Adaptation of Organisms to Estuarine Habitats

Adaptation of Organisms to Estuarine Habitats
By Susan Sobehrad
Fall 97

Summary:

Adaptations in the estuary are many and varied. This survey attempts to note conditions in the estuary which require special adaptations along with representative examples of species that meet the harsh requirements for survival. Many of the species described many live in one or more the estuarine environments because many estuarine environments exhibit characteristics which stress living organisms in similar ways.

The Estuarine Environment:

Conditions. The diversity of organisms that live in the estuary is limited by the harsh conditions. Temperature, salinity and light levels vary along the length of an estuary, and organic accumulation is common.

Variations in salinity are affected by numerous influences, including temperature, dissolved gases, density and viscosity. Salinity in the estuary varies with depth, side to side, and changes with the tide. Floods result in reduced salinity and drought can result in higher salinity. Organisms that can handle a wide range of salinities are euryhaline, and organisms that can survive only a narrow range of salinities are stenohaline. Coping mechanisms include moving out of unfavorable areas, shutting up shells, digging borrows and excretion of excess salts. Estuarine organisms may become stressed if they cannot cope with salinity changes; newly hatched eggs and reproducing adults are more prone to salinity stress than more mature organisms.

Shallow estuarine waters allow great temperature changes. The sun heats up the estuary during the day, and cool waters from rivers and the sea enter the estuary by night. Tides also affect estuarine temperatures--at high tide, the deeper, lower reaches of the estuary remain cool, and only the top layers are heated by the sun. As the tide goes out, heating occurs more rapidly. Eurythermal organisms can withstand the variable estuarine temperatures, while others attempt to escape by burying themselves in the sand. In temperate or polar regions, these high temperature differences sometimes result in a low population density. In tropical areas, water temperature is more stable and population is less affected.

Some estuaries have very low oxygen levels. In highly stratified estuaries, the level of biological activity in the lower levels can deplete oxygen levels. When mixing is low and tidal effects are minimal, replenishment of oxygen in the estuary may be minimal.

Particle size and chemistry of estuarine sediments can make a difference in the organisms that can survive. Infiltration of the estuarine sediments by nutrients, trace elements, sewage and industrial waste can influence the productivity of the estuary.

GULF COAST BAYS AND TIDAL CHANNELS:

Conditions:

Barrier islands protect habitats in the bays from the most sever effects of waves and currents. Due to slower water movement, fine river sediments sink to the bottom creating a muddy substrate. Bays tend to be relatively shallow, heat and cool fast, and undergo abrupt changes in salinity and oxygen content. Tidal flow is strongly related to salinity and temperature in the estuary, and may also affect movement of pollutants. Strong tidal flow may increase feeding patterns, as tides bring in swarms of plankton.

Fauna and their Adaptations:

The stable, soft bottom of the bay and tidal channel is an ideal habitat for burrowing animals. Many faunal organisms in the estuary are osmoregulators. They can regulate the composition of their body fluids by mechanisms which maintain a steady state of water exchange between external and internal factors. They must be able to cope with fluctuating oxygen levels, temperature changes, and salinity levels.

Many estuarine fish maintain water balance by actively drinking salt water, from which much of the water is absorbed by the gut. The solutes are excreted through the gills and the kidneys. Fish can adjust to environment with reduced oxygen levels by decreasing movement, thereby lowering the need for oxygen, and also by simply avoiding low oxygen situations by swimming away. Fish may also adapt by increasing their respiratory water flow. Fish can increase oxygen consumption to compensate for the depressing effects of low temperatures. In the summer, acclimatized fish decrease their oxygen consumption to compensate for the stimulating effects of high temperature. The white sturgeon is a common estuarine inhabitant, and can tolerate a wide range of salinity variations. It comes to the estuary to feed, and spawns upstream in fresh water. Protozoa cope with fresh water inflow by bailing the fresh water out through a small pore as fast as it comes in. Shrimp, crabs, and lobster keep out both water and salt with their shells. Oysters close their shell for a time when the salinity of the waters is not acceptable. The shipworm has a unique fresh water protection system. The shipworm burrows into the wood of an underwater structure, leaving the end of its tail sticking out. The tail has a structure on it called a pallet. If fresh water suddenly enter the area, the shipworm plugs up its burrow by pulling in its tail and sealing it off with the pallet. Salmon have developed highly active kidneys, special salt glands in their gills, and almost impermeable skin to offset saline conditions. To offset low temperatures, the winter flounder has evolved the ability to alter its blood-serum chemistry, and lower the temperature at which its blood will freeze. Flounder also adapt to shallow feeding grounds by swimming on their sides; to improve vision, on of its eyes migrates across its head to joint the other eye on the flounder's top side. The blue crab has heavily muscled claws which it uses to tear up the dead organic matter at the bottom of the estuary. Bacteria in the estuary tolerate salts and heavy metals in the estuary. They are better at retrieving nutrients from the substrate, and can attach to surfaces of particulates to colonize and reproduce. Bacteria also have the capacity for dormancy. When unfavorable chemical or physical conditions occur, bacteria may go dormant to permit survival until favorable conditions return. Mussels feed by filtering out the detritus and minute plants that hang in suspension in the water. Grass shrimp also feeds on the detritus and are good for the flounder and striped bass that follow the tide in to feed.

