DIVERSITY OF LIFE

DESCRIPTION OF THE PHYLUM MOLLUSCA (LINNAEUS 1758)

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Mollusca (mo-LUS-ka) is derived from the Latin word molluscus, which means soft.  Linnaeus (1758) coined the name of this phylum.  

INTRODUCTION TO THE MOLLUSCA

The mollusks are among the most diverse, and well-known of the invertebrate groups and include the clams, snails, tusk shells, chitons, and squids.  Nielsen (2001) identifies 5 synapomorphies that define the phylum: the mantle, the foot, the radula, and pectinate gills.   The central nervous system is made of a brain and a pair of longitudinal nerve cords (three in Bivalvia).  The brain is made of three pairs of ganglia that form a ring around the esophagus.

Brusca and Brusca (2003) suggest that the mollusks arose from within the spiralians before the advent of segmentation and a sister group relationship with the sipunculans (they share similar larval characteristics).   However, recent examinations of the Eutrochozoa [e.g. Struck et al. (2007) and Zrzavy et al. (2009)] suggest that the sipunculids arose from within the annelids; thus any apparent relationship with the mollusks is superficial.

The relationships between the classes of mollusks are complex and somewhat contentious.  Six different phylogenetic topologies are given by Adamkewicz et al. (1997).  Although molecular studies helped to resolve most of them, several competing theories of the evolutionary relationships of higher taxa within the mollusks remain.  Sigwart and Sutton (2007) explored two of the phylogenetic topologies: the Aciculiferan and Testarian models (see Figure 1).  The Aculiferan theory (Figure 1-a) suggests that the mollusks are divided into two unequal monophyletic groups: the Aculifera and the Conchifera.  The Aculifera includes groups that are referred to as the spiny mollusks, that is, they have a hard skin (cuticula) that has calcareous spicules.  The Aciculifera may have multiple calcareous plates (as in the chitons) but they do not have large shells that cover the body of the animal.  A second monophyletic clade, the Conchifera, has animals that make shells.

The alternative Testarian theory (Figure 1-b) has the Neomeniomorpha (Solenogastres) and Chaetodermomorpha (Caudofoveata) as basal groups with Polyplacophora as a sister group to all of the shell-bearing taxa.  Figure 2, a figure from Giribet et al. (2006) presents a very different structure of the mollusks.  The interdigitation of conchiferan taxa with aciculiferan taxa and the somewhat basal position of the scaphopods suggests that the conchiferan structure is primitive and the loss of shells occurred multiple times (also as suggested by octopuses and slugs).  The Monoplacophora and Polyplacophora, groups with serially  repeated structures in the mollusks, emerge as a monophyletic group that is sister to one group of the Bivalvia (Pteromorphia).  A second group of Bivalvia (Heteromorphia) emerges as sister to the other Bivalvia+Polyplacophora-Monoplacophora+Gastropoda.  Clearly, the relationships of the mollusk groups need much more work.

We have chosen to work with a modification of Ruppert et al. (2004; see Figure 3).  Figure 3 most closely approximates the topology of the Testaria theory (Figure 1-b).  The cladogram is modified by information from Pechenik (2005), Valentine (2004), and Sigwart and Sutton (2007).

 

 

FIGURE 1.  This is figure 1 from Sigwart and Sutton (2007).  The main difference between the two views is the monophyly of Neomeniomorpha (Solenogastres), Chaetodermomorpha (Caudofoveata), and Polyplacophora in the Aculiferan hypothesis.

FIGURE 2. This is figure 2 from Giribet et al. (2006).  Monoplacophora is nested within the Polyplacophora.  Bivalvia emerges as a paraphyletic group.  Scaphopoda, Solenogastres (Neomeniomorpha ), Caudofoveata (Chaetodermomorpha), and the Cephalopoda make a monophyletic clade.

 

MAJOR CLADES OF THE PHYLUM MOLLUSCA

 

1.  Molluscan Clade

2.  Aplacoporan Clade

3.  Neomeniomorpha Clade

4.  Chaetodermomorpha Clade

5.  Eumolluscan Clade

6.  Polyplacophoran Clade

7.  Conchiferan Clade

8.  Monoplacophoran Clade

9.  Ganglioneuran Clade

10. Rhacopodan Clade

11. Gastropod Clade

12. Cephalopod Clade

13. Ancyropodan Clade

14. Scaphopod Clade

15. Bivalve Clade

FIGURE 3. MAJOR CLADES OF THE MOLLUSCA WITHIN THE PROTOSTOMATA.  The cladogram was modified from Ruppert et al. (2004) and informed by Pechenik (2005), Valentine (2004), Sigwart and Sutton (2007).  Higher taxa within the Mollusca are in bold and within the shaded box.

