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One of the main functions
of the gastropod shell is to defend against predators (gastropods have yummy,
fleshy bodies). This week, we will examine various defensive shell morphologies
and compare the defenses of Paleozoic and Cenozoic marine gastropods.
If the adult gastropod
shell could be designed from scratch with no limitations, it would be simple
to devise effective defenses. However, the shell architecture bears historical
baggage in two ways: 1) Since the first gastropods had shells that were coiled
cones that grow by marginal accretion, all subsequent gastropods have this type
of shell (though some have secondarily lost the shell entirely). 2) The shell
must be functional as it is enlarged though an individual organism’s lifetime.
The juvenile form limits the possible shapes of the adult form, since the adult
form is simply the juvenile form with some more calcium carbonate added onto
it. We will see how gastropods have worked within these limitations (or, in
some cases, overcome them) as they evolved many interesting and ingenious defenses.
Like the shells
of most mollusks, the gastropod shell is basically a coiled cone. As the animal
grows, it adds new shell to the open end of the cone (the aperture), and the aperture
becomes wider as the cone becomes longer. Since the gastropod often retracts
its body completely within the shell, the volume of the shell limits the animal’s
maximum body size. If we ignore coiling for the moment, there is only one way
in which a cone-shaped shell can vary. For a given body size (or shell volume),
you can either grow a short, fat cone, or a long, skinny cone. There are trade-offs
between these options. A short, fat cone will be more compact and easier to carry,
but the large aperture gives predators and parasites convenient access to the
soft parts. A long, skinny cone will be cumbersome, but it will have a small
opening that limits the ability of predators to attack. However, the skinny cone
may be more susceptible to crushing, since the mouth or claw of a predator can
more easily reach around it.
| These
two cones have the same volume, so the snails that lived in them would have
equivalently sized bodies. |
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With a coiled
shell, the gastropod avoids the weaknesses of both types of straight cone.
Most gastropods have a shell that is a long, skinny cone that is coiled. It
has the small aperture of the skinny cone, but is wrapped around to form a more
compact, easy-to-carry bundle. By coiling helically instead of in a plane,
the shell is made even more compact and globular for a given shell volume.
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Spiraling in a plane |
Helical spiral |
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Paleozoic Gastropods
Straparolus (Amphiscapha)
catilloides, Euomphalus sp., Platyostoma niagaranensis, Platyceras (Platyostoma)
shumardi, Yunnania leavenworthana
Compared to the Mesozoic
and Cenozoic, there was relatively little predation on gastropods in the Paleozoic
Era. Most gastropods were fairly small—the Platyceras is a giant for
a Paleozoic species, though nothing special by Meso- and Cenozoic standards.
Take a look at the Straparolus.
Shells that spiral in a plane (planispiral) are present in Paleozoic gastropods,
but become much less abundant in later marine settings. Which other group of
mollusks commonly has a planispiral shell? Why might planispiral shells be
more common than helical shells in that group?
Look at Euomphalus.
The successive whorls of this shell are not in contact with each other. Again,
this is a morphology that is much less common after the Paleozoic. If there
had been abundant predators that crushed shells at the time, why might the Straparolus
and Euomphalus be relatively easy prey?
Predators on the Rise
In the Mesozoic and Cenozoic,
there was an explosion of predators in the marine realm. In order to understand
the evolutionary response of gastropods, we must know the predators against
whom they are defending, and how the predators attack. The following list is
not comprehensive, as there are many other ways gastropods are attacked. The
following are emphasized because the corresponding defensive morphologies are
well-studied.
• Shell crushers––Some
fish have platy teeth between which they can crush shells, and many crabs and
lobsters can crush with their claws or mandibles. There is a tremendous increase
in durophagous (crushing) crabs and lobsters in the Mesozoic and Cenozoic.
• Shell Drillers––Many
gastropods can drill holes into the shells of other gastropods, bivalves, echinoderms,
etc. The drilling is accomplished by the secretion of an acid which dissolves
a hole in the victim’s calcium carbonate skeleton. The snail then injects digestive
juices, and sucks out the meal. These drill holes are often preserved in fossil
shells. Moon snails and murex snails are the most common drillers. (Drilling
gastropods are also commonly called boring gastropods, to the delight of pun-loving
paleontologists and marine biologists everywhere.)
• Shell peelers––Some
crabs will insert their claws into the aperture of a gastropod and rip off a
bit of the edge. They will continue to “peel” back the edge of the shell until
they hit the meat or give up.
Check out the crab with
the big, muscular claws and the crushing tooth plate of a fossil ray. Also
look at the big moon snails. The fleshy foot of these beasts could extend well
beyond the shell as they plowed through soft sediments looking for prey. They
wrap their big foot around the prey and drill a small, round hole. Look at
the small moon snails (Polinices cateroides), and note the drill holes.
The beveled nature of the holes indicates that these poor critters were eaten
by other moon snails. It’s a cruel world.
