|General Biology Additional Information||Abundance and biomass
The abundance of Spisula solida varies with location. For example, the following abundances and biomass were reported:
In Danish waters the average biomass of Spisula solida was 265 g/m² in the Røde Klit Sand 103 g/m² at Horns Reef (Kristensen, 1996). In Waterford Harbour, (Ireland) the maximum biomass was 600 g/m2 (Fahy et al., 2003).
- 0-240 ind./m² (0-2046 g/m²) at Røde Klit Sand (Denmark) (Kristensen, 1996);
- 0-45 ind./m² (0-632 g/m²) at Horns Reef (Denmark) (Kristensen, 1996); whereas
- 2000 ind./m2in Start Bay (UK) (Ford, 1925).
Growth can be influenced by environmental factors, particularly density. For instance, Weinberg & Hesler (1996) compared growth curves of Spisula solidissima in two areas off the New Jersey and Dekmarva coasts (U.S.) and the Long Island and South New England coasts (U.S.) following a hypoxic event, which resulted in mortalities in the southernmost in 1976. Both growth and maximum shell length declined in Long Island/South New England, whereas in New Jersey/Dekmarva growth and shell length remained constant and had not been affected by the hypoxia. Weinberg & Hesler (1996) suggested that following the hypoxia, the first clams to recolonize grew more rapidly in the presence of a good food supply and without competitors.
The growth of Spisula solida is rapid during its first two years and then slows down (Gaspar et al., 1995; Kristensen, 1996). This rapid increase in size was reported in Waterford Harbour where the number of Spisula solida per kg declined rapidly between the ages of 2-3 (769 - 227 ind./kg) (Fahy et al., 2003). Over the following three years this figure halved again to 101 ind./kg (Fahy et al., 2003).
Clear shell sculpture marks occur on Spisula solida, suggesting annual rings, but their interpretation is not straight forward (Fahy et al., 2003). The shell surface of Spisula solida also exhibits some disturbance lines, that are impossible to distinguish from annual growth lines therefore internal bands are used (Gaspar et al., 1995). Taylor et al. (1969,1973; cited in Fahy et al., 2003) described the shells of the superfamily Mactracea. Their shells are composed of two layers of aragonite: a white, opaque, outer layer, consisting of crossed lamellar crystalline structure, which is separated by the pallial myostracum from a grey, somewhat translucent, inner layer. The white outer shell layer and the chondrophore are streaked periodically with dark lines (internal growth lines). This structure confirms the presence of true annuli, which external sculpture alone might not indicate. During winter, wide growth increments are deposited, which is characteristic of rapid shell growth whilst narrow spaced dark zones are formed in summer (Gaspar et al., 1995).
The maximum length of Spisula solida (5 cm) from Irish waters is similar to that of northern European stocks but growth rates appear to vary geographically. Dimensions attained by Irish Spisula solida differ from those reported from other northern European stocks of the species. In the Danish North Sea, individuals between 2-3 years reached a length of 35 mm. Meixner (1994; cited in Fahy et al., 2003) reported that Spisula solida 35 mm in length from the German North Sea similarly averaged 2.5 years old while in Waterford Harbour individuals were 5.27 years at the same length (Fahy et al., 2003).