Wednesday, December 30, 2015

Monitoring and Marking Techniques for the Comal Springs riffle beetle Heterelmis comalensis

I continue my self promotion:

Huston, D.C., J.R. Gibson, K.G. Ostrand, C.W. Norris and P.H. Diaz.  2015. Monitoring and marking techniques for the endangered Comal Springs riffle beetle, Heterelmis comalensis Bosse, Tuff, and Brown, 1988 (Coleoptera: Elmidae). The Coleopterists Bulletin, 69: 793-798.  PDF

This was the second paper that I worked on that involved the endangered Comal Springs riffle beetle Heterelmis comalensis.  I discussed the natural history of the beetle and its endangered species status, as well as our attempts to culture them in the laboratory in my previous post.

The goal of this particular project was to evaluate various methods for monitoring the population size of H. comalensis at Comal Springs in New Braunfels, Texas.  Two methods were tested: (1) the use of paint for a mark recapture pilot study and (2) using the "cotton cloth lure" technique for trapping.

As you are likely aware, mark-recapture methods are relatively straightforward.  You capture a group of animals, give them some sort of recognizable mark or tag, release these animals, and then recapture these animals at a later time.  The information obtained can help estimate population size and dispersal of the animals you are studying.  Well, in the end it turns out that marking tiny aquatic beetles isn't really that easy.

While there is a good amount of published literature available about marking terrestrial insects, very little work has been done in terms of marking aquatic invertebrates.  Most of the difficulties inherent in marking an aquatic invertebrate are obvious, I.E. water soluble paints won't work, paper tags will be destroyed, glue will dissolve, etc. etc.  This becomes even more complicated when you consider individuals which live in riffle environments, as these abrasive habitats can damage and obscure even water resistant tags (Freilich, 1989).  Whatever to do?  Well Wineriter and Walker (1984) did an evaluation of various marking methods for insects, and they recommended the use of water insoluble paints.  That seemed the best option, so oil-based paint pens were acquired for the job.

Now of course we couldn't just go about slapping paint willy-nilly on our endangered beetles.  I mean what if we poisoned a bunch of the little guys to death?  Killing a bunch of endangered species on accident means paperwork, so we needed a surrogate for the test.  Fortunately, a related species of riffle beetle, Heterelmis vulnerata, is relatively common and widespread in Texas and is around the same size, thus H. vulnerata seemed a good match for the surrogate.

In order to give these tiny (~2mm) aquatic beetles a mark, the following procedure was devised.  Beetles were placed in a water filled tray, with a nylon mesh placed on the bottom.  The beetles like to cling to things, so they always grab onto the nylon mesh.  Then a small drop of paint was liberated from the paint marker into a little blotter.  The mesh was then pulled from the tray and the elytra of beetles were carefully dried with a cotton swab.  Then, using an insect pin placed in a pin vice, a tiny dab of paint was placed on the beetle.  The beetle was then kept out of the water for a minute or so as the paint dried, before being returned to the water. Clearly this wasn't the easiest thing in the world.  You needed to have these beetles dry long enough for the paint to dry, but not so long as for the beetles to die.

Marking Heterelmis vulnerata with oil-based paint using an insect pin in a pin vice. Photo courtesy of J.R. Gibson

After some 20 days, none of the marked or unmarked H. vulnerata had died.  Thus it was thought that the procedure would be safe for H. comalensis in the field.  

So in the field, 100 beetles were collected and separated into 10 groups of 10 with each group being marked with a different color.  After one month only a single marked beetle was re-captured 1.7 M from its original release site.  The second month only a single beetle was re-captured (though a different individual) at its original site of recapture.  I'll cut through the long-winded part of the discussion and say that what this really told us was that the marking techniques would be difficult in the field, but they do have the potential for showing that dispersal is very low for individual H. comalensis.

The other part of this project was the temporal evaluation of the "cotton cloth lure" method.  So what is the cotton cloth lure (CCL) method?

Essentially this methodology was pioneered for the Comal Springs riffle beetle by my colleague and co-author Randy Gibson.  Randy once said that he developed the method based on some techniques used by cave biologists.  Apparently these cavers would throw old mop heads down into cave pools.  As the mop heads decomposed, biofilms would grow upon mops and it would attract cave invertebrates which could be captured when the mop heads were retrieved.

In the CCL method for riffle beetles, pieces of cotton/nylon sheet are cut into squares, folded up, and buried in the interstitial gravel habitats surrounding spring openings and upwellings.  Again, biofilms will grow on the cotton as it decomposes, which seems to attract grazers like riffle beetles.  This method also attracts multiple other species including the common riffle beetle Microcylloepus pusillus and ocassionally the endangered Pecks Cave amphipod Stygobromus pecki and the endangered Comal Springs dryopoid beetle Stygoparnus comalensis.

The goal here was to watch these lures over a long period of time (17 weeks) and see if the "capture" success varied.  Presumably, if part of the lure was decomposing, eventually there would only be inert fibers left, upon which the lure should no longer be useful.  Furthermore, perhaps there would be a peak time in which the lure was the most attractive to H. comalensis (meaning capture success was highest).

Again, I'll cut through the long winded discussion and just show the graphical results:

Three cotton cloth lures (A,B,C) buried 1 M apart from one another in Comal Springs for 17 weeks and the number of Heterelmis comalensis beetles found on each lure during each weekly sampling effort.

As you can see from this graph, it looks like capture success for H. comalensis is best between 7 and 10 weeks after the deployment of lures.  My presumption is this is when the biofilm fauna has really begun to flourish, and there is lots of good food available for the beetles.  Also, it looks like the capture success drops off rather rapidly after 10 weeks, presumable after all the organic matter is gone and only the inert nylon fibers remain.

In conclusion: This project was a pretty useful pilot study into the best methodologies for evaluating Comal Springs riffle beetle population sizes.  The lure method still remains the best technique, because it is minimally damaging to the habitat and beetles are able to be returned alive at the site of capture.


Freilich, J. E. 1989. A method for tagging individual benthic macroinvertebrates. Journal of the North American Benthological Society 9: 351–354.

Wineriter, S. A., and T. J. Walker. 1984. Insect marking techniques: durability of materials. Entomological News 95: 117–123.

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