Post by apple on Dec 28, 2005 18:17:33 GMT
(a) Aestivation
Under culture conditions, that aim at a fairly uniform environment, the normal conditions
aestivation might be presumed to be absent. However, all these Achatinidae burrow
extensively, particularly when they have recently arrived in the country. Archachatina tend to
adapt more readily (3-4 months) than other genera, and most individuals appear to be active
each night. Nevertheless, a few may be found below soil level at any time, although not
usually in true aestivation. It is, therefore, necessary to define aestivation as a condition of
dormancy in which the snail has formed an epiphragm and remains entirely quiescent. This
happens occasionally with Archachatina, more extensively with Achatina achatina, A. fulica,
A. panthera but less with Burtoa. The snails also burrow in order to oviposit, when they
construct a smooth-walled oval cavity within which the eggs are laid, the snail usually
returning to the surface the following evening.
The third condition, of dormancy without forming an epiphragm, may arise casually in
individuals of a group of active snails. It has happened with each of the two Achatina
(Achatina) achatina, possibly because they did not like the company they were forced to keep
(Archachatina). It also happens extensively when conditions in the vivarium become too hot
(above 28°C), too cold (below 23°C) or too dry, but not when conditions become
excessively wet. This kind of dormancy may lead into normal aestivation but it may be
significant in other ways. One A. (Achatina) achatina received in July 1955, spent only 6
weeks of the following 20 months above ground. Yet, during this period (to 18 March 1957)
it had increased its shell length by 8.2 mm (120.5-128.7mm) and its breadth by 5.0 mm
(65.0-70.0 mm). The addition of new shell to the body whorl at its greatest width was 28.5
mm. Most of this additional shell growth occurred while the snail was below ground and
might be related to the need for a balance of development between such parameters as shell
size, shell thickness and body size.
Lastly, in adverse conditions, some snails may aestivate without burrowing. This enables
observation of the process of epiphragm formation. The animal retracts within its shell,
leaving about 2-5 mm of the internal surface of the lip exposed. The collar is extended to
enclose the retracted foot and secretes a yellowish mucous layer. The latter slowly whitens,
finally becoming a thin, tough, shiny and slightly flexible structure with a fine slit that
follows the line of the pneumostome.
Aestivation withdrawal differs from defensive withdrawal in that, although in both cases the
left posterior half of the folded foot blocks the shell aperture, in the former case the mantle
collar is extended as a thin layer over the foot, leaving a very small aperture through which
the foot can be seen and over which the epiphragm is thinner than elsewhere .
«The Life of Achatinidae in London»- R. H. Nisbet
petsnails.co.uk/docs/Nisbet_1974_Presidential_Address.pdf
petsnails.co.uk/docs/Nisbet_1974_Presidential_Address.txt
Under culture conditions, that aim at a fairly uniform environment, the normal conditions
aestivation might be presumed to be absent. However, all these Achatinidae burrow
extensively, particularly when they have recently arrived in the country. Archachatina tend to
adapt more readily (3-4 months) than other genera, and most individuals appear to be active
each night. Nevertheless, a few may be found below soil level at any time, although not
usually in true aestivation. It is, therefore, necessary to define aestivation as a condition of
dormancy in which the snail has formed an epiphragm and remains entirely quiescent. This
happens occasionally with Archachatina, more extensively with Achatina achatina, A. fulica,
A. panthera but less with Burtoa. The snails also burrow in order to oviposit, when they
construct a smooth-walled oval cavity within which the eggs are laid, the snail usually
returning to the surface the following evening.
The third condition, of dormancy without forming an epiphragm, may arise casually in
individuals of a group of active snails. It has happened with each of the two Achatina
(Achatina) achatina, possibly because they did not like the company they were forced to keep
(Archachatina). It also happens extensively when conditions in the vivarium become too hot
(above 28°C), too cold (below 23°C) or too dry, but not when conditions become
excessively wet. This kind of dormancy may lead into normal aestivation but it may be
significant in other ways. One A. (Achatina) achatina received in July 1955, spent only 6
weeks of the following 20 months above ground. Yet, during this period (to 18 March 1957)
it had increased its shell length by 8.2 mm (120.5-128.7mm) and its breadth by 5.0 mm
(65.0-70.0 mm). The addition of new shell to the body whorl at its greatest width was 28.5
mm. Most of this additional shell growth occurred while the snail was below ground and
might be related to the need for a balance of development between such parameters as shell
size, shell thickness and body size.
Lastly, in adverse conditions, some snails may aestivate without burrowing. This enables
observation of the process of epiphragm formation. The animal retracts within its shell,
leaving about 2-5 mm of the internal surface of the lip exposed. The collar is extended to
enclose the retracted foot and secretes a yellowish mucous layer. The latter slowly whitens,
finally becoming a thin, tough, shiny and slightly flexible structure with a fine slit that
follows the line of the pneumostome.
Aestivation withdrawal differs from defensive withdrawal in that, although in both cases the
left posterior half of the folded foot blocks the shell aperture, in the former case the mantle
collar is extended as a thin layer over the foot, leaving a very small aperture through which
the foot can be seen and over which the epiphragm is thinner than elsewhere .
«The Life of Achatinidae in London»- R. H. Nisbet
petsnails.co.uk/docs/Nisbet_1974_Presidential_Address.pdf
petsnails.co.uk/docs/Nisbet_1974_Presidential_Address.txt
