Popular Science Monthly/Volume 45/May 1894/The Sleep of Mollusks
By CHARLES T. SIMPSON.
IT is probable that the sleep or dormant period which mollusks share in common with many other organic beings is brought on not merely by the exigencies of climate, but that it is more or less necessary in building up the wasting physical powers. All organized beings seem to require rest in some form or other. If plants, whether from the tropics or temperate regions, are kept in hothouses, they will not grow the year round, and when forced to do so soon become sickly or die outright.
With the mollusks this sleep in many cases may be prolonged indefinitely, often without the slightest apparent damage, and under some conditions which seem really astonishing.
In the sea the clams (Venus and Mya) have rest periods, during which they sink more deeply into the mud and retreat from the fisherman; the tritons, murices, and ranellas form a shelly growth and mark their seasons of repose by a thickening of the aperture called a varix, which is sometimes guarded by spines or knobs. The littorinas, which are amphibious, pass most of their time on grass or sedges at the edge of the sea in the colder regions and high aloft on mangrove or other trees in the tropics, only occasionally going into the water to moisten themselves. Tryon tells of some West Indian species which survived over a year in his cabinet, and of others that lived for months in the dry air of Philadelphia, though they exhibited but little activity; and the writer has kept specimens of the nearly allied Tectarius alive in his collection for nearly two years.
Most of the fresh-water species of mollusks pass a period of hibernation in cold climates or testivation in the tropics, and many of them are wonderfully tenacious of life when withdrawn from the water. In June, 1850, a living pond mussel was sent to Dr. Gray from Australia which had been kept out of water more than a year, and instances of the survival of unios without moisture for long periods are not rare. While living in south Florida I discovered a colony of unios in a small drain that ran through the pine woods and which only contained water during the rainy season—some three months in the year. Thousands of these mussels were found in the channel among bulrushes, buried vertically an inch or so below the surface in nearly dry soil, with the anterior end downward, and the slightly moist, sandy banks in many places were full of them. The colony extended some ten or a dozen yards along this drain; not a specimen could be found either above or below this space, and the species was not found in the little stream into which it emptied.
A lot of these unios were taken home and laid in the garden, where they remained more than three months wholly unprotected from the hot autumn sunlight during the dry season, and when opened a number of them were found to be alive. Yet ordinarily the want of water causes the Unionidæ to speedily die. The summer of 1886 was one of the least rainfall ever known in the upper Mississippi Valley, and many streams and ponds went dry that had never been known to be so before. At this time I collected in northern Illinois and Iowa, and in every instance where the water had evaporated I found the mussels dying by thousands, though in many cases the mud was too soft to bear the collector. While collecting in the Indian Territory I visited a large pond near McAllister that had just been drained, and, although the water stood everywhere in pools, yet the Unionidæ were apparently all dead and gaping open, and the stench was so horrible that the struggle between duty and comfort was a severe one. For years I have watched the dredging operations in the Potomac at the capital, as the mud was thrown out on the flats, and in every case the mussels were dead before it was firm enough to be trodden on.
I do not believe in building a theory on too slight a foundation of fact, but I am of the opinion that these unios which have been kept dormant for lengthened periods out of water inhabited streams or ponds that were intermittent. The instance I have given in Florida is a good one, and the mussel Dr. Gray received from Australia is no doubt another. The whole island is noted for heat and long-continued droughts, and with scarcely an exception the streams and lakes go dry during the rainless season. Even the Murray River, the largest stream in the country, and ordinarily navigable for hundreds of miles, sometimes ceases to flow altogether.
The ampullarias or idol shells, a noble genus of tropical pond snails, bury themselves deeply in mud during the dry seasons. They are remarkable for their ability to live without water, having been kept out of it for years, and they are often brought to foreign countries alive in mahogany logs.
Guilding first noticed that the species of the Antilles had a double system of respiration, which was further dilated on by Caillaud, who brought these snails alive from Egypt; and D'Orbigny discovered that they had a distinct pulmonary apparatus in addition to their gills. According to Joly, anodons and viviparas survive freezing, and will reproduce on being thawed out; and no doubt many of the species that live in cold climates are frozen every winter and resuscitated with the return of spring.
It is believed that all shell-bearing land mollusks either hibernate or æstivate according to conditions of climate. Most of the snails close the aperture with a membranous or coriaceous covering, consisting of lime and mucus, which is called an epiphragm. W. G. Binney has thus described the operation: "The animal being withdrawn into the shell, the collar is brought to a level with the aperture and a quantity of mucus is poured out and covers it. A small quantity of air is then emitted from the respiratory foramen, which detaches the mucus from the surface of the collar and projects it in a convex form like a bubble. At the same moment the animal retreats farther into the shell, leaving a vacuum between itself and the membrane, which is consequently pressed back by the external air to a level with the aperture or even farther, so as to form a concave surface, where, after becoming desiccated and hard, it remains fixed. These operations are nearly simultaneous, and occupy but an instant." As the winter advances the snail withdraws deeper and deeper, shutting itself out by other epiphragms, like a retiring army covering its front by breastworks as it retreats, until sometimes it has made no less than half a dozen, one within the other. With the snails such as ours, that inhabit moist wooded districts, this protecting wall is thin and nearly transparent, while in those of arid regions it is thicker and often calcareous. Some of the large helices of south Europe secrete a somewhat shelly epiphragm resembling the coating of a turtle's egg, convex externally, with the edge turned in and roughly cemented to the aperture of the shell. In this condition, if not resuscitated by moisture, the snails will remain alive for an indefinite period. Woodward tells of a desert snail (
Helix desertorium) which, after being glued to a tablet for four years in the British Museum, was noticed to have discolored the paper of its label, and on being put into warm water it revived; and Dr. Stearns kept a Helix Vietchi from Cerros Island, Lower California, alive six years without food. Many other such cases are known.
