Grazing by green sea-turtles does not affect reproductive fitness in Thalassia testudinum

Abstract

The negative effects of turtle-grazing on the vegetative plants parts of seagrasses are relatively well-known, but its effects on the reproductive success have received less attention. We studied reproductive frequency and success of Thalassia testudinum under simulated (clipping for 6 months) and natural grazing regimes in the Mexican Caribbean. Simulated grazing did not affect the density (flowers m−2) of female (clipped: 10.9±4.7SD, control: 8.4±4.6SD) and male flowers (clipped: 20.7±12.0SD; control 25.6±16.8SD), resulting in a similar seed-set (overall average 55%) and fruit-set (overall average 44%) in clipped and control plots. Naturally turtle-grazed areas did not present significant differences in fruit size, fruit-set or seed-set from nearby ungrazed areas. Thus, T. testudinum maintained sexual production while grazed, and flowers and fruits were not a targeted food source for the green turtles. We expected incidental ingestion of the flowers, usually positioned above the cropping height of ∼3cm. But in the grazed areas, the flowers were shorter; being 3.6cm for female flowers, and 3.3cm for male flowers. Due to these reductions in length, the flowers and fruits were closer to the sediment than in ungrazed patches; just below the usual grazing level of the turtles. Green turtles can potentially eradicate complete meadows. Re-establishment of meadows can only occur through recolonization by seeds in floating fruits provided by nearby seagrass meadows, which can maintain a stable fruit and seed production when moderately grazed.