31 December 2005

Papers read this week

Musical accompaniment: Rachmaninoff: Piano Concerto No. 2 & Tchaikovsky, Piano Concerto No. 1, played by Felicja Blumental.

I have written about the almost-but-not-entirely-terrestrial snail Littoraria angulifera common in mangrove forests in Florida. The following 2 papers are about its close relative Littoraria irrorata (the periwinkle snail). The latter also lives at the edge of the sea and enters it only to spawn.

Brian R. Silliman, Johan van de Koppel, Mark D. Bertness, Lee E. Stanton, and Irving A. Mendelssohn. 2005. Drought, Snails, and Large-Scale Die-Off of Southern U.S. Salt Marshes. Science 310:1803-1806.
The authors did several experiments in salt marshes in Georgia and Louisiana to understand the causes of the recent large scale die-off of the dominant plant Spartina alterniflora (cordgrass). Their results put the blame on severe droughts (1999-2001) that stressed the plants and the enormous numbers of Littoraria irrorata, which at some sites reached mean densities of 834 individuals/m2. The figures from their paper reproduced below compare Spartina biomass in plots from where snails were excluded (solid bars) with those in lots where snails were present (open bars). The latter are hardly visible.


Interestingly, the snails don't actually eat Spartina alterniflora, but the fungi that grow on it. But the snails' grazing damages live plant tissues and consequently, facilitates their subsequent infection with fungi and thereby creating a circular and indirect process of destruction.

My first thought upon reading this paper was that it wasn't quite normal for so many snails to be in one spot. Clearly, something that would normally regulate snail population densities was obviously missing and that is the predatory control of snails. The predators of L. irrorata were discussed in an earlier paper by some of the same authors.

Brian Reed Silliman and Mark D. Bertness. 2002. A trophic cascade regulates salt marsh primary production. PNAS 99:10500-10505. Full text
The figure below, from the cited paper, is the proposed salt marsh food web. Blue crabs (Callinectes sapidus), mud crabs and terrapins (Malaclemys terrapin) eat the periwinkles, L. irrorata, which feed on the fungi growing on Spartina alterniflora.


The authors believe that over harvesting of snail predators, for example, blue crabs, may be leading to the development of very high population densities of snails and, indirectly, resulting in the disappearance of salt marshes.


Happy New Year everyone!

29 December 2005

Barnacles: Darwin's old buddies

balanus

I am holding in my hand a cluster of fossil barnacles, Balanus concavus1, from the Miocene (5-23 million years ago). I found this specimen at a location along the Calvert Cliffs, Maryland, an area famous for its fossils.


Compiled from Lippson & Lippson2.

Barnacles (Cirripedia) are marine invertebrates. A barnacle starts out as a free-swimming larva, called a nauplius, (plural nauplii) . The nauplius grows a pair of shells around its body. At this stage it is called a cypris larva. The cypris larva attaches itself permanently to a rock, a mollusk shell, an adult barnacle or any other hard structure. The larva, now sessile, starts to develop into an adult. It casts off its shells and begins to secrete several calcareous plates that end up completely surrounding its body. The fossil in the picture above consists of the shells of adult barnacles.


A barnacle feeding. From Lippson & Lippson2.

A barnacle feeds by opening its shell and unfolding its appendages that are covered with fine hairs. The appendages sweep tiny particles of food suspended in the water into the barnacle's mouth.

Because of their calcareous shells, until the early 19th century people thought barnacles were mollusks. It was finally determined around 1819 that barnacles were actually crustaceans.

During the 19th century, the world's leading authority on barnacles was none other than Charles Darwin. Darwin spent 8 years studying, dissecting and classifying barnacles. The outcome of his efforts was a set of authoritative books. (Links to complete texts and scanned plates of Darwin's barnacle books and other publications are available on this page.)

His work on barnacles exposed Darwin to the extent of variation that exists in nature within and between species. Later, in On the Origin of Species, Darwin developed the idea that one species gradually evolves into another one. According to Ernst Mayr3, Darwin's studies of barnacles may have influenced the development of his ideas on speciation. Darwin himself noted this in his autobiography:

"The Cirripedes form a highly varying and difficult group of species to class; and my work was of considerable use to me, when I had to discuss in the 'Origin of Species' the principles of a natural classification."


1. Harold E. Vokes, John D. Glaser and Robert D. Conkwright. Bulletin 20: Miocene fossils of Maryland. Second Edition January, 2000. Electronic Publication No. 00-1. Available as a CD-ROM from the Maryland Geological Survey.
2. Lippson, A.J. & Lippson, R.L. 1984. Life in the Chesapeake Bay. Johns Hopkins University Press.
3. Mayr, E. 1991. One long argument. Harvard University Press.


Also posted at Transitions.

28 December 2005

The tree sees you

beecheye3

I have posted about beech eyes before. These are marks on the trunks of beech trees (Fagus grandifolia) where branches used to be. This is the best one I have so far noticed. It was on a relatively young tree. I suspect that as the trunk gets larger this particular eye will lose its almost perfect shape.

If we could only fit it with a monocle...

27 December 2005

Pardon my Latin

latin1

Yesterday at my favorite used bookstore, this little dictionary from 1900 was too irresistible to return to the shelf; despite the rather high price of $2(!), I bought it. Exitus acta probat*. So then, allow me to put my new acquisition to good use.

I have written about the slug genus Pallifera, native to the U.S. Their name comes from pallium (=cloak; here referring to the mantle) and fero (=to bear) to mean "mantle bearer", not a very distinctive name, because all slugs have a mantle. What about the name of the clausiliid land snail Idyla bicristata of Turkey and Greece? I don't know what Idyla means, but bicristata translates as "two-crested", in reference to the 2 keels on the back of the body whorl of the shell. This too isn't very distinctive, because many clausiliids from where it comes have 2 keels. Vertigo, the genus name of cute little snails, means "turning around"; likewise, quite generic. Quid faciendum?

