Marine Mussels. Elizabeth Gosling

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Figure 1.2 Phylogenetic diagram showing hypothesised relationships between the major clades recognised for the living members of the class Bivalvia. While formal endings imply certain ranks in the Linnaean hierarchy, no attempt was made to re‐rank all hypothesised clades from Bieler et al.’s study (2014) as they considered ‘such a step as premature until a denser family‐level sampling is presented’.

      Source: Based on Bieler et al. (2014).

Evolution and Adaptive Radiation in Bivalvia

      As already mentioned, bivalves, because of their strong shells, provide one of the most complete fossil records of any animal group. The earliest known bivalves are the Early Cambrian (540 mya) Pojetaia runnegari from Australia, the Middle Cambrian (510 mya) Tuarangia gravgaerdensis from New Zealand and the Upper Cambrian (485 mya) Fordilla troyensis (details in Giribet 2008). These were all <1 cm in length and possessed a bivalved shell, a simple ligament, bivalve‐like pedal muscle insertions and well‐developed adductor muscle insertions (Morton 1992). According to Morton (1996), they were probably surface dwellers that used the foot for feeding and locomotion. Protobranchs closely resembling modern‐day representatives (Figure 1.2) were widespread by the early Ordovician (485 mya), as were all extant lineages and feeding types from most other lineages (see later). Protobranchs are clearly separated from the rest of the Bivalvia by a number of distinct morphological traits that are unique to the group (see earlier), but whether these represent an ancestral or a derived condition (i.e. traits that have evolved as a result of the clade's evolution) is not clear (Cope 2000 and references therein). Results from a comprehensive cladistic analysis that incorporated both morphological and molecular characters show that Protobranchia are basal to the entire Bivalvia class (Giribet & Wheeler 2002).

One of the most significant developments in the evolution of modern bivalves from the more primitive form (Figure 1.3a) was moving the site of water intake to the posterior of the animal so that water both enters and exits the mantle cavity posteriorly (Figure 1.3d; see alternative evolutionary pathway in Figure 1.3b). This made it possible for bivalves to penetrate sand or mud ‘head first’, with the posterior end freely communicating with the water above. Extensions of the mantle to form siphons at the posterior enabled the animals to live deeper and deeper under the surface. As bivalves evolved, plankton in the incoming current were increasingly adopted as a source of food, the ctenidia (gills) replacing the palp processes as the feeding organs (see Chapter 2 for details on mussel morphology). The chief modification of the ctenidia for filtering was the lengthening and folding of individual gill filaments. In addition, many extra filaments were added so that they extended as far forward as the labial palps. Both of these modifications greatly increased the surface area of the ctenidia. It is believed that the triangular‐shaped filaments of the primitive bivalve gill progressively changed over evolutionary time to the W‐shaped filaments of the modern bivalve gill (see Cannuel et al. 2009). A notch at the bottom of each side of the W lines up with similar notches on adjacent filaments to form a food groove that extends the length of the underside of the ctenidia. Yonge (1941) suggested that since the food groove was necessary for nutrition, these notches probably preceded folding of the gill filaments. Changes in both ciliation and water circulation followed. The exploitation of filter feeding led to the first increase in bivalve diversity and body plan divergence, so that by the Ordovician period (~450 mya) all extant higher lineages and feeding types were present and had colonised a wide variety of habitats that had hitherto been inaccessible to their protobranch ancestors (Giribet 2008 and references therein).