Coral reefs are interesting places,
but they are wildly misnamed. The most numerous living organisms
on coral reefs are bacteria. Other than bacteria, the most
abundant organisms on coral reefs in terms of weight or mass
of living tissue are algae. If we consider animals, most of
the animal biomass on a coral reef is, indeed, coral, but
in terms of the number of different types of animals, corals
come up short. The animal group with the greatest degree of
diversity, determined by the number of species, found on coral
reefs or in the oceans, for that matter, is the mollusks.
I decided that this column should be a celebration and discussion
of the diversity of this wonderful group that contains some
of the most beautiful, exciting and interesting of all animals.
All coral reef aquarists know what
sort of.., almost.., maybe
most reef aquarists are only vaguely aware of this group,
and what it means to them as aquarists. To most reef aquarists,
the total array of mollusks that they may think about includes
the grazing snails, the herbivorous strombid conchs, a few
specialized whelks such as Nassarius, tridacnid clams,
and little else. However, the total array of molluscan species
found on any given Indo-Pacific coral reef is likely to exceed
2,000 species (by comparison, only 800 or so species of coral
exist in all of the Indo-Pacific), and in some areas is probably
five to 10 times that number. Some of these are widespread
throughout a region; others are endemic to, or found in, only
small areas such as a single archipelago or atoll. On a coral
reef, they fill every conceivable ecological niche, from detritivore
to top carnivore, and in terms of their weights, individuals
of Tridacna gigas may be the largest individual animals
found living their lives on the reef proper. Some massive
coral heads will weigh more than the big clams, but the mass
of living tissue on these corals is relatively small. It is
worth remembering that in the late 1700's, when shipwrecked
on the Great Barrier Reef, Captain Cook fed all the men in
his expedition for two days on the flesh of one Tridacna;
these clams may be huge. Mollusks are found everywhere on
or near a reef; they are found in the waters over it, crawling
on it, burrowing in it, and crawling under it.
The mollusks form a very cohesive
and identifiable group, called the Phylum Mollusca by taxonomists.
They are not simple or uncomplicated; in fact, they are amongst
the most sophisticated of animals. It is generally considered
that only three major groups are at the pinnacle of complexity.
These are: the chordates, such as fishes, mammals and birds;
the arthropods, such as insects, crustaceans and arachnids;
and, of course, the mollusks. All of these animals have complex
tissues and organs, arranged in organ-systems.
Mollusks have numerous well-defined
characteristics setting them apart from all other animals.
- They are unsegmented; in no mollusk
is the body subdivided into discrete segments as it is in
such animals as the Annelids (bristleworms), Arthropods
(shrimp), or Chordates (fishes). Mollusk bodies sometimes
have replicated gills or shells, but these replications
are not reflected in the internal anatomy. In the segmented
annelids, for example, the rings seen on the surface of
the animal divide the animal into a replicated series of
discrete and similar internal compartments. Similarly, the
muscular and skeletal system of fishes, and humans, is made
of a series of repeated similar components.
- Mollusks have a specialized gill
that is probably the most consistent structural character
found throughout the phylum. This gill is called a
and no structure like it is found in any other group of
animals. These gills come in pairs, one on each side of
the animal, and are elongate structures, looking a bit like
fleshy feathers. In primitive mollusks they are found at
the rear of the animal in a cavity, called the mantle cavity,
under the shell.
this link to see where
the gills are on a squid. The link leads to a diagram of a
squid partially dissected. The gills are labeled "B"
(for branchium, which means gill in Latin).
- Mollusks have an exceptionally
complicated anatomy that is probably best evident in the
circulatory system. They have at least one heart (squids
and octopuses have three), arteries, and blood chambers
called sinuses or lacunae. Only the cephalopods have capillaries
and veins. The heart typically consists of one ventricle
and one to four auricles. They are true "blue bloods;"the
respiratory pigment is hemocyanin, a copper-protein complex
that is colorless when deoxygenated and blue when oxygenated.