But, here is something a bit more exciting: Anguispira is a combination of anguis (=snake) and spira (=spire, coil), presumably implying that the shell is coiled like a snake. This one is better: the name rotifer (phylum Rotifera) derives from rota (=wheel) and, once again, fero, to mean "wheel bearer". They were given this curious name, because the early microscopists thought that the ciliated disks surrounding these animals' mouths were turning wheels.

Well, that's it for today. Nec scire fas est emnia‡.

Before I forget, the dictionary indeed fits into the pocket of my vest.

latin2


*The result justifies the deed.
†What is to be done?
‡We are not allowed to know all things.

26 December 2005

Land snails of Turkey: Pyramidula chorismenostoma

Pchorismen1
Pyramidula chorismenostoma from Bodrum Peninsula, Turkey.

In adult shells of Pyramidula chorismenostoma (Pulmonata: Pyramidulidae) the body whorl separates from the rest of the shell. There is no other European land snail that has a similar shell morphology. The shells of P. chorismenostoma are quite small, about 2 mm in diameter. (Rulers visible in these photographs are in millimeters.)

Pchorismen2

The range of P. chorismenostoma extends from southern Greece to Crete and the Aegean islands1. We recently published its first record from the mainland Turkey.


Archive of the Land Snails of Turkey series.


1. Gittenberger, E. & Bank, R.A. 1996. A new start in Pyramidula (Gastropoda Pulmonata: Pyramidulidae). Basteria 60:71-78.

25 December 2005

24 December 2005

Land snails of Bodrum Peninsula, Turkey

In August 2002, we (Zeki, Burçin, Francisco and I) spent 8 days in and around the city of Bodrum, the ancient Halicarnassus, in southwestern Turkey collecting land snails. I have an account of our expedition and some pictures here. The results of the survey, after a few setbacks, finally got published1, ending this year on a high note.

On Bodrum Peninsula, we found 50 species of land snails, 38 of which were native to the area. Two of those species, Pyramidula chorismenostoma and Vitrea sossellai, had never been recorded from Turkey before. P. chorismenostoma is a unique snail and as soon as I get a chance to photograph my specimens, I will post some pictures here.

We found 3 species of Albinaria. Two species, A. lerosiensis and A. munda, are native to the peninsula. The 3rd one, A. brevicollis, is present only in the Castle of the Knights in Bodrum and was introduced there. That is a story in itself and will be the subject of a future post.


1. Örstan, A., Yıldırım, M. Z., Gümüş, B. A. & Welter-Schultes, F. W. 2005. The land snails of the Bodrum peninsula, Turkey. Mitteilungen der Deutschen Malakozoologischen Gesellschaft 73/74:1-15. (Download scanned pages here.)

23 December 2005

Love your hair (and equation too), dude!


Picture from Nature

Einstein's famous equation that equates energy to matter, E=mc2, is one of the fundamental building blocks of modern physics. In yesterday's Nature, Rainville et al.†, reported the results of their tests of the accuracy of this equation.

A direct way to test the validity of Einstein's equation is to independently measure the mass lost (Δm) during a particular process and the actual energy (E) released and then compare the latter with the energy equivalent of the former. By dividing both sides of E=Δmc2 by E, we get 1=Δmc2/E. The rearrangement of the latter gives 1-Δmc2/E=0. In other words, if the energy released and the mass lost during a process are equal as predicted by Einstein's equation, then Δmc2/E=1 and therefore, 1-Δmc2/E=0.

When an atomic nucleus captures a neutron (n) it moves up to an excited state (marked by an asterisk below). Upon returning to the lower energy ground state, the nucleus emits a gamma-ray photon. Using a sulfur (S) atom as an example, this can be represented as follows.

32S + 1n --> 33S* --> 33S + gamma-ray


Rainville et al., measured the mass difference (Δm) between isotopes of sulfur (and silicon) and unbound neutrons and the final isotopes incorporating captured neutrons. The multiplication of Δm with c2 (c=speed of light) gave the energy equivalent of mass lost during the above process. They also measured the energy (E) of the gamma-rays released. Their combined results obtained with sulfur and silicon gave the value of

1-Δmc2/E=-0.00000014


Because of various errors involved in measurements, no experimental result for the above equation would ever be exactly 0. Rainville et al., state that their result is "55 times more accurate than the previous best direct test of E=mc2".

Cool, dude!


†Simon Rainville , James K. Thompson, Edmund G. Myers, John M. Brown, Maynard S. Dewey, Ernest G. Kessler, Jr, Richard D. Deslattes, Hans G. Börner, Michael Jentschel, Paolo Mutti and David E. Pritchard. 2005. A direct test of E=mc2. Nature 438, 1096-1097.

22 December 2005

A tree with a grammatical error


Ant + Mat were here

21 December 2005

Evolution rules

In yesterday's post I introduced some of the evolutionary rules Bernhard Rensch listed in his 1959 book Evolution Above the Species Level. Rensch’s Rule No. 16 relates to the origin of terrestrial animals:

"16. Terrestrial animals could originate only after the development of the following characters: solid tissues and sturdy organs of locomotion (Archimedes' Principle: in water the body weight is decreased by as much as the weight of the volume of water which is displaced by it), organs for direct aerial respiration, and mechanisms preventing desiccation. As sensory epithelia can function only if kept in a moist condition, these organs had to be 'withdrawn' into the interior of the body."

Of the more than 30 known phyla of animals1, only 6 have terrestrial representatives (Platyhelminthes, Nemertina, Mollusca, Annelida, Arthropoda, Chordata). Ernst Mayr2 in Populations, Species, and Evolution (1970), citing Rensch’s book, attributed the relative scarcity of terrestrial phyla to 5 "facts".

