28 February 2011

First Megapallifera of the year

On 2 occasions earlier in this month when the weather was rainy and unseasonably warm, I searched for the local native slug Megapallifera mutabilis in the woods. But the slugs were not out yet. Late this afternoon as another warm and rainy day came to close, we did another search. This time they were out in throngs. In about 20 minutes I collected 12 of them from the trunks of beech trees. This slug was indeed the very 1st one I saw.

I brought the slugs home and weighed them. The data will be incorporated into my ongoing research project on the annual population cycle of this species.

27 February 2011

Vultures at lunch

During my after lunch walk last Friday, I interrupted the lunch of 2 black vultures. One flew away as soon it spotted me. But the other one, because it was either dauntless or stupid, stayed at the carcass and continued to peck at it. I kept approaching while taking pictures. Unfortunately, the only camera I had with me was my iPhone.

When I was within about 10 meters of the 2nd vulture, it too flew away. Then I walked over to the lunch spot and found out what was on the menu: a headless squirrel.

Black vultures apparently feed almost exclusively on carrion. This particular squirrel had no smell that I could smell from my height. It is amazing that the vultures had detected it.

25 February 2011

The 1st earthworm of the year

The warm air and the rain that came during the night had already the brought the earthworms out early this morning when I was walking to work.
This one on the wet sidewalk was indeed the 1st one I saw this year.

To commemorate this occasion, I will ask this perennial question one more time: Why do earthworms come out of the soil during and after rains?

They do so because they can come out without risking desiccation.

But because flat and wide and porous concrete surfaces were not in the original repertoire of earthworms habitats, evolution has not equipped earthworms with the ways and means necessary to deal with the rapidly dehydrating circumstances that arise once the rain stops and the sun comes out. As a result, the worms often get stranded on dry sidewalks and parking lots and die by the hundreds.

23 February 2011

The boquet of 18 chemicals

Smells and other odours are sweeter in the air at some distance, than near the nose; as hath been partly touched heretofore. The cause is double: first, the finer mixture or incorporation of the smell: for we see that in sounds likewise, they are sweetest when we cannot hear every part by itself. The other reason is, for that all sweet smells have joined with them some earthy or crude odours; and at some distance, the sweet which is the more spiritual, is perceived, and the earthy reacheth not so far.

Francis Bacon
From this book
For Valentine's Day, my wife gave me a nice bottle of cologne. The ingredient list on the box includes more than 20 chemicals. If denatured alcohol, water, propylene glycol, which are the solvents, and green 5, yellow 5 and yellow 6, which are the colorants, are excluded, there remains about 18 chemicals that I suspect all contribute to the perceived fragrance of this eau de toilette.

For a chemical to be odoriferous it needs to be volatile: if not enough molecules travel from a substance to the inside of our noses thru the air, we will not smell it*.

While reading an article today on the recreation of flower smells in the laboratory, a question popped in my mind. If many natural and artificial odors are mixtures of several volatile chemicals, how do our noses detect them more or less simultaneously to perceive a single odor that is always the same?

What happens is that as the volatile molecules in a fragrant flower or a layer of perfume applied to one's skin evaporate, they begin to fill the air space above. I suspect that at ordinary temperatures, the relative concentrations of the molecules in a small volume of air above a source producing the molecules, i.e., a flower or a liquid, is always roughly the same. Thus, the nose receives the same gaseous mixture on every occasion. The perceived odor itself results from the simultaneous interactions of all the different molecules with the receptors in the nose and the resultant electrical outputs sent to the brain.

*Of course, we can also smell the chemicals that are not volatile but are in solution. When that happens, for example, during the consumption of a food, the odor is usually perceived as a flavor.

21 February 2011

Beaver's unfinished business

This is the 3rd time I am using this title on a blog post. It is indeed quite common to come across standing trees whose trunks beavers gnawed but left before felling them. Yesterday's post had a picture of a beech tree that had been the focus of interest of a beaver and which I had photographed from afar. Today I went back to the same spot and took better pictures.

Here is a closer look at the teeth marks left by the beaver on the trunk.

The marks of the 2 front teeth are distinct.

The previous posts with the same title as this one were here and here.

20 February 2011

Opportunistic beaver eats beech

Edward Warren's 1927 book The Beaver, does not include the American beech (Fagus grandifolia) among the more than 12 species of trees that are listed as the food of the North American beavers (Castor canadensis). A much recent book, also called The Beaver, by Müller-Schwarze & Sun (2003), puts the beech in the 5th position in the order of tree preference of beavers.

During our walk today we encountered a large beech that had been knocked down by the strong winds we experienced during the past several days. Sections of its bark, where now the light brown wood is showing, had been stripped off by one or more beavers who obviously didn't want to miss out on this free dinner. I suppose in the middle of winter a windfall beech is preferred to a tastier tree that would require expenditure of energy to bring down.

Also notice the still standing younger beech in the back whose trunk had been gnawed by a beaver. The perpetrator will probably come back to finish off the tree.

