| back to shell index |
Cowries
The shell of the adult cowry really doesn't look much like a seashell at all. It's much too glossy, and it's shaped less like a snail than like an egg with a narrow slit along one side. Seashells usually are shaped like caps, as the limpets are, or they are coiled. The cowry shell appears to be neither.
Cowries are coiled
Actually, if you look carefully at the rear end of a cowry shell (if you can figure out which end is the rear end), you may see the remnants of a spire or crown along with subtle evidence of coiling. If you can bring yourself to break the shell apart, you will see even more convincing evidence of coiling within. The cowry shell indeed is coiled. In fact a juvenile cowry, as long as it continues to grow, looks like a conventional snail, possessing an obviously coiled shell with a gaping aperture. The coiling is mostly to the side of the animal, as it is with the cone shell; so the aperture extends along almost the entire length of the shell. As the young cowry grows, it extends its shell by increasing the length of the last coil (the body whorl), analogous to adding a short length to the outer end of a rope or hose lying coiled on the ground. As it completes its maturation, the cowry folds the outer lip of the aperture inward, shrinking the aperture to a small slit. As it does this, the animal also thickens and decorates the shell of the body whorl, which in the end hardly resembles a whorl at all. At this time, lengthening of the body whorl apparently stops, and further growth is limited to a modest amount of additional thickening and decoration of the outer shell. The size of the shell of the mature cowry does not seem to change appreciably, and small mature cowries of a particular species are not destined to grow up some day to become large ones. There are several other types of sea snails that, on reaching the final stages of maturation, stop lengthening their body whorls and begin thickening and decorating their shells, occasionally with spectacular results. These snails include the horse conchs, the strombs (true conchs), the spindles, and the doves.
Having a polished shell
So the shell of the cowry actually is coiled; but why is it so
highly polished? Actually the inner surfaces of all sea-snail
shells are highly polished. The difference with the cowry shell is that
the outer surface is just as polished. The reason for this is
simple; the outer surface of the cowry shell is formed in the
same way as the inner surfaces of other snail shells. So now
we have pushed the question to the inside... why is the inner
surface of a sea-snail shell so glossy? The immediate answer
is that the inner surface of the shell is composed of very flat
crystals laid out either like tiles or like slightly-overlapping
shingles. The surfaces formed by these crystals are extremely
smooth. The answer to the question will not be complete
until we understand how the snail produces such flat crystals
and lays them down in such beautiful order, When the same crystal
species (calcite and aragonite) grow in geological formations,
they tend to be cubic rather than tile- or shingle-shaped. The
same crystal species, with still different shapes, are found
arrayed in very orderly fashions over the sensitive surfaces
of the gravity-sensing organs (the saccule and utricle) of the
inner ear. In our inner ears, and those of other mammals, the
crystals are calcite; in the ears of frogs and other amphibians
they are aragonite. Mineral crystals of various species are
widespread in the animal world, being key ingredients in
skeleton, tooth and shell. Therefore, the control of crystal
formation and the shaping of mineralized structures by animal
tissues is an important general issue in biology, and one that is
far from being settled.
Part of the answer undoubtedly is the organic structure that always
seems to be laid down first and seems to provide a foundation upon or
around which crystals subsequently form. In the seashell, these
organic structures apparently are made largely of the same kinds of
tough proteins (keratins) that are major components of skin, nails
and hair. A typical pattern of seashell construction seems to be
first the laying down of a thick outer skin (the periostracum), then
the formation of a thick layer (the prismatic layer) of column-shaped
crystals just under the periostracum, followed by the laying down of
a very thin skin, then the formation of a very thin layer of tile- or
shingle-like crystals, then the laying down of another very thin
skin, another very thin layer of tiles or shingles, and so on.
When the tiles or shingles are aragonite crystals, the surface
is called the nacre; when they are calcite, it is called the
calcitostracum. The materials for all of these various
layers are secreted by specialized tissue (the mantle) lying
just inside the shell. The actual formation of each layer must be
controlled remotely by the mantle, however, since the layer always is
beyond the cell boundaries of the mantle proper.
Shell growth and repair
Control of the formation of each layer very likely involves well-timed
changes in the chemicals secreted by the mantle.
