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.”
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| 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
[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.
[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:
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:
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.