"(1) Weight. Water is 800 times as heavy as air. Only animals with a strong skeleton or armor can become terrestrial. A jellyfish needs no support in water, but collapses completely as soon as it is brought on land. Ciliary locomotion, so common in water, is useless in air. Locomotion on land requires strong muscles.
(2) Protection against the environment. A land animal must be protected against the danger of drying out and against strong fluctuations of temperature. There is a high selective premium in favor of a tough skin, armor, or scales.
(3) The excretory system. Permanent life on land, in contrast to amphibious existence, requires the excretion of metabolic end products in such a manner as to reduce water loss to a minimum.
(4) The respiratory system. It must be possible for the land animal to take in oxygen directly from the air.
(5) Sense organs. On land, there is much greater need than in water for sense organs that are effective at long range, particularly among the rapidly moving land animals; hence land animals must develop long-distance vision and hearing."

Mayr appears to have extracted Nos. 1, 2 and 3 from Rensch’s Rule No. 16 quoted above, and based his No. 5 on Rensch's rule No. 11 that I quoted yesterday. I am not sure if and where in his book Rensch talks about the evolution of the excretory system in relation to terrestrial life.

I am assuming what Mayr meant by calling these "facts" is that he presumed that they are requirements that must be fulfilled during the evolution of a lineage of animals that are increasingly more adapted to living outside of water. Although I don't agree with everything he says, in future posts I will return to Mayr's facts Nos. 1-4 as they relate to the evolution of grades of terrestriality in gastropods. For the time being, I will only point out that Mayr's fact No. 5 that "land animals must develop long-distance vision and hearing" is not a requirement at all for life on land. Land snails, annelids (i.e., earthworms) and many arthropods don't have long-distance vision and hearing and I don't see why there can't be a completely blind and deaf land animal. Therefore, I am dropping "fact" No. 5 from further consideration.


1. The exact number of animal phyla depends on who is counting.
2. Curiously, Mayr wrote in Populations, Species, and Evolution that there were only 3 terrestrial phyla, missing the platyhelminths, nemertines and annelids. Some nematodes (Nematoda) that I often observe in land snail cultures and which are active at 100% humidity in the absence of liquid water may also be considered terrestrial, bringing the total number of phyla with terrestrial representatives to 7.

20 December 2005

Rensch’s rules

German zoologist Bernhard Rensch (1900-1990) studied birds, land snails and other animals and was one of the contributors to the evolutionary synthesis during the early to mid-20th century. He is best remembered perhaps by one of his books, Neuere Probleme der Abstammungslehre (Newer problems of the evolutionary theory) that came out in 1947, followed by the second edition in 1954. The latter edition was translated into English with some revisions and published in 1959 as Evolution Above the Species Level. A few years ago I inherited a copy of that book from a late friend’s library. For the past week or so, I have had it on my bedside table. Every nite I read a page or 2 before falling asleep.

Evolution Above the Species Level, now more than 50 years old, still offers some relevant, useful insights. Rensch apparently liked to come up with "rules" to explain many evolutionary patterns. In Chapter 4, Section D, titled "Rules of phylogenetic development", he discusses a number of them.

"One of these rules states that large terrestrial vertebrates must develop heavy columniform legs with disproportionately large bones, because with an increase in body size the volume of the body (and thus its weight) increases by the third power, while that of the bones as the supporting structures increases only by the second. Hence, such columniform legs can be seen in the large species of mammals in various orders: in elephants, rhinoceroses…

Moreover, one may state that sessile animals could evolve only in the water, because the eddying of food particles and spermatozoa towards the body cannot occur in the air, but only in a liquid medium."

After mentioning several others, Rensch goes on to list 20 more. For example:

"6. Animals, i.e. heterotrophic organisms, could not originate before the autotrophic organisms (i.e. plants) providing a source of food."

Number 6, like a few others, is quite obvious and derives from common sense. But when you start thinking about it, you realize that it is a very fundamental notion that cannot be violated without stretching definitions.

Likewise:

"11. Locomotion requires receptors which react to stimuli leading to food."

Other rules are more specific:

"15. In terrestrial animals, large body size can emerge only if an internal skeleton is developed, as an external one tends to become prohibitively heavy. Hence, there are not and never have been any really giant types of terrestrial Arthropoda…"

This is how Rensch explained his rationale behind these rules:

"The numerous ecological studies made during recent decades have proved that the structure of animals is definitely correlated with special modes of life and special habitats. These correlations may be summarized as more or less general rules, often applying to many animal groups. There are further rules pertaining to the functional-anatomical consequences of a certain mode of life."

And summarized his purpose as follows:

"These ecological and functional-anatomical rules may in turn be used to show the directedness of the phylogeny, from which certain lines of future evolution can be predicted.

They are intended to show how and why the evolution of organisms had to follow certain main lines."

Ernst Mayr in Populations, Species, and Evolution (another classic, now >35 yrs old, but that is still useful), extracted 5 of Rensch's rules relating to the evolution of terrestrial animals from marine ancestors to explain why there have been so relatively few transitions from the sea to land. That's a topic I have a special interest in and I will return to it in the 2nd post of this series.


Another short biography of Rensch is available here.

19 December 2005

New year (well, almost), new camera

I couldn't wait for Santa (I've been a naughty boy anyway) and bought myself a new camera, this time a digital SLR, the new Olympus E-500. Besides the 2 lenses that come with the camera as a kit, I also got a Zuiko 35 mm macro lens. Here is my meager review of the macro lens with some test shots I have taken with it.

My test object was a stage micrometer (Edmund Optics NT36-121), which is an etching of a millimeter (1000 μm) divided into 10 μm*. The picture below shows a photograph of the micrometer taken thru an Olympus SZ60 stereomicroscope. The 10-μm lines are clearly resolved.



