meiofauna

Question 1

meiofauna overview

Answer 1

• Meiofauna (Greek = smaller) are microscopic animals living in marine sediments
• They are operationally defined based on the standardised mesh size of sieves with 1000 μm and 20 μm as upper and
  lower limits, respectively
• These organisms form diverse and abundant communities extending from the shore to the deep sea
• Very little is known about the life-cycles of meiofauna; a surprisingly large number of species coexist in the tiny
  interstices, but still most questions on their interactions and life strategies await answers

Question 2

meiofauna in numbers

Answer 2

• Meiofauna are operationally defined based on the standardised mesh size of sieves with 1000 μm and 20 μm as
  upper and lower limits, respectively
• Meiofauna are estimated to represent 80% of the metazoan abundance of life on Earth
• They encompass 24 of the 35 animal phyla
• On the average there are a million meiofaunal organisms per m−2 of sediment surface
• The highest abundance values come from intertidal muddy estuarine habitats (6–12 million per m2), lowest values from the deep sea (hundreds to thousands per m2)

Question 3

ecological role of meiofauna

Answer 3

• 1] To serve as food for organisms higher in the food web
• 2] And, to facilitate mineralisation of organic material and enhance nutrient regeneration

Question 4

meiofauna : food for higher trophic levels

Answer 4

• Meiofauna are very important nutritionally to a variety of animals that could not survive without them
• Many predators (macrobenthos) go though an obligatory meiofaunal feeding stage, and copepods appear to be the major meiofauna prey item for most of these predators
• In sediments, the meiofauna are restricted to the upper few millimetres or centimetres of oxidised
  sediment; thus bottom-feeding predators only need to take a shallow bite to obtain abundant food

Question 5

mineralisation and nutrient regeneration

Answer 5

• Meiofauna are important in stimulating bacterial growth, which then enhances remineralisation (the conversion of organic nitrogen, phosphorus, and carbon to their inorganic forms)

Question 6

adaptations to life between the sediment

Answer 6

• Sediments consists of sand or mud particles which are mostly round due to abrasion
• Usually spaces between them occupy 30-40% of the sediment volume, creating a system of interstices which is filed with water, air, detritus and organisms
• The smallest animals living in the sediment, the interstitial communities, move through the sediment, using the film of water which surrounds individual particles
• Interstitial organisms live in a harsh environment and must be able to withstand the marine conditions of inundation by salt water which alternates with exposure to terrestrial conditions

Question 7

adaptations for narrow spaces

Answer 7

miniaturization, elongation and flexibility

Question 8

adaptations to a mobile environment

Answer 8

adhesion, special locomotion and reinforcing structure

Question 9

adaptations or 3 dimensional dark conditions

Answer 9

static organs, reduction of pigments and eyes

Question 10

life history strategy

Answer 10

interstitial organisms show specific adaptation related to reproduction and development, including the production of only a few eggs, direct sperm transfer or internal fertilisation, brood protection, abbreviated larval life and restricted propagation

Question 11

diversity of meiofauna

Answer 11

• Meiofauna are an extremely diversity assemblage,
  representing 24 of the 35 animal phyla
• Dozens of very distantly related eukaryotic lineages
  can simultaneously inhabit a teaspoon of sand
• Evolutionary constraints caused elaborated lifecycles, migration patterns, special reproductive
  behaviours and structural adaptations
• Meiofauna encompass a variety of feeding modes,
  including but not limited to predators, grazers,
  suspension feeders and bacteriovores
• Several convergent morphologies have evolved

Question 12

Nematodes

Answer 12

• Nematodes (Latin “nemat”= thread) are a very diverse
  group with some 20,000 described species
• Species can be very difficult to differentiate from one
  another, but estimates have placed the true figure
  closer to 40,000 species worldwide
• Nematodes are even more remarkable in terms of
  abundance than diversity: there are estimates that
  globally four out of every five multicellular organisms
  are nematodes
• Communities can include as many as one million
  nematodes living within one square metre of seafloor,
  with most of those on the surface where the food is
  most abundant, and others within a few centimetres of
  the surface

Question 13

Nematodes

Answer 13

• Nematodes (Latin “nemat”= thread) are a very diverse
  group with some 20,000 described species
• Species can be very difficult to differentiate from one
  another, but estimates have placed the true figure
  closer to 40,000 species worldwide
• Nematodes are even more remarkable in terms of
  abundance than diversity: there are estimates that
  globally four out of every five multicellular organisms
  are nematodes
• Communities can include as many as one million
  nematodes living within one square metre of seafloor,
  with most of those on the surface where the food is
  most abundant, and others within a few centimetres of
  the surface

