// Twitter Cards // Prexisting Head The Biologist Is In: mimicry
Showing posts with label mimicry. Show all posts
Showing posts with label mimicry. Show all posts

Saturday, February 11, 2017

Unionids Along the Missouri

Figure illustrating the river system of Nebraska. At top right a single position is indicated by concentric red circles.

Green is where Unionids were found in research.
Red is where I collected Unionid shells.
[Figure derived from those at link.]
Several years ago, my brother and I went on an overnight road-trip to Nebraska. Why? ...well, mostly because it was there and it was close enough to make an overnight trip. Among other memories, one of the highlights was hanging out on the banks of the Missouri River at Decatur.

As we wandered around the river edge, we found numerous large mussel shells. I collected a few, with intentions of identifying the species that made them at some later time.


Fast forward a few years and I'm digging through some boxes in the basement. I'm not sure what I was looking for, but the shells grabbed my attention. It was time to figure out what they were.

I had collected two pairs of shells. One pair was thinner and the other was thicker. One of the thinner shells broke while in storage. The remaining shell is 13.8 cm long, 8.0 cm tall, and 2.6 cm deep. After some looking around at various documents, I realized I could identify these thinner shells as a specimen of a Unionid species called the Great Floater (Pyganodon grandis). The species seems to get this name because of the penchant for their shells to float away when one has died and has begun to rot.

Four different views of a large freshwater mussel shell. The shell is very thick, showing signs of damage and healing over its long life.
Thick-shelled Unionid. Lower-right
is a closeup of growth ridges.
The second pair of shells was much thicker. They're similar in size to the previous shell (12.6 cm x 7.7 cm x 3.5 cm). The shells both look scarred and aged. The best way to determine the age of Unionid shells involves destructive dissection of the shells. Instead, I used the less accurate method of counting the yearly growth ridges. I estimated the shell at ~140 years old (which is well within the age range known for these animals), but I still haven't had any luck with an ID.

Through the process of trying to identify these shells, I accidentally identified a shell I had found in central Texas when I was in highschool. This shell is from another Unionid that is called the Threeridge (Amblema plicata). I had long ago given up on finding the name of this shell, so this was a cool bonus.


Unionids have all sorts of interesting biology. Like most bivalves, they make their living filter feeding water as they hide buried in the sediment. The live in freshwater river systems worldwide, with the most diversity present in North America. Adult Unionids can only travel very slowly by shifting their foot, so you would think they'd have a difficult time traveling up rivers. The Unionids have developed a very special trick to get around this limitation. They use fish to transport their babies.

Unionid larvae (called glochidia) spend some time as a parasite in the gills of fish. The fish can travel upstream or downstream, much further than the adult could ever crawl. After some interval, the glochidia drop from their fishy host and start living the traditional life of a c.

Five images showing Unionid mussels with various features they use to attract fish which are hosts to the larvae of the mussel. Most show fleshy extensions that look like worms or small fish. At bottom-right is a shell edge which has sharp inward pointing teeth growing along it.
Images from unionid.missouristate.edu
How the glochidia get into a fish is kinda awesome. The general strategy is to convince a fish (of an appropriate species) to inhale a bunch of the babies. How they do this varies all over the place. The females of some Unionid species develop large flanges/flaps of tissue that are shaped and colored to mimic small fish or other aquatic creatures. These organs have musculature and so can even move in a realistic manner, which all aids to draw the interest of fish. When the fish come close to try and get a meal, they instead get a mouthful of larval bivalves. Other Unionid species release their larvae in sacs (ovisacs) that attach to rocks or the parent shell and are buffeted around by the current. These sacs look like little fish, again drawing the interest of fish looking for a meal. One group of Unionids (the Epioblasma) even bites onto the heads of fish (using tooth-lined shell edges), so they can shove their babies directly into the fishes' mouths.


These species are long-lived, but sensitive to environmental disruption. They can't survive in a river that dries up and are they're unable to get out of the way when water quality is impaired by human activities. Because of this sensitivity, there are legal restrictions on their harvest (in every state I've checked).

All the shells I have were collected on dry land, which sidesteps the legal restrictions designed to protect the live animals from harm. Frankly, I couldn't imagine collecting living animals to get their shells. You have to respect your elders, even if they happen to be living on the bottom of a river.


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Tuesday, July 26, 2016

Artistry of the Insect Kind

I've been posting a lot about mimicry lately, so I figured I would post about a few interesting insects I've come across that appear to take mimicry to another level. They mimic things in the way an artist would - by painting them on a canvas. Of course, the insects aren't really painting anything. Instead, they're growing the images using some complicated developmental processes.


A fly with wings held out perpendicular to its body, along the ground. Each wing has a dark mark that could be interpreted to look like a small spider or ant.
Goniurellia tridens
Goniurellia tridens is a species of small fly which appears to have ants/spiders/somethings on its wings. The group of flies to which this species belongs have many examples with marked/patterned wings, but usually the patterns are simpler. Generally the flies use their patterned wings in courtship displays, dancing with their wings held to the side (like in this photo) while they shift and flap one wing after the other.

