Wednesday, November 27, 2013

Caecum ~ Stages in a Life

In its old age, does the Caecum snail think back on its life in the seas, on the shells discarded, from when it was first enveloped in tiny, tight whirls, a mere “protoconch,” to when it reached some semblance of its adult self having broken free of those curls, and finally to when it enjoyed full maturity, living within a grand shell, all two millimeters or so of it?  I suppose not.

Still, it’s a life history with a wonderful trajectory, though one that is potentially misleading.  Paleontologist William Healey Dall, writing of the family to which the Caecum genus belongs, noted with some wry humor:
Owing to the remarkably different aspects these forms assume at different stages of growth in the same individual, they are particularly well adapted to lead the unwary into error.  (Contributions to the Tertiary Fauna of Florida, Part II, Transactions of the Wagner Free Institute of Science of Philadelphia, Volume 3, Part II, 1892, p. 295)
The error is to identify the different stages in that history as separate species.  Initially, the gastropod lives inside a very small coiled shell.  As the animal grows bigger, the shell extends, still somewhat curved or arcuate.  The whorled end, now vacated, breaks away; the animal moves into the remaining section which is plugged at the break.  In the last stage, a similar transition occurs, as a juvenile section drops off and the mature shell is occupied and plugged.

Just how many stages the Caecum goes through may, perhaps, be in the eyes of the beholder.  Some experts, such as zoologist Harald Alfred Rehder as well as Dall, described three, while geologist Lyle D. Campbell described four.  What appears to be at issue is how many phases separate the protoconch from the adult.  I will come back to this at the end of this post.  (Rehder, National Audubon Society Field Guide to Shells, 1981; Dall, Contributions to the Tertiary Fauna of Florida, p. 295 – 296; Campbell, Pliocene Molluscs From the Yorktown and Chowan River Formations in Virginia, Virginia Division of Mineral Resources Publication 127, 1993, p. 61 – 62.)

My introduction to the Caecum came through an articulated fossil clam shell given to me by a friend.  This shell, some 12 million years old, was found in the Boston Cliffs Member of the Choptank Formation.  Though it always feels a bit sacrilegious to separate any two valves that have endured together for millions of years, it’s the only way to reach the treasures that might be held inside.  So, I opened the clam and, scattered in the sandy matrix, were many minute, white, tubular shells, totally new to me.  One appears below.

This fossil shell is from Caecum patuxentium, an extinct species of the Caecum genus.  The genus itself offers a rich array of living species, perhaps 100 or so, and an additional roughly 25 that have gone extinct.  Scottish naturalist John Fleming first named the genus in 1813 in his Conchology entry in David Brewster’s Edinburgh Encyclopædia (Volume VII).  Though he gave no explanation of his choice of the name, it’s fairly self-evident.  The term caecum (the American spelling is cecum) in anatomy is, generally, a closed-off pouch, or, specifically, the pouch that marks the beginning of the large intestine.  It originates from the Latin caecum which means “blind.”

C. patuxentium, the species in the picture above, is possibly found only in Maryland’s Choptank Formation, and perhaps just in the Boston Cliffs Member of that formation.  In 1904, geologist George Curtis Martin first identified the species, offering only the barest of descriptions:
The only sculpture consists of from 30 to 40 strong, regular, closely-set annulations.
This species bears a strong superficial resemblance to C. floridanum Stimpson, but differs from it in possessing no longitudinal markings.
Length of segment, 2.2 mm; diameter, 0.5 mm.
        (G.C. Martin, Gastropoda, Miocene:  Text, Maryland Geological Survey, 1904, p. 231.)
C. patuxentium was depicted in the accompanying volume of illustrations (Miocene:  Plates, Plate LV, Nos. 11, 12).

I am particularly taken by the small plugs that seal one end of the shells.  These appear, for all the world, to be little stoppers, complete with handles to facilitate removal.  One is shown below in a specimen (and, yes, there is a hole on the side of the shell marking where some predator "pulled the plug" on this little snail).

Geologist Julia Anna Gardner observed that the plug or “septum” closing the end of the Caecum shell differed from one species to another.  (Mollusca From the Miocene and Lower Pliocene of Virginia and North Carolina, U.S. Geological Survey, Professional Paper 199-B, Part 2, Scaphopoda and Gastropoda, 1948, p. 203.)  One source described the plug as composed of “shelly material,” which seems to imply that the gastropod fashions it out of material found in its surroundings.  (Percy A. Morris, A Field Guide to Shells of the Atlantic and Gulf Coasts and the West Indies, Peterson Field Guide Series, 1975, p. 138-139.)  Or is the plug made of secreted calcium carbonate?  The views I've had of the plugs and my reading of the literature leave me agnostic on this.

The C. patuxentium specimen shown in the first photograph above is, I feel certain, an adult version of the shell.  But, mixed among the myriad shells which tumbled out of the clam were tiny shells with a distinct curl at the end (specimen in the first picture below), and shells looking like the adults though on a somewhat smaller scale (specimen in the second picture below, as well as the one I used to illustrate the plug in the picture immediately above).  (Though I believe each of the specimens shown in the various photographs in this post is complete, some may in fact have broken ends.)

The first of the two photographs immediately above may capture a source of some confusion over the number of stages in the Caecum’s passage through life because it apparently shows two of those stages still attached.  Geologist E. Willard Berry, in a note published in 1925, wrote that, while examining some Choptank matrix for microfossils, he had come upon “several tiny specimens of shells with an initial planospiral and a nearly straight annulated cone . . . .”  (Protoconch of Caecum in the Miocene of Maryland, The Nautilus, Volume XXXIX, Number 2, October, 1925, p. 66 – 67.)  Here’s his drawing of same:

These fossil shells he identified as the “protoconch and nepionic stage" of Caecum patuxentium.  In other words, the initial, protoconch section consisting of a tight spiral is here joined to what is in essence a more mature phase, the nepionic section.  For those seeing three stages to the snail's life cycle, this drawing captures the two stages that precede the adult stage.  The tight spiral will eventually drop away from what will become a somewhat arcuate portion which, in turn, will be superseded by the adult stage.

I think that Campbell found the Caecum to have passed through four stages because he considered the one attached to the protoconch in the specimen studied by Berry to be different from a subsequent, more robust “subadult” one to which the protoconch would definitely not still be attached.  In turn, this would be followed by a fully mature adult stage.  But, in the end, Campbell seemed to throw in the towel on being able to distinguish among the stages, warning that “[s]tages 1 and 2 are nearly impossible to identify specifically, and some caution must be used with stage 3.  Only the adult stage can be identified with any confidence.”  (p. 62)

There is inescapable poetry for me in these various incarnations of the Caecum recorded in the beautiful shells it discards as it travels from the beginning to the end of its life.  This life of stages which we struggle to identify, strewn with discarded shells, mirrors ours.
How sweet the silent backward tracings!
The wanderings as in dreams – the meditation of old
                   times resumed – their loves, joys, persons, voyages.
                                               Walt Whitman, Memories,
                                                Leaves of Grass (The “Death-Bed” Edition)

No comments:

Post a Comment

Nature Blog Network