QUANTIFYING DEVELOPMENTAL PROGRESS FOR COMPARATIVE-STUDIES OF LARVAL FISHES

Several convenient metrics for quantifying the ontogenetic state of fi sh larvae (based on age, size, and thermal history) were assessed in t erms of intrinsic variability, the influence of temperature, and varia bility among species. Data for 12 ontogenetic events were collected fr om Atlantic menhaden (Clupeidae: Brevoortia tyrannus) and 15 events fr om red drum (Sciaenidae: Sciaenops ocellatus). Each species was reared at two constant temperatures 4-5 C apart. Precise timing of events an d the variability in timing were calculated using a method analogous t o a ''dose-response'' relationship. The logarithm of total length had the lowest intrinsic variability of all metrics examined; dry weight a nd age had the highest variability. Warmer temperatures accelerated on togenetic events on all measurement scales, especially age-based metri cs. Logarithmic transformations of day.degrees and effective day.degre es were insensitive to temperature, but these metrics require informat ion 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.