In reviewing research on Australian species (Hyriidae), I’ve encountered many examples where sex ratios are clearly biased towards males or females. These often are large samples, numbering in the hundreds, and most of the biases are statistically significant. I know that age and population density are implicated in research on species of Margaritiferidae and Unionidae, and that there may be differential mortality associated with dissolved oxygen levels (in that situation, brooding females are more vulnerable to hypoxia than males). The incidence of hermaphrodites may also need to be factored in.
In our Australian examples, the biases for any one species may differ between different times or places. There is little possibility of biased sampling, as these species are not sexually dimorphic.
Is anyone aware of published studies that shed more light on these seemingly haphazard shifts in favour of one sex or the other?
In some species I have studied a definite male bias was observed. This was attributed to females being more at risk from predation because of exposure while attempting to release glochidia. It is a complex issue, as I imagine the sex ratios in adults are most certainly related to the natural history of the species, and as you indicated observations for some species may be biased because of lack of sexual dimorphsim.
There can also be behavioural differences between the species where one sex ot the other are at the sediment surface in greater abundance at certain times of the season. If sampling methods are biased towards collection of mussels at the surface - you can get skewed sex ratios
Thanks Kevin and Daryl,
Your comments reinforce the impression that while there is a common basic plan, mussel populations respond to their local circumstances by adjusting their population sex ratio to improve their reproductive capability (or to maintain it, to counter losses to parasites or predators). That seems to be a common situation in Australia (and no doubt elsewhere), where even within one species the sex ratio may vary at different times and at different places.
If that makes sense, we need to measure reproductive output in some consistent manner to allow comparisons across species, time and space. As Daryl suggests, we need to avoid the possibility of sampling bias.
One approach could be to develop ideas using a model (e.g. Berg et al. 2008, A conceptual model linking demography and population genetics of freshwater mussels, J. N. Am. Benthol. Soc. 27, 395-408).
I would be interested to know of any studies that go beyond speculation about underlying causes to better define the issues and provide experimental or other critical evidence of cause-effect relationships.
Keith,
A colleague of mine conducted a study showing the differences I spoke about. It has not been published - but I will send you his contact information should you want to discuss it with him
Cheers
While attending an American Fisheries Society conference (I believe 2005) in Ottawa, Canada I remember seeing a presentation about the Mussel population downstream of Toronto, CA. The sex ratio was almost entirely female. The researchers at the time suspected this skewed ratio was the result of elevated pharmaceutical estrogen concentrations within sewage effluent. Not sure if the study was published, but it may provide a lead on some information or insight
Cheers,
JP
Cheers John: a good line of thought. A lot of work of androgenic/estrogenic effects in other species (e.g. Gambusia) and some for mussels (e.g. Elliptio). Many references to sewage, but also agricultural (irrigation, pasture) chemicals. Thanks, I’ll follow up!
An excellent summary/review is provided in
Haag, W.R. (2012) North American Freshwater Mussels: Natural History, Ecology, and Conservation. Cambridge University Press: 538p.
I found female-biased population of S. woodiana and I can recommend you my two articles concerning this issue.
All the best, Ania
Article Continuous reproduction of Sinanodonta woodiana (Lea, 1824) ...
Article Two pathways for spermatogenesis in Sinanodonta woodiana (Le...
@Keith f. Walker,
Hi Keith F. Walker,
I agree with an adjustment of the sex ratio of the population (different from 1: 1, fisher) to improve their reproductive capacity (or maximum gonad expenditure).
Particularly in broadcast-spawning marine invertebrates as many bivalves and other invertebrates, the game-theoretic models and index for gonad expenditure by sexes (RGES) can explain differentiation on sex ratio.
Then, the reproductive adjustment can occur in the ripe/spawning population frequency, in intensity and period of maximum gonad expenditure, among others reproductive aspects, all to leave offspring
I suggest the following articles:
https://www.researchgate.net/publication/319884230_The_evolution_of_gonad_expenditure_and_gonadosomatic_index_GSI_in_male_and_female_broadcast-spawning_invertebrates
https://www.researchgate.net/publication/327916379_Reproductive_traits_and_relative_gonad_expenditure_of_the_sexes_of_the_free_spawning_Chiton_articulatus_Mollusca_Polyplacophora
Cheers,
Omar
Great question! I have also observed this in freshwater mussels, and hypothesise that it may be due to castrating parasites in our recent article here:
Article Population-level effects of parasitism on a freshwater ecosy...
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