The Population Biology of<i>Lacuna Pallidula</i>(Da Costa) and<i>Lacuna Vincta</i>(Montagu) in North-East England

L. pallidula is more or less confined to Fucus serratus. L. vincta is common on F. serratus but is also found on Halidrys, Laminaria and various Rhodophyceae. Neither species occurs in the absence of these weeds on which they both feed and spawn. Lacuna species are the dominant gastropods on weed at M.L.W.S. at Whitburn, County Durham. Both species are annuals. In L. pallidula the growth rates of females and males are markedly different, the former growing both faster and over a longer period. Females attain a mean maximum size of 8–4 mm compared with 45 mm for males. L. vincta males and females grow at approximately the same rate, though females usually attain a greater size. Males predominate in L. pallidula at all times of the year, comprising between 55% and 75% of the population with a mean of 68–6%. Males die slightly sooner than females but there is no more than 4 weeks difference in longevity. Copulation in L. pallidula begins in November and continues to April, with two peaks in January and March. Spawning commences in January in both species and reaches its peak by April in L. pallidula and by June in L. vincta . Hatching occurs in L. pallidula from April to July. The time of first hatching in L. vincta is not certain but is probably about March. Larval settlement occurs from June to October with a maximum in September. Density is high following hatching in L. pallidula (&gt; 2000/m 2 ) and settlement in L. vincta (&gt; 300/m 2 ). Dispersion is similar in the two species, being aggregated at high density and more or less random at low density.