Question 14

Nematodes : benthic environment

Answer 14

play important role in the benthic environment
1. mechanical breakdown of detritus
2. excretion of limiting nutrients to bacteria
3. producing microfilm conducive to bacterial growth
4. bioturbating sediment around detritus

Nematodes eat bacteria and other microbes and a few
have developed complex symbiotic relationships,
providing the microorganisms with a place to live in
their gut and receiving nutrients from their passengers

Question 15

Gastrotrich

Answer 15

• The gastrotrichs (phylum Gastrotricha), are commonly
  known as hairy-backs
• Gastrotrich (Greek = stomach hair) was coined by the
  Russian zoologist Élie Metchnikoff in 1865, “hairy-back”
  apparently arises from a mistranslation
• Gastrotrichs are the third most common organism in
  marine sediments (behind nematodes/copepods),
  reaching densities of 364 individuals per 10 cm2
• They are detritivores and also microphagous, sucking dead or living organic material, diatoms, bacteria and small protozoa into their mouths by the muscular action of the pharynx
• Turbellarians and other small macrofauna predate upon Gastrotrichs

Question 16

Chaetognatha

Answer 16

• While only around 150 chaetognath species are known,
  they are nevertheless hugely abundant, showing the
  success of their life style
• Following copepods, chaetognaths are known to be the
  dominant taxa in terms of zooplankton abundance and
  biomass in the Earth’s oceans, composing 5%–15% of
  mesozooplankton biomass
• Chaetognaths are pelagic predators ranging in size from a few millimeters to a few centimeters
• They feed on fish fry, copepods and other plankton, making them a key a link in the ocean’s food web

Question 17

Gastrotrich

Answer 17

• Gastrotrichs have a simple, bilateral body plan with a
  head region, with a brain and sensory organs, and a
  trunk with a simple gut and the reproductive organs
• They have adhesive glands with which they can anchor
  themselves to the substrate and cilia with which they
  move around
• Little is known about their reproduction and life history
  strategy
• Placement in the tree of life is unknown:
  morphologically they similar to Rotifers and Nematodes,
  phylogenetically close to Platyhelminthes

Question 18

Halacaridae : marine mites

Answer 18

• Halacarid mites (Greek ‘Halos’ = sea, ‘akari’
  = mite), are minute arachnids (150-2000 µm in length) that inhabit marine and freshwater habitats, from tidal shores to the deep-sea
• The majority of species and genera are marine; the Halacaridae are the only mite family completely adapted to permanent life in the sea
• They live on algae, in sediments, in the flocculent ooze on sediment surfaces, in crevice systems in holdfasts, algal tufts, barnacles, hydrozoan- and bryozoan colonies, and on larger animals
• Most species are thought to be predatory and the long needle-like front legs are adapted for catching prey, and they feed by sucking up food material subjected to pre-oral digestion
• Some species eat fungi and dead organic matter, and a few species are known parasites
• Halacarids are thought to be an ancient taxon, with most genera probably having been present since the Mesozoic and with several species having an age of at least 50 million years
• Evidence for their antiquity is found in the distributional pattern of marine and limnic genera and species, in the lack of endemic genera despite low fecundity and lack of dispersal stages

Question 19

Foraminifera

Answer 19

• Foraminifera (Latin = “hole bearers“), informally called
  forams, are amoeboid protists, characterised by
  streaming granular ectoplasm for catching food and an
  external shell (called a “test”)
• Foraminifera are increasingly considered dominant
  members of benthic communities in both shallow and
  deep-sea
• Sedimentation of organic material from phytoplankton
  blooms often represents a considerable fraction of
  annual benthic organic matter input
• This phytodetritus plays a substantial role in the
  nutrition of foraminifera

Question 20

Kinorhyncha : mud dragons

Answer 20

• The Phylum Kinorhyncha (Greek = move snout), also
  known as mud dragons, are an important component of
  the meiobenthos
• Distribution is recorded throughout the world from the
  intertidal to the deep-sea
• They represent typically 1–8% of the total meiofaunal
  abundance, but may account for up to 15–33% of the
  entire assemblage in some places
• The spines around the head are used to help pull
  these animals along the benthos, where they deposit
  feed on diatoms, bacteria, and other organic matter
  in the sediment
• Three body regions are recognised: a head segment,
  a neck segment and an 11-segment trunk. The head
  is completely retractable. When everted, it extends
  its five to seven circles of recurved spines called
  scalids
• They move by extending the head and using anterior
  scalids to move the body forward to burrow in
  sediment or move through the water
• The head and pharynx can be retracted and covered
  with plates (called placids) on the second body
  segment