Below is a photograph of the same using the E-500 with the 35 mm macro lens at its highest (or almost highest) magnification. Admittedly, the micrometer is not a good photographic subject, because the etching is very faint and, therefore lacks contrast. The photo on the left (A) is the original, while the one on the right (B) is after I increased its brightness and contrast in Photoshop (I did not change the sharpness).



The macro lens can't resolve the lines 10 μm apart; the closest lines it can resolve are about 50 μm apart. That is not bad at all when we realize that this is not a microscope. And I think the image quality is pretty good for a camera lens at such a high magnification.

The alternating blue and red lines visible in the background of Fig. B are known as Newton's rings. They are not an artifact of the camera, but are formed on the micrometer itself. Newton's rings are interference patterns created when 2 glass surfaces are pressed against each other (the micrometer is on a microscope slide under a thin cover glass).

When taking such highly magnified pictures the camera must be on a tripod or some other support. Otherwise, no matter how steady your hands are, the image will be blurred, because even the movements that may be imperceptible to you are magnified by the lens. This makes it somewhat impractical to use the camera for taking extreme close-ups in the field unless a tripod is available and the thing you intend to photograph is stationary. The picture below shows the set-up I used to photograph the micrometer.



A: homemade copy stand; B: E-500 with macro lens; C: spirit level; D: camera remote; E: rack & pinion mechanism from an old microscope used for fine focus; F: glass plate to hold transparent objects to be copied (negative, slide, etc.); G: very heavy, shot filled base; H: light source (originally for HP scanner shown on the left); I: screw into the camera's tripod socket (it all depends on that screw!).

I use the spirit level to assure that the camera and the object that is being photographed are both horizontal. Also, the shutter should be released with a remote especially if the shutter speed is slow. An additional potential source of vibrations is the movement of the mirror inside the camera, but that is harder to deal with.

I will review the camera itself on another post.


*This micrometer is used mainly to calibrate microscope eyepiece reticles. It would be impractical to use for direct measurements.

17 December 2005

Papers read this week

Musical accompaniment: Philip Glass: Circles, played by Arturo Stalteri on the piano.

Since I wrote about the possible impacts of deer overpopulation on forest snails and slugs, I have done additional reading, including the following 2 papers. Incidentally, I found pdf versions of both of these papers on the Internet, but didn't note the addresses. If you search for them you can probably find them too.

Rooney TP and DM Waller. 2003. Direct and indirect effects of deer in forest ecosystems. Forest Ecology and Management 181:165 176.
This is an informative review, but I am not going to discuss everything in it. One thing that I found interesting is the authors' suggestion that the response of some forest plants or animals to deer could be non-linear. For example, curve B in the figure from the paper (below) shows a hypothetical taxon whose abundance is at a maximum at an intermediate deer density.



Cote SD, TP Rooney, JP Tremblay, C Dussault & DM Waller. 2004. Ecological impacts of deer overabundance. Annual Review of Ecology Evolution and Systematics 35: 113-147.
This is also a useful review that stresses more or less the same points as the previous paper. The authors point out that although deer populations in North America have undoubtedly been growing, it is not known if the present deer populations are higher than those before European colonization. They also mention that large herbivores, such as deer, through heavy browsing can move forest plant communities from one stable state to another and that these alternative stable states are not readily reversible when the browsing pressure is reduced.

16 December 2005

North is where the snow is


Looking out my office window the other day, I noticed that the week-old snow was still lingering in almost identical patterns facing the same direction at the bases of the trees lining the median strip of the street. The obvious explanation was that the side of a tree facing north was where snow hadn't melted yet, because that side gets less exposure to the sun. To confirm my hunch, I took my compass and went out for a walk.


The white end of the needle points to north; ignore the red arrow.


This was a common sight throughout the neighborhood.


The compass needle points not to the geographic north, the North Pole, but approximately in the direction of the magnetic north, which is at the present located in Canada.


The red square marks the present location of the magnetic north.

According to a report at the National Geographic News, the magnetic north is moving from Arctic Canada toward Siberia at ~40 km (25 miles) a year and its rate of movement is apparently accelarating. At lower latitudes, and especially when determining on which side of a tree the snow is, the difference between the locations of the magnetic north and the geographic north, the magnetic declination, doesn't make much of a practical difference. But as one gets closer to the North Pole, the magnetic declination must be taken into account for navigational purposes.

15 December 2005

Flying mollusks

"Some species of fresh-water shells have a very wide range, and allied species, which, on my theory, are descended from a common parent and must have proceeded from a single source, prevail throughout the world. Their distribution at first perplexed me much, as their ova are not likely to be transported by birds, and they are immediately killed by sea water, as are the adults. I could not even understand how some naturalised species have rapidly spread throughout the same country. But two facts, which I have observed—and no doubt many others remain to be observed—throw some light on this subject. When a duck suddenly emerges from a pond covered with duck-weed, I have twice seen these little plants adhering to its back; and it has happened to me, in removing a little duck-weed from one aquarium to another, that I have quite unintentionally stocked the one with fresh-water shells from the other. But another agency is perhaps more effectual: I suspended a duck's feet, which might represent those of a bird sleeping in a natural pond, in an aquarium, where many ova of fresh-water shells were hatching; and I found that numbers of the extremely minute and just hatched shells crawled on the feet, and clung to them so firmly that when taken out of the water they could not be jarred off, though at a somewhat more advanced age they would voluntarily drop off. These just hatched molluscs, though aquatic in their nature, survived on the duck's feet, in damp air, from twelve to twenty hours; and in this length of time a duck or heron might fly at least six or seven hundred miles, and would be sure to alight on a pool or rivulet, if blown across sea to an oceanic island or to any other distant point."

Charles Darwin. 1859. On the Origin of Species by Means of Natural Selection

A recent post at Research at a snail's pace on land snail dispersal has prompted me to write this. I have already written briefly about the speculations that very small snails may be distributed by the wind. There is, however, quite a bit of evidence to support Darwin's prediction that birds may disperse mollusks.

