Marine Mussels. Elizabeth Gosling
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 in the size of the two shell valves. In scallops, the shell valves are circular, but they may be concave and similar or the left (uppermost) one may be flat. Like oysters, they also lie in a horizontal position on the substrate. However, scallops, far from being fixed, are active swimming bivalves. In early life, they use byssus threads for attachment to algae, but before they attain a size of 15 mm the majority of species have detached themselves to take up a free‐living existence on the seabed.
Cementation is another mode of attachment that evolved during the early Mesozoic era (252 mya). This adaptation arose independently in Pteriomorphia, Heterodonta and Palaeohterodonta, peaking in the Late Triassic and Jurassic periods of the Mesozoic era (252–145 mya) as a possible response to the appearance of many predatory groups (Vermeij 1977; Harper 1991). During the Triassic, another important development occurred when an ancestral unionid (Paleoheterodonta) colonised freshwater environments, thereby gaining access to a bivalve‐free ecosystem. Giribet (2008) suggests that this move may have been triggered by the evolution of a novel mode of development using microscopic glochidia larvae with fish as intermediate hosts.
Burrowing into the substrate is the habit most extensively exploited by bivalves. Contact is maintained with the surface by way of siphons that extend from the posterior end of the animal. During the Cenozoic era (up to 60 mya), soft, nutrient‐rich sediments on continental margins allowed diversification of shallow burrowing, globular, strongly ribbed forms and deep burrowers with smooth, blade‐like shells (Giribet 2008). Cockles (e.g. Cardium spp.) are shallow burrowers, while many clam species (e.g. razor clams) burrow as deep as 60 cm. The geoducks (Panopea) on the West Coast of the United States are among the deepest burrowers, digging down to a depth of over 1 m, aided by a streamlined shell for fast burrowing and fusion of the mantle edges (apart from a small gape for the large muscular foot), which prevents entry of sediment into the mantle cavity. Many bivalves that burrow deeply (>30 cm) live in permanent burrows, moving deeper as they grow larger. This lifestyle is brought to an extreme in bivalves that bore into hard substrates such as shell, coral, wood and rock and are permanently locked in their burrows, which are therefore inevitably dependent on outside sources of food. Both shallow and deep‐sea wood‐boring bivalves excavate ‘sawdust’ as their principal food source with the aid of gill‐associated symbiotic bacteria (Voight 2007).
Phylogeny and Evolution in the Mytilida
Pteriomorphia is monophyletic and contains four well‐supported marine clades: Arcida, Mytilida, Ostreida and Pectinida + Limida (Figure 1.2). The order Mytilida originated in the Cambrian era some 540–560 million years ago and is composed of a single superfamily, the Mytiloidea, which is considered to be representative of and basal to the Pteriomorphia (Morton 2015 and references therein). Information based on shell