Flora and their Adaptations:

Flora must be able to withstand changes in salinity levels and must be adapted for surviving in the soft muddy bottom with its accompanying hydrogen sulfide layer.

Shoal grass and turtle grass help stabilize the muddy bottom of the bay. The grasses trap sediments, building low mounds. They are found only in shallow water because they require light for photosynthesis. They create excellent habitats for the burrowing animals of the bay.

INTERTIDAL ZONES:

Conditions:

As the tide ebbs and flows, the intertidal zone is once exposed to the elements and then inundated by tidal waters. In addition to the alternating wet and dry conditions, organisms must adapt to the waves that are in constant action in this zone.

Fauna and their Adaptations:

Most creatures that live in the intertidal zones are invertebrates, and must be able to adapt to alternating wet and dry conditions, unreliable food sources, and high energy wind and water action.

Sponges can be found under rocks in the intertidal zone, and they feed on tiny plants and animals when covered with water. Chitons are a primitive mollusk with a shell that consists of eight overlapping plates. They feed by rasping off microscopic algae from rocks. Mollusks are also able to accumulate energy reserve when food is abundant as insurance against the times when food is scarce. They can also regulate their metabolism in order to counteract changes in temperature. Some crabs, like the Sally lightfoots of the Galapagos, have spoon-shaped tips on their pincers which are well-adapted to grazing on algae and scavenging. Rock barnacles are resistant to drying and can endure a considerable portion of the day out of water if they are closed up. The letter olive snail and the moon snail glide just under the surface of the sand, leaving trails along the top. These snails have large feet for burrowing--the shell is cylindrical and polished to make burrowing easier. Coquina moves up and down the beach with the tide, cued by the vibrations of the pounding surf. It has a large foot for quick burrowing. The coquina can adapt to temperature change to any marked degree, and can vary its oxygen consumption to match seasonal variations in oxygen levels. During periods when food is scarce, coquina can also lower their metabolism levels to conserve energy and ensure their survival. Mole crabs have short legs and tiny, almost blind eyes. They live at the surface, and feed by digging backward into the sand, and then waving feathering antennae and mouth in the surf as it comes in. Polychaete worms build tubes to live in; the tubes are strengthened with mucous and are firmly anchored in the sand. The owl limpet uses its large shell in bulldozer fashion to push invaders away, and also are able to raise themselves up in order to "stop on" the front of the predator's foot with its shell.

Flora and their adaptations:

Flora have special mechanisms to prevent desiccation and to ensure capture of sun energy for photosynthesis.

Red algae have highly branched and intricate growth patterns to increase the light-gathering surfaces for photosynthesis. Both lichen and green algae can survive in the upper zones of the intertidal zone where the occasional splash and the sea mist provide enough water for survival. Sea sacs are often found in large patches. It has a hollow core that holds a reservoir of sea water to keep if from drying out at low tide. The pepper dulce has a biting, peppery taste which protects it from hungry herbivores. Bull kelp is an annual alga which can grow up to 10 centimeters a day. Air bladders lift the plant to the water's surface where large blades spread out to capture sunlight.

MUD FLATS:

Conditions:

Characteristics of the mudflat are defined by the specific combination of sand, silt, clay and organic matter content. Extreme seasonal variations in fresh water input may occur in tropical mudflats, resulting in seasonal variations in the organisms that inhabit the flats. Mudflats are highly susceptible to erosion, and as the flats are "attacked" by channels and gullying, this sediment can affect the volume of an estuary by as much as five percent, affecting population densities. Mudflats are exposed during low tides, leaving nonburrowing species open to predation.