Cenozoic Gastropods
It has been
suggested that the evolution of more competent predators has driven prey organisms
to evolve better defenses. The predators then evolve better methods of attack
to overcome the defenses, and an ecological “arms race” ensues. Anyone who
is interested in this topic should read Evolution and Escalation by Geerat
J. Vermeij. Cenozoic gastropods show an array of defenses not seen in their
Paleozoic relatives.
Strombus gigas
These hardy shells line
many a garden plot in Florida and the Bahamas. (The shell contents also make
great conch fritters.) They use the thick, outer edge of the aperture to pry
open bivalve shells, as shown below. How could the large size and thick shell
of this gastropod foil many attacks?
Opercula
Many gastropods have an
operculum, a “trapdoor” cover for the shell aperture. Most of these are small
and organic (like the brown example), but some extraordinary opercula are massive
and calcified (the big white one). Against what types of predatory attack would
an operculum be useless?
The Limpet Form: Fissurella
and Crepidula fornicata
Many gastropods have evolved
a flat, cap-shaped shell with a large aperture. This might seem at first like
a rather poor shape for avoiding predation. Why might this be such a successful
form? (Hint: Do you know where limpets live?) Also, what function might the
ridges on the shell of the keyhole limpet (Fissurella) serve?
Pteroceras lambis, Pterocera
chiragra
Many gastropods have a
terminal growth stage. A growing juvenile looks like a fairly ordinary gastropod,
but at a certain point all growth stops and the animal deviates from the normal
pattern of spiral accretion and adds elaborate features. This allows a much
greater range of morphologies than is provided by simple spiral accretion, but
further growth is impossible. What are some possible advantages and disadvantages
of this strategy? How might the spines on this shell deter predators?
Try to identify three other
gastropods on display that show a terminal growth stage. Also identify three
that do not. Why, based on a few specimens, can you definitively identify one
species as having a terminal growth stage, but not definitively establish that
another species lacks it?
Murex pecten
This species belongs to
the family Muricidae, which along with the moon snails are the major gastropod
drillers. Members of the family are often spiny and colorful, making them popular
among collectors. Notice the two rows of spines on the outer whorl of the shell—as
the gastropod continues to grow, the lower row will be overgrown by the aperture!
This snail has the limited ability to resorb (destroy) shell material (e.g.,
spines), allowing it to break the strictures of simple accretionary growth.
Why might a driller need such good defenses?
Many of the Murex's
spines project from a long, thin tube, the siphonal canal. A portion of the mantle
is folded into a fleshy tube (the siphon) that projects from the end of this tube.
The snail sucks water in through this tube and can smell prey in the water. Many
(but not all) predatory snails have a projecting canal, or at least a notch in
the shell in the same position (e.g., Pteroceras). Predatory snails eat
bivalves, snails, fish, worms, etc. Using this clue, find 3 other gastropods
in this lab that were predators. Also, find one that is a predator but lacks
this feature.
Xenophora calculifera
Why are these called “carrier
shells?” Take a look at the pictures of the common carrier shell on Plate
42 of A Field Guide to Shells. This snail’s camouflage is presumably
an anti-predatory device. What types of predators would be fooled by such camouflage?
Oliva sayana
Do the holes in several
of the olive shells look like predatory drill holes (compare with the small
moon snails seen in the predator display)? If not, try to think of some other
reasons why these shells might have holes in them. Compare the aperture of
the olive shells and the cone shell to the apertures found in Paleozoic gastropods.
How is it different, and why might this be useful against a shell-peeling crab?
Conus literatus
Like Murex, Conus
can absorb previously secreted shell material. It absorbs the juvenile shell
whorls that are completely enclosed by the final whorl, thus allowing more space
inside the outer shell wall for the body. It cleverly circumvents a major constraint
on gastropod growth. Conus is also neat because it has a poisonous,
harpoon-like tooth that it shoots at its prey. They attack and devour worms,
mollusks, and even fish! The sting of some cone snails is lethal to humans.
What anti-predatory modifications does Conus display?
Cassis, Distortrix,
Chemopus, and Cypraea
Identify some anti-predation
features on these shells. Also, take a guess at the origin of the genus name
Distortrix.
Trochus fatulus
Not all Cenozoic gastropods
have elaborate adaptations to foil predators—these top shells look much like
some Paleozoic species. Brainstorm—why haven’t all gastropods evolved anti-predation
morphologies?
Buccinum undatum
and Fusus colus
Sometimes a shell-peeler
will fail, and the lucky prey will continue growing. What evidence can you
find of a failed peeling attack on these shells?
Turritella terebralis
First off, do you think
both of these belong to the species Turritella terebralis? Do the Turritella
look well suited to resist predation? What strategies other than shell morphology
might this snail use to resist predation? (Brainstorm!)
Sea Slugs
Look at the bookmarked
pictures of sea slugs in the book. How might these shell-less gastropods resist
predation? (Hint: what does bright coloring often indicate in other organisms,
like butterflies?)
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B16 Course