|General Biology Additional Information||
Pecten maximus are hermaphrodite and, therefore, there is no separate male and female size range or size at maturity. Pecten maximus grows up to 15 cm and will be at least 6 cm when sexually mature.
Pecten maximus normally lies recessed into slight hollows (recesses) in the seabed (Mason, 1983). Recessing is achieved through a series of powerful adductions (valve closures) where water is ejected from the mantle cavity and lifts the shell at an angle to the seabed so that subsequent water jets blow a hollow into the sediment (Brand, 1991).
Mobility Pecten maximus is capable of swimming by rapidly clapping the valves and expelling the water on either side of the dorsal hinge so that the scallop moves with the curved edge of the shell foremost (Thomas & Gruffydd, 1971). Jumping is achieved through the gradual relaxation of the adductor muscle followed by the rapid opening and closing of valves, which jump the scallop hinge forward (Thomas & Gruffydd, 1971).
Swimming is generally limited to escape reactions. Experimental contact with different starfish species elicited distinct, energy adaptive types of response from Pecten maximus. Full swimming response was initiated only by extracts of Asterias rubens and Astropecten irregularis which prey on molluscs, while limited jumping or valve-closing responses were induced by non-predatory starfish (Thomas & Gruffydd, 1971).
Size and growthEyes
Specimens of up to 21 cm have been recorded, although this is exceptional and the size range of scallops caught commercially is usually between 10 and 16 cm (Mason, 1983).
Scallop shells bear distinct and concentric annual growth rings. The shells also bear numerous regularly occurring concentric striae 0.1-0.3 mm apart which are also used to age the scallops (Mason, 1957). Minchin (2003) stated that it took between three and six years to attain 11 cm in shell length. The Minimum Landing Size (MLS) for this species in Britain and Ireland is 10-11 cm (depending on area) and growth to this size is usually achieved within four years (Brand et al., 1991).
Growth rate can be affected by several factors including salinity, temperature, competition, water depth and food supply. For example, Laing (2002) found that the growth rate of spat grown at 13-21 °C was significantly lower at 26 psu than at 28-30 psu. Mason (1957) found that specimens from inshore, shallower waters typically displayed higher growth rates and maximum sizes than those from deeper waters. Even differences in growth rate between different grounds have been reported (Mason, 1983). Growth in Pecten maximus slows down or stops altogether in the winter, starts again in spring and continues through summer when it is most active. Growth also becomes slower in older individuals and consequently the growth rings become closer together and difficult to distinguish (Mason, 1957). In contrast to many other studies on bivalves, Beaumont et al. (1985) found no association between heterozygosity and size in this species, i.e. genetic factors are relatively unimportant compared to environmental controls on growth. However, they also suggested that genetic factors may be more important during the larval stage.
Embedded among the bases of the sensory tentacles around the edge of the mantle are numerous tiny eyes (Mason, 1983). The eyes are a blue green colour no more than ca 1.5 mm in diameter. The eyes bear a superficial resemblance to the camera eyes of vertebrates and have a highly specialized retina (Wilkens, 1991). Light has both inhibitory and excitatory effects and scallops will swim, orient themselves or close their shell in response to shadows or movement (Wilkens, 1991).
Campbell et al. (2001) reported that, in July 1999, the Amnesic Shellfish Poisoning toxin, Domoic Acid (DA), was found in Pecten maximus at levels exceeding the regulatory limit of 20 µg DA / gram across large areas of northern and western Scotland. The risk of human illness resulting from consuming toxic scallops is, according to Shumway & Cembella (1993, cited in Campbell et al., 2001), a significant threat to both public health and the shellfish industry.