xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxDave Barczak
If the only time you’ve ever seen a blue crab was after it emerged flaming red from a steamer, you may wonder how it got its name. The shells, legs, and claws of living blue crabs do indeed have blue highlights. The tips of a female’s claws are red — she is said to “paint her nails.” Another way to tell males from females is to turn them over and look at the shape of their apron, really their abdomen, which has been shortened, folded under, and compressed against their bottom shell. In males, or “Jimmies,” the apron is shaped like an upside-down T. In immature females, known as “Sallies” or “she-crabs,” the apron is triangular, while in mature females, or “sooks,” the apron is more rounded or dome-shaped.

While watermen have contributed much to our understanding of the ways of the blue crab, modern science has been able to fill in some of the gaps. For example, it has long been known that female crabs mate only once, at the time of their last molt, in the brackish reaches of an estuary. They then instinctively make their way to the saltiest waters near the mouth of the bay, where their eggs develop and hatch. However, the fate of the young crabs was a mystery until relatively recently.

Like most crustaceans, blue crabs go through a series of growth stages. The initial stages, the zoeae, are microscopic and bear little resemblance to adult crabs. They are planktonic filter feeders. As they grow they must shed their old shells, or molt, four to seven times before reaching the second stage, the megalops. This stage looks slightly more crablike, with its claws and jointed legs. Megalops swim freely and eat slightly larger prey. With just one molt, megalops are transformed into tiny juvenile crabs, typically about 2.5 mm wide. After a year to 18 months, and about 20 additional molts, crabs reach maturity.

Until they are capable of swimming, larval crabs are at the mercy of currents and wind. A biologist and an oceanographer at the University of Delaware teamed up to demonstrate how these natural forces may affect the success of a spawning season and thus the population of mature crabs later on.

Larval crabs are initially carried out to sea by a coastal current that emerges from the mouth of Delaware Bay in the spring and early summer when fresh water flow is at its peak. Later, as the strength of the current diminishes, summer winds blow the surface waters carrying the tiny zoeae back toward the estuary, where the maturing crabs settle down to their life along the bottom of bays, marshes, and creeks. The UD scientists concluded that the number of larval blue crabs that are able to return to the estuary to mature is greatest in years when the river flow is low, such as during a drought.

Other recent findings have illuminated the important ecological role blue crabs play in the marsh environment. Crabs are not too picky about what they eat, consuming just about anything from carrion to seaweeds, but one of their favorites is the periwinkle, a saltwater snail that is common along the Atlantic shore. A group of researchers at Brown University found that when blue crabs are eliminated from a plot of marsh, the periwinkle population increases to the point that the snails destroy the smooth cordgrass. The scientists theorize that overharvesting of blue crabs is a major contributing factor to marsh die-offs that have occurred in the southeastern United States.

Periwinkles

xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxRyan M. Moody
The relationship between blue crabs, periwinkles, and smooth cordgrass is a familiar one to ecologists, who could cite numerous similar stories. On the Pacific coast, for example, kelp forests have been mowed down by sea urchins when sea otter populations declined. Or witness the damage done by runaway deer populations in areas where wolves and other predators have been driven out. In each case, predators maintain a population of grazers at a level that the habitat can support. When that control is gone, the grazer population gets out of hand, resulting in degradation of the habitat.

In addition to overharvesting by humans, blue crabs face other potential problems. One is competition from new species of crabs, often brought to Atlantic waters as larvae in the bilge water of cargo ships. Species such as the green crab and the Asian shore crab can make it harder for native blue crabs to find food and may even feast on young blue crabs. Blue crabs also suffer from pollution and the resulting oxygen depletion that plague many aquatic species. There are still plenty of blue crabs out there, but such a valuable and beloved resource requires careful monitoring and protection.

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xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxAlan D. Wilson, Wikipedia
Great blue herons stand about four feet tall, and their wingspans can reach six feet across. They have long pointed beaks for spearing fish from the shallow waters in which they wade. Fish are their main prey, but they will eat other animals as well, including invertebrates, amphibians, and small mammals. They swallow their prey whole and have been known to choke on prey that is too large. Because they have the longest legs, blue herons can avoid competition with other wading birds by hunting in slightly deeper waters. They are most active in the early morning and at dusk.