Fauna and their Adaptations:

Organisms best suited for the mud flat are burrowers. Moving on or through the mudflat sediment requires special adaptations. Polychaete worms, burrowing crabs, crawling snails are some of the life forms that are expected in the mudflat habitat. Some feed on the surface, some below the surface, some rework the sediment, and some stabilize the sediment. Productivity of benthic organisms in the mudflat varies seasonally to strong changes of light intensity and temperature. Benthic diatoms grow well here, and can move into the water column when the mudflat is flooded by entrainment.

Flora and their Adaptations:

The primary prerequisite for living on the mudflat is the ability to survive salinity and water level changes.

Seagrasses can dominate the mudflat and in some cases, can virtually exclude marine algae. Cord grass sends roots deep into the mud to tap rich nutrients. The grass sends out underground stems from which new plants sprout. Excess salt is accumulated and discharge through its leaves. Cord grass breaks up and is carried into the tidal channel where plant material is broken down into nutrient-rich detritus.

ROCKY SHORES:

Conditions:

Wind and waves are relentless on the rocky shore. Some rock intertidal zones are flat, while others are steep and irregular with boulders and tidal channels. These irregular and varied surfaces offer organisms sheltered place and anchors in and on which to grow.

Fauna and their Adaptations: Irregular surfaces and high energy wind patterns and waves along with tidal action require strong root systems and methods for dealing with intermittent wet and dry conditions.

Limpets are able to hold onto the rocks with a large muscular foot. They also seek out depression to live in that shield them from direct sun to avoid desiccation. Limpets also secrete a sticky mucus that seals their shells to the rocky surface. Mussels anchor themselves to each other or to rocks, and keep their shells closed to avoid drying out. Aggregating anemones form masses of hundreds of individuals to reduce water loss by exposure. Sea urchins survive pounding waves by excavating pit-like depressions in the rock with their spines. Nudibranch, isopods, and spider crabs all adjust their colors to match their surroundings to reduce the likelihood of being eaten by predators,

Flora and their Adaptations:

Organisms must have mechanisms to deal with high energy winds and waves.

Trees in this windy environment must make several adaptations. The most efficient tree is low, with numerous crowded branches. Other adaptations of the tree may include flattening of the trunk, root and branches in a plan parallel to the wind direction. Growth rings are chiefly active on the leeward side of the trunk, and most branches emerge from or are bent toward the leeward direction. Sea palms grow on wave-pounded rocks where predators like sea urchins are unable to follow. Sea palms have a tough cartilaginous stipe (stem) and holdfast (root) and withstand the pounding waves. Their thick slippery cell walls help prevent drying out. Sea lettuce loses its moisture during low tide and becomes crispy, but lives on to reabsorb water. Coralline red algae secrete calcium carbonate to make themselves tough and crusty. Articulated corallines have a calcified sections with flexible joints that allow them to bend and withstand waves and high energy water movement. Rockweed can tolerate considerable desiccation, which is slowed down by thick cell walls and high concentrations of sugars. At high tide, air bladders on the rockweed cause the alga to float up from the bottom to ensure best exposure to light. Surf grass grows in rocky intertidal zones. Its seeds have pointed projections that help to lodge on coralline alga. It then sends out strong roots to grab hold of the substrate.

SALT MARSHES:

Conditions:

Sediment in the salt marsh often has a high salt content, which means that salt marsh organisms must be able to access fresh water. Salinity levels in the salt marsh vary as river inflow and tidal inflow compete. Soils in the salt marsh tend to be either water-soaked or flooded, and anaerobic. Tangled marsh plant roots help to stabilize the muddy bottom of the marsh and trap debris and nutrient with the tides. Thus, the soil is organic rich. Bacteria thrive in this detrital material, and are food for algae, invertebrate larvae, and other animals. This make the salt marsh about twice a photosynthetically productive as a corn field.

Salt marshes act like giant sponges, and absorb large volumes of water--this can minimize flooding impact, reduce erosion, and recharge groundwater. In addition, salt marsh plants help purify water by absorbing toxins.

Fauna and their Adaptations: Faunal adaptations must include mechanisms for coping with detrital food sources and fluctuating water and oxygen levels.

Littorine snails seal their shells with tiny trap doors and enter a kind of suspended animation during dry periods. The fiddler crab feeds by sifting through sand and water with specialized mouth parts. The amphipod uses the debris trapped in the marsh for both food and shelter. The great land crab has inflated epibranchial chambers which aid in respiratory exchange by increasing the volume of air to which the gills are exposed. They can live up to three days without water.

