Phenotypic plasticity has been demonstrated in fishes but rarely addressed with respect to skeletogenesis. The influence of water velocity on the sequence of chondrification and ossification is studied for the median fins of Arctic charr, Salvelinus alpinus, during a period of 90 days post hatching. Time of appearance, relative position within sequences, and direction of development among serially repeated elements are compared between two velocity treatments. Water velocity has induced changes in the timing of events and to a lesser extent on the relative sequence events in the locomotor system. Ossification is more responsive to water velocity than chondrification, and early-forming elements are less responding than late-forming elements. Directions of development are fairly conservative. It is suggested that a faster sustained swimming (behavioural adaptation to a higher water velocity) could induce differential mechanical stresses on developing skeletal elements involved in locomotion and therefore induce changes primarily in the timing of the ossification. 1. Introduction Developmental sequences are known to be controlled genetically as well as constrained environmentally [1, 2]. A specific type of developmental sequence, the sequences of chondrification and ossification, has been investigated in a large diversity of fishes [3–13], amphibians [14–19], reptiles (including birds) [20–28], and mammals [29–32]; both sequences refer to the specific ontogenetic order in which anatomical cartilaginous and bony structures appear during the early development of vertebrates. Information derived from such sequences could be used to validate homology [12, 13, 33] and phylogenetic position [14, 20], and to infer evolutionary developmental patterns and processes (e.g., heterochrony) [11, 29, 30, 32, 34, 35]. It has been suggested that ossification sequences conform to functional needs [29–31, 35, 36]. In a general way, structures that are required functionally earlier during development will ossify earlier in the sequence although inconsistencies may occur [3, 37, 38]. Among the functional requirements most influential on the ossification, locomotion has been shown to induce differential mechanical stress, which may change the shape, size, or toughness of the bones [39–42]. In order to understand if the observed patterns of ossification are the result of processes controlled genetically or developmentally, as well as a response to environmental constraints, experimental studies on living fish morphology are of primary interest. The effects of many environmental
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