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While herons are solitary hunters, they nest in colonies that frequently include more than 100 birds. They build large platforms of sticks lined with pine needles, moss, and reeds high in trees at the edges of wetlands. Both parents incubate the eggs and care for the young when they hatch. They are particularly sensitive to disturbances during nesting, so their colonies should be given plenty of room and protected from human intruders.

Herons and other wading birds represent the top level in the wetland food chain. That makes them good environmental indicators. Problems anywhere along the food chain will be reflected in the health of the birds and their populations.

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Mosquito larvae

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The female salt-marsh mosquito deposits her blood-nourished eggs in muddy depressions in the high marsh, spots that are flooded no more than eight days a month. Extra-high tides, storms, or rain eventually fill these depressions, prompting the eggs to hatch into aquatic larvae. The development of these larvae through four larval stages, a pupal stage, and finally into adults takes a few days to a couple of weeks depending on the water temperature. A thorough understanding of the life cycle of the salt-marsh mosquito helps mosquito managers control their populations by attacking them at their most vulnerable stages.

Battling mosquitoes requires several different weapons. This approach is known as integrated pest management, or IPM. The goal of IPM is not total elimination of mosquitoes, but instead keeping their numbers to a comfortable level while protecting the environment from the overuse of chemical insecticides. It includes such measures as eliminating standing water whenever possible; encouraging natural predators such as the mummichog, a mosquito-eating fish; managing water levels in marshes to minimize mosquito breeding sites; and selective applications of pesticides on either larval or adult mosquitoes.

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Mummichogs belong to the killifish family whose members have a reputation for hardiness. (Two other members of this group are also common in Delaware tidal marshes.) While they rarely venture more than 100 yards from shore, they can be found year-round in estuaries up and down the East Coast. They can tolerate a full range of salinity, from seawater to fresh. They’re not too picky about temperature either, withstanding temperatures from 43 to 93 degrees Fahrenheit. In colder climates, they may burrow several inches into the mud to avoid freezing in the winter.

Low oxygen? Pollution? Not a problem for the mummichog. It has sometimes been the only remaining fish species found in severely polluted waterways in urban areas. This extreme hardiness makes the mummichog a good subject for scientific experiments in toxicology and physiology. The mummichog even became the first fish in space when it was carried aboard the Skylab 3 mission in 1973.

Mummichogs are omnivores, eating a variety of plant and animal foods. Mosquito larvae are a favorite — a single adult fish has been known to consume as many as 2,000 larvae a day when they are available. Mummichogs often play a significant role in mosquito control efforts in coastal areas. The little fish themselves are an important food source for many larger fish and fish-eating birds.

In Delaware, mummichogs spawn between April and late August. Spawning takes place in a two-week cycle that is coordinated with the extra-high tides associated with the new and full moons known as spring tides. Female mummichogs lay their eggs at night in areas reached only by the spring tides. They hide their eggs in leaves or empty shells to protect them from drying out until the spring tide returns in two weeks, at which time the eggs hatch.

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Scientists look at food chains as a series of steps, or “trophic levels.” At each level or link in the chain, energy is passed along: primary producers (plants and algae) convert energy from the sun into a form that can be consumed by other organisms. These organisms in turn are either consumed by predators or fed upon by decomposers after they die, and so on. The inconspicuous grass shrimp, as it turns out, is an important hub in the flow of energy and nutrients through the marsh.

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Grass shrimp can serve as primary consumers (feeding on plant material, especially the algae that clings to the surfaces of marsh grasses), secondary consumers (predators of various small organisms), or detritovores that help to break down dead plant material in the marsh. The shrimp produce tiny fecal pellets that fertilize the marsh sediment or are consumed by other detritovores. Grass shrimp are also a very important source of food for many fish species, including striped bass, croaker, red drum, and mummichogs. The are like the glue that holds the marsh food web together — their disappearance would have dire consequences for the marsh ecosystem.

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Fortunately, this isn’t likely to happen very soon. They are currently very abundant throughout the East Coast and the Gulf of Mexico. However, they are sensitive to environmental contamination. Studies show their numbers are depressed near sources of pollution. This makes them a good environmental monitor – a “sentinel species” that can be monitored for signs of pollution or other problems.

