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RGIZ#13 Molluscs Aquarium Net 1298

RGIZ#13 Introduction to the Phylum Mollusc by Rob Toonen, Aquarium Net has numerous articles written by the leading authors for the advanced aquarist

Reefkeeper's Guide to Invertebrate Zoology:

Part 13: Introduction to the Molluscs

By Rob Toonen

The last article summed up the end of the section on worms, and now it’s time to move on to the molluscs (snails, slugs, clams, squids and octopuses among others). For those of you that were surprised how long it took to get through the worms, you’d better get comfortable – it will take even more time (and articles) to get through the molluscs, because there is such remarkable diversity within this group.

I would hazard to guess that molluscs are among the best known of invertebrate groups, in large part because shells have been collected and used as tools, containers, musical instruments, money, and decorations since the dawn of man. Virtually everyone knows what a clam or snail is, and molluscs have served as important food items to humans, both historically and today (for example, the world squid and octopus fishery alone exceeds 2 million metric tons caught per year). Lets start off with what exactly is a mollusc? Well, there are over 50,000 living mollusc species described, and an additional 60,000 extinct ones (Brusca & Brusca 1990). This is probably a gross underestimate of the total number of mollusc diversity, however, because there are many undescribed species found every year in underexplored regions such as the deep and arctic seas, and both terrestrial and freshwater habitats in the tropics. For example, E.O. Wilson estimates that only half of the living mollusc species have been described. Obviously the molluscs are a diverse and successful group of animals.

In total, there are eight classes of molluscs. There are the three major and very familiar groups: 1) clams and mussels (Bivalvia), 2) snails and slugs (Gastropoda) and 3) squids and octopuses (Cephalopoda). Then there are the less familiar chitons (Polyplacophora), and four groups of which relatively few people will have heard anything about: 1) Scaphopoda, 2) Monoplacophora, 3) Aplacophora, and 4) Caudofoveata (and the relationships of this last group are still the subject of some debate). I’ll spend the majority of the these articles discussing the ecology and biology of molluscs (primarily the first 3 groups listed above) in some detail, and then I’ll try to discuss some of the specifics of the animals themselves. In this article, however, I’ll start with some of the general characteristics of the molluscs and just mention some of the things that I’ll discuss in more detail in a future article. Again, this series will focus primarily on the ecology/biology of the animals and general characteristics of the groups rather than try to answer the specifics of "what is snail X in my tank?"

The name of the Phylum Mollusca itself derives from the Latin term molluscus , meaning soft, supposedly in allusion to the similarity of some snails and clams to the "mollusca" (which is a soft nut found throughout the Old World, with a thin but hard shell). In the US, the group name is often (mis-) spelled mollusk, but in the rest of the world, the word is spelled mollusc (with the exception of certain German terms, which lack a free-standing "c" such as mulluskenkunde) . Technically, the diminutive form of a word should be derived from the proper Latin name, and thus mollusc is the correct spelling.

Molluscan taxonomy (the identification and classification of species) is, unfortunately, extensive and confusing. Since the time of Aristotle, hundreds of names for different groups of molluscs have come and gone (many of them to reappear several times in the history of the group). The name Mollusca was coined by Jontonus in 1650, but this name was applied to only the cephalopods and barnacles. Linnaeus considered all invertebrates other than insects to be part of the group Vermes (hence the term vermiform , which means worm-like), and grouped tunicates, anemones, echinoderms and polychaetes together with the molluscs. It wasn’t until the mid-1800's that barnacles were identified as crustaceans and tunicates were removed from the Mollusca.

To make matters worse, many species (especially among the gastropods) have several, or even hundreds of synonymous names. This results in part because of the long history of amateur shell collectors involvement with the taxonomy of the group. Shell collectors have been a positive force in the study of molluscs, by being a major force in the collection and identification of molluscs (especially gastropods) since middle ages. Unfortunately, Gastropods (which comprise about 3/4 of the described molluscs) and bivalves are typically described only on the basis of the shell (the animal is boiled or bleached away), and the type specimens (original animals from which a new species is described) are often sold among private shell collectors rather than being deposited into an institutional research collection (those maintained by Universities and Museums). Thus, the strong impetus provided by shell collectors has also been a double-edged sword in that: 1) it is impossible to sample the DNA from the shell alone to test morphological classifications (based on size, shape and presence absence of structures), and 2) these type specimens are often prized members of private collections and are therefore unavailable to the scientific community for study. This leads to snails that exhibit different shell forms in different environments (such as Nucella sp. on this coast) often being described as different species, and without access to the original type specimens, it is difficult or sometimes impossible to confirm questionable species assignments. In the end, this means that the total number of species, and the exact groupings of those species is highly debated. For example, the total number of living mollusc species ranges from 50,000 in Brusca & Brusca (1990), to 60,000 in Ruppert & Barnes (1994), to 100,000 in Kozloff (1990). Depending on the source you check, and which school of thought you prefer, the classification and taxonomy of the molluscs can vary widely.