18 February 2011

Gills and lungs at the intertidal

Respiration—gas exchange between blood and an outside medium (air or water)—always takes place across a wet membrane whether the organ hosting the blood is a gill or a lung. There is no fundamental difference between gills and lungs. Their essential anatomy is the same: a large surface area overlaying blood vessels. To further increase the surface area for efficient gas exchange, gills are usually branched or filamentous and lungs may be invaginated or branchiate.

A gill is not necessarily restricted to an aquatic medium: if it can stay erect and wet, it will also function in the air. Likewise, a lung can also function in water. In fact, in certain habitats animals with gills can live side by side with animals with lungs.

Here is an example. In this picture are 2 species of tiny snails that I found on a rock that I had pulled out of the sea during the high tide at a Florida beach. The 4 snails with shiny shells are Assiminea succinea, a species in the superfamily Rissooidea. They have small rudimentary gills. The snail with the duller shell near the center-left is Pedipes ovalis in the family Ellobiidae. Like its distant relatives the pulmonate land snails, Pedipes ovalis obtains its oxygen via a lung.

We see that in transitionary seashore habitats snail species with gills and lungs live alongside each other, creating an evolutionary mosaic.

17 February 2011

Truncatella's air bubble

Truncatella is a genus of coastal-terrestrial snails that have been the subjects of several posts on this blog (for example, this one and this one).

In the discussion of Truncatella subcylindrica in British Prosobranch Molluscs by Fretter & Graham (1994), there is this statement: "...on occasions, a bubble of air may be seen in the [mantle] cavity."

I have noticed an air bubble in the mantle cavities of other species of Truncatella. Here is a photo of Truncatella pulchella crawling in sea water. Notice the large air bubble in the mantle cavity under the shell.

What is the function of the air bubble? Is it an oxygen reserve for the snail?

15 February 2011

Fish out of water

Why does a fish die if it is removed from the water too long? I presume the fish suffocates even though it is surrounded by air that has more oxygen in it than the water in which the fish lives.

I can think of 3 reasons why a fish outside of water suffocates:

(1) When a fish is in water, either the fish or the water moves constantly and so there is always fresh water passing thru the gills. But a fish out of water cannot move and the passive air flow across the fish's gills, unless it is windy, may not be fast enough to maintain a steady and high enough rate of gas exchange across the gills. If the air layer against the gills is stationary, then it will quickly be depleted of oxygen, while becoming saturated with carbon dioxide.

(2) Gills exposed to air, especially when it is windy, will eventually dry and the gas exchange across them will stop; gasses can't seem to diffuse across dry biological membranes. Because fish originated in the sea, they never needed and therefore never evolved a slime layer covering their gills, which, if it were present, would keep the gills wet until, of course, the fish ran out of water.

(3) The gill filaments and their lamellae float in water and therefore remain separated from each other. But if they are exposed to air, they collapse and stick to each other. This decreases the surface area available for gas exchange significantly.

For all these reasons, the majority of terrestrial animals don't have gills and the membrane surfaces they use for gas exchange are continuously kept moist either in a high humidity chamber inside their bodies or by copious slime production on the external surfaces of their bodies.

09 February 2011

Dependence of shell weights on shell dimensions

In this post, I wrote about the dependence of the volumes of snail shells on their linear dimensions. The theoretical relationship between the shell volumes (V) and a linear shell dimension (L) is given by the power law in the form, V=cL3, where c is a constant.

Tonight, I ask a slightly different question. How does the weight of a snail's shell depend on the linear dimensions of its shell? The answer is easy to derive. Since density is weight divided by volume, the relationship between weight (W) and a linear dimension is also a power law in the form, W=kL3, where k is another constant.

While reading an old paper* today, I came upon suitable data to test the validity of this theoretical relationship. Here are the shell weights of the predatory marine gastropod Urosalpinx cinerea, Atlantic oyster drill, plotted against shell lengths.

The best fit curve is W=5.3x10-5L3.16. Is it close enough to the theoretical relationship?


08 February 2011

Protoconch of Helix aspersa

The protoconch of a snail shell is the apical portion of the shell that forms while the snail is still an embryo. In many species, the protoconch has a characteristic and species-specific morphology different than that of the teleoconch, the rest of the shell. However, it is not always easy to tell where the protoconch ends and the teleoconch begins.

Until tonight I had examined hundreds of shells of the land snail formerly known as Helix aspersa*, but not noticed how distinct the end of its protoconch was. The realization came to me when I happened to look at the following shell under the microscope.

This shell is an exception in that the demarcation between the protoconch and the teleoconch (arrow) is very distinct. Here is another and probably more typical shell.

Also notice that the surface of the protoconch is much smoother than that of the teleoconch.

*Now called Cornu aspersum, etc.

07 February 2011

Cerithidea scalariformis and how far it withdraws into its shell

In the previous post, I wrote about how small the foot of the intertidal snail Cerithidea scalariformis was relative to the length of its shell. Then I hypothesized that having a small foot probably enables a snail to withdraw its body deep into its shell when and if its aperture comes under attack by a predator. At the end of the post, I noted that I didn't know how far into its shell Cerithidea scalariformis could actually withdraw. Last night I went thru my notebooks and found some relevant information.

