Pholadoidea, Lamarck, 1809

Monari, Stefano, 2009, Phylogeny and biogeography of pholadid bivalve Barnea (Anchomasa) with considerations on the phylogeny of Pholadoidea, Acta Palaeontologica Polonica 54 (2), pp. 315-335 : 322-323

Phylogeny of Pholadoidea

Cladistic analysis confirms the interpretation of numerous characters sustained by Hoagland and Turner (1981) and the gross topology of the cladogram recalls that obtained by them ( Hoagland and Turner 1981: text−fig. 5, tab. 11) ( Fig. 5 ) using the characters presumed to be unique and unreversed. In fact, the most consistent characters selected by the reweighting process are indeed those indicated by Hoagland and Turner (1981) as highly significant for the phylogeny of pholadoideans. However, the analysis at genus and subgenus levels shows remarkable differences. These are evident in comparing the structure of the selected cladogram with Turner’s (1969) taxonomic arrangement ( Fig. 3 ). Furthermore, it suggests that obscure areas of the phylogeny deserve further research.

Exclusive synapomorphies supporting the ingroup node are represented by the presence of the umbonal reflection (character 5), the position of the umbo located well to the anterior (character 3), the cylindrical shape of the foot with discoid extremity adapted for suction (character 38), and the presence of imbrications on the commarginal ridges of the anterior slope (character 10), changing to fine denticulations in the wood−boring groups. The presence of apophyses (character 15) and the long ctenidia (character 41) are also important characters showing significant reversals in the ingroup (see below).

The subfamilies Pholadinae and Martesiinae (sensu Turner, 1969) are not monophyletic. As described above, a monophyletic group of Pholadinae is exclusively composed of taxa having the protoplax. The Pholadinae clade is the sister−group of a major clade consisting of taxa having the umbonal−ventral ridge (character 17) and ventral condyle (character 19), the latter secondarily lost in Jouannetia .

The major clade includes two groups, namely Martesiinae – Jouannetiinae and Xylophagainae – Teredinidae . The Martesiinae – Jouannetiinae clade is well−supported by two exclusive synapomorphies, i.e., the presence of the callum closing the pedal gape (character 21) and the consequent atrophy of the adult foot (character 37). However, the Jouannetiinae are monophyletic and well−supported by Majority Rule and bootstrap values, whereas the Martesiinae are paraphyletic. In fact, Martesiinae lack synapomorphies. The states of the characters differentiating the Martesiinae from Jouannetiinae , i.e., the presence of the apophyses (character 15) and the long ctenidia (character 41), are plesiomorphies of the Pholadoidea undergoing reversals in the Jouannetiinae . The Martesiinae – Jouannetiinae clade is subdivided in two main groups, one characterised by the presence of a hypoplax (character 31) and the other, including Jouannetiinae , having the siphonoplax (character 32) as a distinctive character.

According to Haga and Kase (2007), the Xylophagainae are phylogenetically distant from the Teredinidae and, consequently, the symbiosis with xylotrophic bacteria arose independently in these groups. On the contrary, the present study shows that the Xylophagainae and Teredinidae form a well−supported monophyletic group. This corresponds to the results of the molecular analysis made by Giribet and Distel (2003) in which Bankia Gray, 1842 is the sister−group of Xylophaga Turton, 1822 . Thus, the position of Xylophagainae as a subfamily of Pholadidae as proposed by Turner (1955, 1969), is not justified. Exclusive synapomorphies are the gills beyond the posterior adductor (character 40) and composed of a single demibranch (character 39). The Xylophagainae lose the apophyses (character 15) independently from the Jouannetiinae .

The structure of the cladogram here selected is in accordance with Hoagland and Turner’s (1981) opinion in showing that the obligate wood−borers originated from the groups boring into inorganic substrates. However, a parallel evolution of the wood−boring adaptation in the Xylophagainae and Teredinidae cannot be excluded. In fact, the wood−storing caecum (character 45), which is commonly considered as the most important character reflecting the ability of wood−boring groups to use the wood for food ( Purchon 1941; Hoagland and Turner 1981), is an ambiguous synapomorphy. In ACCTRAN optimisation it is a synapomorphy of the Xylophagainae – Teredinidae group which is secondarily lost in Kuphus . This would support a monophyletic origin of the xylotrophy in the pholadoideans. Alternatively, DELTRAN optimisation treats the character as originating independently in the Xylophagainae and in the Teredininae – Bankiinae clades. In this case, the absence of a wood−storing caecum and, possibly, the filter−feeding and mud−boring mode of life of Kuphus are plesiomorphies inherited from the ancestors of the obligate wood−boring groups.

Insofar as the origin of the filter−feeding wood−borers is concerned, Ito (1999) and Haga and Kase (2007) maintained that the anterior boring mechanism of the rock− and wood−borers is primary and the groups with ventral boring mechanisms (sand− and mud−borers) evolved later. This hypothesis is supported by the fossil record which testifies that the wood−borers Opertochasma Stephenson, 1952 and Turnus Gabb, 1864 are the oldest pholadoideans known (Tithonian; Kelly 1988 a, 1988b). According to Haga and Kase (2007), the highly specialised wood−boring groups are the most archaic pholadoideans and the great diversification of the superfamily originated by simplification of the shell morphology. On the contrary, the present analysis indicates that the filter−feeding wood−borers are the most derivative members of two Martesiinae clades. This represents a more parsimonious scenario of the evolutionary pathways of the pholadoideans in which the specialised forms, characterised by a high number of strongly adaptive apomorphies, arose from groups with a more generalist mode of life. As expected, the characters reflecting the adaptation to the wood−boring mode of life, i.e., the finely denticulate ornament of the anterior slope, the small and auricle−like prora giving the shape of an angulate embayment to the wide pedal gape, and the robust ventral condyle ( Hoagland and Turner, 1981) developed independently in the Martesiinae clades and in the obligate wood−boring groups. As underlined by Kelly (1988a), the incompleteness of the fossil record and the lack of detailed studies on some problematic taxa do preclude definitive answers.

Figures

Fig. 5. Phylogenetic reconstruction of Pholadoidea and most significant state changes proposed by Hoagland and Turner (1981: tab. 11, text−fig. 5).

Fig. 3. Phylogeny of Pholadoidea. Most parsimonious tree corresponding to the 50% Majority Rule consensus tree obtained by successive approximation weighting. L 96. Indices under unitary weight conditions:CI 0.54, RI 0.85, RC 0.46. Indices under reweight conditions:CI 0.78, RI 0.94, RC 0.74. Abbreviations: L, length; CI, consistency index; RI, retention index; RC, rescaled consistency index. Numbers in bold indicate the Majority Rule values and, in brackets, the bootstrap support values of the individual nodes. Only bootstrap values higher than 40% are shown. Numbers in smaller type represent the most significant state changes at the respective nodes. Exclusive synapomorphies are in bold. Unambiguous not−exclusive synapomorphies are in normal type. Abbreviations A and D mark the ambiguous changes of character states under ACCTRAN and DELTRAN optimisations, respectively. For changes of character states in the Pholadinae clade see Fig. 4. The classification of Pholadoidea in Turner (1969) is shown besides the tree by comparison.