A Previously Unrecognized Symbiosis?

Editor’s Note – This essay was subsequently published as: Dillon, R.T., Jr. (2019d) Everett's Symbiosis.  Pp 101 - 107 in The Freshwater Gastropods of North America Volume 4, Essays on Ecology and Biogeography.  FWGNA Press, Charleston.

Last month’s post on the aerial dispersal of freshwater gastropods [1] turned out to be one of my more popular in recent memory.  Thank you all for your kind emails.  Winning the award for most charming was our colleague Lusha Tronstad of the Wyoming Natural Diversity Database, who sent me the photos below:

And here is the story, verbatim as I received it from Lusha: 

“My son, Everett Tronstad (he just turned 6) caught this beetle (likely Dytiscus gigantus) in our barnyard this past summer.  We live northwest of Laramie, Wyoming in the foothills of the Snowy Mountains.    My son spends hours every day collecting invertebrates, both aquatic and terrestrial.  He caught this beetle on our car who probably thought the shiny paint was water.  Everett picked it up and we immediately saw the limpets attached.  We had just enough time to snap two photos before the beetle took off.  We watched the beetle fly off into the distance and we marveled at how invertebrates can hitchhike on other critters.  The date was 30 July 2016.”

Everett

I also received a cordial email from Chris Davis of the Pymatuning Lab up in Linesville, PA.  He called my attention to a note published by Andrea Walther and a host of colleagues in 2008 [2], with the striking figure reproduced below.

Andrea and her colleagues collected nine water bugs from a pond at the University of Michigan’s Edwin S. George Reserve, two of which bore five limpets each.  Note that the bugs were swimming at the time of their collection.  Andrea wrote, “Given the positioning of the L. fuscus on the dorsal surfaces of the B. flumineum, it is uncertain if the insects were able to lift their hemelytra to take flight.”

So those of you in my email address book [3] may remember the challenge I issued when I announced last month’s post, back on 15Dec16:  “Quick, quick!  How many reports of freshwater limpets on flying water bugs would you expect to find in the published literature?”  The answer I was looking for was 12, the ten listed by W. J. Rees [4] plus the two I found published between 1965 and 1991.  So here are two more [5], making 14.  At some point, a stack of related observations becomes a phenomenon, don’t you agree?

And here’s another remarkable observation, which I’m not sure has at yet been remarked.  There were three Ferrissia on Everett’s dytiscid beetle.  And there were five Laevapex on the back of each of Andrea’s belostomatid bugs.  On surface areas no more than 4 square cm??  I feel sure the densities of limpets on the insects must be orders-of-magnitude greater than their densities in the general environment.  The limpets are almost certainly aggregated on the bugs, don’t you think?

I have developed an hypothesis to account for the phenomenon of high limpet densities on large aquatic beetles and bugs.  This hypothesis depends on one obvious assumption, one commonplace observation, and one trivial fact, with a couple paragraphs of gratuitous speculation plated on the bottom.

The obvious assumption is that large aquatic beetles and bugs must remain motionless for extended periods of time, in contact with limpet habitat.  I have very little first-hand knowledge upon which to draw here, but my (admittedly superficial) google searches have returned the impression that both the dytiscid beetles and belostomatid bugs demonstrate a wide variety of life habits, but that the big species, in any case, often seem to hunt by stealth.  There are a couple YouTube videos [6] showing periods of both active swimming and quiescent waiting.  Belostomatids seem to rest anywhere, in contact with the bottom or the surface, although dytiscids seem to rest primarily at the surface.  Possibly under lily pads?  Lily pads seem to be prime limpet habitat.

My commonplace observation is that freshwater limpets tend to aggregate on smooth surfaces.  Anybody who has ever hunted for them knows that limpets are more common on smooth rocks than on rough rocks, and less common on the parts of waterlogged woody debris covered with bark than on the parts that have lost it.  When I arrive at a collecting site, one of the first items on my agenda is to wade under the bridge and look for beer bottles.  Limpets reach maximum abundance in old bottles, hard plastics, and smooth litter discarded by fishermen and motorists.  In fact, I have developed the abbreviation “bbl” for my field notes, which means “beer-bottle limpets.”  This means that limpets (of whatever species) were so uncommon as to be found only on smooth litter.