It is not a very good respiratory pigment compared to the
hemoglobin found in such lowly animals as earthworms and
their (very) distant cousins, the humans. Blood is pumped
through the gills prior to being sent to the rest of the
body, ensuring that the tissues all receive freshly oxygenated
blood. The blood leaves the vessels near the tissues and
bathes them directly instead of passing through them in
capillaries. Although such a system sounds "sloppy"and
"inefficient,"in small animals such as most mollusks,
it is an exceptionally efficient system. For them, it is
economical since the animal doesn't need to form capillaries.
Many mollusks take an additional advantage of this "vessel-less"nature
of their circulatory system, and the blood may effectively
be used as a hydraulic fluid to pump out or inflate various
portions of their bodies, such as a tentacle or an eyestalk.
After the blood bathes the target tissue or organ, it flows
into a sinus and then back to the heart.
- Mollusks typically have a well-developed
pair of kidneys which, unlike those found in humans, are
associated directly with the heart. As in the kidneys of
humans, molluscan kidneys are capable of high pressure ultrafiltration,
selective reabsorption and active secretion. These processes
combine to produce a true urine as the carrier for nitrogenous
waste, not just the simple secretion of ammonia found in
- Mollusks possess a food-gathering
structure called a "radula."This is an organ found
in the mouth cavity of most mollusks (except in clams; that
have "lost their heads", along with all the related
head structures, including the radula) that is used to collect
food from the environment. The radula has been called a
"toothed tongue"and that name is apt, as the position
roughly corresponds to the position of tongues in humans.
The surface of the radula is covered with teeth arranged
in rows. There may be as few as one tooth per row or as
many as sixty. In some cases, the teeth are hardened and
the animal uses them as a rasp. In other cases, such as
in the grazing snails common in the aquarium hobby, these
teeth may be fairly soft, and act more like a "leaf
rake"to gather in their food. These grazers actually
apply very little pressure to the substrate, and many researchers
say the snails just "lick"their food off the substrate.
Do a web search on the term "radula"to
find a veritable plethora of images of this remarkable structure.
- Mollusks typically have at least
one calcareous shell, generally found dorsally, or on top
of the animal. This is probably the most variable of the
basic characteristics for the phylum, as the number of shells
may vary from none, in several groups, to eight, in the
chitons. The shell is secreted by the mantle, a specialized
structure that covers the dorsal or upper part of the animal.
Mollusk shells vary greatly in composition throughout the
various molluscan groups, but they typically contain protein
and calcareous components. Both calcite and aragonite are
common, often in the same shell. All of these shell parts
are secreted by the mantle. The outermost layer of the shell,
called the periostracum, is often a wholly proteinaceous
covering forming a varnish-like layer or elaborated into
hairs. The protein continues throughout the shell as the
fiber matrix that the calcareous mineral is deposited upon.
Initial shell secretion occurs at the outer, or growing
edge, of the animal's shell. Subsequent deposition by the
underlying mantle thickens the shell. In large benthic mollusks,
mostly clams and some snails, the shell may get quite thick
and heavy. Those mollusks found in the deep abyss, where
calcium carbonate is very soluble, often have shells made
totally of protein.
- Many mollusks move on a creeping
ventral foot. This foot is well supplied with mucous glands,
and they move by muscular action of the foot using the mucus
as an adhesive, or by the action of fine microscopic cilia
using the mucus as a lubricant. Other mollusks, particularly
some snails, have modified the foot into a swimming organ,
and live some or their entire lives swimming in the plankton.
- The complex molluscan brain consists
of a ring of ganglia surrounding the esophagus. In the more
advanced mollusks, 30 or more separate ganglia or large
aggregations of nerve cells, may be found in the brain.
Additionally, there are connecting nerves and tracts that
run across and between ganglia. Many mollusks are generally
quite capable of learning, and in some, such as the larger
octopuses, the sophistication of the brain is secondary
only to that found in the higher vertebrates.
