GASTROPODS
The gastropods (snails & slugs) are a group of
molluscs that occupy marine, freshwater, and terrestrial environments.
Most gastropods have a calcareous external shell (the snails). Some lack
a shell completely, or have reduced internal shells (the slugs & sea slugs
& pteropods). Most members of the Gastropoda are marine (see modern
examples below). Most marine snails are herbivores (algae grazers) or
predators/carnivores.
CONIDS
The conid gastropods (cone shells) are fascinating marine
snails for a couple reasons - they have attractively-shaped, colorful shells
and they are killers. The conids are predatory, as are many other
marine snails, but they take down their prey in an unusual fashion. The radula
of most snails is a mineralized or heavily sclerotized mass of small teeth that
scrapes across a substrate during feeding. Conid snails have a toxoglossate
radula - one that has been evolutionarily modified into tiny, unattached,
toxin-bearing, harpoon-like darts (see
photo) that can be fired at prey. Each dart is an individual
tooth. The nickname "killer snails" is well deserved (even
people have been killed). Some species have incredibly powerful toxins,
while in other species the toxin has little effect on humans.
Conus gloriamaris (7.8 cm tall) - abapertural (left) and
apertural (right) views. The apertural edge has not been filed (unlike
many snail shells in the retail shell market) & the protoconch is present.
The conid shell shown above & below is one of the
most famous rare seashells in history - Conus gloriamaris, the
glory-of-the-seas cone. Conus gloriamaris (a.k.a. Conus
(Regiconus) gloriamaris; a.k.a. Cylindrus gloriamaris; a.k.a.
Conus (Cylindrus) gloriamaris; a.k.a. Cylindrus (Regiconus)
gloriamaris) is a modern, tropical marine gastropod. The hard shell,
or conch, has a distinctively elongated, gently tapering shape and a more sharply
tapered top. This species’ shell surface coloration consists of a medium
to dark brown colored “tent” pattern on a pale creamy yellow background (many
conid gastropod shells have broadly similar tent patterns). Individual
tents vary in size.
Conus gloriamaris was first named & described by Johann Chemnitz in
1777. Shell collectors treasured specimens of this species, which
remained very rare from the time of its original description up to the late
1960s. Conus gloriamaris is not an abundant species, but shells
are now available in the retail market.
Classification: Animalia, Mollusca, Gatropoda, Neogastropoda,
Conoidea, Conidae
Natural distribution: intertidal to <100 meters depth, western Pacific
Basin.
Conus gloriamaris - apical portion of conch (left) and apical view of
conch (right, ~2.5 cm across) showing dextral coiling. Dextrality is an
almost universal trait in conchiferous gastropods. Only rarely can
counter-clockwise coiled shells be found (left-handed coiling/sinistral
coiling).
Conus gloriarmaris - conch surface showing tent pattern (field of view
~2.7 cm across).
FICIDS
Here’s a graceful fig shell (a.k.a. elongated
fig shell), Ficus gracilis (see
photo of snail inside this shell). This is a modern, predatory, tropical
marine gastropod from the western Pacific Basin that inhabits unconsolidated
sand substrates and preys on irregular echinoids. The shell itself is
relatively thin & lightweight.
Classification: Animalia, Mollusca, Gastropoda, Neogastropoda, Ficoidea,
Ficidae
Ficus gracilis shell - abapertural (left) and apertural (right)
views (10.3 cm tall). The apertural lip has been filed (this is commonly
done to retail specimens).
COSTELLARIIDS
The colorful queen vexillum shell (Vexillum regina)
is spectacularly colored with white, orange, and dark brown stripes (the
species does vary in coloration, however). It is a modern, tropical
marine gastropod from the western Pacific Basin. Vexillum is a
predatory snail that secretes toxins to immobilize & kill prey.
Classification: Animalia, Mollusca, Gatropoda, Neogastropoda,
Volutoidea, Costellariidae
Vexillum regina shell (6.2 cm tall) - abapertural (left) and
apertural (right) views.
WORM SNAILS
Here’s one of the more bizarre gastropod shells
around. This is Vermicularia, also called a worm snail. It’s
one of the few snails that does not have a tightly coiled shell. Several
snails from different families have shells somewhat like this (e.g., the
vermetids & the turritellids). They all resemble the twisted
mineralized shells made by some annelid worms, hence the common name “worm
snails” or “worm shells”.
Despite the superficially very different-looking
shells, malacologists have demonstrated that Vermicularia is very
closely related to the high-spired snail Turritella.
Juvenile Vermicularia are free living, infaunal filter feeders that
position themselves apex-down and aperture-up within the sediment. During
this stage in ontogeny, the Vermicularia shell is tightly coiled, as is
any ordinary gastropod shell. Later in life, the snail becomes an
epifaunal, encrusting filter feeder (assuming hard or firm substrates are
available), and its shell starts uncoiling. The advantage of an uncoiled
shell in Vermicularia is generally inferred to be rapid upright growth
(that’s desirable for a sessile, benthic filter feeder).