 

 
DESCRIPTIONS OF THE CLADES  

The Aplacophoran Clade (2)

The aplacophorans are laterally-compressed marine worms.  Some burrow in the mud or live in interstitial spaces.  Many aplacophorans live among corals and feed on them.  They have no shell, but the mantle, which covers almost all of the body except for a ventral groove, does have several layers of calcareous bodies.  They have a preoral sense organ and a subterminal ventral mouth.  They do not have ctenidia, but sometimes do have secondary gills.  There are no specialized excretory organs.  Some of the aplacophorans do not have a radula.  They have ganglia that are fused with both ventral and dorsolateral longitudinal nerve cords.  

The Neomeniomorpha Clade (3)

Most often they are referred to as solenogasters (from the formal name Solenogastres).  These animals have no gills, and the foot is present but highly reduced (see Figure 4).  They are hermaphroditic.

The Chaetodermomorpha Clade (4)

This group has no foot, but they do have an oral shield and gills (see Figure 5).  Also, they have separate sexes.


FIGURE 4


FIGURE 5

The Eumolluscan Clade (5)

Members of this clade have a radula (Figure 6) with typical complex musculature and with cartilaginous supports.


FIGURE 6

The Polyplacophoran Clade (6)

The chitons (Figure 7) are marine animals, which can be seen on rocks feeding on algae in the intertidal zone.  Elongate or oval and  dorsoventrally flattened, chitons are bilaterally symmetrical.  The most obvious distinguishing feature is the dorsal shell of eight overlapping plates embedded in, and sometimes covered by tissue.  They have a large, muscular, ventral foot and a poorly-differentiated head without eyes or tentacles.  The mantle cavity is a groove around the foot, with 6-88 pairs of ctenidia, which together with the overlapping plates, gives it a segmented appearance.  The animals feed with a radula, and the anus is  subterminal.    The sexes are separate and most taxa have larval stages. 


FIGURE 7

The Conchiferan Clade (7)

The most obvious characteristic of this clade is that the shells, when present, are all one piece, or paired.  

 

The Monoplacophoran Clade (8)

The monoplacophorans (Figure 8) are small, deep sea, snail-like animals.  They are almost bilaterally symmetrical with single cap- or cone-shaped dorsal shell so that they resemble a limpet.  The body has a distinct head, but there are no eyes or sensory tentacles (except around the mouth).  The foot is weakly muscular; anus median, posterior; mantle cavity large, extending laterally and posteriorly around the foot with 5-6 pairs of ctenidia; 8 pairs of pedal-retractor muscles; 6 metanephridia; sexes separate; fertilization external.  Only about 20 extant species are known, but many more are presumed to occur in the deep oceans.

Monoplacophorans are sisters to all other conchiferan groups (gastropods, cephalopods, scaphopods, and bivalves).  Likely, they resemble the basal organisms that gave rise to all other conchiferan taxa.  Furthermore, they were well represented in the fossil record from the Cambrian to the Devonian and thought to be extinct.  Then, some living animals were found off the Pacific coast of Costa Rica in 1952 (cited in Ruppert et al. 2004).


FIGURE 8

The Ganglioneuran Clade (9)

Nerve cell bodies are concentrated in paired ganglia.  The foot retractor muscles reduced to two.

 

The Rhacopodan Clade (10)

The mantle cavity is posterior but through torsion during development can be twisted to the front.  The shell is conical, though it can exhibit torsion.  The head can be retracted and extended.

 

The Gastropod Clade (11)

The snails are the most speciose group in the Mollusca.  These animals, the snails and slugs are decidedly asymmetrical.  The shell usually is spiraled or coiled and the body of the animal can retract into it.  The body is rotated during development so that the mantle cavity is anterior (see the explanation under Rhacopodan Clade).  There is a single pair of bipectinate ctenidia, but they are often reduced or lost altogether.  The head has eyes and tentacle-like antennae.  The mouth often has jaws as well as a radula.  Taxa may be hermaphroditic or have separate sexes and fertilization may be internal or external.  Also, members of the gastropods may have planktonic larva or direct development.  There are three great groups of gastropods: Prosobranchs, Opisthobranchs, and Pulmonates.

