Benthos Soft Bottom

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• Benthos Soft Bottom
• Benthos presents considerable habitat diversity,
  relative to pelagic environment
• Physical Environment
• Sediment characteristics
• Mixture of inorganic particles, organic
  particles, water
• Particle size related to current energy
  (subtidal) or wave energy (intertidal)
• High energy areas contain coarser particles than
  low energy areas
• Grain size determines porosity and geotechnical
  properties, which affect benthic community
  structure
• Sediments containing mostly coarse particles may
  lack fine particles and organic material required
  by deposit feeders
• Sediments containing mostly fine particles may be
  too unstable for large, dense organisms to
  maintain positions
• Abundances of certain types of fauna may be
  related to availability of specific sediment size
  classes

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• Benthos Soft Bottom
• Physical Environment
• Sediment characteristics
• Size distribution can be described in terms of
  sorting
• Poorly sorted sediments are distributed over a
  wide range of size classes
• Well sorted sediments cover a narrow range of
  size classes
• Poorly sorted sediments usually reflect
  heterogeneity in the deposition or
  post-depositional processes
• Well sorted sediments tend to be deposited in
  areas with relatively constant current regimes or
  gradients of current or wave energy (Ex sandy
  beaches)
• May be useful to calculate median grain size (F)

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• Benthos Soft Bottom
• Physical Environment
• Sediment characteristics
• Areas with high current energy may develop ripple
  marks
• Ripples can affect organismal distributions
• Fine particles accumulate in troughs
• Crests usually are coarser grained, erosional,
  and contain less organic material than troughs
• Partitioning of sediment into microenvironments
  can affect community structure on same spatial
  scale as ripple marks
• Ex Organic material deposited in troughs can
  attract deposit feeding organisms
• Areas with ripple marks tend to contain lower
  abundances of fauna than areas without ripples
  Why?

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• Benthos Soft Bottom
• Physical Environment
• Sediment characteristics
• Most temperate sand beaches contain sediments
  composed of irregularly-shaped quartz grains
  mixed with shell fragments and organic detritus
• Sand particle sizes range from energy beaches to 2 mm on high energy beaches
• Organic matter content affects food availability
  as well as sediment chemistry
• High organic content ? oxygen depletion
• Depth of oxic-anoxic boundary on beaches can
  range from few mm to nearly 1 m
• Depth and stability of oxic-anoxic interface can
  affect structure of beach infauna

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• Benthos Soft Bottom
• Physical Environment
• Environmental conditions
• Beaches are dynamic and physically unstable in
  terms of sediment transport
• Unstable conditions exclude large sessile fauna
  and most epifauna
• Three interrelated factors define beaches
• Particle size
• Wave action (most important for biota NB)
• Slope

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Fig. 6.38
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Eagle Cove, WA
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• Benthos Soft Bottom
• Physical Environment
• Environmental conditions
• Fine sand
• Retains more water by capillary action than
  coarse sand
• More amenable to burrowing than coarse sand
• Beach slope results from interaction between wave
  action, particle size and importance of swash and
  backwash
• Substrate movement varies seasonally
• Winter beaches may be very different from summer
  beaches

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Winter Beaches
Summer Beaches
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• Benthos Soft Bottom
• Species Composition
• Primary producers
• No large, attached plants in sand beach
  intertidal zones
• Most primary production by benthic diatoms,
  dinoflagellates and cyanobacteria
• Primary producers restricted to surface sediments
• Primary production typically low (
• System depends primarily on organic detritus
  and/or primary production in surrounding waters
• Subtidally, macrophytes may play an important
  role and can have a substantial effect on
  community composition and structure
• Basis enhanced deposition, stabilization
• Other structures (e.g. tubes, algae) also can
  stabilize sediments

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• Benthos Soft Bottom
• Species Composition
• Animals often categorized by size and location
• Location epifauna vs. infauna
• Megafauna
• No standard definition
• Some infaunal macrofauna would be considered
  megafauna if exposed
• Nearly absent from sandy beaches
• High energy environment
• Pressure from terrestrial predators
• Conspicuous but less important ecologically than
  smaller, more abundant organisms
• More important in low energy environments

