Differences in herbivore preferences, phlorotannin production, and nutritional quality between juvenile and adult tissues from marine brown algae

Juvenile and adult marine organisms differ in their morphology, chemistry, physiology, behavior, and ecology. Because juvenile algae are thinner, smal ler, and have more delicate tissues than adults, they are often assumed to be more susceptible to grazers. We examined within-species food preferences of four common generalist herbivores for juvenile and adult tissues of eig ht common brown algae in two-choice laboratory food-preference experiments. Our results showed that juvenile algae did not tend to be a preferred food of herbivores. Juvenile tissues were significantly preferred over adult ti ssues in only four of the 32 combinations of algae and herbivores tested. I n 12 experiments, adult tissues were preferred over juvenile tissues, and n o choice occurred in the remaining 16 experiments. When sea urchins exhibit ed a preference, it was always for adult tissues. The other three herbivore s, an isopod and two snails, were more variable in their choices, sometimes preferring juveniles, sometimes adults, and sometimes having no preference . We measured nitrogen and phlorotannin concentrations in adult and juvenil e seaweeds to see whether these parameters were correlated with herbivore f ood preferences. Nitrogen levels were similar in juveniles and adults of th ree algal species and were higher in juveniles of two. Phlorotannin levels were higher in juveniles of four species and lower in juveniles of one. The other three species showed no differences in phlorotannin levels. Phlorota nnin concentrations decreased with increasing juvenile size in three specie s and increased with increasing size in one species. Neither nitrogen nor p hlorotannin concentrations explained overall herbivore food preferences for algae of different stages. Our results suggest that preferences of certain grazers for juvenile algae are not as strong as previously assumed and are dependent on herbivore species. Preferences between juveniles and adults a re likely to be determined by a combination of morphological and chemical f eatures of the tissues and the unique responses of herbivore species to tho se features.