Abstract
AbstractLarval development of the thoracican cirripede Balanus eburneus is invariant, consisting of six naupliar instars and a terminal nonfeeding cypris instar. In order to elucidate determinants of its fixed instar molting pattern we tested the following hypotheses: (1) the rate of morphogenesis is correlated with the rate of molting; (2) the timing of the morphogenetic molt to the cyprid is time (age) dependent; and (3) initiation of the molt to the cyprid is dependent upon the accumulation of a minimum amount of energy reserves. Naupliar food supply was manipulated in order to vary rates of molting and accumulation of energy reserves. Total oil cell volume per larva was used to nondestructively estimate energy reserve level.Compound eye development and thoracic appendage development during the six naupliar instar were confined to specific phases of the molt cycle. Suspension of the molt cycle resulted in suspension of morphological development. Initiation of the molt to the cyprid was neither time dependent nor a function of larval oil cell volume. However, the rate of passage through the molt cycle increased abruptly in larvae having an oil cell volume greater than 4–6 × 104 μm3. Nauplii entering the final phase of the molt cycle (D2–4) would attempt to complete development to the cyprid irrespective of their oil cell volume. Correlations between cypris oil cell volume and cypris metamorphic success implied that the naupliar oil cell volume “threshold” of 4–6 × 104 μ3 represented the amount of energy reserves needed to complete covert developmental events rendering the cyprid capable of successfully metamorphosing to the juvenile.These results suggest that molt cycle stage‐specific morphogenesis and growth (energy reserve accumulation) are important determinants of the fixed instar molting of B. eburneus larvae. Comparisons with studies of larval crustaceans which vary instar number indicate (1) greater independence between the processes of molting and morphogenesis among species with a variable instar molting pattern and also that (2) growth is an important determinant of each type of molting pattern.
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