Can tropical seaweeds reduce herbivory by growing at night? Diel patterns of growth, nitrogen content, herbivory, and chemical versus morphological defenses

Abstract

Tropical seaweeds in the genus Halimeda reduce losses to grazing by capitalizing on diel patterns of herbivore activity. These seaweeds produce new, more herbivoresusceptible growth at night when herbivorous reef fishes are inactive. Plant portions more than 48 h old are low in food value, well defended morphologically (calcified and high in ash content), and relatively resistant to herbivory. Younger plant portions represent 3-4.5 times the food value (nitrogen or organic content) of older portions but are only moderately more susceptible to herbivores due to their high concentrations of the terpenoid feeding deterrents halimedatrial and halimedatetraacetate. Halimedatrial significantly deters grazing by both parrotfishes (Scaridae) and surgeonfishes (Acanthuridae) and occurs in high concentrations (2-4.5% of plant ash-free dry mass) in plant portions that are 4-12 h old, intermediate concentrations (0.3-2.3%) in portions that are 16-26 h old, and low concentrations (0.3%) in older plant portions. The related compound halimedatetraacetate is absent from the youngest plant portions, shows a rapid increase in concentration (from 0 to 1%) in plant material that is approximately 16 h old, and then rapidly declines to low levels (0.1 to 0.2%) in older plant portions. Thus, newly produced tissues are nutritionally valuable but contain high concentrations of defensive chemicals. As these tissues age, morphological defenses increase, the tissue becomes less valuable as a food for herbivores, and chemical defenses decrease. Additionally, new growth of Halimeda remains unpigmented until just before sunrise. Thus, the valuable, nitrogen-containing molecules associated with photosynthesis are not placed in the new, and more herbivore susceptible, growth until lights is available and they can start producing income for the plant.Experiments in a coral-reef microcosm, where diel patterns of light and water chemistry could be altered, indicated that Halimeda's growth pattern is cued by the timing of light-dark cycles rather than by co-occurring diel changes in water chemistry. Although the growth patterns of Halimeda seem unusual, similar patterns appear to occur in numerous other seaweeds and in microalgae such as diatoms and dinoflagellates.

Keywords: Chemical defense; Halimeda; Nocturnal growth; Plant-herbivore interactions; Seaweeds.