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 fre- quency, 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 abor- tion 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 hypoth- esized 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 preda- tion 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.

Introduction

Seagrasses can reproduce both vegetatively and sexually, although the frequency and intensity of sexual reproduction in some species can be highly variable (Duarte et al., 1997;Marbà and Walker, 1999). Little is known on the extent and controls of sexual reproduc- tive success in seagrasses, despite their world-wide ecological importance in coastal and estuarine environments (den Hartog, 1970; McRoy and Helfeerich, 1977). Flowering is considered the most critical factor in limiting sexual reproduction in several seagrasses (Marbà and Walker, 1999), but estimates of the potential yield of seeds are often incon- sistent with the number of established seedlings observed in the field, indicating that large losses occur during the seed or seedling stages. Williams (1995) estimated that a consid- erable percentage of the seeds produced in Phillospadix torreyi S. Watson may be lost due to abortion and pre-dispersal seed predation. Other authors have identified physical disturbance and post-dispersal seed predator activities as additional important causes of seed losses in seagrasses (Orth et al., 1994; Fishman and Orth, 1996). Sexual reproduction, even at a low rate, could play an important role in colonization of new sites, recovery af- ter disturbance and establishing new genotypes in existing seagrass populations (Alberte et al., 1994; Orth, 1999). Conservation and restorative efforts should, therefore, consider factors that affect seed production, and extensive studies on this research topic need to be conducted.