The Comal Springs riffle beetle Heterelmis Comalensis pupates underwater

Underwater Pupation by the Comal Springs Riffle Beetle, Heterelmis Comalensis Bosse, Tuff, and Brown, 1988 (Coleoptera: Elmidae), with an Update on Culture Techniques

Ah my second beetle paper.  This one was a product of my time working at the U.S. Fish and Wildlife Service (USFWS) San Marcos Aquatic Resources Center (SMARC) with my co-author on this publication J. Randy Gibson.  As always, here is the citation for the paper and a link to a PDF for your reading pleasure:

Huston, D.C., and J.R. Gibson. 2015. Underwater Pupation by the Comal Springs Riffle Beetle, Heterelmis Comalensis Bosse, Tuff, and Brown, 1988 (Coleoptera: Elmidae), with an Update on Culture Techniques." The Coleopterists Bulletin 69: 521-524. PDF

So who is the Comal Springs riffle beetle?  The little guys look like this and are around 2mm long. 

A Heterelmis comalensis adult.  Photo by J.R. Gibson, USFWS SMARC.

Heterelmis comalensis is only known from the headwaters of Comal Springs in New Braunfels, Texas and San Marcos springs in San Marcos, Texas (USFWS, 2007; Gibson et al. 2008).  These springs originate from the Edwards Aquifer, a massive aquatic limestone subterranean system.  The subterranean parts of the aquifer support a large number of endemic aquatic cave species (called "Stygobionts").  Furthermore, the above ground spring systems fed by these waters also contain a high amount of species endemism.  The Comal Springs riffle beetle, H. comalensis, is just one of these species.  This beetle is found in very close association with the upwellings and spring openings where water exits the subterranean system (Gibson et al. 2008).  The beetle is completely aquatic throughout its life.  This is why H. comalensis is in danger of extinction; reduced spring flow because of drought and excessive groundwater extraction due to human demand could lead to the complete destruction of the beetle's habitat.  Heterelmis comalensis is listed as endangered by the USFWS, and is listed as critically imperiled by NatureServe.

So what we did at the SMARC, was maintain little colonies of endangered species from around the local Central Texas springs, such as Texas wild rice (Zizania texana), fountain darters (Etheostoma fonticola), Devils river minnows (Dionda diaboli), and of course, Comal springs riffle beetles!

Unfortunately, there had been little success in setting up a self sustaining colony of H. comalensis.  We just knew so little about these beetles and what their culture requirements might be.  The true nature of things as simple as what they really eat still eludes us to this day.  It seems like they scrape biofilms that grow on submerged plant roots and decaying woody debris (Gibson et al. 2008), though this still has not been scientifically confirmed.  Therefore, its been quite difficult to figure out how to raise these guys through all life stages in captivity.  In fact, trying to get a colony of these beetles up and running has been going on for over 10 years (Fries, 2003)!  For a long time, it seemed that the majority of the beetle larvae would just never pupate into adults.  The real change in pupation success came when Randy and I designed a new style of culture container, which we call the EasySpring!

Easy Spring aquatic invertebrate culture container

How does it work?  Obviously this thing is built out of a big plastic tote container.  Spring water is pumped into the spray bar through the black tube.  The spray bar has a bunch of tiny holes drilled in it, thus spraying a little line of water along the side of the container at a pretty good pressure.  The water runs down the side into the rock pile.  For riffle beetles, we pile up rock, layering bits of leaf and wood in between the rocks.  The U shaped standpipe also has holes drilled along the length.  It maintains the water depth, while allowing the water to exit the container in a sort of linear fashion.  All this mimics the natural habitat of H. comalensis as best we could in the lab with the materials we had.  When we added H. comalensis larvae to the container we began finding adults within a few months.

This is when we began noticing something strange.  Every 1-3 months I would go through these containers and carefully remove all the rocks while collecting the beetle larvae and adults.  Then I'd sit down for the rest of the day and count.  And it takes forever to find all the little first and second instars, let me tell you.  Well I started to notice that sometimes when I would pick up a rock that was near the bottom of the container, pupae would pop out and float around on the surface of the water.  Why and how were the beetles forming pupae under the water?  This was especially interesting seeing as Elliott (2008) in a review of the ecology of riffle beetles, had made the rather definitive statement, "pupation always occurs above the water line".  Well it looked like H. comalensis had gone rogue and wasn't following the rules.

A Heterelmis comalensis pupa.  Photo by J.R. Gibson, USFWS SMARC.

So we reckoned that the beetles were pupating underwater.  How?  Well the pupae were super hydrophobic.  I'm still not sure if that was because they held an airspace inside the carapace, or if all the tiny hairs on the carapace were serving to trap air themselves, serving like the plastron of the adults.  Hmm, I never described the plastron did I?  Well I'll give a the quick and dirty description.  A plastron is a tight bundle of hydrophobic hairs found on the abdomen of riffle beetles.  The plastron traps air in a little film on the beetle, and the spiracles of the beetle open into the plastron.  This allows the beetle to breath underwater indefinitely. 

While pretty interesting, the hydrophobia presented a problem for us.  We couldn't see the pupae where they were found normally in our containers, they were just too small and were always between rocks.  If we collected the pupae from the container and put them in something else, they floated, so we couldn't prove that they could eclose (escape the pupal carapace) under the water line.  We needed something to get ourselves to believe these guys were pupating underwater.  The solution?  Well we tried a few things, but in the end we kept the pupae in a small re-purposed plastic air lock thing as an aquatic flow through chamber with one of those little steel mesh rings you put in your faucet in order to keep the pupae forced underwater.

Heterelmis comalensis pupa in a plastic flow through chamber kept forcefully underwater with a steel mesh ring.

Well, it worked.  We were able to watch the beetles pupate in these things, and always found live adults in these chambers when we put pupae in them.  Though I haven't demonstrated this yet, I think that the hydrophobic quality of the pupae comes from hairs on the outside of the carapace.  There is evidence for that in this photo.  Can you see the little air bubble attached to the pupae?  It seemed like every single pupae had these bubbles.  I think it quite possible that the adult beetles use these bubbles to establish their own plastron upon eclosion.  But only future research will tell!

REFERENCES

Elliott, J. M. 2008. The ecology of riffle beetles (Coleoptera:Elmidae). Freshwater Reviews 1: 189–203.

Gibson, J. R., S. J. Harden, and J. N. Fries. 2008. Survey and distribution of invertebrates from selected springs of the Edwards Aquifer in Comal and Hays Counties, Texas. Southwestern Naturalist 53: 74–84.

United States Fish and Wildlife Service (USFWS). 2007. Endangered and threatened wildlife and plants; Designation of critical habitat for the Peck’s cave amphipod,Comal Springs dryopid beetle, and Comal Springs riffle beetle; Final Rule. Federal Register 72: 39248–39283.