Taxonomy of the Phylum Mollusca
by Winston Barney, June, 2000
Classification is the arrangement of organisms into groups with regard
to their natural relationships (systematics) and according to established
criteria (taxonomy). The terms classification, systematics, and taxonomy
are often used interchangeably because they apply to the same concept
of arranging things in their "correct" relationship.
We classify things as soon as begin to think... as toddlers we soon learn
the different colors and shapes. We develop tastes for "good" food and
"bad" food, and learn the difference between parents, friends, and "strangers".
In our adult lives we classify many items in our daily routines: department
stores are, by definition, divided into different departments. Grocery
stores have separate sections for different kinds of items. Auto dealers,
electronic stores, fast food chains are all categorized. Most children
can sort a pile of seashells into fairly correct relationships simply
by comparing shapes. But we know that the relationships between living
things go much deeper than shape or size, so let us examine the ways that
scientists classify the subject of this site - the molluscs!
The following Classification Hierarchy chart is a visual aid to understanding
how scientists classify living things. Follow the chart from left to right
and down. There are seven major categories: Kingdom, Phylum, Class, Order,
Family, Genus, and Species (NOTE: You can make up sentences to remember
the order (called mnemonics) - how about "King Phillip Comes Over For
Greasy Sandwiches"?). Each level of classification can be
divided into as many of the next lower level as needed - for example,
some families might have only a single genus (the Family Conidae (the
Cone Shells) has only the single genus Conus, while the closely-related
Turridae are divided into anywhere between 200 and 300 genera (and about
15 sub-families!), depending upon which authority you believe!!) . The
categories beginning with the prefixes "super-" and "sub-" may be used
to further subdivide groups when needed. (in the Phylum Mollusca, the
level of "superfamily" is particularly important: Cones and Turrids, for
example, are in the same superfamily.)
The road to classifying a living organism begins with its placement
in a kingdom. There are five kingdoms of living things: Animalia (animals),
Plantae (plants), Protista (one-celled organisms), Procaryotae (bacteria
and blue-green algae) and Fungi (plural of fungus) (then there are Viruses,
which are semi-alive, since they cannot reproduce by themselves, "Viroids",
which are mini-viruses, and something called "ricketstae" (or something
close!!)). Molluscs are animals so they belong to the animal kingdom.
Within the animal kingdom there are two sub-kingdoms or branches. They
are the Invertebrata (invertebrates), which have no backbone , and the
Vertebrata (vertebrates), which have a backbone (We are mammals, which
are vertebrates of the family Chordata, class mammalia). The number
of living invertebrate species is over 800,000 (possibly up to 20,000,000!
(most experts concede at least 10 million), and the vertebrates number
around 70,000 species.
The invertebrate subkingdom contains many groups called phyla. Examples of some familiar phyla are the Protozoa (protozoans), the Porifera (sponges), the Coelenterata (jellyfish, anemones, and corals ), the Platyhelminthes (flat worms), the Nemathelminthes (round worms ), the Annelida (segmented worms), the Echindodermata (starfish, sea urchins, sand dollars ), the Mollusca (snails, slugs, clams, oysters, scallops, octopuses, squids ), and the largest phylum ("Phylum" is the singular of "Phyla") by a long shot, the Arthropoda (insects and crayfish).
The name Mollusca comes from the Latin word molluscus meaning
soft. The phylum Mollusca is made up of animals that have no backbone
and possess internal cavities containing the heart, kidneys and
gonads. Most molluscs have a hard exterior shell that is produced by
the mantle, a fleshy fold of the body, however some, such as the slugs,
have no shell at all. Some molluscs (squid and seahares (Pteropods)
produce an internal shell.
The division of the phylum Mollusca into smaller groups is based on major morphological (form, especially that of the shell) and anatomical features (i.e., internal organs) that differentiate each class from the others. Here are the ways some of the early naturalists attempted to define the classes of the phylum Mollusca:
Aristotle (384-322 B.C.) divided the mollusca into two groups:
Pliny ( the Elder, 23-79 A.D.)
