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In a post just before I submitted my thesis I promised significance and today I shall deliver it!  Over the next few weeks I’ll be posting some results of the experiments I’ve done during my Masters and explain the rationale behind them and the results I ended up with.  This post is going to outline the study system and show that individual snails have a preference to turn in a certain direction which is repeatable.

The snail I used was Lymnaea stagnalis which is the common pond snail found in much of Europe.  It is a hermaphrodite, meaning it possess both male and female sex organs, and can self fertilise through sexual reproduction, meaning that the gametes (sperm and eggs) go through meiosis and are subject to recombination.  What is relatively unique about L. stagnalis is the shell; a snail’s shell coils in either a clockwise or an anticlockwise direction and most species only contain one chiral morph, that is all snails have the same shell coil direction as mating with a partner that is the exact mirror image of you is very difficult.  In L. stagnalis however, both dextral (clockwise shell coil) and sinistral (anticlockwise shell coil) morphs are present; mating between the two types is still difficult, but not impossible, in fact the most difficult part is finding two snails of the opposite shell coil phenotype as the sinistral morph is very rare (less than 1% of wild L. stagnalis are sinistral).

The shell coil direction of a snail is controlled by a maternal effect gene, meaning that the genetic make up of the mother is responsible for the phenotype of her offspring.  Shell coil direction is controlled by a single locus (a single point on the genome); there are two alleles (gene variations) the D allele and the S allele.  Snails which have the genotype DD or DS produce offspring that are dextral (when playing the part of a female during reproduction) and snails which have the genotype SS produce sinistral offspring (when playing the part of a female during reproduction; see figure below).

I tested a range of populations for their behaviour using the following procedure; snails were placed in a t maze and observed for ten minutes, the direction that the snail first turned in was recorded when the foot (fleshy bit) of the snail crossed either the left or right line (see figure below).  This was repeated twenty times and only snails that had turned more than ten times were used in the analysis.  To control for light and geomagnetism effects I rotated the maze 180 degrees every two observations.  Snails were tested on two separate days, usually the second trial was conducted the day after the first but a few (<5) were conducted three days apart due to the weekend getting in the way. To analyse the data I calculated the proportion of right turns (R turn proportion) each snail had made in a trial (where 0 was all left turns and 1 was all right turns), this measured the direction of lateralisation, and plotted the correlation between this value in one trial with that of a second trial (see figure below).  What I found was that the R turn proportion was significantly correlated between the two trials, meaning that the behaviour was repeatable and not a one off. Other animals have been found to show repeatable behaviour performed to the same degree rather than direction; i.e. moving in the same direction 7 times out of 10 in one trial and moving in the  opposite direction 7 out of 10 times in a second.  I wanted to know if pond snails did this, so I calculated the laterality index which was simply the amount of of deviation from an equal number of left and right turns (0 was a fifty:fifty split of left and right turns and 1 was when a snail turned in the same direction every time).  I plotted the correlation between two trials for this measurement (see figure below) and found that there was no significant relationship. The above results show that it is the direction which a snail turns in that is the important factor in turning behaviour, meaning snails are lateralised at the individual level, meaning that individual snails will move left more times than right or vice versa.  What I will show in my next post is whether there is a population level lateralised behaviour, investigating whether the majority of snails are biased in the same direction.

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