Question 21

Turbellaria

Answer 21

• Turbellaria, a sub division of the Phylum Platyhelminthes (flatworms), are common members of interstitial fauna
• The name “Turbellaria” refers to the “whirlpools” of
  microscopic particles created close to the skins of aquatic species by the movement of their cilia
• Locomotion is snake-like, moving through water-filled
  spaces between sediment grains
• Most other turbellarians are carnivorous, either preying on small invertebrates or protozoans
• Many turbellarians clone themselves by transverse or
  longitudinal division, and others, especially acoels,
  reproduce by budding

Question 22

Loricifera

Answer 22

• Loricifera (from Latin ‘lorica’
  = armour + ‘ferre’ = to bear) is a phylum of minute animals that live exclusively in marine sediments
• The phylum was only discovered in 1983 by Reinhardt
  Kristensen and are therefore among the most recently
  discovered groups of Metazoans - as such, very little is
  known about Loricifera
• They strongly adhere to sediments - normal meiofauna
  sampling techniques do not dislodge them
• They have a very complex life cycle, with larval forms
  (Higgins-larva) which look very different to the adult
  morphology and which can outnumber the adults by
  100 to 1
  Loriciferans are similar in size to large single-celled
  protozoans (< 0.5 mm) but are multicellular, with
  about 10,000 specialized cells creating a tiny but
  complex body
• This includes a brain, digestive and excretory systems,
  specialised appendages, sense organs, musculature
  and locomotory functions, and separate sexes
• They feature a characteristic lorica - an armour-like
  structure consisting of a protective external shell or
  case of encircling plicae
• Loricifera have been found >3,000 metres down, in a deep hypersaline anoxic basin (or DHAB)
• DHABs are characterised by depressions on the seafloor, where long-buried salt deposits become exposed to seawater and dissolve into the sea, forming extremely dense, hypersaline lakes devoid of oxygen
• As some of the most extreme oceanic habitats known, it was previously thought that DHABs were uninhabitable for eukaryotes
• Loricifera are the first multicellular organisms known to spend their entire lives in an oxygen-free environment
• They are able to do this because their mitochondria act like hydrogenosomes, allowing them to respire anaerobically

Question 23

tardigrades

Answer 23

• Most tardigrades are phytophagous (plant eaters) or
  bacteriophagous (bacteria eaters)
• Some are known to be carnivorous and may eat
  smaller species of tardigrades
• They share a broadly similar morphology, making
  species differentiation challenging
• However, the claws and buccopharyngeal apparatus
  (swallowing structure made of muscles and spines
  that activates an inner jaw and begins digestion and
  movement along the throat and intestine) are the
  major taxonomic characters

Question 24

Tardigrade resurrection

Answer 24

• Several molecular modifications, including upregulation of genes involved in DNA repair
  during the rehydration stage
• Wang, et al., (2014). Towards decrypting
  cryptobiosis—analyzing anhydrobiosis in the
  tardigrade Milnesium tardigradum using
  transcriptome sequencing.
• Tardigrades possess a unique combination of
  resistance and repair pathways endowing highly
  stress resistance (Foster 2012)
• Förster, et al., (2012). Transcriptome analysis in
  tardigrade species reveals specific molecular
  pathways for stress adaptations.

Question 25

where are meiofauna found ?

Answer 25

The abundance, diversity, distribution and functional properties of meiofauna can be affected by several abiotic factors, including:
* Temperature, salinity, hydrodynamic and sedimentary processes, sediment grain size, oxygenation level and food availability
* In general, sediment grain size and oxygen availability are the primary factor affecting the abundance and
species composition within a given depth range
* In areas where temperature varies seasonally, meiobenthos abundance and species composition also vary seasonally

Biotic factors, such as predation and competition, also play a role in community assemblage

Question 26

extremeiofauna

Answer 26

• 90% of marine environments are considered
  extreme for life, extreme marine environments
  cover more than half of the Earth’s surface
• Most extreme marine environments host
  abundant, often endemic, and well-adapted
  meiofaunal communities
• Recent discoveries of meiofauna living in hostile
  environments (hypersaline/anoxic conditions)
  have opened new frontiers in ecology and
  biology and their specific adaptations make
  them particularly suitable as biological indicators
• Several fields of knowledge such as assemblage,
  ecology, biology and physiology need further
  and deeper investigations

Question 27

Extremeiofauna: mangroves

Answer 27

( variation of salinity, pH, temp and o2. presence of sulphide)
( meiofauna : nematodes (very abundant) copepods (abundant) Platyhelminthes (abundant) )
* Nematodes are dominant in mangrove sediments, followed by free-living flatworms and harpacticoid copepods
* Mangroves show features that are different from other
estuarine regions and may increase regional meiofauna
richness
* Litter from mangrove leaves is rapidly colonised by meiofauna, and there is a succession of different species related to the different stages of the decomposition process
* However, anoxic and sulphidic muddy sediments, in particular, show sharp declines below the surface layers