Jonstonus (1650) changed the name Mollia to Mollusca
Cuvier (1795) subdivided molluscs into:
Dumeril (1806) divided molluscs into:
Lamarck (1801) divided molluscs into:
Lamarck (1815-1819) removed bivalves and brachipods from the phylum
so that mollusca now included:
By now, the reader has realized that classification is not a static thing,
but an evolving process, changing with our increasing knowledge, and new
theories of how organisms are related to each other. The early naturalists
were trying to contribute to the body of knowledge using only the specimens
on hand and very little prior literature for comparison. During the nineteenth
century scientific voyages were undertaken for the sole purpose of collecting
species of all phyla from both the animal and the plant kingdom. . The
species collected were often immediately transferred to the cabinets of
wealthy collectors and the jobs of describing and naming of these species
were undertaken later.
During this time naturalists described species from widely disparate areas of the world. At times the same species was given two different names by two different authors. This happened when an author discovered and named a new species not knowing that another author, in another place, had already described it. (see note on Linnaeus, above. One of the most extreme examples is Conus mediterraneus Hwass, 1792 (the only cone in the Mediterranean Sea), which was given over 120 names by different people over the years!! All of these names are now "synonyms" of the original name - i.e., they refer to the same thing!) Compare below some of the problems the early workers encountered and note how they are being solved in the twentieth century.
THEN: Early collectors
depended on morphological characters of shells, and gross anatomy
to classify their molluscan species.
THEN: A very
debilitating aspect was the lack of a universal plan to be obeyed by
all when naming new species. Although most followed Linnaeus'
lead in giving species a two-part (binomial) name, there was little
agreement regarding the proper rules for naming new species, until very
THEN: One of
the greatest obstacles to scientific agreement was lack of communication
among scientists in all parts of the world..
|Today we recognize these seven classes of molluscs:|
(1) GASTROPODA (gastropods). The name means "stomach-footed".
These are animals with a well developed head bearing eyes and tentacles, The foot is usually large and muscular and is especially useful for crawling. Most of the gastropods produce an outer shell which is spirally wound, but this class also includes the slugs which have no shell. A unique characteristic of the gastropods is a process called torsion, in which the immature bilaterally symmetrical animal twists itself into an asymmetrical shape, becoming spirally coiled.
(2) BIVALVIA (bivalves), also called PELECYPODA. The names mean "two-valved" and "hatchet-footed" respectively. An older name for the class was LAMELLIBRANCHIATA.
These animals produce two shells (valves) that are connected by a hinge. The animals have no head but with a foot that often extends between the two valves. Common names for some of the bivalves are clams, scallops, and oysters.
(3) SCAPHOPODA (scaphopods). The name means "boat-footed"
Animals without eyes, gills, or tentacles. They are enclosed in tusk-shaped shells, open at each end, which gives them their common name, tusk shells.
(4) APLACOPHORA (solenogasters). The name means "without plates" (without shell).
Worm-like animals having no shell . The animal has no head, mantle or nephridia (excretory organs). The foot has been reduced to a short ridge down the middle of the animal. Instead of a shell the animal is covered with tiny calcareous spicules.
(5) POLYPLACOPHORA (chitons). The name means "bearing many plates".
These are bilaterally symmetrical , flat molluscs with a shell consisting of eight overlapping plates called valves, which are attached underneath and around the edges to a tough, flexible part of the mantle called the girdle. The head has no eyes or tentacles. The animal has a radula designed for heavy-duty rasping.
(6) MONOPLACOPHORA (gastroverms and segmented limpets ). The name means "bearing one plate" (bearing one shell).
Bilaterally symmetrical cap-shaped shells. The animal is without torsion, but with head, foot, and mantle, and possessing a radula. The muscles, external gills, nerve branches, and nephridia are paired.
(7) CEPHALOPODA (cephalopods) The name means "head-footed".
This class is characterized by animals with distinct heads and large,
complex eyes. The foot has been altered into a set of arms that circle
the mouth and parrot-like jaws ("beak"). Except for the chambered nautilus,
the molluscs of this class do not have an external shell. Common names
for cephalopods are octopus, squid, cuttlefish, and nautilus.
For a much more detailed, systematic account of each Class, see the Advanced
Introduction to the Mollusca Article.
