The Population Biology ofLacuna Pallidula(Da Costa) andLacuna Vincta(Montagu) in North-East England

Abstract

L. pallidulais more or less confined toFucus serratus. L. vinctais common onF. serratusbut is also found onHalidrys, Laminariaand various Rhodophyceae. Neither species occurs in the absence of these weeds on which they both feed and spawn.Lacunaspecies are the dominant gastropods on weed at M.L.W.S. at Whitburn, County Durham.Both species are annuals. InL. pallidulathe 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. vinctamales and females grow at approximately the same rate, though females usually attain a greater size.Males predominate inL. pallidulaat 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 inL. pallidulabegins 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 inL. pallidulaand by June inL. vincta. Hatching occurs inL. pallidulafrom April to July. The time of first hatching inL. vinctais not certain but is probably about March. Larval settlement occurs from June to October with a maximum in September.Density is high following hatching inL. pallidula(> 2000/m2) and settlement inL. vincta(> 300/m2). Dispersion is similar in the two species, being aggregated at high density and more or less random at low density.