Meiofauna

I. Definitions
    A. Meiofauna -- pass through a 500 mm sieve, but are retained by a 62 mm sieve
    B. Other terms
        1. meiobenthos -- synonymous with meiofauna
        2. endobenthos – displace sediment (bigger)
        3. mesobenthos – move and live within the interstitial space
        4. epibenthos -- live at the sediment-water interface
        5. interstitial – spaces between the sedimentary particles (where the meiofauna lives)
II. Environmental Characteristics
    A. Where are they found?  wet sediments
            most abundant group of animals on the deep-sea floor
    B. Grain size controls density and diversity of interstitial organisms
        1. coarse  vs. smaller-grain sediments
        2. sediments with median particle size < 125 mm are dominated by burrowing animals
        3. sediments with median particle size  >125 mm are dominated by mesobenthos
        4. grain size is a barrier to movement of meiofauna
        5. grain size also controls the amount of water in the sediment
    C. Oxygen tension --
        1. coarse grained sediments versus muddy sediments
        2. when sediments are anoxic, the redox potential (Eh) becomes a species indicator
    D. Mineral composition of sediment
    E. Wave action
    F. Temperature
    G. Salinity
    H. Light - rarely penetrates more than 5-15 mm into the sediments

III. Composition of the Interstitial Assemblages –
    A. High species richness
    B. Similar in size to mesoplankton and microplankton
    C. Groups of animals -- some are miniature versions of organisms found elsewhere; some are found nowhere else (e.g., gnathostomulids)
        Examples: Annelids, Ascidians, Brachiopods, Cnidarians, Echinoderms, Entoprocts, Gastrotrichs, Loricifera, Mollusca, Platyhelminthes,                                 Priapulida, Protozoans, Rotifera, Tardigrades
        Harpacticoid copepods second most abundant, and nematodes are most abundant;
        Phylum Gnathostomulida - found only in meiofauna
        Phylum Kinorhyncha - all 150 species are restricted to marine muds
    

IV. Sampling and Extracting Meiofauna
    A. Retrieving meiofauna
    B. Can be difficult to extract from sediments
  
V. Adaptations
    A. Organisms have to remain in spaces and avoid being crushed or suspended in the water
        1. small
        2. shape -- elongate (vermiform), flat
        3. in metazoans there is a decreased body complexity and number of organ systems

    B. Body wall reinforcement to resist crushing many have skeletons, spines or armor

    C. Aility to extend and contract quickly
    D. Ability to stay in sediment if resuspended -- adhesives, hooks and claws, statocysts

    E. Most are free-moving
    F. Life history
        1. Temporary vs. permanent meiofauna
            a. temporary -- juvenile animals that have recently settled
            b. permanent -- live their entire life in the interstitial
        2. Produce few eggs
            a. adaptations to increase probability of successful fertilization --
             
            b. adaptations to increase probability of successful survival of embryos --
             
        3. generation times a few days to more than a year; average 3 generations/year
        4. many species have seasonal reproductive cycles
VI. Ecology
    A. Very abundant -- ~106 per m2; 1-2 g/m2
        1. most in intertidal; decreasing abundance with increasing depth
        2. high metabolism
        3. biomass can approach that of macrofauna, but generally 1/10th to 1/5th as abundant

        4. production similar to or greater than that of the macrofauna in the same area
        5. important in nutrient cycling
        6. unknown how much energy makes it to higher trophic levels --
            a. nonselective deposit feeders and some macrofaunal predators consume meiofauna;
            b. probably more energy transferred from meiofauna to macrofauna in muddier sediments than in sands (more concentrated in surface)
    B. Types of organisms
        1. detritus feeders -- gastrotrichs, some nematodes, archiannelid polychaetes
        2. suspension feeders - more rare -- no interstitial plankton
        3. herbivores - harpacticoid copepods, ostracods;
        4. scavengers - some turbellarians
        5. predators --
            a. predators within meiofauna
            b. macropredators of meiofauna
    C. Zonation
        1. depth in substratum -- most in upper few cm; deeper in sand than in mud; vertical migration
        2. salinity
        3. predators
        4. grain size
        5. moisture content
        6. oxygen concentration
            a. most taxa only present in aerobic layers
            b. thiobios is found in the anoxic layers
    D. Seasonal Patterns

    E. Dispersal
        1. most have no dispersing larval phase
        2. transported as hitchhikers with sediment, icebergs, or other organisms
 

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