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

The aim of the present study was to determine the effect of environmental c onditions (controlled and natural) on the duration and sequence of developm ental steps in sofie, Chondrostoma toxostoma, early ontogeny. Few previous studies on the early development of fishes have included relative growth an d none have compared relative growth in the laboratory and the field. Such comparison is important to quantify the morphological development of differ ent parts of fish during their early ontogeny, to determine potential varia tions in growth that may occur under laboratory conditions and to understan d better the plastic nature of relative growth. Early development and relat ive growth of 23 characters were examined in specimens of sofie reared unde r both laboratory and natural conditions in tributaries of the River Garonn e basin (France). The sofie is still present in this basin despite progress ive localised extinction in the rivers of south western Europe over the las t 30 years. Growth of field and laboratory embryos (in degree days, degrees days) was the same up to larva step 1 (9 mm SL), but thereafter was marked ly slower in the laboratory than in the field. Ontogenetic rate in the fiel d was twice that in the laboratory, suggesting a precocial (specialist) for m under natural conditions and an altricial (generalist) form under laborat ory conditions. Stabilisation of relative growth, i.e. end of the remodelli ng 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 th e 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.