The oldest published record on this subject that I have in my collection is an anonymous 1936 note from the Nautilus1 that reported that "a tiny fresh-water mollusk [sic], Succinea" had been found in the back feathers of a Vesper Sparrow, Pooecetes gramineus.

Rees2 cited several records of aquatic gastropods and bivalves recovered from the feet of birds. He also gave records of the land snails Vitrina pellucida in Europe and Succinea species in the U.S. that had been found in the plumage of migrant birds. Interestingly, he also gave records of both the land snail Pomatias elegans and several species of freshwater limpets found on the legs of bees and aquatic beetles, respectively.

Dundee et al.,3 reported finding Succinea unicolor among the feathers of the Woodcock (Scolopax minor), the Common Snipe (Gallinago gallinago) and the Whippoorwill (Caprimulgus vociferous) in Louisiana.

Vagvolgyi4 also discussed the dispersal of land snails by birds and gave citations to several relevant papers some of which I haven't read.

Baur & Bengtsson5 briefly discussed land snail dispersal by birds.

Wesselingh et al.,6 proposed 2 ways birds can transport live mollusks: attached to their feathers or feet or in their digestive tracts. They gave examples of live mollusks recovered from birds' feces. They also implicated dispersal by birds in the distribution of the freshwater snails Tryonia and Planorbarius.


Oxyloma retusa preparing for takeoff.



1. Anonymous. 1936. Succinea carried by a bird. Nautilus 50:31. [Note that Succinea is actually a land snail genus.]
2. Rees, W.J. 1965. The aerial dispersal of mollusca. Proc. Malac. Soc. London 36:269-282.
3. Dundee et al. 1967. Snails on migratory birds. Nautilus 80:89-91.
4. Vagvolgyi, J. 1975. Body size, aerial dispersal & origin of the Pacific land snail fauna. Systematic Zoology 24:465-488.
5. Baur & Bengtsson. 1987. Colonizing ability in land snails on Baltic uplift archipelagos. Journal of Biogeography 14:329-341.
6. Wesselingh, F.P., G.C. Cadée & W. Renema. 1999. Flying high: on the airborne dispersal of aquatic organisms as illustrated by the distribution histories of the gastropod genera Tryonia and Planorbarius. Geologie en Mijnbouw 78:165-174.

14 December 2005

Puzzling Wednesday

A legendary city was said to have been built on seven islands that were connected to each other and the mainland as follows: Each island had the same number of connections as its rank in size. So, the largest island had 7 connections, the second largest 6, and so on until the smallest island, which had only 1 connection. And the mainland had but one connection to one of the islands.

If the legend is taken literally, explain why no such city could have existed.

This should be easy. I will post the answer in comments tomorrow.


Modified from puzzle #83 in C.R. Wylie, Jr., 101 Puzzles in Thought & Logic, Dover Publications, 1957.

13 December 2005

Squirrel tracks in snow


When I first saw these tracks last Friday, I ruled out deer and thought they had been left by a rabbit. But after I started following them, I noticed that they were going from one tree to another, which suggested that they were made by a gray squirrel, Sciurus carolinensis. In some of the tracks I could see faint imprints of toes.


Then I noticed some paw prints near one of the tracks (below). These were so delicate and faint that if I hadn’t been paying attention, I’d have stepped right on them. These prints identified the animal definitely as a squirrel. Compare them with my photo of squirrel prints left in cement.


The drawing below should explain how a squirrel creates its track pattern.


Drawing from Anonymous. Animal Tracks, Stackpole Co., 1954.

12 December 2005

Deer tracks in snow


Drawing from Anonymous. Animal Tracks, Stackpole Co., 1954.



Around here in Maryland the only deer we have is the white-tailed deer, Odocoileus virginianus. Their characteristic 2-toed prints left in mud or snow are easy to identify even when the prints are old or distorded.

It snowed here last Thursday nite. While hiking in the woods over the weekend, I ran into deer tracks everywhere. The picture on the left shows one of the tracks. Although most prints in the snow were nothing more than just elongated holes, there were always some that clearly showed the impressions of 2 toes, and thus, identifying the animal as the white-tailed deer. By comparing these prints with the drawing above, you can tell which way the deer was going.




Below is another set of deer tracks in addition to those of a dog. The dog had walked along the hiking trail visible near the righthand margin and 1 or more deer had crossed the trail at right angles. Again, the 2-toed deer prints are unmistakable.


At some point during Thursday nite, the snow was mixed with rain. You can see the poke marks left on the snow by the raindrops.

10 December 2005

Papers read this week: Zoology in the Middle East 35

Musical accompaniment: Jethro Tull: Thick As A Brick.

Zoology in the Middle East is published by the Kasparek Verlag in Heidelberg, Germany. The Journal specializes on the ecology, zoogeography, biology, systematics and taxonomy of the animals of the Middle East. The coverage area extends from the Arabian Peninsula thru Turkey and Iran to Armenia. The journal comes out twice a year with each issue, about 120 pages long, labeled as a separate volume.

The latest volume, 35, came out in October. The following are some of the articles from that volume that I found interesting.

Yousef, M. A. & Amr, Z. S. 2005. Altitudinal stratification and habitat selection of rodents in Dana Nature Reserve, Jordan. Zoology in the Middle East 35:13-18.
The authors collected or observed 12 species of rodents in a 229-km2 nature reserve in southwestern Jordan. There are 4 types of vegetation zones in the reserve: Mediterranean semi-arid forest, Irano-Turanian steppe, Acacia subtropical and sand dune desert. The Mediterranean semi-arid forests had the most number of species (7), while the Irano-Turanian steppes the least (3). My main criticism is that very few specimens of most species were collected to reach sound conclusions about their habitat preferences. For example, only 2 specimens of Gerbillus gerbillus were collected, both in sand dune deserts. That may very well be the usual habitat of that species, but I would hesitate to make any generalizations based on such a small sample size. Furthermore, 4 of the species were only "observed", but the authors did not indicate how many times each was seen.

