Suspension-Feeding Bivalves: Ecology, Mechanisms & Ecosystem Functions

Overview of Today’s Topic

• Focus: Suspension–feeding bivalves (mussels, oysters, clams) inhabiting soft-sediment estuarine and coastal systems.
• Complements previous lecture on deposit feeders; together, they represent the two dominant benthic feeding guilds.
• Central themes:
  • Particle capture and sorting mechanisms (gills, cilia, labial palps).
  • Ecosystem services: water-clarification, habitat engineering, benthic–pelagic coupling, nutrient regeneration.
  • Environmental controls (flow, patch size, residence time) on filtration and community impacts.
  • Case studies: mussel aquaculture, cockle chamber experiments, San Francisco Bay recovery.

Taxonomy, Key Species & Life-Habits

• Phylum : Mollusca → Class : Bivalvia (two symmetrical shells).
• Representative families / species mentioned:
  • Mytilidae – e.g.
  • Green-lipped mussel Perna canaliculus (≈ 350\,\text{L d}^{-1} filtration).
  • Mytilus edulis (blue mussel).
  • Ostreidae – oysters.
  • Veneridae – New Zealand cockle Austrovenus stutchburyi.
  • Mactridae – trough shell Paphies australis (high palp-to-body ratio).
  • Pectinidae – scallops.
  • Tellinidae – wedge shells able to switch between suspension and deposit feeding.

Core Ecosystem Services & Functional Roles

• Water-clarity enhancement
  • Rapid removal of suspended particulates; lab image showed clear vs turbid tank.
• Habitat / structural engineering
  • Dense beds (1000s m^{-2}) create 3-D complexity; horse-mussel (Atrina) supports sponges & ascidians.
• Benthic–pelagic coupling
  • Filter organic matter → package as faeces & pseudofaeces → fuels microbial & infaunal processes.
• Nutrient regeneration
  • Excretion of NH_4^+ and other remineralised nutrients stimulates primary producers (microphytobenthos, macro-algae, seagrass).
• Food-web links
  • Predation by eagle rays, fish, birds, humans; major cultural resource for coastal Māori communities.
• Biomonitoring role
  • Sessile filterers accumulate metals & toxins, providing site-specific pollution indices.

Particle Capture: Gill Micro-Mechanics

• Ciliated gills create micro-currents drawing water through inhalant aperture.
  • Lateral cilia – high-beat frequency, pump water.
  • Lateral‐frontal cilia – size-select particles (> 0.6 µm).
  • Frontal cilia – secrete mucus, bind particles, guide to marginal groove.
• Filtration window: 0.6\,\mu\text{m} \rightarrow 120\,\mu\text{m} (bacteria → phytoplankton).
• High inorganic load → gill clogging → ↑ energetic cost → ↓ growth & reproduction.

Sorting Organ – Labial Palps

• Located at mouth entrance; decide ingestion vs rejection.
• Low-quality / inorganic particles → rejected as pseudofaeces ("false faeces").
• Palp Size Index (PSI) = (palp area / body size).
  • Correlated with selection efficiency \bigl(r^2 \approx 0.9\bigr).
  • High-energy taxa (e.g. trough shells) → large PSI → superior sorting amid sandy turbulence.

Digestive Pathway

1. Mucus-bound bolus delivered to mouth.
2. Crystalline style secretes digestive enzymes.
3. Gastric shield grinds frustules (diatoms).
4. Prolonged residence in digestive gland for enzymatic breakdown & absorption.

Environmental Regulation of Food Supply

Flow‐Velocity Experiments (Flume Tank)

• Mussel bed exposed to slow vs fast unidirectional flow.
• Slow flow (≈ 0.02 m s^{-1}) → Phytoplankton concentration markedly depleted within & just above bed.
• Fast flow (≈ 0.13 m s^{-1}) → Little depletion; transit time too low for complete extraction.
• Take-home: \text{Food access} \propto \dfrac{1}{U} where U = near-bed current speed.

