Serripes groenlandicus
Serripes groenlandicus, the Greenland cockle, is a common circumpolar bivalve found in up to 100m depth on a variety of substrates. It can grow up to 112mm. It is infaunal and usually inactive, but can use its foot to escape predators. The animal is a hermaphrodite, and after reaching maturity, the cockle spawns in March and April. Larvae are pelagic and settle in late summer. S. groenlandicus is a suspension feeder and can live up to 39 years. It is a main component in the diet of some walrus species as well as an important bycatch in the Atlantic surfclam (Mactromeris polynyma) fishery in Eastern Canada.
Identifyable by their pale white siphon (Ambrose et al., 2006)
In a laboratory test (Legault and Himmelman, 1993), S. groenlandicus escaped from predators with an initial ejection of water followed by violent movements of the foot. Escape response intensity increased with predation risk, except that Solaster endeca provoked a significant response despite the fact that no predation by that species has ever been documented for S. groenlandicus . This behavior is considered striking compared to its normal slow movements. It is never buried more than 7cm deep and is sometimes found on the surface, suggesting that it relies on its escape behaviour to limit predation (Legault and Himmelman, 1993).
The cockle grows within a reported range of 5.4-7.4 mm/year and may not grow its shell during winter; external growth bands are annual (Khim, 2001). However, as the cockle ages the growth rings become closely spaced and difficult to determine, although annuli in the chondrophore/nymph are more easily distinguishable in older animals (Kilada et al, 2007). For this reason, Kilada et al (2007) determine measuring the chondrophore/nymph growth lines to be the most accurate method of ascertaining the age of a specimen. The growth of S. groenlandicus appears to be affected by Arctic climate oscillations, however the relationships are complex and indirect (Ambrose et al., 2006). Petersen (1978) found that growth is best at shallow depths, with the heaviest density of juveniles found in March in depths of less than 6 meters.
Wood (1835) states that S. groenlandicus externally has the appearance of a Venus and might be taken for a shell of that genus except for its hinge, which has four teeth as cockles do; the two primary teeth are small.
A study of gill mitochondria membranes at subzero temperatures (Gillis and Ballantyne, 1999) revealed the primary membrane phospholipids to be phosphatidylcholine and phosphatidylethanolamine, and that the monounsaturated fatty acid 20:1 was abundant, equaling 13% of the fatty acid composition of the clam.
Shell: the shell is thin, brittle, ovate, inflated, somewhat longer posteriorly, and rounded ventrally, up to 112 mm in length with prominent and orthogyrate umbones (Coan et al., 2000). Radial growth lines are evident, but are too closely spaced to determine age on older animals; growth lines are clearer on younger specimens (Coan et al., 2000; Kilada et al., 2007). The interior is generally white, although some reddish brown hue may show through. According to Coan et al. (2000), the periostracum is straw colored to dark brown. Abbott (1968) describes the shell's color as brownish or gray, while Wood (1835) calls it mouse colored.
Siphon: Ambrose et al. (2006) state the siphon is pale white.
Length to 100 mm, adults usually 50-70 mm (Khim, 2001)
S. groenladicus comprises a main component in the diet of the Atlantic walrus (Odobenus rosmarus rosmarus) and of the Pacific Walrus (Callorhinus urshuts) (Kilada et al, 2007). Yakovis et al. (2008) found that empty S. groenlandicus shells produce microhabitats for different sessile taxa.
Circumpolar in Northern Hemisphere (Kilada et al, 2007). In the Atlantic it extends from Greenland to New England and in the Pacific from the Bering Sea and Aleutian islands to Japan (Coan et al., 2000). Abbot (1968) notes that it is very common.
Inhabits the subtidal zone to about 100 m deep (Khim, 2001). They are infaunal (Kilada et al. 2007), and a review of available literature suggests S. groenlandicus inhabits a variety of sediments. It is never buried deeper than 7cm (Legault and Himmelman, 1993).
In Kilada et al. (2007) study, the maximum observed age was 39 years.
Serripes groenlandicus are hermaphrodite, with male tissues reaching maturity faster than female tissues. Minimum size and age at sexual maturity, from a sample of 86 individuals, were 27.92mm and 2.83 years for male tissues and 37.22mm and 3.69 years for female tissues (Kilada et al., 2007). Petersen (1978) lists mature animals as larger than 25mm. The cockle spawns in March and April, and Petersen (1978) suggests this is induced by phytoplakton being swept beneath the sea ice by currents since during these months light cannot penetrate the ice. In one study of spat settlement (Garcia et al, 2003), S. groenlandicus settled mainly between August and November, however, Petersen (1978) found settling to occur in July and August. This is possibly explained by larvae originating in sheltered fjords which experience longer ice coverage. Petersen (1978) found that the nature of the sediment had no obvious influence on settling.
Suspension Feeder (Khim, 2001).
Since 2002, S. groenlandicus has become an important by-catch in the Atlantic surfclam (Mactromeris polynyma) fishery in Eastern Canada (Kilada et al, 2007)
Greenland cockle, Greenland smooth cockle