Bilecenoğlu, M. 2005. Observations on the burrowing behaviour of the Dwarf Blaasop, Torquigener flavimaculosus along the coast of Fethiye, Turkey. Zoology in the Middle East 35:29-34.
The Dwarf Blaasop, Torquigener flavimaculosus, is a species of pufferfish that is a Lessepsian migrant in the Mediterranean Sea. When scared, the fish buries itself in the sand until only the eyes and a portion of the back are exposed.


The Dwarf Blaasop (left) buried in the sand with only its eye showing (right). Photographs are from the cited paper.


Seçkin, S. & Coşkun, Y. 2005. Small mammals in the diet of the Long-eared Owl, Asio otus from Diyarbakır, Turkey. Zoology in the Middle East 35:102-103.
The authors examined the remains of mammals in 211 pellets of the Long-eared Owl collected over a year in one location in southeatern Turkey. They identified 8 species of mammals from the remains recovered from the pellets. The vole Microtus guentheri comprised about 71% of the prey remains.

Haddad et al., 2005. Record of Sphecophaga vesparum, a natural enemy of Vespa orientalis in northern Jordan. Zoology in the Middle East 35:114-116.
The Oriental Hornet, Vespa orientalis, apparently a widespread species whose range extends from southern Europe, thru Middle East to India, preys on various insects, including honeybees and is an apicultural pest. The nests of the hornet in Jordan were infested with the parasitoid wasp Sphecophaga vesparum. The authors emphasize the potential role of S. vesparum for the biological control of V. orientalis.

08 December 2005

Sachem (Atalopedes campestris)


If I am not mistaken, this is a female Sachem (family Hesperiidae), one of the widespread skippers. Its range extends from the eastern U.S. and southeastern Canada to California in the west and to Mexico and Brazil in the south.


Both pictures are of the same individual. I photographed it near Lake Artemesia, College Park, Maryland last September. Females are identified by the chevron pattern on the lower sides of their hindwings and 2 large white spots on the upper sides of their forewings1.

Some good pictures of the Sachem are available here.


1. Glassberg, J. Butterflies through Binoculars. The East. Oxford U. Press. 1999.

07 December 2005

A shell on a log


During a hike in the woods last Sunday, I saw some fungi growing at one end of a large dead tree on the ground. After I took some pictures of the fungi, I was about to leave when I noticed on the trunk what I first thought was a large nut. Then I realized it was not a nut but a shell of Neohelix albolabris, the largest land snail around here.

The shell was empty. The picture above shows it the way I found it, with its aperture up. I don't know how it ended up where it was. But I can think of 3 possibilities:

1. A person found it on the ground and left it there.
2. The snail crawled up on the log and for whatever reason died there. Eventually it rotted, leaving behind its empty shell. They do have a tendency to die with their apertures up.
3. A predator found it on the ground, brought it up on the log to eat it. Then, either it was able to pull the snail out of its shell without having to break it, or the shell was empty and the animal realized it and left the shell on the log.

At this point, your guess is as good as mine.

06 December 2005

Do overpopulated deer influence forest snail populations?

Last Sunday's post by Nuthatch at bootstrap analysis reviewed a recent paper by Allombert et al.,1 on the impact of overabundant deer on forest invertebrates, mainly arthropods, but also on land mollusks (gastropods). Nuthatch's emphasis was on the potential indirect and negative effects of overabundant deer on migratory songbirds. I cannot comment on that aspect, nor on the conclusions of the study regarding arthropods. Instead, I will criticize only the results obtained with snails and slugs.

The subject study1 was carried out on the Haida Gwaii archipelago (Queen Charlotte Islands) 80 km off British Columbia, Canada, where Sitka black-tailed deer (Odocoileus hemionus sitkensis) were introduced in 1878. The authors determined the abundances of various arthropods and land mollusks on 6 islands, 2 of which did not have deer. Invertebrates were collected with pitfall traps consisting of "two plastic cups fitted together with a light gray ceramic tile supported by small sticks as cover". To kill anything that fell in them, the traps were filled with a 1:1 mixture of water and ethylene glycol with a few drops of detergent.

The authors determined that "gastropod species density (significantly) and abundance (markedly but not significantly) decreased with increasing [deer] browsing history". I have 2 objections to this conclusion.

1. The most commonly used, and perhaps the most effective, land snail collection method is leaf litter and soil sieving. (This is also the most labor intensive and time-consuming method.) I have never used pitfall traps for snails and I don't know how efficient they are, compared to soil sieving, to determine the land snail fauna of an area. Arthropods presumably do fall into pitfall traps, but snails and slugs, on the other hand, may simply crawl out of the trap before they contact the liquid on the bottom, especially if they are repelled by ethylene glycol. (I don't know how snails and slugs react to ethylene glycol.)

The point is that the study may have missed some of the snail and slug species present on the islands. I am assuming nobody had surveyed before for land snails on the Haida Gwaii archipelago and so there is probably no other survey data to compare with. (The islands sound like wild, isolated places. I would love to visit them one day).

2. They only identified 9 gastropod species, but, unfortunately, they don't give a full list of names. Nor do they tell which species were found on which islands. I would like to know if any of the gastropod species were themselves introduced to the islands by humans, because if they were, then this would be a factor in determining their presence or absence on a particular island.

My main problem is that 9 species is a low baseline. If one island without deer had 9 species of snails and another with deer had, say, 5 species, could one really be confident that the difference is due to deer? Maybe, maybe not.