The subclasses are exceedingly important divisions in molluscan classification because they represent very significant differences among the animals in each class.
was until recently (about 15 years ago), divided into four subclasses,
as follows. As soon as i look up the diagnostic charactaristics
of the more recent divisions, i will also post them!
is divided into five subclasses:
Polyplacophora, and Monoplacophora are not divided into subclasses by
Every class of shells contains one or more orders. Each order identifies an important characteristic that is common to all the molluscs included in it. Orders are significant groups in the classification process and provide another step in the precise classification of a species.
contains 17 orders and 11 suborders.
All the names
of orders have the ending "-OIDEA". Example: STROMBOIDEA
Referring to the chart you will see that each class can be divided into several families. A family is a level of classification that consists of animals that share all the characteristics of their class, but differ from each other in other ways. For example, the families Olividae (olives) and Muricidae (murexes) both belong to the class Gastropoda, but they are instantly differentiated by their appearance, olive shells being smooth and glossy while murex shells are mostly covered with spines or leaf-like constructions.
It is customary for all the names of families to have the ending "-idae". Example: Strombidae
Here are some of the other characteristics used to determine placement of molluscs into orders, families, and genera:
At the generic level (genus) the animals are sorted into even smaller,
but more closely related, groups. This is the first part of the scientific
name is given is given to a species - the first part of its two-part
name (binomen), by which it will always be known. (example: Homo
sapiens: The genus is Homo).
The "final" step of the classification hierarchy is the specific (species)
level. At this point scientists have developed a definition that applies
to all living organisms. Although authorities do not agree
on the exact wording of the definition, the consensus is that a species
must be a population of individuals that interbreed and are able to
reproduce fertile offspring only when breeding with their own species
A subspecies is an animal that obviously belongs to a particular species except that it has developed one or more specific characters that are different from the parent group. It is geographically separated from the parent group (i.e., it is separated by geographic features such as mountains, land, etc.) but may produce intergrades where their ranges overlap. In this case the animal may be given a subspecific (subspecies) name. For example, Strombus marginatus from the Philippines varies in shape from the typical Strombus marginatus of the India, so it is given a name consisting of the genus, species and subspecies monikers. This arrangement is called a "trinomen". The Philippines subspecies is named Strombus marginatus septimus.
classification of this animal is therefore:
(VARIETIES and FORMS)
When two animals of the same species produce different shapes, colors
or patterns of color with some regularity, these variants are often
given a variation name or a form name. For example, the Strombus
marginatus from India has two forms.
One has a ridge (carina) around the body whorl, the other has no ridge
but is considerably rotund. The latter has been named Strombus
marginatus forma robustus,
whereas the former is called S. m. forma carinata.There
are basically two types of varieties or forms (the terms are often
used interchangeably - their usage in the literature is quite inconsistant!!):
It should be noted that in many cases, it is difficult to determine whether a given variety is the result of "nature" (a genotype), or "nurture" - a response to specific ecological conditions. It is also often difficult to determine if a given form has any adaptive value. However, in situations where a lot of different forms occur together in a population sharing the same environmental conditions (as in the case where there are many color & pattern forms in the same population (e.g.: the Pecten (family Pectinidae) Chlamys nobilis Linne, has literally dozens of named color and pattern forms, which all live together in the same population: this means they are probably the result of random genetic drift, and not a valid response to environmental conditions)), it is easy to infer that they are genotypes of no intrinsic adaptive value (hence useless to all but collectors who want to collect a lot of named varieties and forms!!). HOWEVER, in some cases, a genetically-determined (i.e., genotypical) form or variety may breed true, and may be found in a limited range, geographically-speaking. This occurs quite often in land snails, reaching an extreme in the Caribbean tree-snail Liguus fasciatus Muller, 1776, which has about 200 named forms in Florida and Cuba, some of which occur only on a single small mound of earth (called a "hummock") in the Everglades!