Flora and their Adaptations:

Succulence occurs in most salt marsh plants, with plants having fleshy stems and leaves. Some plants also have water storing tissues. During dry seasons, salt marsh plants may close their stomata to reduce water loss. However, this also stops carbon dioxide intake and reduces photosynthesis. Salt glands can be found in many salt marsh plants, where salt may be excreted through specialized glands on the plant's leaves. Because water moves toward a more concentrated solution, the water of plant cells is drawn into the salty soil. However, salt-tolerant plant, or halophytes, can reverse this osmotic effect. They concentrate slat ions in the their roots, and thus water flows into the roots. To cope with anaerobic conditions, many salt marsh plants have hollow passage through which air passes, connecting to stomata on the leaf surfaces with roots and providing oxygen to roots cells.

The red mangrove is common in estuaries. It lives in soil almost always covered by water. Small breathing pores cover its proper roots to help the red mangrove cope with this very wet environment. Red mangroves deal with estuarine salinity by screening the water and not allowing the salt to enter the roots at all. Seeds of the red mangrove grow small downward spikes which dig into the ground when they fall so they are not washed or carried away by fluctuating estuarine waters. The black mangrove also favors estuarine habitats. Its roots send up small, foot-long extensions called pneumatophores that help the plant exchange gases. It brings salt in through its roots, but excretes it through its leaves. Eelgrass lives in the lowest, most marine zone. It doesn't tolerate fresh water or conditions that would leaves it roots exposed to the air. Cordgrass can cope with salinity and with periodic exposure to the air. Cordgrass filters most of the salt out at the root. Any salt that does seep through is excreted by glands on the leaves. The same pore that ooze salt and served a respiratory function, breathing in supplemental oxygen and passing it back to the roots. When the tide submerges the leaves, the breathing holes close to keep the plant from "drowning. "Pickleweed" has an unusual way of getting rid of excess salt. Pickleweed has joints which allow a part of the plant to be broken off. The plant sends salt to its tips, and these portions break dry up and break off during the fall season.

SANDY BEACHES:

Conditions:

The sandy beach presents numerous problems for the organisms that choose to live there. Pounding waves, shifting sands, strong winds, and saline soils make living on the beach difficult--there is no solid substrate, nothing to hold on to keep from being blown or washed away. Beyond the high tide mark where there is no reliable water supply, the beach is hostile to most creatures. But where the sand is wet by the tides, organisms can thrive beneath the surface. Because of capillary action, each grain of sand retains a film of water around it. This liquid provides a habitat for millions of microscopic creatures. Sand also exhibits the property of thixotropy. Thixotropy is the ability of a gel to become liquid when stirred. The burrowing movements of an animal are sufficient to "liquefy" the sand and allow an animal to dig in quickly.

Fauna and their Adaptations:

Microscopic and larger animals have adapted to life under the sand to escape the harsh conditions at the surface. In their burrows, the organisms are protected from the pounding waves, from the hot, dry sun, and from frost and ice. Animals who make permanent burrows must strengthen them by building tubes or using mucous to strengthen the sand. Finding food underground is an obstacle that burrowers need to overcome.

The annelid worm and the bivalve mollusk have special adaptations to help them burrow into the sand. They can change the shape of their bodies, and push and pull their way through the sand. The worm leads with its head, and the mollusk with its foot. Razorshell clams have a streamlined shape which helps them burrow deeply. Lugworms ingest the sand itself, and then digests the organic particles in the sand. A acre-large lugworm population can process nearly 2,000 tons of sand a year. Clams have siphons, which they push up through the sand to inhale sea water, which is then filtered and microorganisms digested. Ghost crabs live higher up the beach. They often dig burrows at the foot of the dunes, but the crab is not an air breather. They must return to the water several times each day to wet its gills. Ghost crabs can also prevent drying out by the presence of thick setae around the opening to its gill chambers. These setae can take up capillary water from the sand. Sand hoppers also live above the high tide line. They dig into the sand and close the "door" by filling in the hole with sand grains. In the evening, it comes out to feed when it is safe from flying predators.

Flora and their Adaptations: Beach plants have developed many adaptations to survive. Low, sprawling root systems help hold the plants in place as winds blow and sands shift. Thick leathery or hairy leaves help reduce water loss.

The saltbush has developed special cells for salt excretion on stems and leaves. Dune grasses and fiddle leaf morning glories have adapted to the dry and windy conditions on the beach with specialized root systems.

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