Oysters & Mangroves

What are mangroves, and where are they found?

• Mangroves are tropical, intertidal, semi-terrestrial shrubs or trees that tolerate seawater.

• Found in estuaries, lagoons, and protected coasts, often behind coral reefs.

• Grow in salty, muddy, depositional environments where land meets the sea.

• Adaptations include:

  • Shallow, tangled prop roots for stability and oxygen access.

  • Aerial root extensions to breathe in low-oxygen mud.

  • Salt glands to excrete salt.

  • Succulent leaves for water retention.

• Provide crucial habitat and structure for many terrestrial and aquatic organisms.

What is a mangal?

• A mangal is the entire mangrove ecosystem — including plants, animals, sediments, and water.

• It’s a depositional, land-building environment that:

  • Supports diverse infauna, epifauna, and nekton.

  • Acts as a carbon sink and detrital-based food web.

  • Provides nursery habitat for fish, crustaceans, and mollusks.

What is oyster spat?

• Oyster spat are juvenile oysters that have just settled from the plankton onto a hard surface (like shell or reef).

• They attach permanently and form the foundation of oyster reefs.

• Larvae often prefer to settle near adults, promoting reef growth over time.

What is a TMII, and how does it work?

• A TMII occurs when a predator changes the traits or behavior of its prey, which indirectly affects another species.

• It’s behaviorally based — prey alter their actions (like hiding or feeding less) due to fear of predation.

• Example from oyster reefs:

  • Oyster toadfish scare mud crabs, making them feed less on oyster spat.

  • This increases oyster survival — even when no crabs are eaten.

What is a DMII, and how does it differ from a TMII?

• A DMII occurs when predators reduce prey numbers (density), which indirectly benefits another species lower in the food web.

• Example in oyster reefs:

  • Oyster toadfish eat mud crabs, decreasing crab density → fewer oysters are consumed.

• Difference:

  • DMII = prey density changes

  • TMII = prey behavior changes

Explain the trophic cascade in oyster reefs.

• Oyster toadfish → mud crabs → oyster spat

  • Toadfish are top predators.

  • They reduce mud crab predation on juvenile oysters (spat) by:

    • Eating mud crabs (DMII)

    • Causing mud crabs to hide and feed less (TMII)

• Result: More oyster spat survive → reef structure grows → more habitat for other species.

• Complexity of reef structure influences which mechanism (TMII vs DMII) dominates — complex reefs = more TMII effects.

What characteristics do salt marshes, mangroves, and seagrasses have in common?

• Found in intertidal to subtidal coastal zones.

• Dominated by foundation species (cordgrass, mangroves, or seagrass).

• Vegetated systems with detrital-based food webs (decomposed plant matter fuels system).

• Provide nursery habitats for fish and invertebrates.

• Enhance biodiversity by creating structure.

• Carbon sinks – store and sequester organic carbon (“blue carbon”).

• Protect coastlines from erosion and storms.

Why are these plant species considered foundation species?

• They physically structure the habitat, allowing many other species to live there.

• They stabilize sediments, reduce erosion, and buffer waves.

• Their roots, blades, or prop roots provide refuge and nursery space.

• They enhance species richness and abundance by altering the environment in beneficial ways.

🧠 Foundation species create ecosystems — without them, the entire community collapses.

What ecological services do oyster reefs provide?

1. 🌊 Wave breaks / shoreline protection:

   • Reduce erosion by attenuating wave energy.

   • Serve as natural breakwaters.

2. 🐟 Habitat for commensals:

   • Provide structure for fish, crabs, and other invertebrates.

   • Increase biodiversity and fisheries productivity.

3. 💧 Water filtration:

   • Oysters filter plankton, PIM (particulate inorganic matter), and POM (particulate organic matter).

   • Reduce nutrient pollution and eutrophication.

4. 💰 Economic services:

   • Major fishery resource (~$300 million annual value in Louisiana).

   • Job creation and coastal protection value.

5. 🌿 Ecosystem engineering:

   • Create complex, three-dimensional reef structures that stabilize sediments and enhance nearby habitats.

Similarities to Seagrasses, Marshes, and Mangroves:

• All provide habitat, filtration, coastal protection, and carbon storage.

• All are foundation-based coastal ecosystems with high ecological and economic value.

What environmental factors determine where oysters live?

• Salinity: Most important factor.

  • Low salinity: fewer predators & diseases, but reduced growth and reproduction.

  • High salinity: increased growth, but more predation and disease (Dermo, MSX).

• Predation risk:

  • Predators (crabs, snails, fish) more common in higher salinity zones.

• Substrate:

  • Need hard surfaces for larvae (spat) to settle on.

• Tidal exposure:

  • Found in intertidal and shallow subtidal zones.

• Disturbance:

  • Storms, burial, and sedimentation negatively affect survival.

🧠 Oysters find refuge in moderate-salinity zones — low enough to avoid predators and disease, but high enough to grow well.

What limits oysters in different salinity levels?

Salinity Level	Limiting Factors
Low salinity	Physiological stress, reduced feeding and reproduction
High salinity	Increased predation and disease (e.g., Dermo, MSX)

Balance Point: Intermediate salinity zones often have highest oyster abundance because they minimize both stress and predation.

How do salt marshes, seagrasses, mangroves, and oyster reefs compare?

Feature	Salt Marsh	Seagrass	Mangrove	Oyster Reef
Dominant life form	Halophyte grasses (Spartina)	Submerged angiosperms (Thalassia)	Salt-tolerant trees/shrubs (Rhizophora)	Sessile bivalves (Crassostrea)
Location	Temperate, intertidal	Tropical/temperate subtidal	Tropical intertidal	Estuarine intertidal/subtidal
Foundation role	Stabilize sediment	Provide substrate & habitat	Build land, protect coasts	Create reef structure
Main stress	Salinity & flooding	Light & turbidity	Salinity & inundation	Salinity & disease
Main consumers	Periwinkles, crabs	Turtles, urchins, fish	Crabs, insects	Crabs, snails, toadfish
Trophic cascades	Blue crabs → snails → Spartina	Sharks → turtles → seagrass	Rare	Toadfish → mud crabs → oyster spat
Disturbance	Negative	Patch-forming	Negative	Negative
RSP fit	Modified	Modified	Modified	Partial (no monopoly, tolerance refuge)

What do all these ecosystems have in common?

• All are foundation-based, coastal vegetated (or structured) systems.

• Provide nurseries, filtration, protection, and carbon storage.

• Each shaped by stress–competition trade-offs, predation, and trophic cascades.

• All are currently threatened by human activity — overfishing, habitat loss, pollution, and climate change.