SLOWLY, BUT SURELY
We've been a little slow at posting this week's photo essay from Hilton Pond. It wasn't that we were too busy doing other things, it's just that our current subject's general lethargy seemed to be contagious, and everything we did to prepare transpired at increasingly slower speeds. It took us forever to photograph the subject--even longer to try to identify it--but at last we finished our tasks and now present our first ever installment about the sloooooow-moooooving land snails of Hilton Pond Center.
Land snails--along with such creatures as aquatic and marine snails, slugs, and nudibranchs--are all mollusks (also spelled "molluscs"). Their phylum name is pretty straightforward--Mollusca--and comes from a Latin word meaning "soft." An animal gets to join this ancient group if it has a soft unsegmented body plus a mantle--an organ that secretes the calcareous (calcium-based) substance that forms the shell. Every mollusk doesn't have an external shell; take the garden- variety Giant Gardenslug, Limax maximus (below right), an introduced species from Europe that bears a slightly humped back covering a soft internal shell. The vast majority of mollusks live in saltwater habitats.
Mollusk taxonomy is complex and includes eight classes, but the more-familiar shelled mollusks are divided into two major groups. The Bivalves have hinged two-part shells, or valves, whose halves are mirror images--as in the Eastern Elliptio, Elliptio complanata (below right), collected from a drought-exposed bank of Hilton Pond; also included are such tasty items as mussels, oysters, and scallops.
Species in the other big group, generically called "univalves," have a single shell that is spiral- shaped; this includes our recently encountered land snail (top photo, grazing on moss) as well as conchs, whelks, cowries, and the freshwater snails kept in home aquaria. Animals one might not expect to be mollusks are the cephalopods ("head foot")--octopi, squid, cuttlefish, and nautilus--which have the largest invertebrate brains. The nautilus forms an elaborate external shell, while most of the rest have internal ones; that of the cuttlefish is the "cuttlebone" used to provide calcium to cage birds.
Nearly 80% of the 50,000-plus named mollusks are "univalves," known more formally as the Gastropoda. A quick look at any of them will explain where this class name comes from; "gastropod" means "stomach foot" and that's exactly what sticks out of the shell of our land snail. This well-developed, stomach-containing, muscular "foot" is used for creeping along--at a rather slow pace, we might add--and can be pulled completely inside the shell. Unlike marine gastropods, land snails usually have no operculum to cover the withdrawn foot. On the underside of the foot's anterior end is the mouth, complete with a radula (rasping part) that typically allows the snail to shave off thin layers of plant material that comprise the bulk of its diet. Marine gastropods such as Oyster Drills can slowly rasp right through the shell of their hosts, allowing access to the succulent meat within. (All mollusks except bivalves have a radula.) Octopi are high-order predators that catch and eat crustaceans, fish, and other live prey; about this they are not very slow.
Like other mollusks (except bivalves), a gastropod snail has a well-developed head region. It also has extensible tentacles-- two pairs in terrestrial snails, one pair in aquatic species. Each of the longer tentacles of the land snail pictured on this page is tipped by a bulbous structure (right) that is primarily an odor-sensing chemoreceptor. There's also a tiny black eye dot that may be capable of seeing more than just movement or shades of gray. The shorter tentacles apparently sense odors but have no eyespots. Both tentacle pairs are extremely touch- sensitive, enabling the snail to slowly move about to find food or avoid danger even in complete darkness. By the way, taking photos of tentacles is indeed a tedious process, since our snail constantly waved them in and out of focus as it slowly sampled its stimulus-rich surroundings.
We found our "This Week" snail dining slowly inside a wire ground trap we use to capture birds for banding. The floor of the trap was covered with cracked corn and millet, and the snail apparently found this scratch feed to its liking. We carefully extricated it from the trap and placed it in a clear glass jar, where we observed it for a few days. When we offered it a choice of leaf lettuce and a piece of flour tortilla, it slowly chose Mexican and ignored the greens. (We have no plan to experiment further to determine if it likes other ethnic foods.)