The I.C.Z.N. (the body which makes and modifies the rules of taxonomy
for all life-forms on the planet) gives limited approval for varietal
or "forma" names (for example, ecophenotypes are often considered valid,
in cases where they are useful ecophenotypic
designations for describing adaptations to specific
habitats (like the imbricata
form of Nucella lapillus
discussed above), or true-breeding
the many varieties Liguus fasciatus
,whereas albino or other color forms are not, since they can occur in
almost any population of a given species - but collectors and
seashell dealers find these to be extremely helpful in describing the
shells. Cowrie (Family Cypraeidae) collectors take this
to the greatest extreme, for some reason only known to other cowrie
IDENTIFYING A SPECIMEN "FROM SCRATCH"
When a specimen is found, there are several means of identifying it: 1) you can ask someone who knows what it is - for difficult species, such as those in deep water, an expert in the taxa (family or genus, usually), or in a particular area (e.g.: an expert in the molluscs of Florida, such as Dr. Harry G. Lee) , or for easy species, just someone who lives in the area it was found ("Oh, i knows what THAT one be: it's a Common Periwinkle, that's what it is for sure!!" - then you look up in a book and see that "common periwinkle" is Littorina litterea Linne, 1758), or 2) look in a book and compare it to correctly-identified photographs or diagrams of species in its family or the area where it was found, and reading the text of the book when in doubt (this is in fact how most species are identified: it is a matter of a) having the correct literature, and b) being familiar enough with molluscs to notice the details necessary to tell similar species apart (a matter of experience: not always as easy as it may seem, since some species are so variable that they often appear to overlap with other, very similar species!) However, suppose you are an alien, coming to earth and trying to figure out what name our species has assigned to the specimen you happen to be holding! In this case, you must painstakingly compare your specimen to the descriptions and diagrams of each taxonomic level, from phylum on down. Once you determine it is a mollusc (phylum Mollusca), you then tackle the class (the subkingdom level is not really important for identification!!): Does it have a shell? Yes? Does it have one, two or 8 parts to its shell? (if one, it is a Monoplacophoran or Gastropod, if two, it is a Bivalve, if 8, then it is a chiton, or Polyplacophoran.) One part to the shell: is it a deep-water shell shaped like a little cone? (Monoplacophoran) No. Ok, it must be a gastropod. Do you have the animal? If so, you can work on the subclass by looking at the gills or lungs, then work your way down. For some particularly difficult taxa, such as some genera of Turridae, you will find the radula quite useful at many levels, including the species level. If you DON'T have the animal, in practice you must jump right from the class level all the way down to the family level: NOT always easy!!! (although in everyday practice, this is the way those pesky humans normally do it!!) - You must painstakingly figure out just what defines each family the shell could belong to: how is a Muricid different from a Buccinidae species? How is one kind of Limpet family different from confusingly similar families?? - humans seem to be able to learn this rather easily, but it is actually quite tricky when you have to try to put it into words: get a bunch of shell collectors together and start asking difficult questions about how to tell similar families and genera apart, and you'll find out that as the alien you are for the purpose of this "thought experiment", it isn't easy at all!!! So, using all kinds of literature, some of which contradicts other literature, or in the case of the early descriptions, is INCREDIBLY vague at times, you struggle down to genus level: now - what species is it??? You look in books and articles about that genus, and for the area you found the shell, and carefully compare diagrams, drawings, photographs, descriptions in various languages - but with no luck!!! You at last think it might be a particular rare species, which is almost impossible to identify just using the literature (believe it - this happens more often than you might think!!). What now?? You must carefully compare your specimen to the original specimen(s) used in describing the species you think it might be, as well as similar species: these are called "type specimens" (see the article "How to Name a Mollusc" for details on this - it is quite fascinating!!!), and are "always" deposited in internationally-recognized collections: museums, universities, etc. - which means you must get permission to see them, then travel to where they are. OOPS!! - after all that work, you find out it is a new species, unknown to science - i.e., you can't find a type specimen anywhere which looks like it close enough - you must now either give it to an expert to be named (perhaps after yourself - Strombus borgensis (how could anyone refuse? - resistance is futile!!)), or you must become a scientist and describe and publish it yourself - so you decide Earth has too much biodiversity and you go back to your home planet, defeated by carbon-based life-form taxonomy. (and i DO hope the real author (i.e., Winston) of this article will forgive this whimsical exercise by the editor (Ross) - strange things happen after 3 a. m.!!)