It is easy to imagine them using the patterns on their wings to discourage ants/spiders/etc. from attacking them, but it isn't clear if they do this behavior. It would seem to be far less risky to simply take to the air and fly away when faced with a predator. There are quite a few jumping spiders that would be able to leap and grab them out of the air. (As well, they have an impressive visual system that would let them see the images as ants/spiders/etc. in a way similar to how we do.) If such a spider were to jump at one of the high-contrast  "prey" images instead of the fly itself, the fly might gain time to escape. A jumping spider might also see the patterns as a pair of predatory ants and choose to be elsewhere. I would love to see some video footage of how this fly responds to predators as this would help clarify if any of the above hypotheticals have any basis in reality.

A small moth holding its wings open and upright, facing the viewer flat-on. The pattern on the wings could be seen to look like a larger spider with eight long legs.
Siamusotima aranea
Or next mimic is the moth, Siamusotima aranea, that appears to have a bunch of spidery legs painted on its wings. This image, again, would be most useful against a predator with acute eyesight like a jumping spider. The photo at right provides some additional evidence. Moths typically rest flat to the surface, with their wings held close. In this image the moth is holding its abdomen raised from the surface and extending its wings, effectively displaying its image in a way not associated with typical moth behavior.

Other moths (like Callimorpha dominula) will actively flash their bright hind-wings to discourage predators (or photographers) who are expressing too much interest, so it is reasonable to interpret the behavior of S. aranea in this photo as being an active display to discourage predation.

A moth next to a fly. On the moth's wing is an image that looks similar to the fly.
Macrocilix maia
Another moth, Macrocilix maia, appears to be a very good mimic of something different - bird poop and flies. The moth even has behavior to back this hypothesis up, often choosing to rest adjacent to real bird poop. Since it rests flat to a surface, like most moths, the image isn't one presented to an predator coming along the surface. Instead the image is presented continuously to the air above, where birds are using their acute color vision to hunt for insects.


It can take some experimentation to determine if an animal is really using its patterns as mimicry. We can make a hypothesis about what their wing patterns are used for and then test the hypothesis using the "natural experiment" of the moths' behavior. If their behavior is consistent with the mimicry hypothesis, then the experiment has a positive result and we can more strongly state their wing patterns are a form of mimicry. In the moth examples above, each species definitely has behavioral traits that align with the mimicry hypothesis.

In the fly example, I haven't been able to find enough information about their behavior to be able to interpret it as a natural experiment. It could be that female flies like male flies with wing-men, so to speak. (This would be consistent with the the use of wing markings in courtship displays seen generally in the group.) It could be that we humans are seeing a pattern in the image that is essentially random with respect to predation. I definitely like the anti-predation model, but without some form of experiment, we really don't have enough information to determine which hypothesis is better aligned with reality.


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Tuesday, July 19, 2016

More Crop Mimics

I've previously posted about a form of mimicry in plants (called Vavilovian mimicry) where one [or several] wild species end up mimicking a crop species due to the selection pressures in crop fields. Since then, I've come across a few other species that are useful examples for the topic.


At top, a row of wheat seed heads. At bottom, a row of similar seed heads to a plant called darnel.
Wheat (top) vs. Darnel (bottom).
[from link]
Darnel (Lolium temulentum) is a  mimic of wheat. It looks almost identical to wheat plants right up until the seed heads form (see at right). The seeds themselves are large and are indistinguishable from those of wheat after threshing. The seeds are also highly poisonous, leading to the common name of "Poison Darnel". When wheat contaminated with sufficient Darnel is milled into flour, the resulting bitter taste reduces its value.

Darnel control efforts have limited success and the plant is found essentially everywhere wheat is grown. Even though the toxic character of this species (when infected) means it will likely never be developed into a primary crop, it is still a nice example of Vavilovian mimicry.


Another common weed in wheat fields is Small Canary Grass (Phalaris minor). It also produces large seeds that would probably get sorted with wheat seeds at the end of the season and its seedlings look very much like those of wheat.

As a general rule, any random grass species is going to be a much better mimic of the grass we call wheat than any other random type of weed would be. Of the five weed mimics of wheat that I've come across (below), all are grasses and three have become major crops in their own right.


Rice crops are often plagued by weedy forms called "Red Rice" due to the red color of their seeds. They are generally less productive than the main cultivated varieties, so farmers try to keep it out of their paddies. Red-Rice happens to be the same species as the cultivated types. It evolved from (or alongside) cultivated rice so it really isn't a case of Vavilovian mimicry, even though it is a useful example in the discussion of crop weed mimics.


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Tuesday, July 12, 2016

Ant Mimicry

Ants are often unpalatable to insect-eating birds due to the formic acid present in their abdomens, so they're often avoided in favor of other prey items. Any potential prey item that happens to look like an ant will have a better chance to live another day than one that doesn't. This selective pressure seems to have led many unrelated arthropods to take on body forms (and behavior) that mimic those of ants.

