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 phase s: (1) a planktotrophic nauplius, (2) a non-feeding, planktonic cyprid that subsists on energy reserves, and (3) a benthic juvenile and adult. We cond ucted a series of experiments to measure the effects of variable food conce ntration during the naupliar phase on the age, size, and lipid reserves of Balanus glandula cyprids. When food shifted during only the first similar t o 25% of the naupliar phase (the first three instars), the initial food lev el did not affect the timing of metamorphosis to the cyprid. Shifts in food that were restricted to the final similar to 40% of the naupliar phase (th e sixth instar) also did not affect age at metamorphosis. During the interm ediate portion of the naupliar phase, enhanced food decreased the age at me tamorphosis, while reduced food lengthened the naupliar phase. Cyprid size generally correlated positively with changes in food, but a maximal size ap peared to result when food increased during the intermediate portion of the naupliar phase (when the timing of metamorphosis was plastic). Amphibian m odels 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 d evelopment rate. In contrast to effects on size and age at metamorphosis, c yprids' lipid concentration (lipid per unit body size) only responded to sh ifts in food that occurred during the sixth instar. This resulted because s hifts in food during the sixth instar affected both cyprids' size and lipid content (lipid per cyprid). Earlier shifts in food greatly affected a cypr id's size, but had little effect on lipid content or lipid concentration. B ecause the size, age, and lipid reserves of cyprids all have fitness implic ations, the net effects of food variability are complicated and depend on e xactly when the variability occurs. Short-term changes in food encountered after the third instar and before the sixth will alter the length of the na upliar 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.