Because the formation takes place beyond the
cell boundaries of the mantle, it is especially
intriguing. In sea snails, the region of the mantle in the immediate
vicinity of the aperture (or the lip of the shell) apparently
specializes in laying the foundation (periostracum and prismatic layer)
for new shell. The remainder of the mantle apparently specializes in the
much slower process of thickening the new shell from the inside by
depositing layer after layer of glossy nacre or calcitostracum. This
organization is appropriate for normal growth of the shell (by
elongation of the body whorl) or for repair of damaged lips, but it
is not appropriate for repairing damage elsewhere in the shell.
Therefore, as one might expect, sea snails are good at repairing chipped
lips, but they are not good at repairing holes elsewhere in their
shells. As the unexposed mantle thickens the shell of the body whorl,
it simultaneously thins the unexposed shell of the previous whorls,
often leaving coils of delicate, glass-like shell as an inner skeleton.
For the cowry, the pattern of shell formation is modified slightly.
Both juvenile and adult animals have mantles that cover not only
the inner surface of the shell, but can be extended over the entire
outside of the shell as well; and it deposits glossy layers on both sides.
The shell of the mature cowry is thickened and decorated from the outside,
by deposition of many alternating layers of very flat crystals and very
thin skin.
Eating and being eaten
When their mantles are out, covering the outsides of their shells, some cowry species can be spectacularly beautiful, as ornate and colorful as some of the most brilliant sea slugs; other species may be as drab as some of the most cryptically colored sea slugs. Some cowry species can be kept easily in aquarium tanks. In fact some, such as the Honey Cowry (Cypraea helvola) and the Snakehead Cowry (Cypraea caputserpentis) will do well grazing on the small amount of algae that inevitably grow on the walls of the tank. An adult Snakehead Cowry survived on this fare for more than nine years in one of my tanks, maintaining its shell in excellent condition (failure of the edges of the mantle to join at the top of the shell, and subsequent erosion of the glossy surface are signs of stress in a cowry). Many cowries are carnivorous, browsing on coelenterates or other animals, and they probably are food specialists like the Hawaiian cones. Occasionally a carnivorous cowry can be adapted to eating liver or some other easily obtained food. If not, then identifying and providing the appropriate food specialty may be difficult. Cowries often will attack one another if kept in the same tank, apparently reflecting territoriality. Cowries also are eaten by other sea snails (the Penniform Cone clearly is predisposed to prey upon them, for example), so one must be careful in choosing their tankmates. Like some opisthobranchs, cowries may secrete acid when attacked. Their best defense against fish and other visual predators, however, probably is the fact that they generally are abroad at night and hidden during the day.
Cowries learn
The fact that some cowries can be adapted to a new diet is not
something that should be treated lightly. Although they admittedly
are very beautiful, cowries more-or-less continually grazing on
algae or occasionally munching on liver in an aquarium tank may
seem to lack behavior comparable in complexity to the fascinating
hunting activities of cones and some opisthobranchs.
They might be considered the aquarium's cattle-- behaviorally dull
and predictable, whereas the cones are the cats-- alert, active,
interestingly unpredictable. Perhaps this image of the animal
would change if it were known that the cowry can be trained;
it can learn. For nearly five years a juvenile measled
cowry (Cypraea zebra) from the Florida Keys lived in
one of my aquarium tanks with an Hawaiian olive (Oliva
paxillus sandwicensis), three cat cones,
and a small basket (Nassarius gaudiosus),
all from Maui. From the time they were captured, the olive
and basket sought out and ate pieces of liver with no
hesitation, apparently being predisposed to foraging for
underwater carrion; so once each week I placed a few pieces for
them on the sand in the bottom of the tank. When it was
placed in the tank with the other animals, the cowry
at first ignored the liver, choosing instead to graze the
algae-covered sides of the tank.
Knowing that the algae
would not be enough food for such a large cowry, and having read
that this species could be coaxed into eating liver, I
decided to try coaxing. Each time I found the cowry grazing
near the surface of the water, I held a small piece of
liver directly under its mouth, in a pair of tweezers.
