15.2 What communities exist along sediment-covered shores?

life on sediment covered shores requires very different adaptations than on rocky shores bc sediments change shape all the time.

nearly all large organisms on sediment-covered shores are called infauna bc they burrow into sediment. also contian large #s of microbial communities, particularly in quiet environments i.e. salt marshes and mud flats

Physical Environment of the Sediment

sediment-covered shores include:

• coarse boulder beaches

• sand beaches

• salt marshes

• mud flats

these represent progressively lower-energy environments and are consequently composed of progressively finer sediment. the energy level that a shore experiences is related to the strength of waves and longshore currents

along shores w/ low energy levels, particle size becomes smaller, the sediment slope decreases, and overall sediment stability increases —> sediment in a fine-grained mud flat is more stable than that of a high-energy sandy beach

along high-energy sandy beaches, a large quantity of water from breaking waves rapidly sinks into sand and brings a continual supply of nutrients and oxygen-rich water for animals that live there

• supply of oxygen-rich water for the animals that live there

• this supply of oxygen also enhances bacterial decomposition of dead tissue

• sediment in salt marshes and mud flats is not nearly as rich in oxygen —> decomposition occurs more slowly creating a “rotten egg” smell

Intertidal Zonation

intertidal zone of sediment-covered shore consists of supratidal, high tide, middle tide, and low tide zones which are best developed on steeply sloping, coarse-sand beaches and are less distinct on gentle sloping, fine-sand beaches

• on mud flats, the tiny clay-size particles form a deposit with essentially no slope, so zonation is not possible in this protected, low-energy environment

species of animals differ from zone to zone:

• intertidal rocky shores, max # of species and greatest biomass found near low tide shoreline and diversity and biomass decrease toward high tide shoreline

Sandy Beaches: Organisms and Their Adaptations

most animals at beach burrow into sand bc there’s no stable fixed surface —> these organisms are less obvious but get a more stable environment where not affected by fluctuations of temp and salinity or threat of drying out

Bivalve Mollusks

bivalve = animal that has two hinged shells i.e. clam or mussel

mollusk = member of phylum Mollusca, characterized by soft body and either internal or external hard CaCO3 shell

bivalve mollusks well adapted to living w/in sediment:

• single foot digs into sediment to pull creature down into sand

• how deeply a bivalve can bury itself depends on length of its siphons which must reach above sediment surface to pull in water for food and oxygen

  • indigestible matter forced back out the siphon periodically by quick muscular contractions

Annelid Worms

variety of annelids (worms) = well adapted to sediment life

Ex. lugworm constructs vertical U-shaped burrow, walls strengthened with mucus

• worm feeds by extending its snout up into shaft of burrow to loosen sand w/ quick pulsing movements

  • cone shaped depression forms at surface over head end of the burrow as sand continually slides into burrow and is ingested by the worm

  • as sand passes through the worm’s digestive tract, the sand''s biofilm is digested, and the processed sand is deposited back at the surface

Crustaceans

crabs, lobsters, shrimps, barnacles = chracterized by segmented body, hard exoskeleton, paired, jointed limbs

Ex. on most sandy beaches, numerous crustaceans called beach hoppers feed on kelp cast up by storm waves or high tides

Ex. sand crabs: move up and down the beach near shorline, burying bodies in sand leaving long curved v-shaped antennae pointing up beach slope

• filter food particles from water

Echinoderms

sand stars, heart urchin

sand stars prey on invertebrates that burrow into low-tide region of sandy beaches

• well-designed for moving through sediment: smooth back and five tapered legs w/ spines

heart urchins = flatter and elongated

• live buried in sand near low tide line

• gather sand grains in mouths where biofilm or organic matter that coats sand grains is scraped off and ingested

Meiofauna

meiofauna = small marine organisms that live in the spaces between sediment particles

• feed on bacteria attached to surface of sediment particles

Ex. polychaetes, mollusks, arthropods, nematodes

• live in sediment from intertidal zone to deep-ocean trenches

Mud Flats: Organisms and Their Adaptations

eelgrass and turtle grass widely distributed in low tide zone of mud flats and adjacent shallow coastal regions

numerous openings at the surface of mud flats attest to large population of bivalve mollusks and other invertebrates

Ex. fiddler crabs dig burrows within mud flats to live in

RECAP

sediment-covered shores including sandy beaches and mud flats have a nintertidal zonation similar to that of rocky shores but also contain many organisms living w/in the sediment (infauna)

CONCEPT CHECK 15.2

(1) Describe how sandy and muddy shores differ in terms of energy level, particle size, sediment stability, and oxygen content.

• sandy = higher energy level —> lower sediment stability —> higher oxygen content

• muddy = lower energy level —> higher sediment stability —> lower oxygen content

(2) How does the diversity of species on sediment-covered shores compare with that of the rocky shore? Suggest at least one reason why this occurs.

• sediment-covered shores = much less species diversity bc they’re composed of unconsolidated materials that often change shape and so require specific adaptations for organisms —> harder for organisms to live here

(3) In which intertidal zone of a steeply sloping, coarse-sand beach would you typically find each of the following organisms: clams, beach hoppers, ghost shrimp, sand crabs, and heart urchins?

• clams - low-tide

• beach hoppers = high tide

• ghost shrimp = low tide

• sand crabs = middle tide

• heart urchins = middle tide