Population dynamics and reproduction of HermaphroditicLasaea Rubra(Montagu) (Bivalvia, Galeommatacea)

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Abstract
Lasaea rubra occurred in dense populations among Lichina pygmaea on the exposed rocky shore at Carnsore Point in south-east Ireland. The life history of this bivalve was quantitatively investigated over the period August 1976-July 1978. Brood size, population structure and growth rates were studied with emphasis on brooding and recruitment cycles. At Carnsore Point, Lasaea had a seasonal reproductive cycle, brooding adults were present in the population from May to October, and the main brooding period was completed in August. Brooding first appeared in larger individuals. Brood size varied from 1–21 embryosladult and was related to shell length (brood size =1.6 × shell length in mm2.98 , r2=0.726). Few, if any, individuals brood in their first year. A major recruitment pulse was recorded in August 1976 and 1977 following juvenile release. The 0+ group remained distinct throughout the year, reaching a mean size of 1.2 mm in twelve months. The population structure was polymodal, older year classes being difficult to separate. Probability analysis suggested a population of 2–3 year classes, with few individuals entering their fourth year. The reproductive and recruitment cycles of the Carnsore Point Lasaea population appear to be consistent with other Irish and British populations, but differ from Lasaea populations at other localities. Examination of the gonad of Lasaea rubra specimens from Galway Bay using light histology and gonad squashes confirmed earlier studies showing this species to be an hermaphrodite. Sperm were found in small numbers in the ovotestis and their nuclei have a variable rounded form.