Marine Science Chapters


Introduction to Mud Flats

Mud flat coastlines only occur where the shore is protected from waves. This may be found where a river meets the ocean and a sandspit has formed across the entrance. The entire area is called an estuary if the fresh water flow is slow and the ocean flows into the river mouth - in with each high tide and out at low tide. The gentle movement of saltwater inland brings fine sediments and the slow movement of the river also brings fine sediments. These fine sediments settle out as mud. At low tide the intertidal mud is exposed as a mud flat leaving water only in permanent channels. At high tide the mud flat is covered with water.

Overall View of Mud Flats Break between Salt Marsh and Terrestrial Habitat
The mud flat and salt marsh area may cover several miles of protected coastline. The back bay at Morro Bay, California is a large area of mud flat and salt marsh habitat (left). The break between the terrestrial habitat and the salt marsh may be only an elevation of a few inches but the plant species change here dramatically (right). The bright green area near this path is from a few transition plants and the main salt marsh is to the left - a drab green color.
If mud has built up above the high tide line it may be covered with a habitat called a salt marsh. Salt marsh plants are capable of living with their roots in salt water but not their stems or leaves (except a few times a year). These plants stabilize the mud and hold it in place just above the high tide line. The salt marsh provides a transition between the terrestrial habitat and the estuary.

Everlasting, a transition plant Close up of Everlasting
One of the transition plants between the terrestrial habitat and the salt marsh is called 'everlasting' (left) because of its purple flowers that last a long time. It is also called statice, Limonium sinuatum, and is an introduced plant. Its roots have some tolerance for salt water. A close up of its leaves is shown (right).
Sea Blite, a transition plant <i>Frankenia</i>, a transition plant
More common transition plants include sea blite (Suaeda californica) seen on the left and Frankenia grandiflora seen on the right. Frankenia blooms with tiny purple/pink flowers.
A few plants, like Frankenia and Suaeda, live right along the edge of the salt marsh. Their presence indicates that saltwater is in the soil near the surface. Common terrestrial plants (like grasses and chaparral) cannot live where saltwater is in the soil.

Salt Marsh Overview Break between Salt Marsh and Mud Flat
Salt marshes may extend for miles where sediment has built up above the high tide line (left). The edge of the high tide line is where the salt marsh ends because the pickle weed can tolerate salt water on its roots but not much on its stems (right). It is a matter of an inch or two of elevation that makes this distinct line between the salt marsh and the mud flat. When you visit such a place you can tell where the average high tide will come by this edge of the salt marsh.
Pickle Weed Pickle Weed Flowers
Pickle weed, Salicornia sp. (left) and a close up of its inconspicuous flowers (right).
The dominant salt marsh plant is Salicornia. This plant may turn a red color during some seasons. It was actually eaten in Europe as a 'pickle.'

Dodder, Cuscuta salina, is the orange plant parasitizing the Suaeda in the image above. It also parasitizes Frankenia and Salicornia in the salt marsh as well as many other terrestrial plants.
Pickle weed may be parasitized by Cuscuta, also called dodder or witch's hair. The Cuscuta begins life as a tiny seed, germinating to a thin vine. As the thin vine grows upward it wraps around the first plant that it touches - sending extensions into the host plant and tapping its food and water source. The Cuscuta then breaks off its connection to the soil and becomes a complete parasite on its host. Cuscuta is a flowering plant with pretty tiny flowers that produce the tiny seeds.

Dodder flowers
The tiny white flowers of dodder are some of the smallest known flowers.

Crab Tunnels in Mud Flat Lined Shore Crab in Mud Flat Tunnel
On the edge of the salt marsh and mud flat the tunnels of the crabs can be seen (left). A lined shore crab disappears in its tunnel (right).
Lined Shore Crab Mud Flat Crab
A lined shore crab (left). A mud flat crab (right).
Two crabs build tunnels under the salt marsh. These are the lined shore crab (Pachygrapsus crassipes) and the mud flat crab (Hemigrapsus oregonensis). At first glance it would appear that the two crabs could have some competition going on but the lined shore crab is more active during the day and the mud flat crab is more active during the night so they rarely interact. Both of these animals are scavengers and will eat anything they can get their claws on. They can easily be told apart by looking to the outside of each eye - lined shore crabs have two spines behind each eye and mud flat crabs have three.

Pools in the Salt Marsh Snails in Salt Marsh Pool
Pools in the salt marsh (left) may be populated by horn snails. At close inspection (right) there may be many horn snails in the same pool (seen below).
Horn Snails Two Horn Snails
Hundreds of horn snails may be found in the salt marsh pools (left). Each snail shell is an inch or two long and well camouflaged in the muddy pools (right).
The horn snail lives in open pools in the salt marsh. This snail, Cerithidea californica, can often be found in large groups in the shallow open pools. They are deposit feeders - ingesting the mud and digesting the organic material in the mud, then excreting clean mud as small compact mud fecal pellets.

Salt Marsh Birds
Salt marsh birds include egrets (the white birds above) and marbled godwits (the brownish birds above) as well as many other species.
Salt marshes and mud flats are some of the most productive marine habitats. They are nutrient traps because they are so shallow with slow moving water. While visiting the salt marsh or mud flat one should be thinking about the critters that live there and take some care while tramping through the salt marsh or digging in the mud. It is best to try to disturb the habitat as little as possible. If you must dig a hole you should fill it back in when you finish - because it may take many tidal cycles to return your hole to its original level. The presence of acres of pickle weed creates a wonderful habitat for many birds, some of whom may nest there.

 Copyright and Credits
(Revised 21 December 2004)
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