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First published online October 10, 2003
Inside JEB |
EGG COUNTING SNAILS
kathryn{at}biologists.com
Laying eggs is hard work, and risky with the next generation's future invested in a single clutch. So snails only ovulate when the time and place is right. But how land snails control this costly process wasn't clear until Ronald Chase began investigating a branch of the intestinal nerve that runs close to the mollusc's ovotestis. Working with Tomasz Antkowiak, he discovered that the nerve effectively counts the number of ripe eggs a snail is carrying, triggering the snail's egg laying preparations when the clutch is large enough to warrant the investment (p. 3913).
Chase explains that the snail's tiny nerve was first discovered shortly after the turn of the 20th century, but no one had clearly pinned its function down. Almost a century later, Chase noticed that the nerve passed close to the mollusc's gonad, in fact it looked as if it might innervate the snail's seminal vesicle. But on closer inspection, he realised that the nerve innervated the mollusc's ovotestis. Knowing that most mammalian ovaries are innervated, Chase decided to test whether the nerve might play a sensory role or trigger some of the physical activities needed as the snail produces its eggs, but first he needed a steady supply of snails. Fortunately several volunteers from California were perfectly happy to rid themselves of the gardener's scourge, keeping Chase well supplied with the voracious molluscs.
Testing the nerve's sensory capacity, Antkowiak gently prodded a snail's ovotestis, to see whether the intestinal nerve detected the disturbance and produced a signal. Chase remembers that the work was very fiddly, as the nerve is only 20 µm wide, and covered in tissue that Antkowiak had to gently remove before making his nerve recordings. Antkowiak's patience was rewarded; the nerve began sending signals as he gently prodded the ovotestis.
But how did the nerve respond as eggs matured and the ovotestis swelled? First Antkowiak recorded the neural activity in the nerve and then counted the number of mature eggs in the snail's gonad. As the number of eggs increased, the nerve's response grew stronger. And when Antkowiak simulated oocyte maturation by gently expanding the gonad with miniscule injections, the nerve's sensory signals increased again. The nerve was `counting' the number of ripe oocytes that the snail was carrying.
Pleased that the nerve was effectively counting, Chase wondered whether it also triggered the snail's preparations to produce a clutch? Suspecting that the snail's heart rate rises to maintain its hydrostatic pressure during oviposition, Antkowiak tested whether signals sent by the ovotestis' branch of the intestinal nerve might affect the heart rate regulating pericardial nerve. Electrically stimulating the ovotestis branch, he recorded the pericardial nerve's activity and found an immediate and dramatic increase. `The ovotestis influences the heart rate' says Chase, which is an essential behaviour for snail egg laying.
Chase explains that although other molluscs regulate their egg laying behaviour with hormonal signals, this is the first time that a single nerve has been found to be responsible for sensing the state of the snail's ovotestis, as well as triggering some egg laying reflexes.
References
Antkowiak, T. and Chase, R. (2003). Sensory
innervation of the ovotestis in the snail Helix aspersa.
J. Exp. Biol. 206,3913
-3921.
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