, the jingle shell. The delicate and colorful jingle shell secures itself with byssal threads that pass through its lower shell. It then grows along the contours of the shell or rock it calls home. With a geographical range that extends south to the Yucatan, there are probably many other bivalves that take up residence on them and hopefully, I'll eventually hear about them from readers.
Another mollusk that is form fitted to its substrate is the slipper shell. Three species the common, convex and flat- are regularly found attached to the underside of the crab. They are so abundant and such a regular fixture on Limulus; people often mistake them for part of the crab's shell. To insure that there are males and females living in clusters, slipper shells are said to use pheromones to attract larva out of the plankton to settle on their own kind. The crab is such a good home to slippers that several generations are usually present in one stack.
Snails are well represented on horseshoe crabs. I've found periwinkles, basket and mud snails and that old nemesis of the oyster, the drill. The periwinkles graze algae, common on crabs that spend time in shallow water. Basket and mud snails are scavengers and swarm to dead crabs for food. Snails also lay eggs on the back of the crab, and in the spring we find specimens that are literally carpeted from head to tail with drill and mud snail egg capsules.
Algal growth on the shell is a clue to where the crab has been for the last few weeks. Green seaweeds like sea lettuce and Enteromorpha, and brown weeds like popweed (Fucus) thrive best in shallow waters where the light is intense. They also endure dissication, so we find them getting established on the crab's back in the warmer weather when spawning and feeding in shallow waters. The more delicate red seaweeds, Agardhiella and Gracilaria, are more likely to be found on crabs in the fall when they are dredged up from the cooler water in the bay.
Bryozoans, the colonial "moss animals", are a difficult group to recognize, but they are often the most abundant hitchhikers on horseshoe crabs, sometimes forming a coating over most of the crab's shell, even on parts of its underside. The most noticeable bryozoan, Bugula, is a bushy, golden seaweed like creature. It is probably the most attractive ornament found on a horseshoe crab. Harvested elsewhere, dried and dyed green, it is familiar to us as the "plant" sold by a florist as "everlasting" Irish sea ferns.
Other bryozoans include calcified types that form a delicate lacy pattern on the top of some shells. A third type, the spongy bryozoans, often spread across most of the upper shell of crabs, even covering their eyes.
No one is quite sure how long horseshoe crabs live (although they might live well into a second decade) or on which birthday they reach maturity (although it probably takes about ten years). However, it is suspected that they stop growing at that point, and the spreading growth of the bryozoan colony, especially on adult males, seems to confirm that they indeed stop molting at maturity.
One of the things I look forward to the most each spring is the crabs moving inshore to spawn, because they always bring along something new and interesting to show me. Over the years, I have noticed that the most sluggish are likely to have most of the shell and eyes covered with fouling growth.
Such a thorough covering across the surface of the crab may cause it more than a little inconvenience, by interfering with light detection. The thick coating of hitchhikers seems to presage the final demise of the crab. This and their sluggishness are clues that they are suffering and probably won't survive another season.
I pity them and always peel off the growth from the eyes of heavily infested crabs, but it probably doesn't help them for long, since they undoubtedly grow right back. In spite of it all, the crabs struggle on, ambling off into deeper water, disappearing beneath a thickening blanket of bryozoans, barnacles, and whatever else issues from Triton's wreathed horn.
Perhaps, like their terrestrial counterparts, old soldier crabs never die, they just fade away.