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Tardigrade
evolution
Fossil Tardigrades
• Tardigrades have a long evolutionary
history
• Specimens have been recovered from
Siberia that are thought to be 530myo
• This has given tardigrades time to develop
a variety of forms as they have invaded a
range of habitats (marine  terrestrial).
Cope’s Rule
• Body size tends to increase in an animal
lineage during its evolution.
– eg. horses
– eg dinosaurs
Cope’s Rule - tardigrades
Body length (µm)
400
300
200
100
A
B
C
Assumes evolutionary sequence from A  C and that
examples drawn are representative of each group.
No support is available from the fragmentary fossil record.
Terrestrial invasion
Terrestrial habitats are less regular and
fluctuate more than marine habitats:
• Seasonal changes
• Diurnal changes
• Unpredictable changes
Resistant states
• Anoxybiosis
• Reaction to reduction in oxygen tension
• Encystment
• Reaction to slow changes in environment
• Osmobiosis
• Reaction to increased salinity
• Cryobiosis
• Reaction to freezing
• Anhydrobiosis
• Reaction to loss of water
Anhydrobiosis
• Reduction in body size – ‘tun’ formation
• Invagination of extremities - legs
• Folding of intersegmental cuticle to reduce
exposed surface area
• Loss of water – synthesis of trehalose
Tun formation
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 Intersegmetal folds
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Trehalose
•
•
•
•
•
Disaccharide
Inert
Stable
Replaces water
Reversible reaction
Morphological adaptation
Marine species exhibit a number of
extravagant appendages that would
be an impediment in a changeable
terrestrial environment.
During desiccation, they would be
difficult fold up to reduce surface area
and would be difficult to protect in the
dry state – they could become brittle.
The cephalic sensory organs would be
the key organs – these are reduced or
even internalised in terrestrial species.
In summary
Tardigrades have made the transition from
marine habitats (where constant environments
have supported diversity) to terrestrial habitats
(where fluctuating environments have had an
inhibiting effect upon Cope’s Rule as a result of
the structural and physiological requirements for
anhydrobiosis). The question remains: why have
the ‘ancient’ marine groups not developed larger
body size?
References
Kinchin, I.M. (1994)
The Biology of Tardigrades.
London, Portland Press. pp. 1 – 186.
Kinchin, I.M. (1995)
Evolutionary trends in the tardigrades.
The Quekett Journal of Microscopy, 37(6): 493 – 498.