To identify the factors that might contribute to limiting the sexual reproduction of Posidonia oceanica we examined: (1) flowering and fruiting phenology; (2) variability in flowering frequency, seed production and reproductive success (i.e. the proportion of flowers setting mature fruits per unit area) over a 2-year period; and (3) losses of potential seed production to fruit abortion and/or pre-dispersal seed predation. The flowering frequency of P. oceanica varied among years, ranging between 7.9 and 19.8%. Despite the large number of flowers and ovules, few of these produced mature fruits owing to abortion. Moreover, about 84% of developing inflorescences were damaged by herbivores. Fruit production varied among years, ranging from 4.7 to 13.5 fruits per m2, but reproductive success remained constant (2.0-2.4%). Exclusion of herbivores showed that abortion alone was reponsible for the loss of about 87% of the reproductive potential. Since reduced fertilization did not completely explain the observed frequency of abortion, it was hypothesized that post-fertilization factors could affect seed formation. The higher reproductive success of protected plants (11.2%) compared to herbivore-exposed plants (3.1%) indicated that predation significantly reduced the number of seeds available for establishment. We concluded that pre-dispersal seed losses to abortion and predation may seriously reduce the reproductive success of the species.
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