Reproductive cycles, egg production and recruitment in the Indo-Pacific intertidal gastropod Umbonium vestiarium (L.)

Abstract

Umbonium vestiarium (L.) was found to dominate intertidal sand with a mean 11 808 m −2 (95·4 g tdw m −2), excluding virtually all other invertebrate fauna. Whilst spawning occurs from mid-March to August, substantial recruitment seems limited to March–May, first on the lowest levels, and then upshore as the young grow to 5–6 mm by June and as the growing young move upshore. The number of ovarian eggs varied in broad accordance with total tissue dry weight: both numbers of ovarian eggs and total dry weight in females and males increased in periods between bouts of spawning but fell during substantial spawning which was mostly close to neap tides. The relationships between planktonic-egg counts, ovarian-egg counts and tissue dry weight, together with direct weighings of ovarian eggs, suggest (a) mature eggs weigh about 2·5 μg each (dry weight), (b) 10-mm females each produced about 8800 eggs (22 mg) in the 79-day observation and probably 17 000–19 000 eggs (44 g) in the total spawning period (March–August), (c) the population as a whole produced about 30 × 10 6 eggs m −2 (75 g m −2) in the 79 days and probably around 60 × 10 6 eggs m −2 (150 g m −2) plus a similar quantity of semen in March–August, giving a total reproductive output of aproximately 300 g m −2 year −1. As Umbonium lives for only about one year with apparently one annual major period of spawning and recruitment, it is proposed that high reproductive output, large eggs and reduced larval dispersal may be adaptively related to the short lifespan on comparatively unstable and isolated habitats. These features are likely to have significant effects upon the genetic heterogeneity of Umbonium populations across its great Indo-West Pacific range.