Patch Size × Flow in Aquaculture

• Floor-based mussel patches (3 m vs 5 m diameter).
  • In 5-m patches under low flow, edge individuals monopolise food → centre mussels show reduced shell-length / condition.
  • High-flow sites mitigate depletion; optimal farm layout must marry hydrodynamics × species filtration rate × stocking density.

Impacts on Community Structure

1. Recruitment inhibition by adult cockles
   • Field plots: high cockle density ⇒ lower densities of juvenile bivalves, polychaetes, etc.
   • Likely mechanism: adults filter larval pool (top-down predation) prior to settlement.
2. Biodeposition & Organic Enrichment
   • Sediment-trap study beneath mussel farm vs reference sites (1-yr series):
     • Deposition rate under farm ≈ 2\times that of controls.
     • Leads to lower diversity; dominance by tolerant opportunists.

Nutrient Cycling: Benthic-Chamber Manipulation (Cockles)

• Clear (light) vs opaque (dark) chambers; densities 0–2000 cockles m^{-2}.

Oxygen Flux

• Dark chambers: Net O_2 uptake increases linearly with density (all respiration).
• Light chambers: Net positive O_2 production maintained because MPB photosynthesis utilises cockle-released nutrients.

Ammonium Flux

• NH_4^+ efflux ∝ cockle density in both treatments, yet slope smaller in light chambers (nutrient re-uptake by MPB).
• Demonstrates positive feedback between suspension feeders and benthic microalgae when light is available.

Large-Scale Top-Down Control: San Francisco Bay

• Historic time-series (1970s–1990s):
  • Pre-1987: Summer low-flow periods → algal blooms (high chl-a).
  • Post-1987: Re-colonisation by two native bivalves; chl-a collapses despite same hydrology.
• Consequences:
  • ↑ Water transparency → revival of seagrass / macrophytes.
  • Illustrates capacity for bivalve clearance time < water residence time to govern pelagic biomass.

Residence Time vs Clearance Time Framework

• Water residence time (RT): days a water parcel remains in system.
• Community clearance time (CT): time for resident bivalve population to filter equivalent volume.
• Regulation threshold: If RT > CT, bivalves can control phytoplankton.
  • San Francisco Bay: RT \approx 11\,\text{d}, CT \approx 1\,\text{d} → strong control.
  • Harbour mouth (tidal flush): low RT, control weak.

Global Declines & Management Implications

• > 90 % loss of natural oyster reefs worldwide (e.g., Chesapeake Bay) → shift to turbid, eutrophic, low-oxygen states.
• Restoration requires:
  • Re-seeding / aquaculture with attention to hydrodynamics.
  • Protection from over-harvest & seabed disturbance (dredging).
  • Maintenance of water clarity to sustain positive benthic feedbacks.

Key Numbers, Definitions & Formulae

• Filtration capacity (green-lipped mussel): FR \approx 350\,\text{L d}^{-1} \; (\approx 6\,\text{L h}^{-1}).
• Particle size range captured: 0.6\,\mu\text{m} \leq d \leq 120\,\mu\text{m}.
• Labial Palp Selection Efficiency (conceptual): SE = \dfrac{\text{ingested chl-}a - \text{rejected chl-}a}{\text{ingested chl-}a}.
• Residence vs clearance criterion: Control achievable when RT/CT \gg 1.

Synthesis / Take-Home Messages

• Suspension-feeding bivalves are keystone ecosystem engineers in soft-sediment systems.
• Their physiology (gill ciliary pumps, labial palp selection) allows high-volume, high-quality particle extraction.
• Dense beds:
  1. Clarify water, enhance light, promote primary producers.
  2. Transfer pelagic organic matter to seafloor as bio-deposits, fuelling benthic microbes & infauna.
  3. Influence community structure (both via larval consumption and sediment geochemistry).
• Effectiveness is modulated by flow velocity, patch size, species-specific filtration rates, and system residence time.
• Large-scale losses of bivalves compromise water quality and ecological resilience; restoration/aquaculture must integrate hydrodynamic context to re-establish positive feedback loops.