- The mollusks have a characteristic
embryological development with several features peculiar
only to them, such as an arrangement of cells in the early
embryo called the "molluscan cross."Primitive
mollusks have a larva called a trochophore, similar to that
found in several invertebrate groups including annelid worms.
More derived animals have a larva called a trochophore,
found only in the mollusks.
There are few other animal groups
as economically important as mollusks. We harvest many of
them as food, others compete with us for different foods,
and introduced species are often ecologically disastrous pests
in areas as diverse as Yellowstone National Park or tropical
islands. Others are vectors or secondary hosts for some of
the most debilitating human parasites. Their effects and relationships
serve to structure and maintain many marine ecological communities
and habitats, including coral reef assemblages, as well as
large areas of the temperate marine environment.
Major Variations on the Molluscan Theme
The number of mollusks is estimated
at between 50,000 and 150,000 species. The lower numbers were
in vogue in the 1980's and 1990's and represent a reaction
to such practices as the naming of different snail color forms
and minor shell variations as separate species by nineteenth
century scientists. For example, William Healy Dall (his name
rhymes with "gal") for whom the Dall sheep and the
Dall porpoise were named, was the first curator of mollusks
at the Smithsonian. He named over 5,000 species, many of them
mollusks. Some of his species descriptions are just one or
two sentences long, such as: "It is a small white snail,
found living in Alaska."He would then show a drawing
of the animal. His next description might be, "It is
a white, small snail from Alaska" with another drawing.
As there are a lot of small white snails from Alaska, such
names or descriptions are essentially useless.
Most of the workers in the latter
part of the twentieth century thought that most of these color
forms could be "lumped" together into a few species.
Recently, however, genetic investigations are starting to
show that many species that were considered to be cohesive
"good" species, may actually consist of as many as five to
ten reproductively isolated, essentially identical "sister
species."For example, the common small blue mussel found
intertidally on beaches throughout the Northern Hemisphere
used to be considered to be one, widespread species,
It is now apparent that this "species"
is actually a group of at least four species found intermixed
throughout the range of the" lumped"
Mytilus edulis. I think it is likely that when all
of this shakes out, probably by the turn of the next century,
that molluscan diversity will be shown to be far in excess
of 150,000 species. For the present, however, it is best to
be aware of the variation in the estimates.
Much recent research has lead to a complete revision of molluscan
taxonomy in the last few years. About 30 years ago, we considered
that there were about five major subgroupings, called "Classes",
of mollusks. Present taxonomy, based largely on molecular
genetic studies as well as a better understanding of the fossil
record, recognizes at least nine classes. In the following
list, the key characters - those that can help you distinguish
what kind of mollusk you may hold in your hand - are in
bold font. The other characters are important, too, but
are secondary in the identification process. The molluscan
groups follow, and those found in aquaria
are indicated in blue:
- estimated to have about 250 species.
They lack a shell, and instead are covered in calcareous
o They are hermaphroditic.
o They lack gills.
o Solenogastres have a ventral ciliated groove that is
used to grip gorgonians as they climb on them. This is thought
to be their "foot".
o They climb on and eat gorgonians and hydroids, but there
have been no detailed ecological studies.
o Occasionally they are found in aquaria,
coming in on uncured live rock.
This non-prepossessing creature is a Solenogastre or
Neomeniid mollusk found living on the spines of a pencil
urchin collected in the Bahamas. The pencil urchin's
spines were covered with zoanthids, which this animal
was eating. The mouth is extended at the left end of
the animal. It was about an inch long. Solenogastres
found in aquaria are longer, tend to be more "wormlike",
and eat gorgonians.
Class Caudofoveata (or Aplacophora) -
estimated to have about 250 species.
o They lack a shell, and are covered in calcareous "slivers"
o They have separate sexes.
o They have gills.
o They are without any remnant of the foot, and they look
and act like worms.
o Caudofoveatans live in sediments and oozes, where they prey
upon and eat foraminiferans and other things in the sediments.