A few years ago, I attempted to keep a small number of Cerithidea scalariformis in captivity. But the snails didn't seem to do well and remained in their shells most of the time. Nevertheless, this gave me a chance to note how far into their shells they were withdrawing. Here is one live snail whose body was about one half whorl behind the aperture. This was indicated by how far into the shell light shone thru the aperture could penetrate.

When forced, Cerithidea scalariformis can probably withdraw even deeper into its shell. I suspect it can withdraw at least one whorl away from the aperture. Curiously, though, among the several hundred live snails I examined in Florida, only a few had repair scars in the body whorls of their shells. This may mean that they either don't have major predators, or, despite being able to withdraw deep into their shells, only a small fraction of snails survive predator attacks and repair their shells.

05 February 2011

Cerithidea scalariformis and its little foot

The 3-eyed, intertidal and almost-terrestrial snail Cerithidea scalariformis has been the subject of several posts (for example, this one and this one).

The picture below shows one snail photographed from below while it was crawling on a glass plate. Note the snout all the way in the front and the foot behind it, both in contact with glass.

Also note how short the foot is relative to the shell. One consequence of the discordant sizes of the foot and the shell is that the former cannot lift the latter above the substrate. The snail drags its shell behind it. Look at the picture below.

To pull its shell through wet sand and mud, its usual habitat, Cerithidea scalariformis must spend quite a bit of energy. There must, therefore, be a counterbalancing advantage to having a shell much longer than the foot that is supposed to pull it. Otherwise, evolution would have long ago done something about it.

One possible advantage of having a foot much shorter than one's shell is that when the snail is withdrawn into its shell, the foot, the last part of a snail to enter its shell, can withdraw far behind the aperture. And that is good when the aperture is under attack by a predator like a crab that can break open the aperture.

I don't actually know how far into its shell Cerithidea scalariformis can withdraw. I intend to find out in the near future.

Here is the follow-up post.

02 February 2011

An elevated bagworm

I was walking under a bare tree yesterday when I noticed that there was one leaf still clinging to a branch. Then I decided it was not a leaf, but most likely the case of the larva of a bagworm moth (Psychidae). Here is the cropped shot. The case was quite high and, unfortunately, I couldn't get any closer to it with my iPhone camera.

Come spring, a caterpillar, if it survives the winter, will come out of it. Two previous posts about bagworms are here and here.

01 February 2011

What if there was a significant difference?

An Anatolian folk hero named Nasrettin Hodja* was once seen spooning yogurt culture into a large lake. When the onlookers reminded him that lake water wouldn't turn into yogurt, his response was "What if it did!"

Nasreddin was testing a hypothesis that was highly unlikely to be true. Yogurt forms when certain types of bacteria grow in milk and the acid they produce causes the milk proteins to precipitate. Compared to milk, lake water has an insignificant concentration of proteins. Thus, a lake cannot be made to turn into anything comparable to yogurt or cheese. Had Nasreddin known the chemistry and microbiology behind the formation of yogurt, he might not have bothered to carry out his test. But that's besides the point. The moral of Nasreddin's story is that we should not be afraid to test unlikely hypotheses. Not all the time, of course, but sometimes.

A couple of weeks ago, I thought of an experiment involving oppositely coiled, i.e., dextral and sinistral, snail shells. All I can reveal now is that by doing this experiment I am hoping to find an explanation for why most snail species have dextral shells. However, I don't exactly have an explanatory hypothesis, but only a vague notion. If at the end of the experiment I see a significant difference in the outcome between oppositely coiled shells, I will then try to come up with an explanatory hypothesis and then carry out further tests.

I ran the idea by my friend and colleague Tim Pearce. Tim objected to the experiment, which he called a "what if" experiment, on the grounds that such experiments have one interesting and one non-interesting outcome and that the latter is usually the more likely result. In other words, in my planned experiment a positive outcome, that is, a significant difference between oppositely coiled shells, is highly unlikely and, therefore, the experiment would be a wasteful undertaking.

Another concern Tim had is that negative results that are the more likely outcome of such experiments rarely get published. As a result, the same "what if" hypotheses may get tested over and over again, because we don't have a way of knowing that they have already been tested.

My attitude is that "what if" experiments should be done if they can be done without the expenditure of too many resources, money included, and effort and, of course, provided that they are ethical. So, if I can obtain enough number of dextral and sinistral snail shells, which is the largest hurdle in this case, I intend to perform the experiment. I did promise Tim that I will do my best to get the negative results published. In return, he will try to get me the shells I need.

And if I end up getting the unlikely positive outcome, so much the better.

Is it yogurt or what? Nasrettin Hodja may have been onto something.

*Nasrettin Hodja was apparently a real person who lived in the 13th century. But many of his tales were probably posthumous attributions.