There are a large number of ant mimic spiders, that often look remarkably like the ant species they're associated with. Because they look like the ants, they are free to go about their business hunting the ants without intervention by birds. The ant mimic mantis probably gains the same freedom to hunt near ant colonies, though I haven't read anything specific about its life-style.

Though I have seen ant mimics in real life (spiders and the occasional beetle), I don't have any of my own photos for illustration. Because they mimic the behavior of ants (in addition to the look), they tend not to stay still for my camera. I've never really gotten a good photo of an ant either, for the same reason.

The photos that follow here were found around the web and I have no legal claim to them, though I feel their educational use here should provide me some protection as fair-use under US copyright law.

Spider that looks very much like a black ant.
Spider
Cricket that looks very much like a black ant.
Macroxiphus sp cricket.
Predatory true-bug that looks very much like a black ant.
Dulichus inflatus (Hemiptera)
Beetle that looks like a red and black ant.
Euderces beetle.
A small mantis that somehow looks like a large black ant.
Asian Ant Mantis (Odontomantis planiceps)
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Tuesday, March 1, 2016

Vavilovian Mimicry

Vavilovian mimicry is a form of mimicry where an agricultural weed begins to take on characteristics of a domesticated agricultural crop due to the selective forces present in the agricultural system.

The first example is involving wheat. Well, really, it involves both wheat and barley. During the early phases of domestication of these grains, they weren't distinguished as separate plants. Initially, the large seeded grasses were collected from the wild and planted closer to home. The now-farmers would then collect the large seeds from the grasses they grew, eat most and protect some, and then plant what was left the following year. You could say that wheat and barley are Vavilovian mimics of each other, because we really don't know which was the first grass to enter domestication.

At some point while the wheat/barley agricultural system was developing, other weedy grasses invaded the prime growing habitat found in the fields. Two of these weeds evolved to become what we call "rye" and "oats". They developed larger, non-shattering seed heads and an annual life-cycle. This allowed their seeds to be collected, saved, and planted as contaminants to the main crop. Both rye and oats are more tolerant of cold conditions and poor soils. Because they had become mimics (and contaminants) of the major crop, when farmers tried to establish the crop system in marginal conditions, these mimics can come to predominate as the major crop.
Wheat/Barley -> Wheat (Triticum spp.)
Wheat/Barley -> Barley (Hordeum vulgare)
Wheat/Barley -> Rye (Secale cereale)
Wheat/Barley -> Oats (Avena sterilis)

Another often-cited case of Vavilovian mimicry is found in the agriculture of lentils (Lens culinaris). A common weed in lentil fields is the Common Vetch (Vicia sativa). The Common Vetch seeds are bitter, so farmers are able to sell their crop for less if there is too much vetch contamination. As farmers have increased the selection pressure on the vetch by mechanical (and computer-vision) assisted seed sorting, strains of the vetch have evolved so that their seeds mimic the lentils in color and size, as well as the characteristic flattened lens-shape.

A. Lens culinaris. B. Vicia sativa, wild and mimic.
(from: www4.ncsu.edu/~fgould/pdfs/Gould1991.pdf)
Lentil (Lens culinaris) -> Common-Vetch (Vicia sativa)
Lentil (Lens culinaris) -> Black-Pod-Vetch (Vicia sativa subsp. nigra)
If farmers could impart some selective force on the mimic vetches such that they would lose their bitter flavor, they would have effectively created a new crop. This new crop might grow better in some conditions where lentils don't thrive, thus spreading the useful area of agriculture.


The selection force involved in the development of Vavilovian mimicry can be mechanical (as in Flax weeds) or manual (as in Rice weeds). What is key is that the selection force separating weeds from the crop has to progressively get more and more stringent over time. This allows the weed population to always have some individuals that will escape the selection force applied to them.
Flax -> False-Flax (Camelina sativa linicola)
Flax -> Flax-Dodder (Cuscuta epilinum)
Rice (Oryza sativa) -> Early-Baryard-Grass (Echinochloa oryzoides)

An interesting case that I think is related to Vavilovian mimicry is the complex of Andean tuber crops. I don't know which crop was first domesticated in this region, but since before modern history, five species of tuberous crops have been traditionally grown together in fields. Growing several different crops together in this agricultural system mean that there will always be production, even if any given plant doesn't produce in some year (due to weather, disease, or other factors).
Potato (Solanum tuberosum) -> Maca (Lepidium meyenii)
Potato (Solanum tuberosum) -> Oca (Oxalis tuberosa)
Potato (Solanum tuberosum) -> Mashua (Tropanolum tuberosum)
Potato (Solanum tuberosum) -> Ullucus (Ullucus tuberosus)
Though I doubt any of these species entered the agricultural system as weeds, I expect that each species will undoubtedly have evolved towards a set of traits similar to those of the most common plant grown in the fields. Any individual plants that didn't prosper in the agricultural system would have contributed less to the next generation and the species would shift to a form that did prosper. This shifting of the traits of one species to align with another, due to the selection forces favoring the majority plant species, is a characteristic common between Vavilovian mimicry and whatever this case should be referred to as.


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