FEEDING RADIUS, BURYING DEPTH AND SIPHON SIZE OF MACOMA-BALTHICA AND SCROBICULARIA-PLANA

This paper investigates whether siphon weight limits the elongation of the siphon of deposit-feeding benthic bivalves under natural conditio ns. Were this to be so, it would imply that foraging and predator avoi dance place conflicting demands on these animals, since an increase in the feeding radius on the surface would be associated with a decrease in the burying depth, and vice versa. The paper presents methods with which to measure siphon length and equations to transform siphon weig ht to siphon length in two benthic bivalves, Scrobicularia plana and M acoma balthica. Relatively heavy siphons are longer, but also thicker, than lightweight ones. We conclude that most individual bivalves stre tch their siphons fully while feeding. However, bivalves with heavy si phons keep part of the siphon in reserve within the shell. Siphon crop ping therefore results in an immediate reduction in siphon length, unl ess the siphon is heavy and a reserve is available. The feeding radius of M. balthica and S. plana is a linear function of shell size. S. pl ana use half of their siphon length to feed on the surface and the oth er half to bury themselves, but if the siphon weight is below average, the proportion extended over the surface decreases with siphon size. However, among the animals with a short siphon, those with a good body condition take no risks and live as deeply as possible. A comparison between species reveals that the weight of the extended inhalant sipho n per cm amounts, on average, to 0.6% of the total body weight.