The authors cite a study by Suominen2 that obtained somewhat similar results. In the cited study, Suominen studied the effects of reindeer and moose grazing on land mollusks in the Finnish Lapland in 23 paired plots each up to 30x30 m. In each pair, one plot was fenced to keep large animals out. One confounding factor is that the fences had been placed at different dates, one dating from the 1940s, while others from the late 1980s. Suominen too used pitfalls to collect gastropods, although he was aware that pitfall trapping “is not the optimal method for sampling terrestrial gastropods”. Thirteen species of gastropods were found, but less than 20 individuals were collected for each of 6 species3. Of the remaining 7 species, the abundances of 2 did not differ significantly between grazed and ungrazed plots, 4 species were more abundant on ungrazed plots and one species (Zoogenetes harpa) was actually more abundant on grazed plots. From these results, I would hesitate to derive any generalizations as to whether or not grazing by large mammals is impacting land mollusk populations.

Nevertheless, what these studies demonstrated may indeed be true as far as the interactions of large grazing mammals and the snail faunas of high latitude forests are concerned. But I don't think they are relevant, as far as snails and slugs are concerned, for lower latitude forests, in which I am mostly interested.

In 2nd growth Maryland forests, where the white tailed deer (Odocoileus virginianus) is native and overabundant, I routinely find more than 20 species of snails and slugs with the total approaching 30 in good places. Of course, we have no way of knowing which species lived in the virgin forests that were there 200 years ago. But based on the distribution patterns of all species and some old records, I would guess that the land mollusk species compositions of virgin forests were probably close to what we find today in 2nd growth forests. In other words, deer overabundance hasn't probably had any effect on snails and slugs. In any case, even if we knew that there were differences in the faunal compositions of 2nd and old-growth forests, it would be difficult to single out the responsible factors.

Moreover, most snail and slug species of 2nd growth forests concentrate on and around deadwood (large fallen trees or snags), where they find both shelter and food (rotting wood and fungi). I can't think of a mechanism how deer overabundance could affect the quality and quantity of large dead trees.


Note added 7 December 2005: While I was writing this post yesterday, I sent an e-mail to Otso Suominen and asked for a copy of his 1999 paper. Later I went ahead and posted what I had written without waiting for his response. Suominen has since kindly sent me pdf copies of not one, but 2 of his relevant papers. Therefore, today I have revised parts of my post dealing with Suominen's study. My conclusions from yesterday remain unchanged.

For another update, please read this post.


Appreciations to Nuthatch at bootstrap analysis for bringing the paper by Allombert et al., to my attention and Tim Pearce for quickly sending me a pdf copy of it and Otso Suominen for copies of his papers.


1. Sylvain Allombert, Steve Stockton & Jean-Louis Martin. A Natural Experiment on the Impact of Overabundant Deer on Forest Invertebrates. Conservation Biology 19:1917-1929
2. Suominen, O. 1999: Impact of cervid browsing and grazing on the terrestrial gastropod fauna in the boreal forests of Fennoscandia. Ecography 22: 651-658.
3. I am hesitant to attach any significance to results obtained with less than about 20 individuals, which is obviously an arbitary cut-off point.

05 December 2005

Benchmark No. 20300


This survey marker is in Black Hill Regional Park, near Boyds, Maryland. I discovered it in April 2000 while exploring the meadows near the southern boundary of the park. I returned last Friday to take these pictures. The marker itself, a metal disk embedded in concrete, is less than a meter† in front of the so-called witness post.


The engraving on the marker was difficult to read in the field, because the marker had gotten soiled from being buried under a thick layer of grass. By enlarging the picture on the computer screen I was able to make out most of the pertinent information, which states that this is a W.S.S.C. (Washington Suburban Sanitary Commission) First Order Control Station, number 20300. The abbreviation "B.M." stands for "benchmark". I don't know what "T.S." and "MC" stand for.

I also measured the coordinates at the spot with my Garmin eTrex, visible in the first picture. The data sheet‡ for this mark gives its coordinates as 39° 11’ 07.27002’’ N and 77° 18’ 29.63964” W, while I measured 39° 11’ 07.3” N and 77° 18’ 29.6” W. They seem to be pretty darn close, but it is difficult to relate to latitude and longitude. I find it easier to work with UTM coordinates, which are in meters. Again, the data sheet gives the UTM coordinates (for zone 18) as 4,339,883.768 m north and 300,637.576 m east. In comparison, I measured 4339884 m north and 300,639 m east, which place my location about 1.42 m east and about 0.23 m north of the mark.

Finally, the data sheet gives the elevation as 125.8 m, while I measured 120 m.

When I am doing land snail surveys, I try to measure the GPS coordinates of every one of my collecting stations. I am hoping that the coordinates will help me or others to return to the same spots in the future. This exercise shows that my coordinate measurements are accurate enough for my purposes. In this case, what contributed to the accuracy of my measurement was that the sky was unobstructed by trees or nearby hills. Under tall trees, especially when they have leaves, or in deep and narrow canyons, the accuracy of GPS measurements goes down; sometimes I can't even get a fix on satellites.

You can download a list of all the registered survey marks in your area from the National Geodetic Survey’s database. Many of the marks around here seem to be in residential neighborhoods. There are, however, a few in more rural and isolated locations, this being one of them. Whenever time and weather permits, I will try to locate more of them.


Little Seneca Lake photographed facing north from near the survey marker.



†The data sheet gives the exact distance from the witness post to the mark as 0.65 m.
‡If you live in the Germantown area and want to find this survey marker, its data sheet provides detailed directions to the location. Alternately, you may let your GPS receiver, if you have, direct you to the spot. It would be a fun activity.

03 December 2005

Papers read this week: Triton No. 12

Musical accompaniment: Steve Reich: The Desert Music.

Triton is the journal of the Israel Malacological Society. It is published twice a year, usually around March and October.

Triton publishes in English articles of interest to malacologists and shell collectors. To subscribe or to submit a manuscript to Triton contact E. L. Heiman, Editor-in-Chief, e-mail: heimel AT netvision.net.il

The last issue of Triton, No. 12, was published in October. My copy arrived about 2 weeks ago.

