Quantifying developmental progress for comparative studies of larval fishes

Several convenient metrics for quantifying the ontogenetic state of fish larvae (based on age, size, and thermal history) were assessed in terms of intrinsic variability, the influence of temperature, and variability among species. Data for 12 ontogenetic events were collected from Atlantic menhaden (Clupeidae: Brevoortia tyrannus) and 15 events from red drum (Sciaenidae: Sciaenops ocellatus). Each species was reared at two constant temperatures 4-5 C apart. Precise timing of events and the variability in timing were calculated using a method analogous to a "dose-response" relationship. The logarithm of total length had the lowest intrinsic variability of all metrics examined; dry weight and age had the highest variability. Warmer temperatures accelerated ontogenetic events on all measurement scales, especially age-based metrics. Logarithmic transformations of day.degrees and effective day.degrees were insensitive to temperature, but these metrics require information that is often not available. An ontogenetic index, which compares species on the basis of their size or age at metamorphosis, performed best in interspecific comparisons.