At first the animal turned away, presumably disturbed by
being touched. After three or four sessions, it began
to scrape the liver with its file-like teeth. Soon after
that, I could let go of the piece of liver and the cowry
would hold it with its mouth and continue scraping. It still
would not take the liver left on the sand, and if it
dropped the piece it was holding it would not attempt to
retrieve it. After a few more feedings, however, it would
retrieve the dropped liver; and very soon it began to go
directly for the liver placed on the sand for the olive
and basket. Even if it were high on the wall of the tank,
the cowry would turn downward and head for the sand as soon
as a piece of liver was dropped into the tank. Clearly,
the cowry had learned a new search image (probably olfactory)
associated with food.
It seems obvious that learning ability has great
survival value for any animal living in a dynamic,
constantly changing community, and that learning
ability therefore is to be expected. Nevertheless, learning
in mollusks remains contoversial. J.Z. Young convincingly
demonstrated it in the octopus long ago, but convincing
evidence for learning in sea slugs was found only recently,
and learning in sea snails remains virtually unexplored.
Looking for cowries
The glossy shell of the cowry is especially susceptible to erosion
by the sea, so good beached shells are rare. Living snakehead
cowries are abundant and easy to find in Hawaiian tidepools
and many other places. Members of the other cowry species are
more difficult to find in their daytime hiding places. One
good place to look is under large, flat boulders of reef
rubble in shallow water. A snorkeler with a pair of gloves and
modest breath-holding capacity can turn over such a boulder
and inspect both the sand or hard surface beneath and the
undersurface of the boulder itself for sea snails (and other
animals). Occasionally a small moray will dart out from
beneath the boulder when it is lifted; to avoid being startled,
lift the edge farthest from you first. A diver that turns over
a boulder and fails to replace it in its original
position becomes a strong outside agent in local predation.
Small fish rapidly converge on the site and devour many of
the uncovered animals as they scurry for cover. Probably
even greater impact results from predation on newly exposed
egg masses. The fish often are so bold that they move in
as soon as the boulder is overturned, ignoring the waving
of hands, shaking of fists, and growling of the diver.
This boldness can be used to advantage by the collector of
aquarium fish.
Another place to look for cowries is in small pockets and
crevices in a solution bench, reef or basalt formation.
Here you also will find urchins and should remember to
be especially careful of the wana (long-spined urchins).
If you happen to
brush against one, even very gently, it will leave a
few spines in your skin. When this happens,
you will see a purple substance spreading outward from each
broken spin, forming a small spot in your skin and stinging
slightly as it does so. The most annoying feature, however,
is the brittleness of the spines-- they seem impossible to
remove. A small puka poker (such as a long screwdriver or
kitchen tongs) can be used to remove cowries from crevices or
from behind wana. Even with such devices, you inevitably
will swim away frustrated at times, leaving a much-desired,
beautiful cowry gleaming deep in a hole or crack. On steep,
waveswept basalt formations, large cowries occasionally
can be found in crevices very close to the water line.
Even a small surf can make the search for these animals
very difficult; larger surf can make it downright dangerous.
Endemism
Although it is not abundant anywhere, there is one cowry species
that we have found in nearly every type of underwater habitat we
could reach-- beneath small, isolated boulders on a sandy bottom
at 45 feet, inside dead coral heads on patch reefs, inside pockets
in solution benches, and inside cracks in basalt formations at the
waterline in heavy surf areas. That species is the groove-toothed
cowry (Cypraea sulcidentata). This is one of
seven cowry species presently believed to be found only in
the waters of the Hawaiian Islands, and never to have occurred
elsewhere. By virtue of these two constraints, it qualifies
as an endemic species. Applied to a species, endemic
means that throughout its existance that species has been
restricted to a particular locale, such as a particular
range of mountains, a particular valley, or a particular
group of islands. To the evolutionary biologist, endemism
implies that the species arose locally, in response to
conditions prevelant in the locale sometime in the past and
perhaps persisting to the present. How does an endemic species
arise?
More than one hundred years after Darwin revolutionized
the science of biology by addressing this very question, its
answer remains incomplete and definitely controversial.
Do new species arise explosively in response to sudden events
such as widescale opening of niches (e.g., by cataclysmic destruction
of the beings that previously occupied those niches),
or do they arise slowly, by gradual shifts in the gene
pool perhaps brought on by changes in environmental
pressures and leading to gradual shift to a new niche?