I feel sure this is an adaptation for defense.  Limpets are better able to make a seal with the lips of their shells, and indeed better able to create suction with their foot, on a smooth surface than a rough surface.  It is very nearly impossible to remove a limpet from a beer bottle.  They may slide around on the surface of the bottle, but they do not come off.  For this reason, I keep a scalpel in the pocket of my collecting vest (sheathed in an open plastic sample vial).  The only way to remove a limpet from a really smooth surface without damaging it is to insert a scalpel blade and lift.

My obvious assumption that big aquatic beetles and bugs must hold still, plus my commonplace observation that freshwater limpets tend to aggregate on smooth surfaces, plus the trivial fact that the backs of big beetles and bugs are smooth, can yield a symbiotic association between freshwater limpets and large aquatic insects.  I feel pretty sure that the relationship is positive for the limpets, at least initially, since smooth surfaces yield better protection from predators.  I imagine the relationship is slightly negative for the insects, since the limpets must add a bit of weight, and a bit of drag.  More negative for the bug if attached limpets interfere with its ability to fly, as Andrea suggests.

What might the unusually high densities of limpets we sometimes observe on the backs of aquatic insects eat?  It would be lots of fun to imagine that the limpets are commensal with the bugs – sharing or somehow benefiting from the meat diets of their hosts.  Dytiscid beetles seem to be such sloppy eaters [7] that one might hypothesize increased concentrations of bacteria and fungi on their bodies.  The excellent studies of Calow [8] from the mid-1970s, however, convince me that European Ancylus populations graze rather exclusively on periphytic algae, especially diatoms.  I just cannot find any warrant to imagine that limpets straying onto the backs of carnivorous insects might reasonably switch to anything else.

Which means that the limpets rapidly become hungry.  My gratuitous speculation is that they graze every last diatom cell off the backs of their hosts in a matter of hours, and subsequently ride around in sullen misery, regretting their decision to hitchhike, looking for any opportunity to get off.  Which brings us back to the phenomenon that brought us here, which, I seem to recall, was not symbiosis, but rather transport.

So after this brief but (I hope you’ll agree) interesting digression, next month we’ll return to the theme I introduced last month, which was the aerial dispersal, broadly, of freshwater gastropods, generally.

Notes

[1] Freshwater Gastropods Take To The Air, 1991 [15Dec16]

[2] Walther , A. C.,  M. F. Benard, L. P. Boris , N. Enstice , A. Tindauer-Thompson & J. Wan (2008) Attachment of the Freshwater Limpet Laevapex fuscus to the Hemelytra of the Water Bug Belostoma flumineum.  Journal of Freshwater Ecology, 23:2, 337-339, DOI: 10.1080/02705060.2008.9664207.

[3] If you’d like to receive regular alerts from the FWGNA, email me at DillonR@fwgna.org

[4] W. J. Rees (1965) The aerial dispersal of Mollusca.  Proc. Malac. Soc. Lond. 36: 269-282.

[5] Okay, fine, for you sticklers out there!  I understand that the first 13 were records of aquatic Coleopteran beetles, not technically Hemipteran “bugs.”  And I aso realize that the fourteenth wasn’t literally a “flying water bug,” because Andrea collected her Belostoma as they were swimming in the pond.  So technically, the answer to my query of 15Dec16 might still be zero.  I’ll bet you all really irritated your tenth-grade Biology teachers, didn’t you?

[6] Here’s a montage of videos of large belostomatid bugs hunting.  The first one shows a bug holding very still on a bottom of smooth stones (interestingly) waiting for its prey to swim by.  There’s also some footage of another belostomatid hunting from the surface, and then a sequence suggesting ambush from submerged vegetation:

https://www.youtube.com/watch?v=YEPvXZXfEK4

As a bonus, around minute 2:30 there’s a sequence showing a belostomatid capturing and eating a big individual Melanoides.  The bug apparently lunges right by a fat, juicy Helisoma to snatch the bony Melanoides.  I can’t imagine why.

[7] Here’s a YouTube video of a Dytisid making very sloppy work of a minnow:

https://www.youtube.com/watch?v=peYliiwWWPY

But we should probably also note simultaneously that belostomatids seem to be very neat eaters, inserting their proboscis and sucking their prey clean from the inside.

[8] Calow, P. (1973) The food of Ancylus fluviatilis Müll., a littoral stone-dwelling, herbivore. Oecologia (Berl.) 13, 113–133.  Calow, P. (1975) The feeding strategies of two freshwater gastropods, Ancylus fluviatilis Müll. and Planorbis contortus Linn. (Pulmonata), in terms of ingestion rates and absorption efficiencies.  Oecologia 20: 33-49.