Deposit-Feeder Lecture – Comprehensive Study Notes

Introduction & Context

• Lecture focuses on “deposit feeders” observed during the recent Tupiro Point field trip.
• Contrasted with suspension (filter) feeders (covered in next lecture).
• Habitat choice tightly linked to hydrodynamic regime & grain size:
  • Slow‐flow, finer sediments ⇒ deposit feeders dominate.
  • Very muddy zones unsuitable for filter feeders (gill clogging).
• Most deposit feeders are cryptic (live inside sediment); surface clues = burrows, mounds, pits, casts, tracks.

Deposit-Feeder Diversity

• Major phyla represented
  • Annelida: polychaetes (e.g.
    Arenicola marina, maldanids).
  • Mollusca: bivalves (Macoma/Macomona), gastropods (Hydrobia).
  • Arthropoda: crustaceans (crabs, amphipods).
  • Echinodermata: heart urchins (Echinocardium), sea cucumbers, etc.
• Many are classed as ecosystem engineers—modify habitat, alter community & biogeochemical processes.

Field Signs & Bulk Feeding Example

• Arenicola marina (lugworm)
  • Classic surface cast-mounds; bulk feeds on surrounding sediment.
  • European manipulation (Netherlands, Riese & colleagues): bulldozed 400\,\text{m}^2 plots to remove worms → quantified ecosystem-scale effects.
  • Sediment in inhabited area passes through lugworm gut 3\text{–}7 times yr⁻¹.

Feeding Modes

• Two functional subgroups
  1. Surface/near-surface browsers
  • Tentacular pickers (e.g. Hampsonina).
  • Siphonate rakers (Macoma baltica, Macomona liliana): extend siphon, sweep surface.
  1. Head-down (subsurface) feeders
  • Live head-first in sediment; defecate at surface (predator avoidance).
  • Variants:
    • Pit excavators (polychaete tubes, amphipod Corophium).
    • Ventilating pumpers (Arenicola fluidises sediment via peristaltic water pulses).
• Many can switch or combine modes (e.g. facultative deposit/filter feeding in bivalves).

Food Sources & Nutritional Challenges

• Main targets
  • Benthic microalgae / microphytobenthos (MPB) in coastal shallows.
  • Organic detritus: algal & seagrass fragments, phytoplankton carcasses, faecal pellets.
  • Bacteria & associated biofilms.
• Offshore quality/quantity declines ⇒ animals must ingest huge bulk to obtain nutrition.
• Typical sediment has high carbon : nitrogen (C:N) ratios:
  • Coastal detritus \mathrm{C:N} \approx 20 (poor food).
  • Living tissue optimum \mathrm{C:N} \approx 7.
• Paradox: How do deposit feeders grow fast on poor diet?

Microbial Gardening & Biochemical Upgrading

• Hypothesis: animals stimulate microbial growth that upgrades food value.

1. Seagrass decomposition experiment (J. Levinton)
   • Initial detritus \mathrm{C:N}\sim20.
   • Incubation with natural bacteria lowered C:N (↑ N) faster than sterile controls ⇒ microbial decay enriches substrate.
2. Hydrobia ulvae case study
   • Snail crawls, leaves faeces on surface.
   • Bacteria colonise pellets (N rises over ~4 d).
   • Snail revisits, re-ingests pellet (“microbial strip”); cycle repeats.

• Concept termed “microbial gardening”.

Particle Selection & Physiological Adaptations

• Many species actively select fine grains (larger surface area = more microbes).
• Sorting mechanisms:
  • Tentacles, ciliated palps, siphonal winnowing, fluidisation.
• Gut retention time plasticity: slows when high-quality food detected to maximise absorption.
• Specialized enzymes detach microbes / digest refractory carbon (interest for anti-biofouling tech).

Sediment Chemistry & Bioturbation Framework

• Sediments exhibit steep redox gradients (oxic → anoxic).
• Penetration depth depends on grain size (advective sands vs diffusive muds).
• Bioturbators alter gradients via two trait groups:
  1. Burrowers (ventilators): create lined tubes, ↑ sediment–water interface, couple oxic–anoxic zones.
  2. Bulldozers (reworkers): plough surface layers, deepen oxic zone, add fresh OM to depth.

