Ontogeny of yolk-feeding fish: an ecological perspective

Abstract

Ontogeny is a continuous process with temporaryaccelerations. The embryonic period from eggactivation to hatching, and the larval periodthereafter, are considered. Advances in studieson the ontogeny of yolk-feeding Europeanfreshwater and Antarctic marine fish arecompared. New techniques and approaches aresummarized. A method for exact quantificationof the time to any developmental event isrecommended. Four attempts to quantify anindividual's ontogenetic advancement arereviewed, of which Fuiman's ontogenetic indexseems to be the best choice. The relationshipbetween the time to any ontogenetic event(τ, days) and temperature (t, °C)has been quantified by exponential, power law,Bělehradek's, Leiner's, and polynomialmodels, whose common weaknesses are that theparameters have no biological meaning, and theydo not allow comparison of temperaturerequirements between species. The ontogeneticrate (V = 1/τ, days−1) was welldescribed (r2 = 0.92 − 1.00) by a straightline V = a + bt (linear model) in 44 fishspecies over a broad low-mortality temperaturerange. The linear model produces biologicallymeaningful parameters: the temperature ofbiological zero t0 = −a/b, effectivetemperature teff = t −: t0, andeffective day-degrees D°eff = τ(t − t0) = b−1. From t0 andD°eff the time to any ontogeneticevent can be computed as: τ =D°eff/(t − t0). In coldwaterspecies low t0 is accompanied by highD°eff, whereas in warmwater speciesthe opposite is true. The linear model wasvalidated. Day-degrees (D° = τt)and physiological day-degrees (PD° =τt/q, where q is Winberg's temperaturemetabolic correction factor) are based onincorrect assumptions. Their use as temperature-independent measures of ontogeneticadvancement is not advised; by contrast,effective day-degrees (D°eff)are temperature-independent and arerecommended. The remaining extrinsic factorsaffecting ontogenetic rate during yolk feedingare: oxygen, salinity, pH, light, dissolvedbiotic compounds, and anthropogenic factors.The main intrinsic factor is egg size, whichpositively affects the time to particularontogenetic steps at inter- and intra-specificlevels. Attempts to quantify the combinedeffects of temperature and salinity, and oftemperature and egg size, are reviewed.