Ron's Musings

those bloodsuckers, bar none – Pyramidellid snails.   

Whether or not I develop this article fully depends on a lot of things, not the least of which the direction my muse (and I do have one – a characteristic I find “amusing”…  🙂 ), takes me.  And also, of course, whether or not, I can find an editor who will accept it.  Anyway I am still in the very early stages of this thing, and I am trying to find some more background information about Pyrams. 

Now, like Yogi, I figure I am a bit “better than the average bear” with my knowledge of the group, but I really need to know more before I do anything that can be considered to be a reasonable and – hopefully – entertaining article about them.

Basically, the Gastropod family Pyramidellidae is a very large group of small… white… snails without any really nice identifying characteristics.  There are, quite literally, thousands of names applied to these snails, and how those names relate to the actual species – if any – of those snails is really open to question.  As with many marine groups – probably as with MOST marine groups – of animals, they are a taxonomic mess.  There are three basic reasons for this, the primary one is that they are all – dare I say it, again – small,  white, and feature-less snails.  If you think about it, once you have the basic constraints of a small, helically-coiled, shell, there are NOT a lot of ways that those constraints can be varied.   In point of fact, back in the 1960s, a paleontologist named David Raup, wrote a series of nice papers noting that virtually all marine animals with a “shell” can be easily described mathematically (Raup, 1962; 1966; 1969). 

The term “shell” in this sense is restrictive and excludes arthropods – even though they are “shellfish” they don’t have a “shell” in the way that Raup meant: “a non-living external covering to the animal.”  What arthropods have is an “integument” – a covering that is made of an intricate and complex fusion of living and secreted elements that is an integral part of the animal’s body, interiorally and exteriorly.  If you try to remove a shrimp’s “shell,” you are left with a mass of dead flesh.  If you try to remove a snail’s or a clam’s shell, you will find that, in many cases, it is quite possible to do this and still have a living animal.  And that animal may remain alive – in the case of some snails – indefinitely.  Clams without a shell perish in short order, as they can’t feed.  But snails without a shell can generally do everything shelled animals can do.   Everything, that is, except withstand the bite of a predator’s jaws.  Virtually every biologist who studies large populations of marine snails occasionally finds a shell-less one; probably an animal whose shell was eroded away by a sponge, or whose shell was cracked by a crab and, by a miracle, the shell was removed.  As an example, over the course of three years, I encountered two almost-naked specimens in one species, Ophiodermella inermis, that I worked on many years ago.

Ophiodermella inermis, photographed in Dyes Inlet, Washington.

Anyway… Raup found that virtually all mollusk and brachiopod shells were basically helical in shape, and that all fundamental shell shapes could be described by only three parameters, 1) the shape of the shell’s aperture, 2) the rate at which the shell’s aperature coiled around the central axis, giving the width of the animal, and 3) the rate at which the coiling moved along the central axis which determines the animal’s length.  As all of the functions occur simultaneously, how each of the parameters varies in relationship to the others changes both the absolute and relative proportions of the shell’s shape.

This diagram below shows how the shell parameters determine the difference in shell shapes for some snails commonly found in marine reef aquariums. The top shows a Nerite; here the shell aperture just moves in a spiral around the axis of coiling and moves outward at a rate that is not very large.  This results in a spirally-coiled shell where the whorls appear to overlap.  The other shells are trochoideans.  And here the parameter of “aperture shape” is circular.  The aperture moves around the axis of coiling at various rates – giving a turbinate or trochoid shape.  Additionally, the aperture moves along the axis of coiling determining whether the shell is squat or elongate.  Although the flat Stomatella seems to be very different from a conical Trochus in structure, with a little thought, it is easily possible to see how those shell shapes are related.

.Shell parameters illustrated by various snail shells; all shells grow in three dimensions but the position of the aperture in each successive whorl may change due to only 3 parameters. A. Growth of a shell where the aperture shape moves in a spiral within one plane; the nerite shell is an example of this. B. Here the aperture moves along the axis of coiling but remains tangent to that axis; the illustrated turbinid is an example of that growth. C. Here the aperture moves along the axis of coiling, but also translates or moves outward from that central axis. An opening called an umbilicus in the center of the shell is the result. The illustrated trochid shows this growth form, with the earlier positions of the growth generating aperture shown in gray. The shell of Stomatella is auriform, an extreme example of the prominence of the whorl translation rate.

Given that all molluscan shells can be basically described in this manner, if shells lack much distinctive coloring, sculturing, or extra ornamentation, it should be apparent that there really are not a lot discrete characteristics that can be used to uniqually differentiate any given shell from all others that are basically similar in shape.

That lack of potential characters is the first factor that has caused taxonomic problems with pyramidellid shells.

The second and third problems were two malacologists, William Healey Dall and Paul Bartsch.  Dall was the first curator of mollusks at the United States National Museum (aka the Smithsonian Institution).  Bartsch was his “disciple” and successor.  As curators at the Smithsonian, a lot (really A LOT!!!) of specimens (= dead shells) were sent to them by various collectors.  As they tried to identify these shells, they often found that the shells were not “quite” like shells in the museum’s collection – or in other museums’ collections.  This meant that these shells were then new to science and needed to be taxonomically, or scientifically, described.  Virtually all animals that are described from United States regions or by American authors have representative or “type” specimens deposited in the Smithsonian’s collections, so they had a lot of comparative material.  Presently, that is a huge number of specimens, several millions.  Back in Dall’s time, the collections were a lot smaller… but still relatively very large…  So, they had a lot of material with which to compare any given specimen in their descriptions.  Thus, if they decided a given specimen was a “new” species the odds would seem to favor the fact that it was new. 

