Is where a snail is found related to its tolerance?

One of the big questions we have about springs is why is it that some species ONLY live in springs and not rivers or billabongs, and why are those from springs only found in some springs and not all?  Is it because they are stuck in springs and can’t get anywhere else? Or is it because they have specific requirements that only the springs provide?  This is one of the key questions of my thesis, and I’m starting to get some ideas to help answer these questions.  

In this post, I’ll quickly summarise some of my newest findings using one of my Edgbaston favourites – Glyptophysa.

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Glyptophysa are one of the rarest snails on Edgbaston

Not all springs of Edgbaston are occupied by all the species present there – in fact there are particular springs that are more diverse and have been for at least the last 20 years (see my previous post on our mapping of diversity).  This might be because some snails have a harder time getting around than others and thus better ‘connected’ springs are more diverse [1] however its also very likely that the springs they don’t occupy aren’t quite up to scratch.

It could be that Glyptophysa are one of the rarest snails on Edgbaston because it can’t find its way to new unoccupied springs. However it may ALSO be because it needs very particular things from a spring it will occupy.

Map of springs on Edgbaston occupied (black) or unoccupied by Glyptophysa in 2015 (map) and in past surveys (right side)

Map of springs on Edgbaston occupied (black) or unoccupied by Glyptophysa in 2015 (map) and in past surveys (right side)

So how particular are they?

I’ve been trying to figure this out in two ways.  The first is by looking at whether there is something special about the springs they live in compared to the ones they don’t.  The second, is by testing how they handle different kinds of physiological stressors in experimental situations.

And both are telling us that Glyptophysa are sensitive little souls.

The springs they occupy are particular – they are generally large (>100m2) with big deep pools (max pool depth >10mm and >10m2).  And within these springs, they stick to the bits that are deep.  This association is probably not by chance, these areas are the most permanent, and most stable in terms of the kinds of things that could stress out a snail (salt, pH, temperature – see my previous post on how springs change through the year).

The springs that Glyptophysa occupy (black squares) are a particular subset of those that are available.

The springs that Glyptophysa occupy (black squares) are a particular subset of those that are available.

Two examples of springs that Glyptophysa are found within (E509 - top and E524 - bottom).  Colours represent abundance, graduating from red (highest) to yellow (lowest) and open (not found there).

Two examples of springs that Glyptophysa are found within (E509 – top and E524 – bottom). Colours represent abundance, graduating from burgundy (highest recorded) through red (high) to yellow (lowest) and open (not found).

However, there’s something paradoxical about them when we test them under laboratory conditions. Whilst their distribution across Edgbaston and within the springs they occupy suggests they are very sensitive and only occupy the most delightful springs (and bits of springs), they are not the MOST sensitive and could technically live in a whole lot more places. They can handle salt levels DOUBLE the maximum found on Edgbaston and being completely dry for 12hr.

physiology

The proportion of Glyptophysa alive after x hours exposed to four levels of salinity (left) and to three types of drying exposure (right).

So what gives?

When it comes to physiological tolerance, the story may be a little more complex than what we know so far.  Glyptophysa as adults seem like they should be one of the least restricted by environmental conditions, a species that could technically handle half a day out of the water, where it’s salty or hot.  But does that mean they can stay there forever? And does that mean they can raise happy healthy snail families in those kinds of places?  This is a question we have to answer before we come to conclusions about the relationship between physiological tolerance and distribution, and represents an important situation within ecology and ecological enquiry in general.

When you think of an organism, you usually think of the ‘dominant’ life stage, the bit that YOU see more often.  But individuals of a species have different life stages that they have to go through to persist in a place.  And for each one of those life stages, the conditions have to be right.  For some species this means they move around to find the right conditions at the right time (e.g. humpback whales, monarch butterflies in the Americas) however for the less mobile (i.e. springs snails) you need to make sure the right conditions for every life stage are found all in the same place.

So what we need to know now is what happens when Glyptophysa are exposed to the more ‘harsh’ conditions, the conditions that they seem to be able to handle but don’t live in, in the long-term?  Are the adults strong but only up to a certain point?  Will adults still reproduce in places were conditions are ‘harsher’?  And if they do, can the eggs and hatchlings survive?  Can we assume that what we know about how the adults responds tells us something about all life stages?

Work by Ben Kefford on other freshwater species shows that particular assumption is not a sound one, especially for snails [2].  Whilst the adults may be super tolerant and able to hack short-term exposure to some pretty crumby conditions they don’t grow or reproduce at those levels.  And even at levels that, relative to their maximum tolerance, are pretty benign, their eggs or hatchlings may not survive.

The response of Physa acuta - another freshwater snail invasive to Australia with a relatively similar body size to Glyptophysa - at three life stages.  For the adult, the pink line represents optimal growth conditions and the blue line represents optimal reproduction.

The response of Physa acuta – another freshwater snail invasive to Australia with a relatively similar body size to Glyptophysa – at three life stages. For the adult, the pink line represents optimal growth conditions and the blue line represents optimal reproduction.

What now?

In this final part of my PhD I will be trying to work out some of these things.  I will be extending my physiological experiments to see if adult Glyptophysa from different springs and at different times all respond the same way.  To do that, Sasha and I are heading to the field this month.

I am also trying to see if I can get Glyptophysa to reproduce in the lab.  If I can, then I can start looking at how they respond across the different life stages.  Fingers crossed.

Lab Glyptophysa and the first egg mass

Lab Glyptophysa and the first egg mass

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[1] A lot of the existing literature looks at this possibility by looking at how similar the populations of animals in each spring are (i.e. how ‘related’ they are) and whether this has anything to do with how far away these springs are from each other.  A great read on this topic is: Murphy, N.P., Guzik, M.T., and Wilmer, J.W. (2010) The influence of landscape on population structure of four invertebrates in groundwater springs. Freshwater Biology 55(12), 2499-2509.

[2] My favourite studies on this subject are: Kefford, B., Nugegoda, D., Zalizniak, L., Fields, E., and Hassell, K. (2007) The salinity tolerance of freshwater macroinvertebrate eggs and hatchlings in comparison to their older life-stages: a diversity of responses. Aquatic Ecology 41(2), 335-348 and Kefford, B.J., Papas, P.J., Metzeling, L., and Nugegoda, D. (2004) Do laboratory salinity tolerances of freshwater animals correspond with their field salinity? Environmental Pollution 129(3), 355-362.

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