Metamorphosis of barnacle nauplii: effects of food variability and a comparison with amphibian models

Like many animals, barnacles have a complex life cycle with shifts in both diet and habitat. The life cycle of most barnacles has three distinct phases: (1) a planktotrophic nauplius, (2) a non‐feeding, planktonic cyprid that subsists on energy reserves, and (3) a benthic juvenile and adult. We conducted a series of experiments to measure the effects of variable food concentration during the naupliar phase on the age, size, and lipid reserves of Balanus glandula cyprids. When food shifted during only the first ∼25% of the naupliar phase (the first three instars), the initial food level did not affect the timing of metamorphosis to the cyprid. Shifts in food that were restricted to the final ∼40% of the naupliar phase (the sixth instar) also did not affect age at metamorphosis. During the intermediate portion of the naupliar phase, enhanced food decreased the age at metamorphosis, while reduced food lengthened the naupliar phase. Cyprid size generally correlated positively with changes in food, but a maximal size appeared to result when food increased during the intermediate portion of the naupliar phase (when the timing of metamorphosis was plastic). Amphibian models adequately describe the effects of variable food on the size and age at metamorphosis to the cyprid, provided that the model specifies an upper limit to size at metamorphosis when food is enhanced prior to fixation of development rate. In contrast to effects on size and age at metamorphosis, cyprids' lipid concentration (lipid per unit body size) only responded to shifts in food that occurred during the sixth instar. This resulted because shifts in food during the sixth instar affected both cyprids' size and lipid content (lipid per cyprid). Earlier shifts in food greatly affected a cyprid's size, but had little effect on lipid content or lipid concentration. Because the size, age, and lipid reserves of cyprids all have fitness implications, the net effects of food variability are complicated and depend on exactly when the variability occurs. Short‐term changes in food encountered after the third instar and before the sixth will alter the length of the naupliar period and the size of cyprids. Variability restricted to the sixth instar will not alter the age at metamorphosis, but will affect the cyprid's size, lipid content, and lipid concentration.