Land snails can eat and breathe slowly at the same time, since the mouth is anterior and the lung opening is on the side of the foot near the edge of the mantle and just anterior to the edge of the shell (left). In an apparent design flaw, the anus lies adjacent to the breathing hole, just above the head and mouth. This arrangement works just fine for snails, of course, but may explain why some folks don't like escargot.
Although we have referred to our snail as "it," we might be more correct in saying "he/she" because many gastropods are true hermaphrodites, serving slowly but simultaneously as both male and female. Regardless, they still pair up to copulate--all in slow-motion, of course--and in some species each snail stimulates the other with a needle-like, calcareous "love dart" during the embrace. This seemingly sadistic and/or masochistic practice is thought by some authorities to inhibit self-fertilization, while others believe it increases storage of sperm from the dart shooter --sometimes for years after copulation!--thus insuring that the intial shooter fathers more offspring than future partners. Fertilized eggs from each snail are slowly deposited singly or in a jelly-like mass in some moist, dark locale, where each zygote slowly develops and hatches into a tiny snail--complete with an even tinier and transparent shell of its own.
As novitiates into the world of mollusk identification, we admittedly found it difficult to key out the land snail we found at Hilton Pond Center; for us, it was really slow-going. As might be expected, terrestrial gastropods are classified in part on shell characteristics, but finding all these traits on a live snail isn't as easy as it might sound. After narrowing things down a bit, we then had to consider whether the snail's shell was elongate, globose, or depressed; lipped (and reflected?); higher than wide, or vice versa (in this case, about .75" in diameter by .5" thick); and whether the back side of the shell was imperforate or with an obvious or half-hidden umbilicus (a belly-button-like depression, below left). And, by the way, it was also important to know whether the pedal grooves of the mollusk's foot were conspicuous or, as in the case of our snail, hidden by the foot as it glided slowly along on its trail of slimy mucus.
Using John Burch's How to Know the Eastern Land Snails, we slowly worked through unfamiliar mollusk terminology and the above comparisons--along with many, many more--to finally conclude our Hilton Pond gastropod is the White-lipped Globe, Mesodon thyroidus, a species common throughout most of the eastern and central U.S. (At first we thought it could be M. clausus, the Yellow Globelet--also found in the East from North Carolina northward--but then it apparently would be a South Carolina state record.)
We may have taken a wrong turn in the identification key, however, so we're certainly willing listen to malacologists (shell specialists) who have better understanding of gastropod taxonomy. If we need to, we can ship the organism to any expert--by snail-mail, of course--but please correct us immediately, lest we come to believe that everything associated with land snails is as sloooooooooow as what we've experienced so far at Hilton Pond Center.
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Hummingbird Super-stimulus Experiment 2003
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Please report your sightings of
Color-marked Ruby-throated Hummingbirds
This fresh-plumaged male banded on 5 April was the first spring migrant of its species to arrive at Hilton Pond; two more followed on 7 April. This is the third consecutive year that the first RTHU arrived locally on 5 April.
SPECIES BANDED THIS WEEK
* = New species for 2003
(with original banding date, sex, and current age)
White-throated Sparrow (2)
01/09/99--after 5th year unknown
11/06/00--4th year unknown
WEEKLY BANDING TOTAL
YEARLY BANDING TOTAL
BANDING GRAND TOTAL
(since 28 June 1982)
OTHER SIGHTINGS OF INTEREST
--Two female Wood Ducks continue to incubate in nestboxes on Hilton Pond.
--Continuing spring rains and excessive wind made mist-netting impossible almost every day this week; even trapping did not fare well. The good news is that Hilton Pond remains full and the underlying aquifers undoubtedly are beginning to be replenished after four years of drought.
Current Weather Conditions at Hilton Pond Center
Hilton Pond Center for Piedmont Natural History
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