Detailed ecological studies are lacking.
o They can be found in the sediments around coral reefs, but
are rare in those areas.
o They are not reported from aquaria.
This is a Caudofoveatan or Chaetodermid mollusk, Chaetoderma.
The mouth is at the bulbous right end, and the gills
and anus at the left. The body is covered in calcareous
spicules, although they are best seen near the left
end. Chaetodermids move through sediments and eat foraminiferans
and other small animals.
Monoplacophora - estimated to have about 25 to
o They have a single limpet-like shell; similar to
fossils found at the beginning of the Paleozoic Era, 450
million years ago.
o These are classic living fossils; almost all the living
mollusks are thought to have descended from Monoplacophoran
o Their major distinguishing characteristic is that virtually
all organ systems are found in multiples,
unlike all other mollusks.
For example in Neoplina:
10 pair of lateral pedal connectives in the nervous system.
pair of pedal retractor muscles.
pair of nephridia.
pair of gills.
pair of gonads.
pair of auricles.
pair of ventricles
o Living examples are small, deep-water limpets, and their
ecology is unknown.
o They are not found on coral reefs or in reef tanks.
|Figure 3. This is the fossil
shell of a monoplacophoran mollusk, Helcionella walcotti,
that lived almost 500,000,000 years ago. Such animals
are thought to be very similar to the ancestors of most
shelled mollusks. Living monoplacophorans have smooth
shells, and tend to be found in deep water.
Polyplacophora - Chitons - estimated to have
about 500 species.
o They have eight shells embedded in a broad mantle,
called a girdle.
o They have a broad, strong foot, specialized for gripping
in areas of breaking waves.
o The head is reduced and lacks eyes; although they all
have photoreceptors that pierce the shells.
o The radula has 17 teeth per row, and some of these teeth
are highly mineralized with iron salts.
o Their gills are replicated and lie in grooves beside the
posterior half of the foot.
o Most chitons are shallow water animals specialized to
eat coralline algae.
o Generally, they have separate sexes.
o They are commonly found in reef
This chiton, Tonicella lineata, is more colorful
than are most other chitons. Many chitons found in aquaria
tend to have the shells largely covered by the tough
tissue girdle on the sides. The shells often appear
as a series of diamond-shaped white spots or plates
on the upper surface.
Gastropoda - Snails, slugs, nudibranchs - estimated
to have between 40,000 and 150,000 species.
o All gastropods have one shell as a larva, but the adults
may have one, two, or no shells.
o They have undergone torsion (the body and guts rotate
180° during embryological development), so that the
anus comes to lie over the head.
o The shell shows coiling. This coiling is independent
of, and not related to, torsion.
o There is extreme diversification of shapes and ecological
roles within this immense class. This is the most diverse
group of non-insect animals, and they have come to occupy
every habitat on the Earth's surface and are dominant animals
in many of them.
o They are very common on coral reefs
and commonly found in reef tanks.
Click Here for a Gastropod
Class Cephalopoda -
Nautilus, Squids, Octopuses - estimated to have about
o Typically, cephalopods have one shell, but a shell is
totally lacking in octopuses.
o The head is elaborated into 8 to 90 tentacles.
o They have a closed, high-pressure circulatory system.
o The cephalopod respiratory system is the most efficient
respiratory system found in the marine environment.
o They have well-developed sensory structures. Their
eyes, like those of the vertebrates, are based on the principal
of the pin-hole
camera. Essentially, as in the human eye, such eyes are
spherical fluid filled chambers with a front lens and posterior
retina. Such eyes are capable of forming excellent images.
The cephalopods have the best eye of this design in the
invertebrates, and it is as good as any vertebrate eye of
the same size.
o They possess an exceptionally well-developed brain, particularly
in the largest octopodes. They are capable of learning,
have long-term memory, and have been shown to play
using criteria developed for mammals.
o Squids are the largest of the invertebrates. The deep-sea
giant squid, Architeuthis lux may be over 60 feet
long and weigh over 2 tons. Enteroctopus dolfleini
of the Pacific Northwest Coast of North America may have
a tentacle span of about 40 feet and weigh as much as 500
o Most cephalopods are predators, generally on fishes or
o Small species may be kept successfully
in reef aquaria.