If hard or firm substrates aren’t available, Vermicularia
generally doesn’t grows an unwound shell during growth, and they end up looking
like typical Turritella shells. The degree of uncoiling also
depends on the nature of the hard substrate (e.g., a ramose scleractinian coral
vs. a hemispherical scleractinian coral vs. a bivalve shell). So, shell
uncoiling is an ecophenotypic character.
Classification: Animalia, Mollusca, Gastropoda, Mesogastropoda,
Cerithioidea, Turritellidae
Vermicularia (worm snail) (9 cm tall)
Most info. synthesized from Morton (1953) and Gould
(1968, 1969):
Morton (1953) - Vermicularia and the
turritellids. Proceedings of the Malacological Society of London
30: 80-86.
Gould (1968) - Phenotypic reversion to ancestral form
and habit in a marine snail. Nature 220: 804.
Gould (1969) - Ecology and functional significance of
uncoiling in Vermicularia spirata: an essay on gastropod form. Bulletin
of Marine Science 19: 432-445.
XENOPHORIDS
The xenophorid snails (a.k.a. carrier snails),
especially members of the genus Xenophora, are remarkable in
their tendency to pick up other shells, skeletal fragments, rock fragments, or
corals (sometimes still alive) from their surrounding environment and cement
these objects to their own shells. The result looks like a pile of shells
on the seafloor. Often, sponges and serpulid worm tubes are found
encrusting the xenophorid shell - they contribute to the illusion that a
xenophorid is simply a patch of seafloor. Xenophora carrier shell
snails do this as a camouflage defense against predators. Decorator
crabs are arthropods that do this as well.
Xenophorids are principally detritivores on
unconsolidated, fine-grained to coarse-grained to rubble-bottom substrates.
Classification: Animalia, Mollusca, Gastropoda, Mesogastropoda,
Xenophoroidea, Xenophoridae
Xenophora carrier shell (above & below; 12.7 cm across; above: apical
view; below: umbilical view) with numerous attached shells. Most
of the objects that have been picked up by this individual are snail shells,
but there are also a few clam shells, and a long worm tube.
Some info. from Harasewych & Alcosser (1991) and
Hill (1996).
MURICIDS
Many muricid snails have highly spinose shells.
The high degree of spinosity in such snails is usually considered an anti-predation
feature. Spinose muricids typically have three axially-oriented rows of
spines per whorl, so that each spine row is ~120º from the next.
Conchologists have pointed out that such spine row distributions provide
orientation stability to the snail and prevent sinking on unconsolidated,
fine-grained, high-water-content sediment substrates. Another suggestion
holds that well-developed spine arrays could act as traps for potential
prey. Muricids are predatory gastropods, but they principally prey
on encrusting, conchiferous organisms (e.g., bivalves, barnacles) by boring
through the shells. It's more likely that the spine arrays protect the
snail from predatory arthropods or fish while engaged in boring & feeding
on prey.
Classification: Animalia, Mollusca, Gastropoda, Neogastropoda,
Muricoidea, Muricidae
Murex pecten Lightfoot, 1786 shell (12.8 cm tall) - abapertural view (left) &
apertural view (right).
Murex troscheli Lischke, 1868 shell (11.8 cm tall) - abapertural view
(left) & apertural view (right).
Some info. from Morris & Clench (1975), Paul
(1981), Harasewych & Alcosser (1991), and Hill (1996).
TURBINIDS
The turbinids are common, tropical, herbivorous
snails. One of the more visually intriguing turbinids is the star
turban. This snail tends to inhabit unconsolidated, fine-grained
substrates in moderately deep, low-energy settings. The long spines
projecting from the final whorl's keel are throught to help prevent the snail
from sinking (a common problem among epifaunal, benthic invertebrates
throughout geologic history) and to help prevent predators from easily flipping
the shell over.
Classification: Animalia, Mollusca, Gastropoda, Archaeogastropoda,
Trochoidea, Turbinidae
Guildfordia yoka Jousseaume, 1888 shell (above & below; 8.3 cm
across)
Above:
apical view. Below: umbilical
view.
Some info. from Morris & Clench (1975), Harasewych
& Alcosser (1991), and Hill (1996).
LAMBIDS
The spider conchs, or lambid snails (often grouped
with the true conchs - the strombids), are herbivorous gastropods in tropical
photic zone environments, feeding on benthic filamentous algae. Lambids
can have large, thick shells that are often quite colorful, especially in the
apertural areas.
Classification: Animalia, Mollusca, Gastropoda, Caenogastropoda,
Stromboidea, Lambidae (or Strombidae)
Lambis chiragra (Linnaeus, 1758) shell (the chiragra spider conch)
(22.5 cm tall) - abapertural view (above).
Lambis chiragra shell - apertural view (above). Note the large,
rounded drill hole (1.3 cm diameter) near the left margin of the shell.
The boring hasn’t penetrated through the thick shell wall - evidence for an
unsuccessful predation event.
Lambis scorpio (Linnaeus, 1758) shell (the scorpion spider conch)
(13.5 cm tall) - abapertural view (left) & apertural view (right).
Some info. from Harasewych & Alcosser (1991) and
Hill (1996).