Marine
by Lynn Scheu
When we think of shells, it is
seashells that probably come to mind first, in a multiplicity
of shapes, colors and patterns. Scallops, cowries, conchs and
cones in rainbow hues. Lacy frills, elongate spikes, gleaming
egg shapes, huge bowls and tiny rice grains, all characterize
marine mollusks.
Mollusks first evolved in the
sea and have been adapting to its changing ecological niches
for nearly 600 million years. So it is not surprising that marine
species exhibit the phylum's greatest diversity. Mollusca in
general exhibit more morphological diversity than any other
phylum, including arthropods. The vast difference between the
giant squid and microscopic nuculid clams is unmatched in the
animal kingdom.
All seven of the existing classes
of the Mollusca are present in the ocean; only the Bivalvia
(clams) and Gastropoda (snails) have moved into freshwater;
Gastropoda is the sole class to adapt to life on dry land.
Invertebrates, animals without
backbones, are commonly presented with a special survival problem
-- soft bodies, easily snapped up by a hungry predator. The
shells which mollusks have evolved function to help them with
this difficulty by presenting a tough exterior to predators
in search of a meal. It bears mention here that many marine
mollusks get along fine without a shell, having adapted in other
ways which help them escape being eaten, at least long enough
to reproduce their kind. Squid are fast swimmers and many species
travel in schools. The octopus jets out a blob of "ink"
to distract and fool a predator. Many sea slugs are able to
consume stinging cells of other animals like coral. They transfer
these natural deterrents to their own body cells where they
function to instruct predators to avoid their kind. But in general,
the phylum has flourished through epochs of geologic time wearing
hard shells that afford excellent protection against all but
the most determined foe.
Mollusks have also adopted an
amazing array of life styles and habitats. Some groups are carnivores,
some are strict vegetarians, others are scavengers or parasites
or commensals; the bivalves, for the most part are sedentary
filter feeders, but some are predacious. Marine mollusks burrow,
creep, tunnel, float or swim. They make their homes in mud,
sand, silt, coral grit, rock, shell, tidal pool or grass.
With such a multitude of lifestyles,
it is not surprising that any number of evolutionary experiments
and accidents have proved successful, explaining in part the
fabulous diversity of pattern, size, and form among marine mollusks.
Pattern often functions as camouflage. Wide flaring lips on
conchs can help protect extended, grazing soft parts. Elongate
anterior canals can likewise protect a mollusk's vulnerable
siphonal "nose," stretched out to detect food or foe.
Extended teeth on some muricids function as pry bars to pop
open the protective plates of their food, barnacles. Low, flattened
shells on limpets can withstand wave pressure. The high spires
on many species aid in streamlining. Convoluted apertures function
to keep out would-be predators. Thickened ribs add strength
to shells of many species without adding commensurate weight
and "construction costs." Specialized slits on abalones
and others send waste products away from their water intake.
In some cases mollusks from many different lineages have produced
similar shells, probably because of a similarity of lifestyles,
and similar constraints on how to build shells using only a
logarithmic spiral, using only what their ancestors gave them.
It seems to most of us that we'll
never become familiar with all the types of mollusks in the
sea. And then, just when we begin to feel comfortable, we discover
the entire array of microscopic mollusks waiting for us to discover
them, or the hundreds of millions of years worth of extinct
shells, preserved as fossils, that occur in our rocks and wash
out in streams. Or we find that we'd like to narrow our scope
somewhat by specializing in a particular group of mollusks.
There's always something new to hold our interest, some fresh
discovery, or new scientific advances leading to changes in
the taxonomy of mollusks. Some new family of bivalves catches
our attention, or the intricacy of chiton plates (Class Polyplacophora)
fascinates us, or we decide we'd like to specialize in the tusk
shells, Class Scaphopoda, for a while. And we're off again,
on a new pursuit of the magnificent diversity that is marine
mollusks.
FRESHWATER
| TERRESTRIAL
