Application of Effective Day Degrees in the Assessment of Stable Isotope Patterns in Developing Seahorses under Different Temperatures

SIMPLE SUMMARY: Temperature affects fish development, with especially strong influence on juvenile growth rates and metabolism. The present study provides new insights on stable isotopes (δ(13)C and δ(15)N) for the understanding of growth and food assimilation in early developing European long-snouted seahorse Hippocampus guttulatus under different temperature levels. The effects of feeding status, ontogeny and temperature regimes on stable isotope patterns were assessed and modelled as function of development. We argue that chronological time is not a convenient developmental scale and we encourage the use of D°(eff) as temperature-independent developmental index in stable isotopes studies involving temperature comparisons. ABSTRACT: Relations between nutrient assimilation and growth rate in fishes may vary with abiotic factors such as temperature. The effects of feeding status, ontogeny and temperature regimes (15, 18 and 21 °C) on stable isotope (δ(13)C and δ(15)N) patterns were assessed in juveniles of the seahorse Hippocampus guttulatus. The use of effective day degrees (D°(eff)), day degrees (D°) and chronological time (age) were compared as development progress indices. Newborn seahorses were maintained at three temperature levels both deprived of food (5 days) or fed (30 days) on copepods or/and Artemia. Isotopic signatures in fed seahorses clearly differed from those in unfed juveniles. Temperature had a significant effect on δ(13)C values in fed juveniles throughout the experimental period. δ(15)N values also varied significantly with age, but not with temperature level. Faster growth and food assimilation in seahorses held at 18 and 21 °C were supported by faster variations in isotopic values. Our findings demonstrate that effective day degrees should be preferred over chronological time as index of developmental progress in temperature fluctuating scenarios or for comparative studios.