Comparison of growth plasticity in the laboratory and field, and implications for the onset of juvenile development in sofie, Chondrostoma toxostoma

Synopsis

The aim of the present study was to determine the effect of environmental conditions (controlled and natural) on the duration and sequence of developmental steps in sofie, Chondrostoma toxostoma, early ontogeny. Few previous studies on the early development of fishes have included relative growth and none have compared relative growth in the laboratory and the field. Such comparison is important to quantify the morphological development of different parts of fish during their early ontogeny, to determine potential variations in growth that may occur under laboratory conditions and to understand better the plastic nature of relative growth. Early development and relative growth of 23 characters were examined in specimens of sofie reared under both laboratory and natural conditions in tributaries of the River Garonne basin (France). The sofie is still present in this basin despite progressive localised extinction in the rivers of south western Europe over the last 30 years. Growth of field and laboratory embryos (in degree days,°days) was the same up to larva step 1 (9 mm SL), but thereafter was markedly slower in the laboratory than in the field. Ontogenetic rate in the field was twice that in the laboratory, suggesting a precocial (specialist) form under natural conditions and an altricial (generalist) form under laboratory conditions. Stabilisation of relative growth, i.e. end of the remodelling process (metamorphosis), occurred well after all larval characteristics (remnants of finfold, rapid allometric growth) had disappeared and all the juvenile structures had appeared (nasal septa, complete scale cover). In the field, this stabilisation occurred in specimens of approximately 50 mm SL, suggesting that metamorphosis ends and the juvenile period begins at the end of the sofie’s first (0+) year of life.