Strangely enough, the slugs undergo no such period of hibernation, as they only cease activity in temperate climates during the coldest weather, and when a warm spell occurs in winter they are thawed out into new life. It is indeed curious that these naked fellows should be so much more hardy than their relatives, who wear their great overcoats of shells into which they can wholly retreat, but so it is. Binneya, a Mexican snail, whose shell is not large enough to cover its body, attaches itself to a spot where it æstivates and forms a parchmentlike epiphragm from the edges of the shell to the place of attachment, and when it returns to activity often carries with it this queer addition to its house. Most snails dissolve this when they awaken from their long sleep.
Nature has kindly relieved the operculated land snails from the trouble of making this protection, and when the time for retiring arrives they simply retreat into the shell and close the door behind them. In this condition they are "not at home" to any callers. The long winter's sleep proves disastrous to many of the snails, and in the spring quantities of dead shells will be found huddled together in hollow trees, under rocks, and in their crevices, or buried beneath the leaves and ground with a few survivors among them. Why they thus assemble together to hibernate is difficult to tell, unless it is because "misery loves company."
The succineas are a somewhat amphibious family of air-breathers, and on the approach of winter often crowd together into tussocks of grass or rushes by the edges of streams and ponds. In eastern Colorado and western Nebraska I have counted from two hundred and fifty to three hundred of them thus tucked snugly away in a single tuft of grass. It is indeed fortunate for them that they are wrapped in unconsciousness during the dreary winter of that shelterless, desolate country, with its howling blizzards and snows drifted wildly over the prairies, and it is marvelous that so large a proportion survives. In that dry region water is a luxury that even a fresh-water snail can not always afford; hence their shells are found strewed over the highest table lands, miles horizontally and hundreds of feet vertically from moisture; and I have gathered numbers of them in winter under projecting rocks high upon the bluffs of the South Platte River.
In the tropics the process of æstivation is analogous to hibernation, but there is not so complete a cessation of the functions. The same epiphragm is made, and the rest is taken for the same purpose—to avoid the vicissitudes of climate; only in this case it is to escape drought instead of cold. And sometimes the same gregarious habit is observed, and the snails crowd in closer than the occupants of a cheap lodging house. On some of the west Florida keys I have seen Helix Carpenteriana æstivating under grass and logs in such vast numbers that one might scoop them up by the quart; and in the Maritime Alps I have found other species of the land snails piled together by hundreds in hollows of limestone cliffs during the dry season. The strophias either cling to the stems of low bushes or lie at their roots, as do many species of Bulimulus, often in great numbers.
The arboreal species firmly attach themselves to the bark of the trees on which they live and on whose foliage they subsist, and form a solid epiphragm of the consistence of sole leather. On the lower part of Florida and on the keys the magnificent Orthalicus and Liguus, the latter gaudy with bands of yellow, brown, and green, the former a soft cream color, with markings of jet black and brown, live often on such trees as the Jamaica dogwood (Piscidia erythrina) and the Bursera, which shed wholly or in part their leaves during late winter and spring, the dry season. The sight of one of these trees without foliage, and loaded with this strange, glittering fruit, is enough to thrill the heart and stir the blood of any collector, and I shall never forget my first experience with them at Cape Sable. In my eagerness to possess the beautiful things I broke several specimens, as the epiphragm adhered so firmly that the shell crushed before it would loosen, and I could only save them by cutting away the bark.
One wonders why these snails so freely expose themselves during æstivation, when they are utterly powerless to escape from their enemies. Many of these trees, which were full of them, were isolated more or less and were without foliage, and every shell could be seen hundreds of feet away. That they have enemies I discovered afterward as I wandered broken-hearted among the thick scrub of Key West to find quantities of fresh broken Orthalicus lying on the ground, but not one alive. Many of them appeared as though a hole had been picked in them by birds large enough to get out the snail and utterly ruin the shell. In this case death came swiftly and painlessly, no doubt, while they were wrapped in sleep. Such, is the summer repose or festivation of the tropical tree snails. For months of bright, sunshiny weather they cling motionless, perched aloft on their favorite trees that at once are home and food for them, firmly attached by a leathery epiphragm that neither sun nor rain nor wind, or anything but themselves, can dissolve; and on the coming of the first showers of the rainy season they awaken to new activity and life.