Temperature and developmental plasticity during embryogenesis in the Atlantic cod Gadus morhua L.

Atlantic cod (Gadus morhua L.) embryos were reared at 4 degreesC, 7 degreesC, and 10 degreesC, and the relative timing of developmental events was characterized, with particular reference to myotomal muscle. Embryos started to feed at an apparently equivalent stage of development, so comparisons were made between temperature groups on the basis of percentage of time to first feeding and somite stage. No differences were found in the time of hatching or timing of appearance of the otic placode, unpaired median fin fold, gut lumen, otic vesicle, lens of the eye, otoliths, first muscular contractions, swim bladder, and hindgut, or in the rate of development of somites, myotubes, myofibrils, and acetylcholinesterase activity over the temperature range studied. In contrast, closure of the blastopore occurred late with respect to segmentation at higher temperatures, at the 3-somite, 10-somite, and 12-somite stages at 4 degreesC, 7 degreesC, and 10 degreesC respectively. Muscle cellularity was also markedly altered in the 10 degreesC group relative to the 4 degreesC and 7 degreesC groups. Larvae reared at 10 degreesC had significantly more ( +14%) deep white fibers at hatch (P<0.001), whereas numbers of superficial red fibers remained unchanged. It is suggested that differences in muscle cellularity might be related to changes in the relative timing of epiboly, through differential proliferation of presomitic myogenic cells and/or their relative exposure to inductive signals.