In the previous tutorial we introduced the general characteristics of mollusks, and listed the eight
classes within the phylum. Class Gastropoda is by far the largest and most diverse class of
phylum Mollusca. In addition to the familiar terrestrial and freshwater snails and slugs,
it contains the limpets, abalones, whelks, conchs, periwinkles, sea slugs, sea hares,
nudibranchs, sea butterflies, and sea angels. They range in size from microscopic forms to
large marine animals like the giant horse conch and the sea hares of genus Aplysia,
some of which can grow to over a meter in length. Gastropods have colonized a wide range of
environments. Different species have been found living in gardens, woodlands, deserts, rivers,
lakes, estuaries, brackish waters, polar-regions, mudflats, intertidal zones, the abyssal zone,
hydrothermal vents, and many other locations. Some gastropods are even parasites.
Speaking of parasites, recall the earlier tutorial on the parasitic flatworms, gastropods are often
intermediate hosts for flukes. In addition, many species play important roles as detritivores and
active hunters, and are important food sources for a wide range of predators. In order to examine
gastropoda, we will discuss classification, form and function, including torsion,
feeding habits, locomotion, respiration and circulation, and reproduction and development.
To begin, gastropod classification is still a subject of some debate. Traditional classification
thus far has generally recognized three major subclasses. First there is prosobranchia,
the largest subclass, nearly all of which are marine. This includes the conchs, whelks,
periwinkles, moon snails, oyster borers, which bore into live bivalves and suck out their juices,
and rock shells. Then there’s the Opisthobranchs which include numerous species of sea slugs,
sea butterflies, and bubble shells. Finally, the Pulmonates which include land and most
freshwater snails and slugs, which have a modified vascularized mantle wall that serves as a lung.
However, this classification has more recently been widely accepted as evolutionarily inaccurate.
A newer, alternate classification includes the following five subclasses: Patellogastropoda,
Vetigastropoda, Neritimorpha, Caenogastropoda, and Heterobranchia. Yet another alternative includes
at least seven different clades. Only time and further research can resolve this debate.
In the meantime, let’s move on to gastropod form and function,
starting with one of their most distinguishing characteristics, their shell. Gastropod shells,
when present, are always in once piece, known as a univalve. Some gastropod shells are well
known for being ornate and highly prized by shell hunters. All of these shells are gastropod shells.
You’re probably familiar with a few of them. The whelk and conch shells are perhaps some of the
most easily recognizable gastropod sea shells. Many people recognize the characteristic shells,
but far fewer people recognize the animals that create them. This is what a live conch looks like.
Remember that mollusk shells are secreted by live mollusks. The sea shells that people hunt
and the shells that people put up to their ears to “hear the ocean”, which is actually just the
sound that's produced when the air entering the seashell bounces about in the hollow cavity of
the seashell, are the hardened excretions of what are essentially marine snails. It
is worth noting that an animal most people would likely try to avoid creates structures
that those same people might buy as gifts. Now, let’s look at shell structure. Gastropod
shells begin at the apex, which contains the oldest and smallest whorl. The whorls
become successively larger and spiral around the columella, or central axis.
Since these shells are asymmetrical, they possess a quality called chirality or "handedness" that
depends on the direction of the spiral. Over 90% of all gastropod species are always dextral or
right handed and less than 10% are always sinistral or left handed. A few species,
like Amphidromus perversus, can be right or left handed. The chirality of a gastropod
is evident early in the process of coiling, or spiral winding, that occurs in the shell
and visceral mass. The shell aperture is generally covered by a protective operculum.
In addition to their univalve shell, another major defining characteristic of gastropods is torsion,
the 180 degree twisting of the visceral mass during development that brings the anus to
the anterior end. Most gastropods begin developing as transparent eggs that hatch
into trochophore larva that develop into veliger larva where the shell first forms.
These larval swimming forms usually look very different from their adult forms. At this point,
the mouth is located on the anterior end and the anus is located on the posterior end. However,
this doesn’t last long. First, the shell rotates 90 degrees, then the anus and mantle continue to
rotate another 90 degrees until the anus and mantle cavity open directly above the mouth
and head. This allows for extra space above the animal’s head, meaning it can quickly retreat into
its shell when threatened, but it also means that it literally defecates on top of its own head,
and gills could potentially cause serious fouling, a term that refers to excreted waste washing back
into the gills, which can be detrimental. This unique arrangement has, unsurprisingly,
intrigued and baffled scientists for generations. Several explanations have been proposed,
but the one that seems the most likely deals with coiling and bilateral asymmetry. Look closely at
this picture of a common freshwater snail. The shell is clearly asymmetrical, but the body,
demonstrated by this slug, is symmetrical. Notice that due to the asymmetry of the shell,
the body is also shifted over for better weight distribution. Internally, most gastropods lack
the gill, atrium, and kidney of the right side and this helps them avoid fouling.
