In this article we will discuss about the phylogeny and phylogenetic relationship of phylum Mollusca.
Phylogeny of Mollusca:
The phylogeny of Mollusca is a controversial issue. It was previously argued that Mollusca originated from annelids, generally assumption based on the evidence of fossilized monoplacophorans.
But this theory was largely rejected because the body structure of Neopilina galatheae has revealed a semi-segmentation (metamerism) that weakens the argument that mollusc has evolved from the annelid. But Sigwart and Sutton (2007) concluded that the ancestral mollusc was metameric and it had a foot for cereeping and a shell was mineralized.
Another school of zoologists argue that molluscs have evolved from turbellarian flatworm ancestry because both groups lack a coelom and body segmentation.
With the discovery of living mono- placophorans the relationship between Mollusca and Annelida has revived and are being discussed. The exact relationship among different classes of Mollusca on the basis of evolution, has not yet been clearly determined.
According to opinions of modern authors like Stasek (1972) and Yochelsen (1978), two evolutionary lines propose the origin of different classes of Mollusca. One line indicates that monoplacophora is the nearest relative of earliest ancestor of Mollusca and another line includes gastropods, bivalves, scaphopods and cephalopods, those have evolved from a monoplacophoran-like ancestor.
But Ruppert and Barnes (1994) have proposed that the aplacophorans were probably an early offshoot of the ancestral molluscs and polyplacophorans, and monoplacophorans may have diverged from some common shell-less ancestor.
Analysing both morphological and molecular phylogenetics it can be concluded that the molluscs are not monophyletic, in particular that Scaphopoda and Bivalvia are both separate, monophyletic lineages and the rest molluscan classes follow another lineage. In other words, the traditional phylum Mollusca is polyphyletic and it can be monophytic if scaphopods and bivalves are excluded.
Phylogenetic Relationship of Mollusca:
Tunicates, Brachiopods, Bryozoans and Annelids were long regarded to be closely related to Molluscs. Excepting Annelids, relationship with other groups cannot be taken with confidence. The structural and functional similarities with the Tunicates specially in the respiratory and feeding mechanism may be regarded as an instance of physiological convergence without having any phylogenetic significance.
The affinities with the Brachiopods and Bryozoans do not warrant any attention, as their structural organisations differ widely. Indications of relationship between Molluscs and Annelids are quite evident from the similarity in the developmental history of the two groups. Existence of Trochophore larva in the two groups is quite significant.
The discovery of Neopilina galatheae has added more support to the annelidan relationship of Molluscs. Neopilina forms a sort of connecting link between Molluscs and Annelids. Based on the typical molluscan plan, Neopilina exhibits segmental arrangement of structures, like shell muscles, auricles, nephridia, gills and gonads.
This occurrence cannot be regarded as a chance of coincidence, but must be due to a phylogenetic relationship. Further research on this line is necessary to establish a definite phylogenetic connection between Annelids and Molluscs.
As regards the inter-relationship of the different groups of Molluscs it can be said that the Solenogastres (= Subclass Neomeniomorpha) and the Protobranchiate Bivalves occupy the simplest rank.
But the primitive position is occupied by the class Monoplacophora which is represented by Neopilina galatheae. The Solenogastres, the Polyplacophora and the Gastropods may have evolved from a common ancestor as all of them possess paired ctenidia, paired kidneys and paired auricles.
The relationship between the Bivalves and the Gastropods leads to confusion. Presence of bivalve shell in Bivalvia and the asymmetrical anatomical configuration in Gastropoda cannot be interpreted.
The Cephalopods possess certain primitive features in their anatomy, but the presence of highly specialised features, such as the transformation of the foot into oral arms and funnel, well-formed nervous system with better-developed eyes, development of cranial cartilage make the Cephalopods completely isolated from the other groups. The Cephalopods demand the highest rank amongst the Molluscs.