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xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxRobert Cohen
If you see a hole in the marsh sediment surrounded by a lot of little balls of mud, it is almost certainly a fiddler crab burrow. The mud pellets are left behind when the crab eats. Fiddler crabs use their front claws (in the case of males, the single small front claw) to scoop mud into their mouths. They sift through the mud for edible bits and pieces of algae and decaying plant matter, or detritus, then spit the leftover mud balls into their claws and put them back in the mud.

As the tide comes in, fiddler crabs retreat to their burrows and plug the openings with mud. This traps a small pocket of air inside the burrow for the crab to breathe until the tide goes out again. Fiddler crabs have gills, which must remain moist, but unlike blue crabs, they get oxygen from the air rather than the water.

The marsh fiddler crab is the smallest of three species of fiddler crab that are common to the marshes of Delaware and Chesapeake Bay. Worldwide there are nearly 100 species. They all share a common characteristic: males have a single enlarged front claw. They can be either right- or left-clawed. In fact, if a fiddler male loses his large claw, it will usually regenerate on the opposite side. He uses this claw to attract a mate and defend himself against male rivals.

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The shad is an anadromous fish — that is, it spends most of its adult life in the ocean but returns to fresh water to spawn. Most shad fishing takes place during spring spawning runs as the shad make their way up East Coast rivers from the Atlantic. The Delaware River run begins in April and is one of the longest, with shad often traveling more than 300 miles upstream to lay their eggs. Unlike salmon, the adult shad do not always die after spawning. They can return to the same spawning areas as many as three times.

The young shad hatch within 15 days and remain in fresh or brackish water until fall. They often seek the shelter of tidal marshes where they feed on small crustaceans, insects and insect larvae, and other young fish. Most remain in the estuary throughout their first winter before heading out to sea where they stay until they are ready to spawn in three to five years.

xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxBob Michelson, NOAA
The life cycles of anadromous fish are easily disrupted by humans. Large concentrations of fish at a single place and time make harvesting them a cinch. Dams present significant obstacles to travel, both upstream and downstream. Pollution in populated areas can also stop a spawning run in its tracks if it results in depleted oxygen levels in the water, a phenomenon known as a pollution block. All of these factors have contributed to declines in American shad runs on the Delaware over the years. Pollution block became an especially serious problem in the mid-1900s when shad were unable to move farther upstream than Wilmington.

The resulting severe decline in the shad population was one of the major factors that led to the formation in 1951 of a marine fisheries laboratory at the University of Delaware, the precursor of today’s College of Marine and Earth Studies. By 1975, efforts to clean up the river and install fish ladders at dams began to yield results. Shad populations have rebounded quite a bit and the Delaware River run has been restored to a reasonably healthy level. The fish have also probably benefited from a decline in the local commercial fishery, which has never recovered to 19th century levels, although recreational shad fishing remains popular.

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Dave Barczak
Diamondback terrapins display a lot of variation in color and pattern. The top shell, or carapace, is light brown to gray or black with scales or plates called scutes that are marked by roughly diamond-shaped growth rings. The bottom shell, or plastron, ranges from yellow to olive, often with darker blotches. The whitish skin of the turtle's head and legs show a pattern of squiggly black marks that are unique to each individual. Its beaklike mouth is well adapted for crunching up hard-shelled prey, including snails, clams, and crabs. It also consumes carrion, fish, worms, and some marsh plants.

Diamondback terrapins were hunted to near extinction in the 1800s to satisfy the public’s taste for terrapin soup, a delicacy served in many of the finest restaurants. The reptile has since rebounded somewhat, but it hasn’t been easy. Terrapins are slow to reproduce, and the odds are against females surviving to reproductive age, around eight years old. Those that survive long enough crawl from marsh creeks onto beaches and dunes in the summer to lay up to 20 eggs in the sand. After about three months, the inch-long hatchlings emerge from the nest to face a number of other threats: loss of habitat, crab and eel traps without Turtle Excluder Devices, pollution, boat propellers, and predators that eat turtle eggs and young. Only about 20 percent of hatchlings survive the first year.

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xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxUSDA-NRCS PLANTS Database
Smooth cordgrass builds the marsh. It spreads out into the water at the leading edge of the marsh through underground stems called rhizomes. Its dense stems and blade-shaped leaves capture sediment, which accumulates at their base, eventually raising the level of the land. Interestingly, the height of the plant varies with its position in the marsh. A tall form that grows up to seven feet high is found along creek banks and in the lowest areas of the marsh that are flooded by tides twice a day. At higher elevations, a short form that is less than two feet tall dominates.

