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Sex ratios of snubnose darters

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#1 fundulus

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Posted 30 April 2015 - 03:03 PM

I'm looking for any feedback on an odd thing we've found with a research project in the Flint River of north 'bama. Since last September we have been making monthly collections of two species of snubnose darters at a single site in the river, Etheostoma simoterum (tennessee snubnose) and E. duryi (blackside snubnose). Our interest is what differences might be found between the two very similar species in general population structure, and also with reproductive schedule and effort. With eight months of data, I feel I can confidently say that the blacksides are a little longer than the tennessees as measured by standard length, often by as much as 3 mm in some months (both average in the mid to upper 30s mm).

The odd thing we've found is that for the four months where sex is easily determined by external observation, January - April, both species have a strong female bias. For those four months, the tennessees have a M/F ratio of 64 to 146, and the blacksides have a ratio of 41 to 88. I don't think I've encountered this in the past; has anyone seen something similar with darters, or have a possible explanation for it? Our collections have been made with a 4 m seine with fine mesh, working up and down a 300 m stretch of the river in riffles and flow pools. I know gear bias is often a factor, but I can't really see it here. Thanks for any thoughts.
Bruce Stallsmith, Huntsville, Alabama, US of A

#2 Dustin

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Posted 30 April 2015 - 03:11 PM

Is it possible that this could be due to immature or poorly colored males?

Dustin Smith
At the convergence of the Broad, Saluda and Congaree
Lexington, SC

#3 fundulus

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Posted 30 April 2015 - 03:20 PM

That's a good thought. I've determined sex in these fish primarily by looking for an ovipositor, which at ~2 mm long is a really pronounced feature in females while males just have a little vent flap. In the eight monthly collections I mentioned, the only one with more males than females was in January when ovipositors first appeared. And certainly in March and April males are not subtle, even the smaller, drabber ones. But.........
Bruce Stallsmith, Huntsville, Alabama, US of A

#4 pjc237

  • NANFA Guest

Posted 07 May 2015 - 09:50 AM

Hi Fundulus,


I've noticed a similar trend in snubnose darters (E. barrenense, E. tallapoosae, E. entieri, E. flavum, and several other species) that I've been collecting the past few years. We usually try to get male and females for behavioral analyses and definitely have a harder time getting decent sample sizes for males. I'm not sure why though...



#5 fundulus

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Posted 07 May 2015 - 11:09 AM

Thanks, I feel better that someone else has noticed this too. With our May collection the other day the trend continues, and we hit all sorts of available habitat within the riffle/pool system we sample. I'm still without a clue.
Bruce Stallsmith, Huntsville, Alabama, US of A

#6 blakemarkwell

  • NANFA Member

Posted 07 May 2015 - 02:42 PM

Sampling bias, perhaps? I've always seemed to land far more female E. barrenense and E. etnieri than males but, other species, like E. pyrrhogaster and E. flavum, have been acquired easily with roughly congruent sex ratios.


Then again, in light of recent phylogenetic work, snubnose has to be used in a broader context. I'm pretty sure that the E. barrenense, E. rafinesquei, and the E. simoterum complex (Ulocentra) is more closely related to E. blennioides types (Neoetheostoma) than to all the rest of the classical snubnoses, such as E. duryi and E. flavum, which are housed in the Adonia clade.


Regardless, all these groups are very closely related, I just always found it puzzling that E. blennioides types (classically part of the subgenus Etheostoma) are wedged between the snubs, which were all traditionally placed in Ulocentra

Blake Markwell
Sangamon River

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