Snails and slugs crawl at a speed of .006-.03 miles per hour- what could a creature so slow even be good for? Most of our lives move too fast to pay them much attention, but they make a whole other world at our feet; approximately 40,000 different species (very slowly) roam the earth!

The class Gastropoda, including slugs, snails, limpets, sea hares and sea butterflies, belongs to the phylum Mollusca, classified as soft-bodied animals bearing an external shell (although many animals without shells belong to Gastropoda). There are approximately 150,000 different species on earth! Gastropods are the largest group of mollusks, and as such it is difficult to pinpoint or characterize the life history of any one species. They have been in existence for approximately 480 million years dating back to the Cambrian era; fossils are easily preserved due to their shells being made of a hard calcareous material that holds together well over time.

 

Gastropod fossils.

Fossils of Nerinea marine gastropods of late Cretaceous (Cenomanian) age, in limestone in Lebanon. Photo by Laurence Comte.

 

Gastropod means “stomach foot,” relating to Gastropoda’s unusual anatomy; in their mature larval stage, they undergo torsion, a process that repositions their organs near their head, including the stomach. Their foot- you guessed it- resides where their stomach once was! It is a muscle that expands and contracts to allow movement.

 

Internal anatomy of a snail.

The anatomy of a common air-breathing land snail. Note that much of this anatomy does not apply to gastropods in other clades or groups. Diagram from Wikipedia.

 

They are most commonly either herbivores or scavengers, feeding on dead animal matter and plant material such as leaves, stems, and algae. Certain species are known to be carnivorous and prey upon other smaller invertebrates. Being so diverse, gastropods can be found in nearly any habitat; both in saltwater and freshwater, on land in marshes, forests, mountains, and even deserts, at almost all altitudes and latitudes. Due to such a wide range, specific choices of sustenance per species are a wildcard.

One of Gastropoda’s most defining characteristics is their “slime trails.” This mucus is released by glands on the sole of their singular “foot” that makes up the whole underside of their body, alleviating friction between the sole and the ground. It is even possible for certain gastropods to crawl along a knife’s edge unscathed! This muscular foot is characteristic of nearly all gastropods.

 

Slime trail.

Remnants of a slime trail on asphalt. Photo by Hans Braxmeier.

 

Gastropoda’s other most obvious and unique trait (pertaining to only some species) is the shell. The pattern of the shell is usually coiled and can be sinistral (spiraling counter-clockwise) or dextral (spiraling clockwise). It is created by a cell layer located along the rim of the shell mouth that secretes calcareous matter (meaning mostly or partly comprised of calcium carbonate) which will then harden into a prism or plate crystals. In this way, the shell grows in size as it grows in thickness (talk about DIY homes!). Gastropods will withdraw into this shell to avoid predators or conserve water.

Many gastropods are hermaphrodites, meaning they bear both male and female reproductive organs, which are usually located near the head; however, not all are capable of self-fertilization. Most gastropods reproduce sexually, yet the method of copulation is greatly dependent on their environment. Some fertilize their eggs outside of the body in water, others penetrate their mate with “love darts” (don’t worry- they don’t hurt!). Leopard slugs, depicted in the below image, have got it twisted-literally. After an inexplicable circling ritual, they hang from a branch using mucus as a lifeline; that translucent knotted substance is their penises exchanging sperm, which will then fertilize the eggs in their respective female reproductive structures. These reproductive knots have often been seen to “flower” or “bloom”. How beautiful…There have even been observed species, such as the slipper limpet (Crepidula fornicata) that can undergo environmentally mediated sex changes in order to manage a sexually-imbalanced population. For the most part this is a male-to-female transition given that that most organisms of these species have a slightly higher male distribution.

 

Two slugs mating.

Mating of Limax maximus. Photo by T. Hiddessen.

 

“Shell art” is a staple in westernised beach culture- anything from earrings to chandeliers are created from the shells of gastropods (as well as other molluscs). While beautiful and fascinating to be able to observe an otherwise hidden aspect of nature so up-close, shell-collecting is a notoriously unsustainable practice. Shells are used as shelter for algae, material for bird nests, and homes for hermit crabs. Many shells used in shell art come from snails harvested for consumption – an age old practice worldwide – but this too yields environmental consequences when done recklessly. Nonetheless, gastropods are a resilient and biodiverse class of organisms that will continue to amaze us (and gross us out) so long as current global conservation trends continue.

 

By: Tom Gregg

Feature Photo by: Jussi, Flickr

Sources:
Kennedy, J. (2016). Characteristics of the Gastropoda. Retrieved November 09, 2016, from http://marinelife.about.com/od/invertebrates/p/Class-Gastropoda-Snails.htm.
Gastropod – Reproduction and life cycles. Retrieved November 09, 2016, from https://www.britannica.com/animal/gastropod/Reproduction-and-life-cycles.
Gastropoda – slugs and snails. Retrieved November 09, 2016, from http://www.ento.csiro.au/education/allies/gastropoda.html.
Nordsieck, R. Snails and Slugs (Gastropoda). Retrieved November 09, 2016, from http://www.molluscs.at/gastropoda/index.html?/gastropoda/morphology/locomotion.html.
Nordsieck, R. Snails and Slugs (Gastropoda). Retrieved November 09, 2016, from http://www.molluscs.at/gastropoda/index.html?/gastropoda/morphology/shell.html.
Nordsieck, R. Snails and Slugs (Gastropoda). Retrieved November 09, 2016, from http://www.molluscs.at/gastropoda/index.html?/gastropoda/morphology/reproduction.html.
How fast does a slug move per hour? (2000, March 30). Retrieved December 11, 2016, from http://www.madsci.org/posts/archives/2000-03/954420651.Zo.r.html.
The Gastropoda. Retrieved December 11, 2016, from http://www.ucmp.berkeley.edu/taxa/inverts/mollusca/gastropoda.php.
Images:
Jussi (Flickr). Snails. https://www.flickr.com/photos/53434152@N00/14038928588/.
Nérinées dans le Crétacé libanais. (2011, July 11). Retrieved November 09, 2016, from https://www.geodiversite.net/media551.
Gastropoda. Retrieved November 09, 2016, from https://en.wikipedia.org/wiki/Gastropoda.
Free Image on Pixabay – Snail, Slime Trail, Shiny. Retrieved November 09, 2016, from https://pixabay.com/en/snail-slime-trail-shiny-474286/.
Hiddessen, T. (2010, October 7). File:Limax-maximus mating 2.jpg. Retrieved November 9, 2016, from https://commons.wikimedia.org/wiki/File:Limax-maximus_mating_2.jpg.
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