Genetic disorders and heritable traits
A heritable trait is one that can be selected for or against by breeding. It is always present in individuals that have the trait, and manifests itself the same way in other individuals that also possess it. Phenotypic plasticity is exhibited when a "trait" only occurs under certain conditions, and offspring raised under different conditions will not manifest it. Phenotypically plastic traits are generally not heritable, but rather environmentally induced. The following is a list of traits and their apparent heritability:
Heritable Traits - these conditions show a strong heritability trend. This means that if a parent snail exhibits the condition, some of the offspring in the vast majority of or all same-trait crossing display that trait.
Thin Shell Disorder. This is a condition in bridgesii that affects the thickness of the calcium-based layer of the shell that gives the shell it's strength. The shell is perpetually very thin, extremely fragile, and breaks easily. It tends to display the following pattern across generations:
Parents: one or both parents may show some tendency towards shell breaks, but not significantly enough that they cannot breed.
Offspring: many offspring may show the same trends as the parents, but some (and in rare cases most) will display severe shell problems and will crush easily. These ultra-fragile snails usually die before they reach reproductive age, die from mating damage, or die shortly after reproduction. There is a strong correlation between these two levels of severity, and often little occurance of snail groups that show a continuous gradient of the condition; there are primarily only two levels at which the condition is expressed.
This pattern suggests that the trait may be a simple Mendelian trait, where the number of copies of the recessive gene determines the severity of the condition:
TT = normal (hypothetically)
Tt = parent condition
tt = severe condition that is usually fatal
Snails that crush easily do not have offspring with thick shells, further indicating that the trait is heritable.
Other shell deformities
There are other, rarer types of deformities in bridgesii that affect the shell and may be heritable. The heritability pattern of these is less certain since mortality rates tend to be high and therefore it is difficult to study such conditions across generations. These conditions often cause the mantle to be improperly shaped and therefore the shell grows strangely. Generally these snails suffer growth complication and do not survive to adulthood, and the parent snails may not show the condition obviously. The mantle may be unevenly shaped, causing the shell to bluge or twist oddly. It has been observed to affect entire clutches of snails raised under different conditions, and the parents may consistently produce clutches of offspring that show the problem, indicating that it is likely not an environmentally induced problem.
Potentially Heritable Conditions - these traits need further study to determine if they may be heritable or constitute a true genetic trait.
Forked Siphon
There have been some reports that snails posessing forked siphons have offspring that later go on to develope a "fork" in the same place on the siphon.
Dysfunctional Siphon
Some snails have siphons that are kinked and do not extend properly. It is questionable whether this is an environmentally induced problem, but offspring of a snail with a dysfunctional siphon may suffer from the same problem (although not always).
Conditions that are not Heritable - these conditions do not constitute true genetic traits because either they do not show a consistent heritability trend, or show a strong correlation with environmental effects (known as phenotypic plasticity--the environment affects characteristics that appear to be genetic traits but arn't in fact genetic at all).
Fissures. The term fissure applies to a sharply V-shaped wedge that develops as a snail grows. Injured or healing tissue can be generally observed at the tip of the V. There are characteristic growth lines radiating out from the tip of the V where subsequent shell layers are put down. It is a problem that does not occur unless the snail damages the mantle in some way to affect the production of shell in that area. Damage in most snails usually occurs from mating or clamping down on an uneven or sharp object. Pressing the shell firmly against the intersection of two aquarium tank walls can be enough to cause this injury if the snails shell is thin (either due to TSD or rapid growth). This cannot be considered a true genetic trait because of the following that is easily observable in different bridgesii gene pools: two fissure-bearing snails are crossed and produce fissureless offspring that are healthy, and these offspring later go on to produce healthy offspring.
Because two fissure-bearing snails can produce 100% healthy offspring that have no shell problems, even when injuries have occured, it indicates that it is not a heritable treat. This does not rule out the fact that fissures may be a side-effect of other genetic disorders (namely TSD, especially in the Tt form), but it does not mean that the fissures themselves are heritable, since it is an injury-induced problem. It is possible that fissures may be a case of phenotypic plasticity, whereby some conditions produce snails that are prone to fissures, since low calcium levels and high temperatures can produce the problem. However, phenotypic plasticity is not an indicator of heritability (it is the opposite, in fact).
Shell Breaks. For the same reasons as fissures, shell breaks as an injury are not heritable traits. Repeated severe shell breaks may, however, be a sign of TSD.
Misc. Deformities. The majority of strange shell deformities seen in apple snails are actually due to an old injury somewhere in the snail's growth history. Bad shell breaks, and even healed fissures can cause the shell to twist or distort in ways that may even cause the snail to appear to be a different species. These cases are not heritable because the offspring do not exhibit the same condition.