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• Benthos Soft Bottom
• Species Composition
• Macrofauna
• Large enough to be retained on 0.5 mm sieve
• Low diversity on beaches compared to less dynamic
  areas
• In terms of biomass, most important taxa are
• Burrowing bivalves
• Polychaetes
• Crustaceans
• All these taxa either are
• Mechanically resistant to sediment movement
  (bivalves)
• Highly mobile (polychaetes)
• Both (crustaceans)
• Typically display zonation on shorelines

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• Benthos Soft Bottom
• Species Composition
• Macrofauna
• Supralittoral Zone
• Occupied mostly by scavenging, semi-terrestrial
  crustaceans
• Temperate regions Amphipods (e.g. Orchestia) and
  isopods (e.g. Ligia)
• Tropical regions Ghost crabs (e.g. Ocypode,
  Sesarma)
• Animals avoid daytime heat by burrowing into
  sediments, coming out at night to feed on
  detritus, e.g. decaying vegetation washed up on
  beach

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• Benthos Soft Bottom
• Species Composition
• Macrofauna
• Littoral Zone
• Supports higher macrofaunal diversity than
  supralittoral zone
• Bivalves
• High densities of burrowing bivalves may be
  present
• Three basic strategies
• Many small clams (e.g. Donax, Tellina) are
  wedge-shaped (Why?) and can burrow rapidly when
  exposed
• Migrate up and down beach with tides
• Small numbers of large burrowing bivalves (e.g.
  razor clams like Ensis, Siliqua) may be present
• Razor clams are highly mobile have smooth, thin
  shells w/narrow profiles, facilitating rapid
  burrowing
• Some burrowing bivalves (e.g. Cardium, Macoma)
  less streamlined but capable of anchoring in
  sediments
• Aided in part by pronounced ridges on shells

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• Benthos Soft Bottom
• Species Composition
• Macrofauna
• Littoral Zone
• Bivalves
• Littoral bivalves tend to be suspension feeders,
  deposit feeders, or both
• Generally, deposit feeders dominate in finer
  sands while suspension feeders are more abundant
  in coarser sands
• Gastropods
• Numerous species, e.g. olive shells (Olivella)
  and large moon snails (Natica, Polinices)
• All have smooth, undecorated shells (Why?)
• Most gastropods are highly mobile predators,
  feeding on small mollusks (primarily bivalves)
• Predation often involves using radula to drill a
  hole through bivalves shell to gain access to
  prey
• In some areas, predatory littoral gastropods can
  have an important effect on community structure
• Experiments with removing moon snails from beach
  led to increases in populations of clams and
  other infaunal prey

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• Benthos Soft Bottom
• Species Composition
• Macrofauna
• Littoral Zone
• Polychaetes
• May form permanent structures on sandy beaches
  (unusual)
• Some species line burrows with mucus or adhesive
• Many sand-dwelling polychaetes are deposit
  feeders, although some are either passive or
  active suspension feeders, ingesting primarily
  resuspended organic material or small zooplankton
• Some littoral polychaetes (e.g. Glycera) are
  mobile predators or scavengers that move through
  sand actively seeking food

Glycera
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• Benthos Soft Bottom
• Species Composition
• Macrofauna
• Littoral Zone
• Crustaceans
• Littoral species tend to be good burrowers but
  often less mobile than supralittoral forms
• Ex Mole crabs (Emerita) typically bury just
  below sediment/water interface, extending only
  antennae into water to strain out particles
• Swift burrowers but often prey for shore birds
• Use a strategy called swash riding to maintain
  position within desirable portion of littoral zone

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• Benthos Soft Bottom
• Species Composition
• Macrofauna
• Littoral Zone
• Crustaceans
• Other crustaceans that inhabit this environment
  include decapods and mysids that burrow
  temporarily but emerge to feed, as well as
  epifaunal crabs that are opportunistic predators
• In winter, when much of a beach is eroded,
  intertidal open-beach invertebrates often migrate
  to subtidal portions of beaches, because erosive
  forces are too great

Mysid
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• Benthos Soft Bottom
• Species Composition
• Macrofauna
• Littoral Zone
• Echinoderms
• May be present at lower tide levels
• Include burrowing holothuroids and echinoids
  (heart urchins, sand dollars)
• Most are deposit feeders
• Sand dollars have shortened spines, facilitating
  burrowing
• Juveniles of some species selectively ingest and
  accumulate dense sediment particles (e.g. iron
  oxide) in digestive tracts to reduce likelihood
  of being swept away during periods of intense
  wave action
• Littoral asteroids are uncommon in temperate
  areas but range from upper subtidal to lower
  littoral in tropical areas
• Littoral asteroids are mostly detritivores