Contents of Triton, No. 12

1. Marine molluscs

Jean & Janine Demartini
Erosaria turdus thrives in the Mediterranean Sea.

B.S. Singer
Thais sacellum and Ergalatax obscura, new immigrants to northern Israel.

Hadas Lubinevsky & Henk K. Mienis
A first record of Nanostrea exigua Harry, 1985, another exotic mollusc species from the eastern Mediterranean.

Y. Sharon, Y. Benayahu & H.K. Mienis
First record of an exotic oyster: Alectryonella crenulifera, from the Mediterranean coast of Israel.

The alien species that have entered the Mediterranean Sea from the Red Sea via the Suez Canal are called Lessepsian migrants. This species, a native of northwest Indian Ocean, is one such species.

E.L. Heiman
Mauritia maculifera hawaiiensis new subspecies.



E.L. Heiman
Intraspecific variation in Bistolida stolida (Linnaeus, 1758) .


2. Land snails and fresh-water molluscs

Alberto Girod
New data on quaternary freshwater and land molluscs in the Sahara.

Motti Charter & H.K. Mienis
Snails in pellets and prey remains of kestrels (Falco tinnunculus) in Israel.

Hartwig Schütt
A buliminoid land snail from the east Anatolian high mountains (Pseudochondrula maden n.sp.).

About half of the specimens of this species are dextral and the other half is sinistral. Based on the measurements of 10 shells of each type, the sinistral shells appear to be slightly larger. This species may provide a good model to study the appearance and maintenance of oppositely coiled snail populations in the wild.

H.K. Mienis & Uri Bar-Ze'ev
On the presence of Buliminus glabratus in the northern and western Negev.

02 December 2005

Something close to normal (for a change) in the backyard

In yesterday's post I presented some shell measurements I had obtained with the land snail Vertigo pygmaea that lives around my house. To construct a histogram of shell heights, I combined all 3 sets of data†. The resulting histogram is below. I haven't done any statistical tests, but visually, the distribution of the shell heights of V. pygmaea seems close to being normal.


The blue curve is my combined measurements of Vertigo pygmaea; the red curve is a normal curve with the same mean and standard deviation. I constructed the normal curve using a normal probability calculator. The total number of specimens for each curve is 132.


The graph below shows the area under a theoretical normal distribution. For our purposes, this graph tells us that in a sample of snail shells, 99.7% of the measured heights are expected to be within plus or minus 3 standard deviations (SD) of the mean. In more practical terms, one could expect to find on the average 3 shells outside the (mean±3xSD) range out of about every 1000 shells collected. However, this is not a hard rule; it only tells us what things will be like on the average in the long run. In fact, on a few occasions, I have found very small or very big shells in samples much smaller than 1000 specimens. It is, however, good to keep in mind these theoretical limitations expected from a normal distribution. For example, if I happened to find, in a sample of 100 shells, say, 3 shells whose heights were <(mean-3xSD), I'd strongly suspect that something was going on. For example, the measurements might be wrong, the sample might be biased (the small shells might have been selectively taken from a larger sample), the distribution might not be normal, the shells might have been broken and repaired‡, the small shells might belong to another species, etc.


Area under a normal curve. From Brase & Brase, Understanding Basic Statistics, Houghton Mifflin, 1997.


My combined sample of 132 shells of V. pygmaea has a mean and SD of 1.89 mm and 0.085 mm, respectively, giving a (mean±3xSD) range of 1.64-2.15 mm. The shortest (1.65 mm) and the longest (2.12 mm) shells are within the range and quite close to the "limits".

I have written about miniaturization in land snails (here and here) and the smallest land snails. I don't know how small V. pygmaea can get. I will collect another sample for measurement about 2 years from now or sooner, if I chance upon large numbers of them. It follows from the argument above that in a large enough sample, I should find a shell smaller than what I have so far measured.


†Although this is in general a questionable practice, I am justifying it in this case, because all the specimens were from more or less the same locality (I don’t have a large yard), the time span was relatively short and the mean and the standard deviation values of the samples were close enough to each other.
‡Repaired shells sometimes end up being smaller than they were before. I have an example
here.

01 December 2005

Data from the backyard

Measuring snail shells, not just one or two, but lots and lots of them, is a passion of mine. Measuring as many specimens as possible (up to a limit) provides statistically more meaningful data. But I don’t like killing large numbers of snails and refrain from doing so if I can. One good argument against permanently removing large numbers of live snails (or any other animal) from a colony that is under long-term surveillance is that the removal of animals could unnaturally influence the future values of the characteristics that are being studied. So I usually measure empty shells and wherever there are large numbers of empty shells of one species, I try to collect as many as I can carry.

I have written about the tiny snail Vertigo pygmaea that lives in my backyard. Actually, they live not just in the backyard, but all around the house. Having them near my house is convenient, because I can collect even live snails, measure them and then return them within a day or two to where I got them from. That way, the snails are not harmed and I have my data. The graph below shows some of the data I have obtained with V. pygmaea since 2001.


These are "box and whisker" plots for shell heights of Vertigo pygmaea from my yard measured on 3 occasions. The top and bottom of each box are the 75% and 25% quartiles and the box encloses the middle 50% of the data for a sample. The horizontal line inside a box shows the median. The horizontal lines (“whiskers”) end at the minimum and the maximum values (there are other ways of determining where the whiskers will end). The numbers above the boxes are the number of specimens measured in each sample. I drew this plot using PAST, a free and user friendly statistics program specifically designed for paleontologists.


This graph shows that the median shell length has been stable during the study period. Although, because of the enlarged span of the horizontal axis, the median values seem to vary, the actual values, 1.89, 1.90 and 1.87 mm, can all be rounded off as 1.9 mm.


Continued here.