In either case, most biologists would agree that
critical ingedients in the formation of a new species seem to be
isolation from established populations of different
species that already are well adapted to the target niche
and isolation from members of the potentially evolving species
that are not facing the same selective pressures.
Just how such isolation has occurred often is unclear. Probably
the most obvious isolating mechanisms are geological or geophysical,
such as stretches of desert between mountain ranges or stretches
of ocean with unfavorable currents between island groups.
In this respect, the Hawaiian Islands presently seem to be
well isolated.
Where did they come from?
How did they get there?
Most of the inshore species that inhabit the sea around
the Hawaiian Islands also are found elswhere in the tropical waters
of the western Pacific Ocean and the Indian Ocean. Because
its marine life is more-or-less uniform and distinct from that
in other regions, this vast tropical oceanic area has been designated
the Indo-West-Pacific province. The Hawaiian Islands occupy the
extreme northeastern edge of the province; and their links to the
rest of the province are tenuous. The islands are swept by surface
currents coming from the east, driven by the tradewinds and parts
of two major oceanic circulations. One of those circulations is the
great North Pacific Gyre, which flows clockwise around the edges
of the North Pacific and thence westward across the mid-Pacific,
north of the equator. This flow leaves the Indo-West-Pacific in
the vicinity of southern Japan, and before reentering the province
at the Hawaiian Islands, it passes the Aleutians and the
west coasts of North America and Mexico.
The other circulation
lies principally to the south of the Hawaiian Islands and is
counterclockwise, so that its northern arm, the North Equatorial
Current, joins the southern arm of the North Pacific Gyre to
form a very broad, westward-flowing stream. The eastward
segment of this counterclockwise circulation is part of
a much less extensive stream, the Equatorial Counter Current,
which crosses the Pacific south of Hawaii, where the tradewinds
have yielded to the equatorial doldrums. Before reentering
the Indo-West-Pacific province from the east, both
of these circulations flow past another tropical marine province,
the Panamic. With a few exceptions, such as the Moorish Idol,
the inshore marine species of the core of the Panamic province
are distinct from those of the Indo-West-Pacific,
evidence that neither of the two circulations is
an effective transporter of marine organisms in either direction
across the Pacific Ocean. If this is true, then how are
Hawaiian inshore waters linked to those of the rest of its
province?
A possibility might be shortcircuiting one of the
two circulations. The likely candidate would be
the southern, counterclockwise flow. Its water
is warm and should be hospitable to drifting larvae
of tropical marine animals, and the distance separating
the westward segment from the eastward segment is
not nearly as great as that of the North Pacific Gyre.
The trick would be to ride the Equatorial Counter
Current to a position east of the Hawaiian chain, then
cross over to the westward North Equatorial Current,
and drift back into the Hawaiian water. With perhaps
enormous numbers of planktonic larvae begining this
journey from the islands to the west, one
might expect pure chance to provide the Hawaiian chain
with a continuous shower of young immigrants.
One problem with this scheme is the pool of nearly stagnant
water that inevitably lies at the center of any eddy and
somehow must be crossed by the drifting or swimming larvae.
Although an occasional large storm might penetrate that barrier,
there is another, perhaps even more potent barrier, and that is
time. Because the Hawaiian chain is so far from
its nearest island neighbors to the west, the distance that any
immigrant must travel on its circuitous path to the Hawaiian
Islands is enormous. Many Hawaiian inshore animals have
planktonic phases of very short duration, making the probability
of surviving the journey in that form overwhelmingly small.
Truly pelagic species, those professional oceanic peregriners
that sucessfully spend their lifetimes at sea, can and do
visit the waters around the Hawaiian Islands, and perhaps
occasionally support a nonpelagic hitch-hiker. Floating debris
could accomplish the same thing. However, for many
inshore species, the Hawaiian Islands may be totally
cut off from the rest of their province.
Even if that is true, it cannot always have been so.
Inshore Indo-West-Pacific animals are present in the Hawaiian
Islands. In previous geological times, seamounts protruded from
the surface between the Hawaiian Islands and the Marshall Islands,
and Professor Kay suggests that these may have provided
stepping-stones along the immigration route from the west.
