M17 Meiafauna and Bryozoans

Question 1

Meiafauna

Answer 1

Defined by sizes between < 500-1000 μm but >45 μm
-Larger than macrobenthos (protists, unicellular algae, bacteria
-Diverse + abundant communities (shore-deep sea), known to live between grains of marine sediment
-Not a taxonomic group but represented by 19 phyla

Question 2

5 exclusively meiofaunal phyla

Answer 2

-Gnathostomulida
-Kinorhyncha (mud dragons)
-Loricifera (Jewel animals)
-Gastrotricha (hairy bellies/backs)
-Tardigrada (moss piglets/water bears)

Question 3

Ecological role

Answer 3

Marine sediments = biggest habitat on earth by SA. Key for sediment dynamics, nutrient cycling + food web dynamics.

Question 4

Adaptations

Answer 4

Sediment = round sand/mud particles due to abrasion with interstitial spaces. Move through sediment using the film of water around particles. Adapted to:
- narrow spaces (miniaturisation, elongation, flexibility)
- mobile environment (adhesion, special locomotion, reinforcing structures)
-3D dark conditions (reduction of pigments and eyes)

Question 5

Key meiobenthos taxa

Answer 5

• Nematoda (worms, most common meiafauna species)
• Copepods (some are benthic)
• Turbellarians (flatworms)

+2 phyla with meiafaunal individuals:
- Rotifera - rotifers
- Bryozoa – moss animals

Question 6

Nematodes

Answer 6

-Mechanically breakdown detritus, excretion of limiting nutrients to bacteria, produce microfilm conducive to bacterial growth (bioturbation)
-Eat bacteria/microbes and few have developed complex relationships. Give a place to live in their gut whilst receiving nutrients.

Question 7

Gastrotrich

Answer 7

800+ species, hairy bellies
-Looks like a flat-worm, bilaterally symmetrical, unsegmented with cilia on ventral surface. Bristles spines and scales on dorsal surface.
-Freshwater, marine, planktonic and deep sea
-Detrivores that eat diatoms, bacteria and small protozoa
-Eaten by turbellarians and other small macrofauna
-Body has a head region with brain and sensory organs, and a trunk with a gut and reproductive organs, and an anus.
-Adhesive glands which they use to anchor themselves to substrate with, and cilia which help them move around.

Question 8

Rotifera (wheel bearer)

Answer 8

-Bilaterally symmetrical, non-segmented
-Two corona of cilia which passes food particles into the mastax (for mastication) with trophi (jaws) where food is masticated
-Short digestive tract, reproductive system (some classes are obligate asexuals (no form of sexual reproduction)
-Foot has 2 or 3 ‘toes’, used to hook onto surfaces
-Cryptobiosis refers to hidden life of adults or embryos of some groups.
-Can be permanently or temporarily sessile (don’t move)

Question 9

Kinorhyncha (mud dragons)

Answer 9

-Throughout the world from intertidal to deep sea
-Segmented with a cuticle with spines
-Can burrow into muddy sediments due to spines around the head (fully retractable, when everted it extends its circles of recurved spines (scalids)) which they use to pull themselves along benthos
-Deposit feed on diatoms, bacteria and other organic matter
-Body includes a segmented head, a neck and 11-segment trunk.
-Head and pharynx are covered in plates called placids when retracted (on 2nd segment)

Question 10

Loricifera

Answer 10

38 species (only marine sediments)
-5 main segments (mouth cone, head, neck, thorax + abdomen which are the trunk).
-Abdomen is covered by lorica which is armour-like with a protective external shell or case of encircling plicae
-Can bear specialised locomotory and chemosensory appendages (scalids)
-Head, neck and thorax can be withdrawn into lorica
-Sometimes sexually dimorphic, some hermaphrodites
Have been found >3000m down in a deep hypersaline anoxic basin. They can do this because they swapped their mitochondria for an organelle which can produce ATP in a new way. They are the first animal to survive in extreme anoxic conditions.

Question 11

Tardigrades (moss piglets/water bears)

Answer 11

+1265 species
-Found everywhere, they have even been seen in outer space (resilient to most extreme conditions)
-They have a clearly defined head and a 4-segment body with a cuticle made of chitins (which can be ornamented/spiky or smooth)
-Sucking pharynx with styles. Oesophagus, stomach, hindgut, excretory systems. Reproductive system with no circulatory system.
-They can do cryptobiosis, which is when they enter a cyst or tun stage where they seem dead, and in that they can survive anything.

Question 12

Chaetognatha (arrow worm)

Answer 12

150 species (not-quite meiofauna, but related to them).
- Very abundant, make up 10% of plankton biomass
- Carnivorous, voracious planktonic predators.

Question 13

Bryozoans (made of zooids)

Answer 13

6300 extant + 15000 extinct species
-Largely sessile, often found encrusting on hard substrates and kelps, colonial filter-feeders (live in zooids). In all aquatic ecosystems.
-Body enclosed in a calcareous, chitinous or gelatinous box, tube or communcal matrix.
-Lophophore is a special filter feeding structure with ciliated tentacles, that pass food to their mouths.
-U-shaped gut with anus on outside of lophophore (ectoprocta)
-Lacks circulatory structures, respiratory pigments.