Authorities still disagree on the systematic arrangement of some families, e.g., Cypraeidae, Muricidae, or Pectinidae. That is why it is important to access the most current literature. One of the problems of identifying just a shell is that some juvenile shells look nothing like their adult parents. For example, some species of Strombus are shaped like cone shells in the juvenile stage but grow expansive outer lips as adults. When one species has two different forms (dimorphism), it is difficult for the casual collector to determine the correct name if he trying to identify a juvenile. So, before you start to identify a shell without the body, you should ask yourself: Does this look like a fully-developed adult?? (is the lip thick or thin? Is the shell extremely light and fragile (usually means it is a juvenile) - if you are either our friendly, determined alien above, or a beginner in Conchology (the study of shells), or simply unfamiliar with the taxon the shell belongs to, you might be fooled rather badly, and never get to the correct family, let alone the right species (cowries are the worst! - a juvenile cowrie can challenge even world-class experts!!!)
BUT, in case you are despairing of ever being able to easily identify shells, 7 times out of 10, it is just a matter of looking at pictures in a book, until you find the name of the species whose representative you are holding (the other 3 are what experts and shell dealers are for!!).
Even after finding the correct species name, you may still have to make a choice between one classification system or another. Cowry collectors are torn between placing all their prizes in one genus (Cypraea) or in any of several systematic arrangements, the most complicated of which contains 35 genera!!! Authorities who study one family (often to the exclusion of other families) tend to split it into more genera due to details they discover through extensive comparisons of thousands of specimens. They thus earn the nickname "splitters". Splitters are also prone to use many form or variety names to identify minor differences between groups of specimens, even when these differences are of the "not useful" kind, as discussed above. Indeed many verbal wars have been waged over the validity of names. Collectors or researchers who prefer to list fewer genera in each family (often, these are "generalists" - not specializing in one narrow field, but studying or just collecting a variety of families), mainly because the shells are obviously related (as in the case of Cypraea), are labeled as "lumpers". (NOTE: For a great discussion on the "lumpers vs splitters" debate, see http://coa.acnatsci.org/conchnet/ac-lump.html.)
In the end, you will have to decide for yourself which way you want
to label your specimen, based on your study and your need or desire
for correctness: if you are a very casual collector, you could just
name your specimen "Sammy the Snail", but if you are a specialist in
a particular family, you might go with Strombus marginatus
septimus forma carinata (G. B. Adams, 1856) (ok,
i just made the date and author up - so sue me!).
It is inevitable that some scientific names will change as scientists
continue to delve into their fossil histories and study their detailed
anatomy, reproductive methods, and of course their DNA. It is also
clear that many families have not received the close scrutiny of scientists
that the more popular families have experienced. (Turridae, for example,
even though it is by far the largest family of molluscs in the world (thousands
and thousands of species so far!), is still a taxonomic "wilderness",
where you can rather quickly go insane if you aren't quite steely-minded!!)
A case in point is the family Marginellidae, which until a few years ago
was classified almost completely on shell characters. But a dedicated
malacologist Gary Coovert, and his wife, undertook the job of
analyzing the shells and animals of all species and found many characters
that begged the placement of many genera in another family. As a result
many species formerly assigned to the Marginella family are now placed
in the family Cystiscidae, which most collectors and many museum curators
completely ignore, since they know that in 20 years, another system of
taxonomic classification will probably come along, with the arrival of
easy and quick DNA sequencing and comparison (for example!), and they
will just have to change all their labels and records once again!!
It is evident that the classification of species is complicated.
But the difficulties are eased by the rigid structure of the classification
hierarchy. Everything must fit in its niche. One who wishes to pursue
the study of classifying molluscs must resolve to read books , periodicals
and internet postings, and to actually use his or her brain rather often,
no matter how hard it hurts. A daily dose of the Conch-L list server
(if you've made it this far, you really SHOULD consider joining!! - go
easy-to-follow directions on how to join this on-line discussion group.)
will heighten your enthusiasm and acquaint you with questions, answers
and discoveries from personalities throughout the conchological world.
For those who wish to get REALLY scientific, the Mollusca internet list
is the way to go: follow the easy instructions at http://www.ucmp.berkeley.edu/mologis/subscribe.html.
See you there!
Joining a local shell club and an international group,such as Conchologists
of America (C.O.A.) is another way to broaden your knowledge and
keep you current.
Happy Shelling To All !
This is a new counter system set up by Globel on
December 01, 2002