The journey still would have been so difficult for some
species that arriving alive in Hawaiian waters would be
extremely improbable, and immigrants of those species would
have been exceedingly rare. However, as island biogeographers
are quick to point out, under appropriate conditions a single
pregnant female could be enough. Furthermore, improbable
as it may seem, very-long migrations apparently even
without stepping-stones have been accomplished by some
mollusk species in the flow of the Equatorial Counter Current.
Clipperton Island, at the extreme western edge of the
Panamic province, apparently has been colonized by
several Indo-West-Pacific sea snails. Among them are
the Hebrew Cone, the Chaldean Cone, and
six of the thirty-three cowries listed by Kay in
Hawaiian Marine Shells: C. helvola, C. maculifera,
C. moneta, C. rashleighana, C. teres, and C. vitellus. The
presence of these animals at Clipperton and their
apparent absence elsewhere in the Panamic province
is a puzzle whose solution may lie in the structure
of the Countercurrent, in the dynamic competition
of colonization, or elsewhere.
Genetic variations
Just as the human species has many physical and behavioral
varieties, so do other species; and when a particular variety
is determined genetically, it very likely will be the
consequence (expression) of several genes rather than one.
The combinations found in a generation of offspring clearly
will be shaped by the pool of genes available among the
parents. With a given gene pool, certain gene combinations
will be likely to occur, others will be relatively rare,
and still others will be so unlikely to occur that the
corresponding physical or behavioral variety is never
seen among the limited number of offspring. In a given locale,
conditions may favor certain physical or behavioral
traits. For example, there might be more secure daytime hiding
places for small cowries than for large ones, so that a
larger proportion of smaller members of a species would survive to
contribute their genes to the next generation; or there might
be more-nutritious food available in deeper water, so that
those members of a species that instinctively favored deeper
water might be better nourished, consequently able to produce
more and healthier offspring, thus contributing more
genes to the next generation.
Therefore, in a given locale,one would expect the gene pool to change
slightly from
generation to generation. In other locales, conditions might
favor different traits. Migration and interbreeding among members
of a species from diffrent locales would reshuffle the gene pools in
all locales and slow the shaping of the local gene pool by local
conditions. Isolation would allow local shaping to move rapidly.
As the gene pool shifted in response to local conditions, some
combinations of genes that previously had been impossibly rare
would become relatively probable, and heretofore unseen traits
would emerge to be tested against the environment. As this
process continued, generation after generation, the members
of the isolated population could become so drastically changed,
physically and behaviorally, that the casual observer might
guess that they were not closely related to other members of the
species from which they arose.
Island biogeography
The conditions shaping the local gene pool of an animal or plant species
in a young group of islands would be determined by two major factors,
one being the physical setting itself and the other being the chance
colonization of the islands by the various immigrants to them. Because of
the tenuous route from the other Indo-West-Pacific islands, even
with stepping-stones available, some inshore animals and plants
abundant elsewhere in the province never would establish colonies
in Hawaiian waters. Thus, any newly arriving immigrant would find
not only a new physical setting but also a mixture of established
inshore plant and animal life thoroughly unlike that found elsewhere
in the province. Because of these differences, survival would
be easier for some species, more difficult for others in the new
environment; and the competitions between species for food,
territories, and the like would have new outcomes. While some
species thrived, others, perhaps abundant elsewhere in the province,
would establish colonies in the new place only eventually to become
extinct there. In fact, extinctions seem to be commonplace in
island communities of animals and plants,
and they definitely appear to have been so in the Hawaiian
Islands. As one might imagine, extinctions could
produce geographical distributions of a species that would
be exceedingly puzzling (e.g., extinctions in the center
of a province could lead to widely-dispersed pockets
of a species at the periphery).
Species that successfully colonize a locale will occupy
a particular niche in the local community. Niche
in this ecological sense is a multidimensional entity
that one might conceive as being bounded by the tolerance
limits of the members of a species. Since members with
different genetic composition (different genotypes)
may have different tolerance limits, the boundaries
of a niche must be considered to be fuzzy. It
is this fuzziness that would allow the sort of
gradualistic evolution that I described in the
previous paragraphs. The upper and lower temperature
tolerance limits, the upper and lower pH tolerance
limits, the upper and lower oxygen-tension tolerance
limits, the upper and lower salinity tolerance limits,
and so forth, all would define boundaries of the
niche (but note that probably none of those tolerance
limits is likely to be independent of the other
environmental variables; e.g., the upper pH
tolerance limit very likely will depend on the
temperature).