Case Study – Austrohelice crassa (NZ Mud Crab)

• Burrow behaviour substrate-dependent:
  • Sandy flats: shallow, U/J-shaped burrows collapse after ~1.5 tides ⇒ crab acts as bulldozer.
  • Muddy flats: stable, branching burrows (≥10 cm vertical) persist weeks ⇒ acts as burrower.
• Epoxy-resin casts quantify geometry; changes in behaviour translate to contrasting nutrient fluxes & sediment stability.

Case Study – Arenicola marina & Biological Pump

• Planar optode imaging (N. Volkenborn) shows rhythmic O₂ pulses along lugworm gallery.
• Process:
  1. Water drawn down to depth.
  2. Sediment fluidised, particles sorted.
  3. O₂-rich water & faecal sand expelled to surface.
• Effects: accelerates nitrification–denitrification, diagenesis, sediment transport.

Case Study – Maldanid “Bamboo” Worms

• Head-down selective feeders; prefer grains <1\,\text{mm}.
• Fine particles ingested; even finer fractions ejected at surface ⇒ patchy redistribution.
• High densities create soft “traps” you can step through on tidal flats.

Sediment Stability & Transport (Macomona liliana Experiment)

• Benthic microalgae exude EPS ⇒ glue grains, resist erosion.
• Field density manipulation (Thrush et al. 2004)
  • Densities: 0, 6, 38, 75, 175\,\text{ind m}^{-2}.
  • Critical shear velocity for erosion decreased with increasing bivalve density.
  • Mechanisms: siphonal raking strips MPB, bioturbation loosens bed.

Case Study – Echinocardium cordatum & Nutrient Fluxes

• Heart urchin bulldozes just beneath surface, leaves trackways.
• Lohrer et al. Nature (2004): benthic chamber incubations.
  • Dark (respiration only): ↑ urchins ⇒ ↓ O₂ (community respiration).
  • Light (GPP possible): despite O₂ drawdown, overall net O₂ production ↑ with urchin density.
  • Explanation: ammonium excretion (higher in dark) fuels MPB photosynthesis when light available.
• Demonstrates positive feedbacks—bioadvective species can enhance primary production even while respiring.

Community-Level Interactions & Trophic Group Amensalism

• Rhoads & Young (1970s): With increasing fine mud (% silt–clay)
  • Deposit feeders ↑; suspension feeders ↓ (clogging, reduced food quality).
• Woodin (1976) additions:
  • Adult–larval interactions (suspension feeders consume deposit-feeder larvae).
• Reality is more complex:
  • Spatial refuges from erosion/stability gradients.
  • Facultative feeders switch modes.
  • Larval supply variability.
  • Predation can override trophic effects (Gray & Elliott reviews).

Ecological Significance (Synthesis)

• Deposit feeders
  • Regulate sediment biogeochemistry: O₂ penetration, nutrient cycling (NH₄⁺, NOₓ, PO₄³⁻).
  • Modify physical properties: grain sorting, compaction, erosion thresholds, bed form.
  • Mediate primary production via MPB disturbance & nutrient release.
  • Influence community assembly (engineering, competition with filter feeders, prey for higher trophic levels).
• Many are vulnerable to bottom trawling/dredging → loss of ecosystem functions.

Key Numerical & Formula Highlights

• Lugworm sediment turnover: 3\text{–}7\;\text{times yr}^{-1}.
• Large-scale worm removal: 400\,\text{m}^2 plots.
• Optimal vs natural C:N: \text{Living tissue} \approx 7 \quad | \quad \text{Coastal detritus} \approx 20.
• Macomona density experiment: 6, 38, 75, 175\;\text{ind m}^{-2} → progressive fall in critical shear velocity.
• Hydrobia faecal N content rises after \sim4 days microbial colonisation.

Suggested Further Reading (as cited)

• Levinton, J.
  S. – Marine Biology: Function, Biodiversity, Ecology.
• Rhoads & Young – Trophic group amensalism.
• Woodin, S.
  – Larval interactions.
• Gray & Elliott – Benthic community ecology reviews.
• Lohrer et al.
  (2004) Nature – Echinocardium nutrient cycling.
• Volkenborn & Reise – Arenicola bioturbation experiments.
• Thrush et al.
  (2004) – Macomona & sediment stability.