PHYLUM TARDIGRADA = water bears

PHYLUM TARDIGRADA = water bears

1. Taxonomic diversity ≈ 800 species (10 fam., 3 ord.)

2. Size range: 0.1 – 1.7 mm long (8-legged bear-shaped)

3. Habitats: mostly freshwater & marine interstitial benthos, many terrestrial but in semi-aquatic micro-habitats (mosses, lichens, soil, leaf litter etc)

4. Anatomy: no blood vessels, respiratory structures or nephridia (some Malpighian tubules present) ; coelom limited to gonadal cavities; well- developed nervous system (annelid-arthropod type); mainly separate sexes → single gonad above the gut, but some hermaphroditic genera

5. No larvae: direct development → 3-6 juvenile instars; growth by molt

6. GENERAL BODY PLAN

   • head with sensory cirri & clava (hollow)
   • body covered by thin, uncalcified cuticle → molted
   • no circular muscle layer → separate bands with sub-cuticular attachment (like arthropods!)
   • 4 pairs of ventro-lateral legs with 1-12 toes
   • toes with: a) adhesive pads or discs; or b) claws
   • cuticle at times divided into dorsal & lateral plates

7. Movement

   • No longer hydrostatic system (waves), but use of antagonistic groups of muscles + claws & pads used for purchase/clinging to objects (step-by-step gait)

8. Feeding

   • Sucking fluids from plant/animal cells → piercing of cell wall with oral stylets
   • In soil → bacteria, algae, detritus or small inverts
   • Some marine species → commensals on isopod pleopods or mussel gills
   • Others → parasites on epidermis of holothurians or barnacles

9. Resting stages & annual cycle

   • Cyclomorphosis → alternation of different forms, normally a summer and a winter (pseudosimplex → resistant to freezing temperatures) form
   • Anabiosis → formation of double-walled cyst → loss of water + very low basal metabolism
   • Cryptobiosis → formation of single-walled tun stage → untetectable metabolism
   • Can survive: temp of +149 to -272 oC; vacuum; ionizing radiation; total anoxia; long time (120 y)
   • Swell and become active within few hours of re-hydration