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• Benthos Soft Bottom
• Species Composition
• Macrofauna
• Littoral Zone
• Chordates
• Relatively few fishes impact littoral zone in
  soft bottom areas
• Shore birds feed in this area, and may impact
  benthic communities through predation

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• Benthos Soft Bottom
• Species Composition
• Meiofauna
• Pass through 0.5 mm sieve but large enough to be
  retained on a 62-µm-mesh sieve
• Very diverse group vs. others inhabiting sand
  beaches
• Sometimes termed interstitial fauna because they
  live in spaces between sand grains
• Many individuals move among sediment grains but
  may or may not displace them in bulk like
  burrowing macrofauna
• Endobenthic forms are larger than interstitial
  spaces and displace particles while moving
• Mesobenthic species move within interstitial
  spaces and do not displace particles while moving
• Most meiofauna are mesobenthic, especially in
  medium to coarse sediments, but in very fine
  sediments most meiofauna are endobenthic

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Loricifera

• Benthos Soft Bottom
• Species Composition
• Meiofauna
• Many taxa represented Mollusks, crustaceans,
  worms from several phyla, etc.
• Some groups entirely or almost entirely
  restricted to this habitat (e.g. kinorhynchs,
  gastrotrichs, loriciferans)

Kinorhyncha
Nematoda
Gastrotricha
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• Benthos Soft Bottom
• Species Composition
• Meiofauna
• Because they share an interstitial habit,
  meiofauna from many phyla have evolved
  convergently in terms of body design

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• Benthos Soft Bottom
• Species Composition
• Meiofauna
• Body trends in meiofauna include
• Reduced body size
• Especially striking in groups whose members
  typically have large body size (e.g. Mollusca,
  Echinodermata)
• Vermiform or flattened shape
• Provides flexibility and maneuverability
• Strengthened body design
• Protects against abrasion and crushing
• Adaptations may include development of protective
  spines or scales (gastrotrichs), well-developed
  cuticle or exoskeleton (nematodes, crustaceans),
  internal skeleton of calcareous spicules
  (ciliates, sea slugs)
• Many soft-bodied animals can contract strongly to
  protect against mechanical damage
• Adhesive and gripping structures
• Adhesive glands, hooks, suckers, claws
• Statocysts
• Sensory organs that detect gravity and help
  animals to orient correctly within sediments

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• Benthos Soft Bottom
• Species Composition
• Meiofauna
• Vermiform body shape, especially when coupled
  with flattening, increases organisms
  surface-area-to-volume ratio and can facilitate
  uptake of dissolved organic material for food
• Nematodes inhabiting dysaerobic portions of sandy
  beaches are thinner than aerobic counterparts
  (thought to be an adaptation for DOM uptake)
• Most meiofauna are mobile, though some, e.g.
  foraminiferans, may cement themselves to sand
  grains

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• Benthos Soft Bottom
• Species Composition
• Meiofauna
• Most meiofauna are
• Deposit feeders (gastrotrichs, nematodes)
• Predators (hydroids, flatworms)
• Microherbivores (scraping diatoms or algae off
  sand grains ostracods, harpacticoid copepods)
• Some suspension feeding species, primarily
  sedentary animals like bryozoans and tunicates
• Reproduction most meiofauna have low
  fecundities, due primarily to small body size
• Many species produce only 1-10 eggs at a time,
  and 98 of meiofaunal species lack pelagic larvae
• Young are brooded or eggs may be attached to sand
  grains young hatch as benthic juveniles
• No pelagic dispersal phase dispersal through
  entrainment in water currents, attachment to feet
  of mobile organisms (e.g. seabirds)

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• Benthos Soft Bottom
• Distribution and Zonation
• Many factors may affect organismal distributions
• Sand beach communities may be affected by
• Tidal regime - Determines range over which water
  occurs and schedule on which a given portion of
  beach is wet
• Wave intensity - Determines nature of environment
  (erosional vs. depositional).
• Sandy beaches exposed to high wave energy are
  relatively devoid of life, since few species can
  burrow rapidly enough to counter continual
  uprooting process
• Some suspension feeding species, primarily
  sedentary animals like bryozoans and tunicates
• Community structure on exposed beaches seems to
  be due primarily to physical factors and not to
  biological factors such as competition for space
  or food