OK, I’ve ranted about taxonomic concerns for long enough, now I’ll tell you what the major groups of the molluscs are, and what characteristics make each group unique. As I said above, there are 8 Classes of living molluscs and a variety of subclasses, superorders, orders and suborders. Unfortunately, as I explained above, the taxonomy is confused and contentious, so depending on the authority from which you get the taxonomy, it may be completely different than that from another source. To top problems off, there are several groups, such as the Prosobranchs (snails), which include some traditional orders (archeogastropoda & mesogastropoda) that are obviously not monophyletic (the goal of taxonomy is to group all related groups into a single classification unit and exclude any non-related groups. If the unit of classification includes all members of a group, and only members of that group, it is said to be monophyletic . If it is missing some members that should be included within it -- usually because their body form is surprisingly different than the rest – or includes some members that do not belong -- usually because they have similar body forms despite being unrelated – it is not a monophyletic taxon). The classification that I follow (Brusca & Brusca 1990) is presented below (Figure 1), but remember this is likely to be somewhat different from that presented in a variety of other sources, and is likely to change, in any event, as some of the problems with the classification of the Molluscs gets worked out. Following each groups name, I have tried to include the common name for that group, or a number of the common members with which people might be familiar ( underlined ). Obviously those groups that live in the deep sea, or are found primarily at depth will be of little interest to aquarists, because you will never encounter one in a petshop. I mention all the groups, however, for the sake of completeness. Hopefully the classification will make a bit more sense to people if you have an animal with which to associate each name...

Figure 1. Living groups of Molluscs: (After Brusca & Brusca 1990)

  • Class Caudofaveata ( no common name – deep sea )
  • Class Scaphopoda ( tusk shells )
  • Class Aplacophora (= Solenogasters) ( no common name – most > 200m depth )
  • Class Monoplacophora ( no common name – most > 200m depth )
  • Class Polyplacophora ( chitons )
  • Class Bivalvia (= Pelecypoda, = Lamellibranchiata) ( all the 2-shelled critters )
    • Subclass Protobranchia ( no real common name )
    • Subclass Lamellibranchia ( just about every bivalve you can think of )
      • Superorder Filibranchia (= Pteriomorpha) ( mussels, oysters & scallops )
      • Superorder Eulamellibranchia (= Heterodonta)
        • Order Paleoheterodonta ( freshwater clams & a few Aussie species )
        • Order Veneroida ( cockles, tridachnids & surf clams )
  • Class Gastropoda ( snails, slugs & their kin )
    • Subclass Prosobranchia ( snails )
      • Order Archeogastropoda" ( abalones, limpets, trochids, & nerites )
      • Order Mesogastropoda" ( periwinkles, vermitids, ceriths & conchs )
      • Order Neogastropoda ( whelks, miters, muricids, cone & olive shells )
    • Subclass Opisthobranchia ( sea slugs )
      • Order Aplysiomorpha (= Anaspidea) ( sea hares )
      • Order Notaspidea (= Plerobranchomorpha) ( sea slugs )
      • Order Sacoglossa (=Ascoglossa) ( sea slugs )
      • Order Cephalaspidea (= Bullomorpha) ( sea slugs )
      • Order Nudibranchia ( sea slugs )
        • Suborder Doridacea ( dorids )
        • Suborder Aeolidacea ( aeolids )
        • Suborder Dendronotacea ( dendronotids )
        • Suborder Arminacea ( sea slugs )
    • Subclass Pulmonata ( land and freshwater snails and slugs )
  • Class Cephalapoda (= Siphonopoda) ( cepahlopods )
    • Subclass Nautiloidea (= Tetrabranchiata) ( chambered nautilus )
    • Subclass Coleoidea (= Dibranchiata) ( squids, cuttlefishes & octopuses )
      • Order Sepioida ( cuttlefishes )
      • Order Teuthoida (= Decapoda) ( squids )
      • Order Octopoda ( octopuses )

Now I certainly don’t expect anyone to remember the details of this figure, but I just wanted to give you an idea for the level of diversity in the molluscs. The only group among which there is greater diversity is the arthropods (which is next on the list), but it’s going to take quite a while before I get there. You shouldn’t take this list too seriously, though (this is basically what we expect our students in Invertebrate Zoology to know for their exams), especially because I expect the membership and definition of these groups will change in the near future. Primarily, I am giving you this list for two reasons: 1) because I am now going to go on to discuss them each in some greater-or-lesser detail, and 2) I wanted to have a single place where all the groups which I will discuss are listed for people to refer to later. It may take me a while to get through all this information, but if you see a name on this list, I’ll eventually discuss that group in more detail in a future article.