Click here for a Cephalopod
(all fossil) - estimated to have 1,000 or more species.
All rostroconchs are extinct; they are the probable ancestors
of living bivalves.
o They had a shell that was similar to the shells of
a clam, in that the rostroconch shell grew down over the
sides of the animal. Unlike the shells of a clam, there
was no hinge connecting the two sides. Instead, a solid
mass of calcareous shell material connected the two sides.
The shell appears to have grown by repeated breakage
at the juncture between the shells.
o Their ecology is unknown, but they are presumed to be
o Some species were found in fossil coral reefs.
For an image of a couple of Rostroconch
fossils, follow this link.
Bivalvia - Clams, Mussels, Scallops - estimated
to have about 10,000 species.
o All have two shells.
o All lack a head, or any of the associated structures
such as a radula or brain.
o They have reduced sensory and locomotory capabilities
compared to most other mollusks.
o The shells are connected together dorsally by a rubber-like
hinge that is a derivation of the protein of the shells.
o The shells enclose the body in a hydrodynamic space
used for suspension feeding in most of them.
o The gills are elaborated into large organs used for respiration
o One group of predatory species is common in the deep sea
and rare elsewhere.
o Most bivalves are specialized to live in sediments or
in rocks. In coral reef areas they commonly burrow into
coral heads. A few species, such as mussels, scallops, and
tridacnids are found on the surface of substrata. Hermaphroditism
o They are common on coral reefs and
common in reef tanks, although most species cannot be easily
or successfully kept for the long term.
Click here for a Bivalve
Class Scaphopoda - Tooth
or Tusk shells - estimated to have about 600 species.
o All have a singular tubular shell open at both ends.
o They lack gills.
o They possess several hundred specialized feeding tentacles
used to capture their prey.
o The head is present, but reduced, and cannot extend from
o They have a large radula used as a grinding organ.
o The foot is large and used to pull the animal through
o They have separate sexes.
o They are specialized to live in sediments.
o All are predators, and most seem to specialize as predators
o They are found in coral reef areas and some have symbiotic
relationships with solitary corals.
o They are not reported from reef tanks.
Much information about scaphopods may be found at this most
excellent site: http://www.rshimek.com/Scaph1.htm
|Figure 5. This scaphopod, Fissidentalium
actiniophorum, generally carries a brownish sea anemone
on the top of the shell (animal "F" lacks the
sea anemone, and the yellow arrows point to the retracted
mouth opening of two of the anemones). Some scaphopods
found in sediment patches around coral reefs carry solitary
corals in a similar manner but, unfortunately, I don't
have any photos of those. Scaphopods have a conical tubular
shell open at both ends, and normally live oriented horizontally
in sediments. These animals are about 3 inches long and
were collected from sediments under water 13,500 feet
Mollusks with names of note:
Abra cadabra: a clam.
Bufonaria borisbeckeri (marine snail); named after
the tennis player.
Amaurotoma zappa; Anomphalus jaggerius (fossil snails);
named after the musicians.
Aaadonta and Zyzzyxdonta (terrestrial snails)
both named by the late Alan Solem with the idea of being the
first and the last entries in any list of endodontoid snails.
He also named, for you fans of Charles Dickens, Ba humbugi;
a terrestrial snail from Mba island, Fiji.)
And last, but not least
. Trivia (a genus of
The abundance, diversity, and economic importance of mollusks
is reflected in the amount of information about them available
to the casual searcher and the serious student. Quite literally,
there are libraries devoted to them. I hope I have given enough
information to pique interest about them. I have listed some
easy to find treatments discussing mollusks in the Suggested
References section that follows. In addition to the links
listed in this article, there are literally thousands of others
leading to various molluscan topics.