Locomotion

How did single-celled organisms move themselves?

They used cilia for self-propulsion (particularly protists) (the last common ancestor of the crown eukaryotes had cilia)

• Rowing = cilia perform a power stroke and a lighter recovery stroke to create net thrust

• Undulation = a transverse wave moves along the cilium, drag forces are resolves into a parallel component and a perpendicular component (perpendicular > parallel), producing thrust

These movements are powered by dynein proteins, which cause microtubule sliding within the cilium

Cilia close together naturally coordinate into metachronal waves, which improve efficiency (and no need for chemical or electrical signals) - the coupling (movement of one cilia influencing the movement of the nearby cilia) is caused by the movement of surrounding fluid

How do multicellular organisms move?

In multicellular organisms, cilia is less effective due to reduced surface area to volume ratios e.g. in sponges, ciliary propulsion works for tiny sponge larvae, but cilia in sponge adults are only used to generate feeding currents (water currents that bring food particles)

Some sponge adults undergo whole-body contractions, likely powered by actin/myosin interactions in the pinacoderm
These contractions can clear sediments and in some species, they can be triggered by glutamate (possibly released by ciliated sensory cells in the exhalant pores)
The sponge will reinflate due to tissue elasticity and water currents generated by chaonocytes

Many eukaryotic cells use the actin-myosin system (including amoeba) - for this to work, cells must not have a cell wall

This shows that coordination without a nervous system is possible

What is the benefit of a neural system compared to a non-neural system?

Some animals, like sponges and placazoans will use chemical signals (hormones) to coordinate movement - this may represent a proto-neuromuscular system

As animals evolved, neural projections may have developed

• Division of labour model (leads to the specialisation of cells)

• Improve the coordination of a group of effector cells

• To increase surface area for chemical release (part of the chemical brain hypothesis) - once these projections grew long enough, they evolved synapses to improve coordination

These required cells without cell walls

Chemical brain hypothesis = early nervous systems evolved

Neural benefits over non-neural equivalents

• E.g. some sponge larvae use many ciliated photoreceptor cells for phototaxis, while others use only two photoreceptor cells which make synaptic contact with ciliated cells (this neural setup is more efficient)

How can we describe locomotion in cnidarians?

Early cnidarians probably had a ciliated larval phase (the planula) and a sessile adult phase (polyp)

Adult polyps have neurally controlled muscles made of actin and myosin, coordinated by a diffuse nerve net that allows precise and rapid muslce contractions (some anemones can swim using these, even though they mainly aren’t used for locomotion)
The re-extenstion of contracted muslces is provided by elastic recoil of the mesoglea or by the contraction of other muscles (this only works if the mouth is closed, the gut contents can act as a constant-volume hydrostatic skeleton)

The box jellyfish (cubozoa) represents the most advanced cnidarian neuromuscular system (nerve net plus a nerve ring linking sensory structures - rhopalia, which have complex eyes and balance organs which enable control of swimming speed and direction)

Ctenophore (comb jellies) propulsion is driven by many fused cilia (comb plates)
Genomic analysis suggests that ctenophores may be the most basal extant animals, which would suggest that nerves and muscles evolved twice independently in animals, or that sponges lost them (the unique ctenophore nervous and muscle systems support the idea of independent evolution of neuromuscular systems in this group)