Size regulation in larvae of the crustacean Balanus eburneus (Cirripedia: Thoracica)

We tested the hypothesis that larval size in the acorn barnacle Balanus eburneus Gould (Cirripedia Thoracica) varies in relation to food availability. In March–November 1980, and March–July 1981, larvae were obtained from adult Balanus eburneus collected in the Newport River, North Carolina, USA. Carapace length and width of larvae reared at three different food concentrations were measured. Mean naupliar instar size was independent of food concentration. Mean size of the cypris instar increased with increasing food level. Greater cypris size could be attributed to increased food reserves in the preceding naupliar stage, and was coinciden with inmarked increase in metamorphic success. Variation in instar size remained constant or declined during naupliar development, but increased sharply at the molt to the cyprid. Naupliar size regulation involved: (1) conservation of a molt increment specific for each naupliar-naupliar molt, (2) an inverse relationship between premolt size and the molt increment during the first five naupliar instars, and (3) an increase in the precision of the molt increment at the molt to the sixth naupliar instar. Experimental evidence implies that size regulation in Balanus eburneus limits variation about a fixed final naupliar size (e.g. volume). Measurement of naupliar size, accumulated energy reserves, survival and development time, and cypris metamorphic success indicated that naupliar cuticular growth is the most conservative feature of larval development. The data suggest that maximum naupliar size is limited by escalating metabolic costs during development, while minimum naupliar size is limited by size-related feeding effectiveness.