Question 28

Extremeiofauna: submarine caves

Answer 28

( low light, low food, presence of sulphide)
(nematodes (very abundant), copepods (abundant), tardigrades and kinorhyncha)
* Submarine caves are oligotrophic environments due to insufficient light for photosynthesis and reduced input of organic material and limited accessibility for surface marine fauna
* Some caves can be sulphur-rich and deoxygenated environments where chemoautotrophic microorganisms take advantage of the sharp redox interfaces
* Submarine caves may share some similarity with abyssal meiofauna due to the comparable environment (i.e. no light, weak hydrodynamic conditions, limited food availability)

Question 29

Extremeiofauna: polar ecosystems

Answer 29

(low temp, salinity fluctuation)
(foraminifera (very abundant), rotifers (abundant), nematodes (abundant), copepods (abundant))
* Polar environments are characterised by low and stable temperatures and are influenced by significant seasonal variation in light intensity and, consequently, primary production
* Changes in salinity and ice thickness can also negatively affect meiofauna (summer melts, brine channels)
* Polar ecosystems host a diverse community characterised by endemic species, specialised for extremism
* The bulk of benthic meiofauna feeds on degraded organic matter, a food source available throughout the year
* Meiofaunal organisms living in the melting ice have osmoregulatory mechanisms allowing them to survive across a wide spectrum of salinities

Question 30

Extremeiofauna: hydrothermal vents

Answer 30

(high temp, low o2, sulphide and heavy metal)
(nematodes (very abundant) and copepods (very abundant)
* Meiofauna may contribute up to 20% of the total diversity known from deep-sea hydrothermal vents
* Life in these environments requires the organisms to develop specific adaptations to cope with the extreme abiotic conditions and the patchy nature of vent ecosystems
* Other challenges include high concentrations of reduced compounds such as hydrogen sulphides, low oxygen levels and the presence of heavy metals

Question 31

Extremeiofauna: whale and wood falls

Answer 31

(high organic matter, presence of sulphide)
(nematodes (very abundant) and copepods (very abundant)
* Cetacean carcasses and sunken wood that fall to the ocean floor represent an important source of organic matter for the oligotrophic deep-sea benthos
* Sulphur production induced by the carcass/wood fall degradation supports a rich fauna of chemoautotrophic organisms
* Wood falls and whale carcasses have been considered as evolutionary stepping stones for the colonisation
of chemosynthetic environments

Question 32

Extremeiofauna: cold seeps

Answer 32

(low o2, reduced chemical compounds)
(foraminifera (abundant) nematode (abundant), copepod)
Characterised by the presence of reduced chemical compounds (hydrogen sulphide, methane and hydrocarbons), local hypoxia/anoxia, and high microbial
abundance
* Foundation species (such as tubeworms and mussels) may shape the meiofaunal community abundance and composition

Question 33

meiofauna bioindicators

Answer 33

• Sediments are the ultimate repository for most of the persistent pollutants released to the ecosphere
• Upon entering aquatic environments, most toxicants associate with dissolved organics, suspended silts, clays, and organic particulates and eventually accumulate in sediments
• Meiobenthic communities are extremely sensitive to pollution and are therefore considered sentinel species

Question 34

anthropogenic impacts on meiofaunal communities

Answer 34

• Meiofauna are good indicators of the health of coastal
  marine ecosystems
• These organisms reflect changes derived from
  environmental disturbance and display specific responses to different types of anthropogenic disturbance
• Diversity and abundance of taxa are lower in polluted and stressed environments, due to the disappearance of more sensitive groups (e.g., ostracods, gastrotrichs, hydrozoans, tardigrades), leaving an assemblage dominated by tolerant organisms, such as nematodes
• Meiofauna are characterised by a high sensitivity to
  environmental changes due to their short generation time and the lack of pelagic larval dispersion for the dominant meiofaunal groups

Question 35

Oxygen availability drives meiofaunal assemblages

Answer 35

• Blue - red shows the minimum and maximum bottomwater oxygen or sulphide concentrations
• Diverse benthic communities inhabit the sediments
  (macrofauna, meiofauna and microbes), driving
  different functions in the ecosystem
• Under stable oxic conditions, faunal abundance and
  bioturbation activity are high, leading to high aerobic
  respiration rates in all size classes
• At the onset of hypoxia, faunal respiration decreases
  and lack of bioturbation favours anaerobic microbial
  communities and processes such as sulphate
  reduction