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Smooth cordgrass shelters the marsh. Held fast by its tangled roots and rhizomes, a thick stand of cordgrass absorbs wave energy, preventing erosion and storm damage. It also slows down floodwaters from upstream areas. As the flood decelerates, it drops much of its sediment load, cleansing the water and fertilizing the marsh. The substrate built by the cordgrass provides a home for worms, mollusks, and crustaceans. Birds nest and feed safely among the tall grasses, while young fish improve their odds of survival by staying hidden in the marsh until they are old enough to fend for themselves in open water.

Smooth cordgrass also feeds the marsh. When scientists speak of the productivity of an ecosystem, they are usually referring to its biomass — the total mass of organisms in a given area. A low salt-marsh area the size of a football field can produce 21 tons of plant tissue a year (and that’s just the aboveground portion). Few places on Earth, including our most fertile croplands, produce so much food energy. More organisms feed on dead cordgrass than living — the leaves die back each winter, forming the detritus that anchors the marsh food chain.

Smooth cordgrass, like most salt-marsh plants, is called a halophyte (pronounced HAL-oh-fight) because it can tolerate salt levels that would kill most terrestrial plants. In fact, smooth cordgrass can excrete salt onto the leaf surface; the crystals are often visible when there has not been a recent rainfall to wash them away. By studying how smooth cordgrass deals with excess salt, scientists may learn how to produce salt-tolerant human food crops that can be grown in regions where the soil has become unproductive due to salt build-up.

Smooth cordgrass is an amazing plant, but it is not equally admired everywhere. On the U.S. West Coast, smooth cordgrass is considered an invasive pest whose spread needs to be controlled. The presence of smooth cordgrass in marshes bordering the Pacific changes the quality of those marshes in ways that are not beneficial for the plants and animals native to that area.

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xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxU.S. Geological Survey
Like other bivalves (mollusks with two shells hinged together), the ribbed mussel is a filter feeder — that is, it collects food from particles suspended in the water. It is one of the few bivalves able to strain particles as tiny as bacteria from the water. Most bivalves are only capable of consuming larger phytoplankton. When the tide is in and the mussels are submerged, they open their shells so water can pass over their gills and soft body. Tiny hairs on their gills, called cilia, keep the water moving along, whereas most bivalves actively siphon water through their shells. The hairs push edible particles into the mussel’s digestive tract and expel the inedible ones, which settle out around the mussel.

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When the tide goes out, you can often find groups of ribbed mussels half buried in the mud at the base of smooth cordgrass plants. They close their shells to prevent drying out, but they may occasionally “gape” a bit to breathe, since their gills are capable of getting oxygen from the air. The mussels have a muscular “foot” that can be extended from their shell for digging. However, they really prefer to stay put. They secrete a clump of tough, sticky threads called a byssus, which holds them tight to cordgrass stems, rocks, and other solid surfaces, and even to each other.

Compared to other mussels, ribbed mussels are relatively large, growing up to four inches long. You can estimate their age by counting the annual growth rings on their shells, which run opposite to the radiating ribs that give them their name. Ribbed mussels are edible, although they are not considered as good to eat as blue mussels. However, because they feed on bacteria, some of which produce toxins, ribbed mussels should only be collected for eating when they are submerged and actively filtering.

Ribbed mussels are capable of withstanding very high temperatures and a wide range of salinities. Their hardiness has enabled their populations to continue to thrive in the Mid-Atlantic. They are susceptible to certain parasites, however, which can cause local populations in limited areas to suffer dramatic losses from time to time.

]]> Phragmites. This tall, tufted grass is a common sight in roadside ditches, storm water management basins, along the banks of ponds and rivers, and in marshes. Some say the common reed is entirely too common. Without a doubt, its range has been expanding in Delaware for several decades. Tens of thousands of acres of marsh that were once a mix of plant species are now entirely occupied by common reed. Is this yet another case of an exotic plant species taking over new territory to the detriment of the native ecosystem?

