Species-specific effects of prior grazing on the palatability of turtlegrass

Abstract

The induction of morphological, physiological and chemical defenses against herbivory is well known in seaweeds, but relatively unstudied in seagrasses. To determine if grazing history affected turtlegrass (Thalassia testudinum) palatability, choice and no choice feeding experiments were conducted with turtlegrass and the variegated sea urchin (Lytechinus variegatus) from two north Florida bays, and turtlegrass and bucktooth parrotfish (Sparisoma radians) from the Florida Keys. Results of choice experiments indicated that urchins exhibited a weak preference for ungrazed turtlegrass. Parrotfish, alternatively, consumed more ungrazed than grazed turtlegrass when only offered one tissue type, but when offered a choice, preferred grazed turtlegrass. The mechanisms underlying urchin feeding behaviors remain unresolved, as analyses of total reactive phenolics, nutrients and tissue toughness did not indicate that any of these factors significantly influenced results over the time scale tested. Parrotfish feeding patterns, however, were likely driven by total reactive phenolic content of turtlegrass. Effectiveness of induced defenses may be grazer-specific and dependent on the feeding mode and chemoreceptive abilities of the grazer.