Prosobranchs:  More than half of all snails are prosobranchs whose defining character is the anterior mantle cavity containing the gill or ctenidium.  Of the more than 20,000 species, most are grazers, but a few have become carnivores and even parasitic.  The prosobranchs are the sister group to all other gastropods, and they can be identified by the occurrence of an operculum.  

The cone shells (Conus sp., Figure 9) are carnivorous prosobranchs that can inject powerful toxins into their prey by means of a specialized hollow radula tooth.  Their venoms are under intense study that may lead to new treatments for pain, depression, and epilepsy.

Opisthobranchs: These are all marine and have a lateral (or even posterior) mantle cavity.  The group includes the sea hares and sea slugs, groups that have lost their shells.  They have limited torsion during development and a reduction or loss of ctenidia.  The nudibranch sea slugs have no ctenidia and gas exchange occurs across elaborations of the dorsal surface.  At least one species of nudibranch is able to separate the chloroplasts from the algal cells that it consumes and move them into the dorsal extensions, thus becoming a photosynthetic animal.  Similarly, some species are able to use nematocysts and move those into the dorsal extensions as a defensive strategy.  Many of the nudibranchs are quite colorful (e.g. The Spanish Shawl, Figure 10).

Pulmonates: The pulmonate snails are somewhat speciose (~17,000 species) and most of them are freshwater and terrestrial.  They include the most common snails of streams, ponds, and lakes as well as slugs and terrestrial snails (e.g. escargot).  The most obvious defining feature is that the mantle cavity serves as a "lung" which can ventilate both air and water, depending on the species.  Some have evolved gills secondarily as specialized folds of the mantle.  The Garden Snail, Helix (Figure 11), is native to Europe, but it has been distributed throughout the world where it has become a garden pest in many places.


FIGURE 9


FIGURE 10


FIGURE 11

The Cephalopod Clade (12)

Cephalopods (octopods, squids, and nautili) are bilaterally symmetrical with a linearly-chambered shell that has characteristic sutures between the chambers.  However, often the shells are reduced or lost.  When the external shell is present, the animal inhabits the last chamber, and a thin filament of living tissue (the siphuncle) extends through the older chambers.  The head is large and well-defined (Figure 12).  It has large, complex eyes and a circle of prehensile tentacles around mouth, which is equipped with a radula and a beak.  The mantle is muscular and has a large ventral cavity, which contains the gills.  The gill cavity opens to the outside by means of a reduced foot forming a siphon through which water forced by contraction of mantle, providing jet propulsion.  The sexes are separate, and some tentacles of males are modified for copulation.  Cephalopods are benthic or pelagic, and entirely marine.

Cephalopods are represented by about 700 species; however, they were much more dominant in the past.   Straight-shelled nautiloids were some of the most common predators of the Paleozoic.  They were supplanted in the Mesozoic by ammonites (Figure 13), large, generally coiled-shelled, cephalopods with elaborately developed sutures between the chambers.  Likely, they were the primary food for mosasaurs, and, like the mosasaurs and non-avian dinosaurs, became victims of the Cretaceous-Tertiary extinction catastrophe.

Unlike all other mollusks, cephalopods are relatively intelligent, social animals with elaborate behaviors.  Squids and octopuses can change color rapidly, which likely is a means of communication, and they perceive such color changes with nearly vertebrate-like eyes (Figure 14).  The intelligence of these animals can be attested to by anyone who has tried to keep an octopus as a pet.


FIGURE 12


FIGURE 13


FIGURE 14

The Ancyropodan Clade (13)

These animals have a foot adapted for digging and are bivalve (the two have fused in the case of the scaphopods).  The shells hinge dorsally and open ventrally.

 

The Scaphopod Clade (14)

The Tusk Shells (~500 extant species) are bilaterally symmetrical, with an elongate body in a tubular one-piece shell, that is tapered and open at each end (Figure 15).  Often the shell is curved like an elephant-tusk.  The mantle cavity is large and extends along the whole ventral surface.  They have no gills.  The head of the Tusk Shell has no eyes, but it has paired clusters of clubbed contractile tentacles (capitula).  The head and cylindrical foot can emerge from the larger end of the tapered shell.  The sexes are separate, and fertilization is external.  All Tusk Shells are benthic and marine. 


