Anomalodesmata, a deep‐sea group of marine clams with unusual morphology, and the remaining Heterodonta, to which Bieler et al. (2014) have applied the name ‘Imparidentia’, a sister group to Anomalodesmata. The new clade Imparidentia, the relationships of which are not fully resolved (Bieler et al. 2014; González et al. 2015), includes some of the best‐known bivalves and most of the commercial species, excluding mussels, oysters and scallops, all of which are members of the Pteriomorphia.
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).
Figure 1.3 Evolution of the heteromyarian form, and ultimately of the monomyarian form, from an isomyarian ancestor. (a) Postulated primitive isomyarian bivalve such as the protobranch Nucula or the heterodont Glycymeris, with water capable of entering the mantle from anterior and posterior directions. (b) Selection of the anterior inhalant stream by representatives of such groups as the Lucinida (shallow burrowers in tropical mud) can only result in the process of heteromyarianisation, leading to (c) a modioliform shell found in representatives of the Arcida. (d) Selection of the posterior inhalant stream can result in full expression of (e) the heteromyarian form (e.g. in the mussel Mytilus) and ultimately (f) the monomyarian form (in pteriomorph oyster and scallop species). See also Figure 1.2.
Source: From Morton (1992). Reproduced with permission from Elsevier.
An important factor in this diversification was the development of a larval byssal apparatus in the basal Autobranchia; this was absent in the Protobranchia. Byssal threads provided an effective method of attachment on to hard substrates, enabling bivalves to adopt an epibenthic lifestyle in new adaptive niches (Giribet 2008). The byssal apparatus is seen as a persistent post‐larval structure that evolved for temporary attachment of the animal to the substrate during the vulnerable stage of metamorphosis. While the byssus is subsequently lost in many bivalve species, in mussels it persists into adult life. In byssally‐attached forms, there has been a tendency for the anterior (head) end of the animal to become smaller, with a corresponding enlargement of the posterior end, thereby allowing free access, posteriorly, to the water above (Morton 1992). Accompanying this change, there has also been a reduction of the anterior adductor muscle and an increase in the size of the posterior adductor muscle. This led to the evolution of the heteromyarian form, a pronounced triangular shape brought about through reduction of the anterior and enlargement of the posterior ends of the shell, which is very marked in the Mytilida (Figure 1.3c and e). The heteromyarian form, coupled with secure byssal attachment throughout life, allowed mussels to anchor themselves and live in high population densities in wave‐exposed habitats (Morton 1996). These two adaptations were so successful that they have evolved independently among otherwise unrelated lineages (e.g. in freshwater dreissenid mussels), a perfect example of convergence (Seed et al. 2000).
The heteromyarian condition has been seen as a stepping stone from the isomyarian form of primitive prosobranchs (Figure 1.3a) toward the monomyarian form and the adoption of a horizontal posture (Figure 1.3f). Monomyarian bivalves, such as the Pectinida (scallops) and Ostreida (oysters), have largely circular shells, no trace of the anterior adductor muscle and a body reorganised around the enlarged and more or less centrally placed posterior muscle. Water enters around two‐thirds or more of the rounded margins of the shell. Shell attachment has led to varying degrees of inequality