Maybe…

Typically what these two ol’ boys did, was to let specimens accumulate until they had enough to write a short (or sometimes very LONG paper describing them all).  At the time, the custom was to describe a molluscan species on the basis of one shell. 

ONE. 

THE

TYPE.

The type was supposedly an “average” or representative shell that “typified” the species.  By the way, Dall and Bartsch were by no means alone in the way in which they described “their” species.  It was the standard method of the era.  They were, however, especially prolific, and it seems, especially “gifted” with the inability to find unique precise and useful descriptive terms.  Simply put, many of their descriptions are “precisely ambiguous,” they are written in ways that seem to precisely describe the specimen, but which don’t allow a reader to determine if a given questionable shell that they are holding in their hand is from that unique species.  I think this was because they didn’t include much or any information addressing variations between specimens of the same species.

And what about those variations?  Well, Dall, at least paid lip service to the concept of variation, but he generally didn’t include any useful way to describe variations in the species he described.  Any good field biologist knows that variation is quite literally “the stuff of life.”  For example, for little white snails, such as pyrams, if one collects one hundred specimens from a known species, they will vary in length, width, the number of whorls, slight color variations occur, the proportion of length to width will vary a bit, the number of ribs on whorl may vary, and on and on and on…  One specimen really can’t do it.

Nonetheless, the conception of a species at the time, which was reinforced by rules of Linnaean taxonomy implied that there was no variation in a species.  This was a result of considering all species to be divinely created.  If a creator designed each species, he/she/it/they would obviously get it right the first time and there could be no variation.  That was all fine and dandy up until the publication of The Origin of Species in 1859.  Once the concept of evolution became established, the concept of a species HAD to include variation.  And so it did.

In concept.

But in practice…  Well, let’s just say the idea of variability in a species was not an easy one to get across.  Modern descriptive statistics is just that, modern, so at that time there was no formal way to estimate variability.  The concept of a standard deviation wasn’t there.  Even including a range of sizes in a species description was rarely done.

In essence, for any species – the idea of that species was “crystalized” within a single typical specimen, the type.

Over his lifetime, Dall described 5,302 species in every group of animals defined at the time, from mammals to mollusks.  Most, however, were snails.  Bartsch described an additional 905.  The heyday of these descriptions extended from the 1870s through about 1925 for Dall (several years after he died, actually, as Bartsch published some of Dall’s descriptions after his death).  Bartsch retired in 1945 and died in 1960, and I think his last taxonomic publications were in the 1950s.

Modern day molluscan taxonomists who work in the Pacific where these men had the largest taxonomic effect have a love-hate relationship with them.

Love…about 0.000001%,  Hate… well, do the subtraction.

Simply put, it is essentially impossible to differentiate most small snail species described by these men.  They gave lip service to the concept of species variability, but using the typological approach and their writing style such variability was impossible to descibe.  Interestingly enough, that is not the case with many other prolific describers of molluscan species that were writing at the same time.  Henry Pilsbry was another malacologist who described a lot of species; according to his article in Wikipedia, he wrote over 3,000 scientific papers and described over 5,000 species.  His descriptions have stood the test of time well, so the problems with Dall and Bartsch were due to Dall and Bartsch.

How bad is the situation, really? 

Pretty bad.   Really!

For example, if a large collection of pyramidellids or other small snails is taken from one bay or locality, their parameters will vary, of course.  If those specimens can be determined by other means (ecological parameters, for example) they are all found to by one species, it would be nice – satisfying even – to put one valid name on them.  And… by examining the works of Dall and Bartsch, one can often find numerous – perhaps several dozen – entirely satisfactory species names that will fit within that single collection of specimens from a single species from a single small bay.

Amongst the references I downloaded (thanks to Google’s digitizing, many books in the public domain are downloadable) was:

Dall, W. H. and P. Bartsch. 1909. A monograph of West American Pyramidellan Mollusks.  United States National Museum, Bulletin 68. 1-258 pp, 30 pl.

This monograph contains 258 pages of species descriptions…  And 30 plates of illustrations…. As an example , take a look at this one.

This is one of many plates of illustrations showing species of Turbonilla.  Turbonilla is one of the genera of pyrams that has species that will attack Tridacna.  Do you think you could use such images as these to differentiate between any two them reliably?

Could anybody?

When I discuss specific animal groups in my articles for the reef aquarium hobby, I like to give examples of those species…

CORRECTLY IDENTIFIED examples of those species.

And, please excuse me, so that I may now go and start beating my head against the wall.

Because it will feel soooo good when I quit. 

An aside… 

Oh… for those of you who might know mammals, the Dall sheep and the Dall porpoise were named after William Healey… AND here is something I will bet you probably didn’t know.  His name “Dall” was pronounced rhyme with “Gal,” NOT “Gall,” as it is most often used.   See… I even give you a piece of “party” trivia to amuse your friends with.  Of course, if you do so, you risk never being invited to such parties again.

References:
Raup, D. M. 1962. Computing as an aid in describing form in gastropod shells. Science. 138:150-152.

Raup, D. 1966. Geometric analysis of shell coiling: general problems. Journal of Paleontology. 40:1178-1190.

Raup, D. M. 1969. Modeling and simulation of morphology by computer. Proceedings of the North American Paleontological Convention. September 1969:71-83.

Until later,

Cheers,

Tags: aquarium, Bartsch, Dall, ecology, gastropoda, identification, invertebrates, mollusca, pyram, pyramidellidacea, resesearch, taxonomy

This entry was posted on Tuesday, January 11th, 2011 at 07:51 and is filed under Reef Aquarium Questions Answered. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.