So with that out of the way, let’s talk about the general characteristics of the Phylum Mollusca. What is it exactly that makes a mollusc unique? Well, this list is going to be even tougher to digest at first than the taxonomic diversity, but bear with me, and I’ll discuss a number of characteristics that link the Mollusca together. First, although they are coelomate animals (possess a body cavity), the cavity is generally reduced to small vestiges around the metanephridia (the molluscan equivalent of kidneys), heart, gonads (reproductive organs) and part of the intestine (humans are also coelomate, but unlike molluscs our body cavity is large and well-developed). The principle body cavity is instead the hemocoel (an open circulatory system – I’ll come back to this in a minute). Second, the body itself is covered by a thick epidermal/cuticular layer of "skin" (the mantle ) which forms a cavity (the mantle cavity ) in which are housed the ctenidia ("gills"), osphradia ("nose" for lack of a better term), nephridopores (the site of waste excretion form the nephridia – this is the molluscan equivalent of peeing), the gonopores (the site of gamete release), and the anus . The mantle contains shell glands which secrete calcareous epidermal spicules, shell plates, or shells to protect/support the animal. In addition to the mantle, there is usually a large, well-defined and muscular foot (a snail crawls around on it’s foot). In gastropods, the foot has a large flattened "sole" for crawling, whereas in bivalves it can be modified for burrowing or attaching byssal threads (the "hairs" that attach a mussel or clam to a rock). Third, with the exception of bivalves, all molluscs possess a muscular buccal region ("throat") with a radula . The radula is (in its simplest form) a toothed ribbon that the animal uses to collect it’s food. The form of the teeth, and how the radula is used may be highly modified in many species (I’ll come back to this later), but in many snails it works more-or-less like a chainsaw to lift particles from the substrate into the mouth. Finally, the developmental pattern of virtually all molluscs is similar. The fertilized egg develops into a ciliated ball (termed a trochophore larva), which typically develops into a veliger larva (again, I’ll come back and discuss this in more detail in the next article).

So, what exactly do I mean when I say that the circulatory system is open? Well, imagine a design that was more-or-less like the circulation pattern in your reef tank. You have a sump in which water from the main tank collects and is run through a skimmer which aerates the water, before a pump of some sort pushes that water back up into the tank. Well, the circulatory system of molluscs is pretty similar to that. They have a hemocoel (open cavity) in which hemolymph (blood which carries specialized cells such as amebocytes) is pumped. Oxygenated hemolymph is collected from the ctenidia ("gills") and pumped into a variety of sinuses (open spaces) where the tissues and organs are bathed in the oxygen-rich "blood" (sorta like the critters in our tanks are bathed in the oxygen-rich water). As the hemolymph passes over the tissues in the sinuses, it pools into the ctendial vessels (this would be the equivalent of overflowing into the sump) where gas exchange occurs and is once again drawn into the atria to be pumped out of the heart. I’ll discuss the circulatory system in a little more detail next time, when I’ll go through all the characters I just mentioned above.

OK, like I said, I realize that this detail is really a lot to digest all at once, but it is really more of an outline for the sections that are to follow. I will take some time in the next couple of articles to try to explain the details of each thing that I just mentioned above, and then will spend several articles discussing the biology and ecology of each of the groups I listed in Figure 1 (which is probably the bottom line for which most people are waiting).

But... I figured that this article already had more than enough information crammed into it (and to digest in one sitting), so until next time, good luck, and happy reefing!

Literature Cited:

Brusca, R.C., & G.J. Brusca, 1990. Invertebrates. Sinauer Associates, Inc. Sunderland, Mass.

Kozloff, E.N. 1990. Invertebrates . Saunders College Publishing, Harcourt Brace College Publishers, Philadelphia, PA.

Ruppert, E.E. & R.D. Barnes, 1994. Invertebrate Zoology, 6 th Edition . Saunders College Publishing, Harcourt Brace College Publishers, Orlando, FL

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Last modified 2006-11-23 01:39