Lifestyles (Niches) of Salt Marsh Organisms

Plants

Halophytes

            Most plants that live in salt marshes have adaptations to deal with the salty environment and are called halophytes.  A halophyte is a “salt-loving plant”.  In actuality, most halophytes tolerate salty conditions, but many of them grow better in lower salt environments. Halophytes often grow most rapidly after heavy rains when the salt content is lower in their environment.  This is also the time when most of them tend to germinate.  Both of these adaptations show that most halophytes only tolerate salty conditions.   They are said to be facultative halophytes.  An example of a group of plants that must have salty conditions (obligate halophytes) are many kinds of mangrove trees and shrubs.  They occur in intertidal areas in tropical climates.  

Salinity is the major problem encountered by salt marsh halophytes. The salty conditions are similar to desert conditions where water is in short supply.  There are a number of similar adaptations among salt marsh plants and plants found in deserts. Halophytes have evolved adaptations that help them obtain fresh water from their saline environment.  Since water generally moves toward a more concentrated solution, the water of plant cells would normally tend to be drawn out into the salty substrate. Halophytes have mechanisms for reversing the osmotic effect. They concentrate salt ions in their roots, so that the salt concentration is greater there that in the surrounding soil, and water moves into the roots.  Some halophytes increase their cells’ solute concentration by producing organic solutes such as malate (malic acid).  This allows them to absorb water from a saline environment.  High salt levels stimulate the synthesis of nitrogen-containing solutes such as proline and glycine betaine in smooth cordgrass (Spartina alterniflora).  These substances also aid the plant in being more salt tolerant by increasing the solute concentration within the plant and allowing more fresh water to enter the plant. 

Halophytes also remove excess salt from their tissues by various strategies. Cordgrass (Spartina) and saltgrass (Distichlis), two dominant grasses of salt marshes, both have glands through which salt is excreted. Salt crystals are  visible on their stems and leaves.  Glasswort (Salicornia) rids itself of excess salt by means of jointed stems which allow a part of the plant to be break off.  The plant sends salt to its tips and, in the fall, these joints dry up and break off.

Many halophytes are also succulent which means their tissues store water in specialized cells.  This water is stored in separate cells from those which contain the organic and salt solutes.  This reservoir of freshwater can be used by the plant in times of stress.

Animals

Deposit Feeders

Salt marsh animals have some relatively unusual lifestyles.  A major food supply for animals in the marsh is dead plant material.  This detritus is broken down mechanically by the action of waves and tides and fed upon by animals sorting through the sediments for the pieces of organic material.  These animals are called deposit feeders.  They also eat other things in the sediment such as bacteria, diatoms, fungi and small animals.  Examples of deposit feeding organisms are fiddler crabs, worms, and various kinds of snails.  A very large volume of the sediment in a salt marsh is processed by deposit feeders on an annual basis.  As they feed, the sediments are mixed, sorted by particle size and have large amounts of fecal material deposited in them. 

Filter Feeders

            Filter feeding organisms remove mostly plankton and some particulate matter that is suspended in the water.  Some actively more water over or through their bodies and filter it.   Others passively filter the water as it is moved over them by the current flow.  Most have some kind of brush-like structure that catches material and removes it from the water column.  It is then transferred to the digestive structures.  Examples of filter feeding organisms are clams,  mussels, and barnacles. Material that can be digested is transformed into fecal material.  Material that can’t be digested is returned to the water in particle clumps called pseudofeces.  Both feces and pseudofeces are important fertilizer sources for both microscopic and macroscopic plants growing in the marsh.

Herbivores

            Herbivores eat living plant tissue.  Most herbivores that live on cordgrass (Spartina)  are plant hoppers or leaf hoppers that suck the sap out of the plant using piercing mouthparts.  Smooth cordgrass contains relatively large amounts of silicates which makes the grass tough and difficult to eat.  Few organisms chew on smooth cordgrass. 

Other herbivores eat living algae growing on the sediments and other surfaces.  Examples are some of the snail species. 

Predators

            Predators eat other animals, their prey.  Examples of predators in the salt marsh are fish that come into the marsh on the tides, swimming crabs, some worms, birds, and some mammals such as raccoons.

Plankton and Microorganisms

            Microscopic plants, animals, bacteria and fungi all play significant roles in the function of a saltmarsh.  Unlike most environments, where herbivores eat a substantial portion of the green plants, in a salt marsh few animals feed on living cordgrass.  Most of the cordgrass is instead broken down as dead material (detritus) by the action of bacteria and fungi or eaten directly when it is dead by animals known as detritivores.  Examples of detritivores are snails, crabs, amphipods, fiddler crabs, and worms.  During the winter months the dead tops of the cordgrass are broken off, often by the action of ice slabs moving in the marsh with the tidal fluctuations.  As the material is being broken down mechanically and then digested by organisms it becomes raw material for microscopic autotrophs, primarily algae.  The algae grows as free-floating forms (phytoplankton) and attached to larger plants and on the surface of rocks and the sediments. These organisms in turn becomes food for microscopic animals and filter feeding species such as mussels. Floating microscopic animals known as zooplankton also feed on the planktonic algae.