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As it turns out, genetic studies have revealed that there are two different varieties of Phragmites. To the untrained eye, they are very difficult to tell apart, but the more aggressive variety is a relatively recent import from Eurasia. Now plant biologists at the University of Delaware have discovered that this variety secretes a toxic acid from its roots that literally disintegrates the roots of neighboring plants, clearing the way for the nonnative Phragmites to take over, which it is able to do rapidly through underground stems called rhizomes.

But, you may well ask, what’s so bad about common reed? It is arguably one of the most successful plants in the world. It’s found on every continent except Antarctica — and after all, isn’t nature about “survival of the fittest”? The problem is that the biodiversity of the marsh suffers. Not only does Phragmites poison and shade out other plants, it offers little of value to the animals that depended on those other plants for food and habitat. Many of them also become displaced. About all you have left is a giant field of Phragmites. Native Phragmites had a balanced place in the complex tidal marsh community, which has been upset by the new variety. Given the potent weapon used by the invader, we face quite a challenge in restoring that balance.

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xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxDave Barczak
The water filtering capacity of a marsh is related to the area it occupies. Unfortunately, tidal marshes in Delaware and elsewhere are being squeezed from both sides. On one side, ground is being lost due to sea-level rise, while development in coastal areas continues to encroach on marshes from the opposite direction. In the past, marshes have been able to adapt to changes in sea level, which have taken place slowly enough for the marsh sediements to build up or for the marsh to retreat inland. With the pace of sea-level rise increasing due to global warming and with the marsh’s retreat blocked by human development, the marsh has nowhere to go.

Bombay Hook, near Smyrna, Delaware, has been protected as a national wildlife refuge since 1937. Its 16,000 acres include one of the largest expanses of pristine tidal marsh in the Mid-Atlantic region. Since 1979, however, about 1,400 acres of vegetated salt marsh in Bombay Hook have been lost to open water. Scientists are working to explain this transformation, which is probably the result of several factors. But the primary hypothesis is that sediments flowing into the marsh are not keeping pace with rising sea level.

Invasive species are another impact on marshes. Marshes edge the estuaries that are the most advantageous locations for ports, where ships from all over the world release bilge water that sometimes contains the larvae of aquatic species that are new to the area. Those that are able to thrive in their new location may gain an advantage over local species. And invasive species aren’t limited to animals: the story of Phragmites, or common reed, in Delaware marshes is being repeated up and down the East Coast.

U.S. Environmental Protection Agency: Tidal Marshes

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West Nile viruses

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Another common complaint about marshes is that they smell bad, and they do sometimes emit an odor reminiscent of rotten eggs. The source of this odor is certain decomposer bacteria that release hydrogen sulfide gas as a byproduct when oxygen is depleted. This condition most often occurs deep in the marsh sediments. The stinky gas may seep out of the sediments gradually or remain there until disturbed by burrowing animals or digging humans.

Different bacterial species have evolved to break down (that is, use as food) a wide variety of substances in a wide variety of circumstances — oxygen or no oxygen, high or low salinity, hot or cold. Each species has a niche to exploit. What is toxic to one species may be the source of life for another. There are bacteria capable of “eating” the various components of petroleum products. These naturally occurring microbes help process the many wastes the make their way into the marsh and their numbers can be enhanced in the case of a catastrophic spill to help restore wetlands to health. As you can imagine, mechanical methods such skimming spilled oil from the water’s surface are more difficult in a marsh than on open water, so microbial clean-up methods take on greater importance. One of the best things about such bioremediation methods is that once the oil is digested, the bacteria run out of food and die naturally, producing no long-term effects.

Thanks to new molecular tools that scientists can use to make direct counts of bacteria and viruses in aquatic environments, we are learning that these microbes are more numerous than we ever imagined. For example, one drop of water from a highly productive estuary like the Chesapeake Bay, can contain up to 10 million virus particles. Many times smaller than the bacteria and algae cells that they infect, viruses nevertheless play a huge role in the environment. Since viral infection usually results in death of the host cell, viruses have an significant impact on the number and type of bacteria and algae present, and thus an impact on the entire aquatic food chain.

Bacteria and algae in the marsh are food for slightly larger, but often still microscopic, organisms. These include the larvae, or the early life stages, of snails, shellfish, and fish. As they find food and shelter from predators in the marsh, these larvae will change into their macroscopic forms, adjusting their food preferences as they grow. Without abundant microbial life, however, these larger organisms wouldn’t get a solid start.

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