Natures wonders: statocysts for sensing balance (Introduction)

by David Turell @, Friday, September 02, 2016, 00:49 (931 days ago) @ David Turell

An early ancient way to keep balance using gravity:

" Invertebrates rely on a simpler structure known as a statocyst to sense their own movement and body position relative to the Earth's gravitational pull. Even comb jellies (ctenophores), which may have been the first multicellular animals to evolve, have a rudimentary statocyst—essentially, a weight resting on four springs that bend when the organism tilts in the water.

"The comb jelly's single statocyst sits at the animal's uppermost tip, under a transparent dome of fused cilia. A mass of cells called lithocytes, each containing a large, membrane-bound concretion of minerals, forms a statolith, which sits atop four columns called balancers, each made up of 150-200 sensory cilia. As the organism tilts, the statolith falls towards the Earth's core, bending the balancers. Each balancer is linked to two rows of the ctenophore's eight comb plates, from which extend hundreds of thousands of cilia that beat together as a unit to propel the animal. As the balancers bend, they adjust the frequency of ciliary beating in their associated comb plates. “They're the pacemakers for the beating of the locomotor cilia,” says Sidney Tamm...

"Sensing gravity's pull and the subsequent ciliary response is entirely mechanical, Tamm notes—no nerves are involved in ctenophore statocyst function. Most other animals with statocyst sensing, on the other hand, do employ a nervous system. Statocysts exist in diverse invertebrate species, from flatworms to bivalves to cephalopods. Although the details of the statocyst's architecture vary greatly across these different groups, it is generally a balloon-shape structure with a statolith in the center and sensory hair cells around the perimeter. As the statolith, which can be cell-based as in the ctenophore or a noncellular mineralized mass, falls against one side of the sac, it triggers those hair cells to initiate a nervous impulse that travels to the brain.

"The complexity of the statocyst system appears to correlate with the complexity of a species' movement and behavior, says Heike Neumeister, a researcher at the City University of New York. Squids and octopuses, which move rapidly around in three-dimensional space, for example, have highly adapted equilibrium receptor organs. Likewise, the nautilus, whose relatives were among the first animals to leave the bottom of the ocean and begin swimming and employing buoyancy, has a fairly advanced system. Each of its two statocysts is able to detect not only gravity, like the ctenophore's, but angular accelerations as well, like those of octopuses, squids, and cuttlefishes (Phil Trans R Soc Lond B, 352:1565-88, 1997). “[Nautilus] statocysts are an intermediate state of evolution between simpler mollusks and modern cephalopods,” says Neumeister."

Comment: Vertebrates have semicircular canals related to their ears. If you read the description carefully the statocyst may be an evolutionary early system but it is very complex and one is left to wonder how it developed bit by bit. Once again saltation is a better answer.

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