Other dimensions of the niche
may include background color or pattern (e.g., the range of
colors or patterns over which the the animal is adequately
camouflaged either to avoid intense predation or to
be an effective predator), substrate types (e.g., the
range of substrates into which the animal can effectively
burrow), foods (e.g., the range of organisms that
the animal can exploit for adequate nutrition),
and so forth. The niche also must incorporate
the tolerance limits (of various sorts) for the animal's
egg-, larval-, and juvenile-forms. Clearly, even without
genetic variability, the notion of the niche is
sufficiently fuzzy. There is an old ecological principle
of questionable value that states "no two species
can occupy the same niche in the same locale at the same
time." Given the fuzzy and multidimensional quality of the
definition of niche, most modern ecologists probably
would restate the principle as follows: "the intensity of the
competition between two species (in the same locale at the
same time) will increase as the degree of overlap of their
niches increases."
There are many possible
outcomes of such competion, among them being-- extinction
of an unsuccessful competitor, coexistence and sharing
of resources by competing species without shifts in their
genetic pools, coexistence and sharing of resources by
competing species after reduction in niche overlap through shifts
in the genetic pools. The last alternative corresponds to the
gradualistic evolution described previously. It has been
proposed as the mechanism that led, among other things,
to the adaptive radiation among the Galapagos finches.
The purpose of this paragraph, however, was to lead up to
another principal, one described in MacArthur and Wilson's
Island Biogeography, namely that a new immigrant
species very likely will be
unsuccessful in its attempts to colonize an island locale
where its niche will overlap greatly with that of an
already-established colonizer. This might be the reason
so many Indo-West-Pacific Cowries were able to obtain
footholds on Clipperton, but were unsuccessful elsewhere
in the Panamic province; Clipperton may have been the
only place with relatively open niches.
Following the logic of MacArthur and Wilson,
imagine that the immigrants of a given species of cowry into this
setting are very rare, but that two of the early immigrants
managed to establish a successful colony, whose population
eventually became large. From that time on, any tiny influx
of new immigrants very likely would have no effect on the
local gene pool, and effective isolation would have been
achieved even if the stepping-stone islands were still
present. Their absence makes isolation even more certain.
The setting now would be ripe for rapid shifts in the gene
pool of the colony and the arising of an endemic species.
Oceanic islands typically are rich in endemic species,
particularly among terrestrial animals. Occasionally, a cluster
of such islands will have an entire group of endemic species
that are closely related to one another, but are so different
from the members of any other group that they are given the
status of a seperate taxonomic grouping, such as a genus
or family. For example, the Seychelle Islands have an
endemic group of frogs that has been designated as a
distinct family (the sooglossids); and the Hawaiian Islands
have an endemic group of birds given the same status (the
drepanids, or Hawaiian honeycreepers). The arising of
entire endemic groups such as these is believed to
result from a process called adaptive radiation,
whereby early colonizers of a single species give rise to
a whole array of new species. The major puzzle in this
process is how the arising species manage to stay
sufficiently isolated from one another to avoid
genetic shuffling. Each instance of effective isolation may
very well involve a different set of solutions to the
puzzle. With birds and frogs, important ingredients
undoubtedly are the advertisement songs or calls that
help establish breeding territories and attract mates.
In some instances, slight variations in song or call patterns
are sufficient to produce breeding segregation among
individuals of a single bird- or frog-species.
Among the eight cowry species listed by Kay as being Hawaiian
endemics, two are relatively common and easy for the snorkeler
to find-- the Granulated Cowry and the Groove-toothed Cowry; three are uncommon
or more difficult to find, but live within easy reach of the
snorkeler-- Gaskoin's Cowry, the Halfswimmer Cowry, and the Maui
Cowry; one is most common at depths that stretch a snorkeler's breatholding capacity-- the
Checkered Cowry; and two are rare, deep-water animals-- Rashleigh's Cowry and
Ostergaard's Cowry.