FIGURE 15

The Bivalve Clade (15)

The Bivalves, also called Pelecypods (~8,000 extant species), are bilaterally symmetrical benthic freshwater and marine animals.  They have laterally compressed bodies enclosed within two calcareous, lateral shells, each usually with a beak-like umbo, hinged dorsally by an elastic ligament and closed by large adductor muscles.  The mantle cavity is large, and the posterior edges of mantle sometimes fuse to form siphons (incurrent and excurrent).  They have one pair of ctenidia, which are relatively large in most species and used for filter-feeding.  They have almost no head, which has only a mouth with palps.  There are no eyes or radula.  The foot is laterally compressed, often greatly reduced.  Some have modified the foot to form a burrowing organ.  Sexes usually are separate and fertilization is external.  Larval stages are aquatic, benthic, sedentary or sessile.  The unionid clams with semi-parasitic larvae called glochidia.  

Palaeotaxodonta (Protobranchia): The gills are very small and similar to those of the gastropods.  The primary feeding structures are the palps that surround the mouth.  They probe the mud and capture food particles with mucus.  They also take in sediment and digest food materials of the organic fraction of the mud.  They are among the dominant benthic marine animals in the deep oceans. The members of the group called the Nut Clams (Nuculids; ex: Acila, Figure 16) are typical in having a row of short teeth on the shell.

Cryptodonta: They have elongate shells without hinges and have gills similar to those of the Palaeotaxodonta.  Most are extinct.

Pteriomorphia: The gills are modified as filters for food as well as gas exchange organs, the lamellibranch condition.  They also are able to secrete a strong attachment thread, the byssus, that holds the animal to a substrate surface.    Also, most marine taxa are lamellibranchs.  

Oysters (Ostrea and relatives, see Figure 17) include a group of warm water lamellibranchs that are economically important as food and a source of pearls.  They occur in brackish and freshwater environments and typical lamellibranch in their form and biology.  They are filter-feeders and cement themselves to any hard surface at hand.  Oyster fisheries have suffered in estuaries like Chesapeake Bay where high levels of sedimentation and periodic anoxia have caused the local populations to collapse.  Food oysters and pearl oysters are different from each other.

Mussels (Figure 18) also are economically important as food sources.  They occur in colder marine waters, and attach themselves to substrates by byssal threads.  They can tolerate periodic exposure to air and, therefore, grow in the tidal zone.

Scallops have shells that have distinctive ridges and are symmetrical.  The animals are benthic but not attached.  The leading edge of the mantle that emerges from the shell has a series of simple eyes (Figure 19) that allow minimal detection of motion.  They can escape from a potential predator by opening and closing the valves rapidly.

Paleoheterodonta: These are lamellibranch clams that have teeth in a single row.  

Unionid Clams are the most common bivalves in freshwater.  They can be common in certain areas, but they are succumbing to the requirements of their life cycles.  Mature unionids retain the young in a marsupium until they develop into small larvae that look like staple-removers called glochidia.  The glochidia upon release move passively in the water until they are pulled into the mouth of a fish.   If it is an appropriate species, the glochidium clamps onto the gill of the fish and continues to develop.  The fish does not provide nutrients, but brings food particles to the developing larva.  In addition, the fish moves about the stream and allows for the dispersal, especially upstream, of the clam population.  The problem is that fish which tend to do the most migration (e.g. eels and shad) are impeded by dams and other human-made obstructions of the stream.  Thus, clams without appropriate host fish, are growing as geriatric populations in many areas.  Some species of unionids evolved marsupia that resemble small fish.  A large fish attacking what it thinks is prey will then get a mouth full of glochidia.

Heterodonta: These lamellibranch clams have teeth that are two sizes: short cardinal teeth and long lateral teeth.  

These include clams and cockles, many of which are commercially important.   Many can stay under the sand or mud and reach the water by means of a long siphon.  They also include the Giant Clam (Tridacna, Figure 21) which occur in the tropics of the Indian and western Pacific oceans and can grow to 200 kilograms.

Zebra Mussels are natives of the Volga Basin in Russia, but have become invasive exotic animals in North America.   They are lamellibranchs that resemble mussels, including the obvious byssus and somewhat triangular shape.  They reproduce like most mollusks with a free-swimming veliger larva (Figure 22).  The mussels are small, but they do tend to clog water intakes.  They also are quite prolific in that a female can produce up to one million eggs per year!

Anaomalodesmata: These bivalves have shells that do not communicate articulate by a hinge.  They are similar to lamellibranchs in their distribution (not in freshwater) and modes of feeding.  However, some have evolved to become carnivorous. 


FIGURE 16


FIGURE 17


FIGURE 18


FIGURE 19


FIGURE 20


FIGURE 21


FIGURE 22

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By Jack R. Holt.  Last revised: 02/01/2015