Arctica islandica (Linnaeus) is the only extant species of an ancient genus (Arcticidae), which has its roots in the early Cretaceous (> 65 Ma) (Lutz et al., 1982). Arctica is a marine bivalve mollusc with a wide Boreo-Atlantic range. Its range extends from the western fringes of Europe (northward from the Bay of Cadiz, Spain, around the islands of Ireland and Britain, into the North Sea, and off the Norwegian coast to the White Sea in Russia and into the Baltic), to the East coast of North America (north of Cape Hatteras to St. George's Bay, Newfoundland, Canada,), (Merrill and Ropes, 1969; Ropes, 1979). Described by Linnaeus in 1767 under the name ‘Venus islandica’ it was reclassified by Schumacher in 1817 within the genus ‘Arctica’. Lamark suggested ‘Cyprina’ in 1818 and this name still appears in the literature, though ‘Arctica’ has priority (Witbaard, 1997). A detailed taxonomy of Arctica islandica is included with those of Mercenaria sp in Table 1
Table 1: Taxonomy of the three bivalve molluscs used in the present study. Arctica is known by many common names, the most widely used being the Ocean quahog (often spelled quahoc, quahaug, quahag) a name that derives from the Native American Narragansett tribe’s word ‘poquauhock’ (literally means hard/dark shell). Other common names are: ‘Iceland Cyprina’, ‘black clam’, ‘ocean clam’, ‘black quahaug’, ‘mahogany quahaug’ and ‘mahoganies’. Names, which derive, in the main, from the distinctive colouring, found on the periostracum (outer shell surface of Arctica and other bivalves) i.e. pale yellow to deep brown in juveniles and dark brown to solid black in adults (Witbaard, 1997). The evolution from the lighter colour to the darker is thought to be a result of the deposition of iron complexes (Brey et al., 1990)(Fig. 2)
Figure 2: The right hand valve of an adult A. islandica, indicated is the oldest part of the shell (umbo) and the youngest (ventral margin). Clearly visible is the black periostracum, worn away through abrasion on some portions of the shell. Visible at the umbo end of the shell are some of the annual GLs. Arctica are thought to be dioecious, though Mann et al., (1982) has reported hermaphroditism. The females are typically larger than the males and may reach sexual maturity earlier (6 and 5 years for male and female respectively). Spawning usually occurs in summer or early autumn, though May spawning has been observed within Arctica’s range. Temperatures of ~ 15° C are optimum for spawning (Landers, 1976). Arctica are filter feeders (Merrill, 1969) and favour shallow burial (as evidenced by their relatively short siphons) in a range of sediments, though they prefer sand and mud mixtures. They are limited by a temperature regime of ~ 0 - 19o C, though can survive extremes of this regime, and a depth range from below lowest low tide and ~ 500 m (Nicol, 1951) Growth continues throughout life, relatively rapidly for the first 20 years (Murawski et al., 1982) then slowing as the bivalve ages. This is reflected in relatively wide growth increments (GIs) within the shell of Arctica in these first years of precipitation (20 – 30mm). As sexual maturity is reached, growth increments narrow as the organism develops to maturity, until eventually, growth increments as small as 10 - 15 µm are encountered (Fig. 3).
Figure 3: Digital photo of the growth increments sampled within a shell fragment of the bivalve Arctica islandica sampled by laser. The ablation pits are ~ 15 µm across and run from right (younger shell) to left.
Arctica have become an important food resource in the U.S.A., where it is replacing the larger bivalve Spisula solidisima, whose stocks are falling rapidly. It is also thought to be a staple source of food for Atlantic Cod (Gadus morhua) and many seabirds (Arntz, 1974; Meisner and Brager, 1990). When individual Arctica, survive predation they can grow to >100 mm in length. Arctica like many other bivalves precipitates its shell, with an annual periodicity, and like other biogenic carbonates, in isotopic equilibrium (with respect to temperature and the δ18Owater) and elemental equilibrium with the surrounding water (e.g. Epstein et al., 1951, 1953; Mook and Vogel, 1968; Krantz et al., 1987; Romanek et al., 1987; Klein et al., 1996; Kirby et al., 1998). This fact, allied with Arctica’s longevity (It has been reported to live for > 200 years (Ropes, 1985) makes it the perfect tool for environmental and palaeoenvironmental reconstructions within the Boreo-Atlantic
A rctica posses a relatively thick shell (can be > 10 mm), that is precipitated by the mantle at the ventral margin, adding length to the shell, and on the inside of the shell, adding thickness (Fig. 4 and Fig. 2). Arctica shells are made entirely of aragonite (CaCO3) crystals, a meta-stable polymorph of calcium carbonate (Gill et al., 1995) with an organic matrix within and between the crystals (Rhoads and Lutz, 1980; Wheeler, 1992). When acetate peels of prepared shell sections are viewed under a light microscope or when the same sections are viewed with the built-in microscope of the laser ablation systems used within the study it is clear that aragonite within Arctica precipitates in two distinct layers within the shell. In addition, two thin layers exist; the pallial myostracum (pallial muscle-scar and mantle attachment layer) separates the inner layer from the outer layer and the periostracum (a dark coloured layer on the outer surface of the shell) (Fig. 2). The periostracum acts as a shield from carbonate dissolution by seawater and as a barrier, when the valves are closed, between the epithelial cavity and the external seawater.
Figure 4: Stylised cross section through a seven-year-old Arctica shell detailing the major layering and structures within. Passing from the interior of the shell the first distinct layer is a nacreous layer, deposited by the mantle on the inside of the shell. The structure of this layer consists of tabular plates of aragonite, laid in sheets, arranged such that succeeding sheets of aragonite are laid one on the other adding thickness to the inside of the shell. This layer extends from the pallial line (the scar formed where the mantle is attached to the inside of the shell) to the umbo. The outer shell layers are separated from the inner layers by the relatively thin (can be 10’s µm) prismatic, pallial myostracum. The outer layer is precipitated by the mantle at the ventral margin, and adds length to the shell. This layer contains the annual growth increments (GIs) that are used for both stable isotope and elemental analyses within the present study. Each annual growth increment within this layer encompasses two distinct structures. When viewed under a light microscope these appear as a relatively narrow, dark, translucent layer, with an irregular simple prismatic microstructure that is adjacent to a relatively wide, light, opaque layer, with an homogenous microstructure consisting of irregular complex crossed lamellar and crossed acicular-crossed lamellar (Jones, 1980) (Fig. 5).
A B C
Figure 5: Microstructure of aragonite crystals; A. prismatic, B. Lamella, C. acicular . The dark translucent layer has been attributed to slow/suspended growth indicative of winter conditions or environmental disturbance (Klein et al., 1996; Arnold et al., 1998). The proteinaceous framework of shell construction may at these times constitute the greater percentage of shell material and alter its appearance. Covering the outer surface of the shell is the dark coloured periostracum. This is an organic rich layer composed mainly of conchiolin (an albuminoidal protein), it is hydrophobic and thought to protect the shell aragonite from dissolution and separate the extrapallial fluid from the surrounding seawater.