Water is brought into the left side of the mantle cavity and out the right side. As the water exits,
so do the waste products expelled by the anus and nephridiopore, a part of the urinary system,
which are located on the right side. Next up, the feeding habits of gastropods are
quite varied, but all involve some modification of the radula, the tongue-like feeding apparatus
that is actually a minutely toothed, chitinous ribbon. Most gastropods are herbivores that rasp
particles of algae off hard surfaces. They do this by extending their radula which scrapes
the surface and removes tiny particles that are deposited into the esophagus. Other herbivores,
like the abalone as well as common garden slugs and snails, hold their food in their
foot and break off pieces using their radula. Others, like the true whelks of genus Buccinum,
are scavengers, while still others are carnivores that tear into their prey with
radular teeth. Some, like the oyster borers, drill holes into the shells of oysters using a
combination of their modified radula and caustic chemicals, and then feed continuously for days,
tearing apart the soft flesh of its prey. Still other carnivores, like the infamous
cone snails, are extremely venomous and feed upon fish, worms, and other mollusks using a modified
hollow radula that injects a unique mix of toxic peptides known as conotoxins. Other gastropods,
like the queen conch, feed on organic deposits in the sand or mud, and still others,
like many species of sea butterflies, feed by catching planktonic food by entangling
it in a mucous web that coats their bodies. But gastropod feeding strategies get weirder
still. The adorable “leaf sheep” is a species of sacoglassa, or “solar-powered sea slug,”
that is capable of kleptoplasty, where the sea slug retains the chloroplasts from
the algae they feed on, thereby enabling the animal to indirectly perform photosynthesis.
However, the sacoglassans aren’t the only symbiotrophic gastropod. The scaly-foot gastropod,
or “volcano snail” is a species of deep-sea hydrothermal-vent snail,
that is found in hydrothermal vents at depths of about 2,400–2,900 meters. It is the only
known animal to use iron(II, III) sulfide, or greigite, in its skeleton. Its feeding habits
as a juvenile are unknown, but when it settles as an adult it obtains all of its nutrition
from the chemoautotrophy of the endosymbiotic bacteria that live within its body. It uses
no other mechanism for feeding. It is possible that there are many other species of deep sea
gastropods just waiting to be discovered. Moving on to locomotion, gastropods use a
single blood-filled appendage, the foot. For many gastropods the power for locomotion is
provided by muscular waves that undulate along the ventral surface of the foot. The force of these
waves is then coupled to the substratum by a thin layer of pedal mucus. This mucus acts as a glue,
allowing the animal to adhere to the substratum on which it crawls while muscular waves ripple
through its body. Some gastropods, like the sea angels and sea butterflies, “fly” in the water
using their modified foot as “wings” that they clap together at the tips and drag them apart,
which sucks fluid into a gap, creating low-pressure vortices that propel them forward.
Respiration in most gastropods is performed by the ctenidium which is essentially a
specialized gill that is made up of an axis with a row of projecting filaments,
while most land-dwelling species have a highly vascularized area of their mantle cavity that
serves as a lung. Most gastropods have a single kidney in addition to well-developed circulatory
and nervous systems. They also have eyes or other simple photoreceptors, statocysts for a sense of
balance, tactile organs like tentacles for a sense of touch, and chemoreceptors for taste and smell.
No analysis of a class would be complete without a look at reproduction, and some gastropods
are renowned for their intricate courtships and rituals. Though some gastropods simply discharge
sperm and ova into the water column, most practice internal fertilization. Some gastropod species
are dioecious, meaning they are male or female. Others are monoecious, meaning they are both male
and female at the same time, and are capable of self-fertilization. Still others are protandric,
meaning their testes develop first, then wither and ovaries develop as the animal matures.
Most interestingly, many gastropods practice internal fertilization and perform prolonged
courtship ceremonies, sometimes involving a “love dart.” Love darts are not used to transfer sperm,
and there is no organ to receive it, so the copulating animals instead merely
stab each other with them. The mucus on the dart contains a pheromone-like compound that promotes
sperm preservation in the receiver, thereby favoring the snail that “fired” the dart.
Other copulation rituals involve the formation of circles where each snail follows the other
and they then rotate clockwise. Others, like the common leopard slug hang off of branches
and form glowing structures with their elaborate penises that emerge from the side of their head
and simultaneously transfer sperm. With that image we conclude our
summary of the gastropods. Let’s move forward and examine bivalves next.