- Understanding the origin and nature of intra specific biodiversity enables us to better conserve and manage animal populations. Biological diversity is seen at different scales and for different traits such as behavior, morphology, physiology, and life history. Behavior is especially important since behavioral changes are believed to precede changes in morphology or physiology among fishes. Salmonids display great diversity in terms of behavior, life histories and morphology within and among populations. Thus, differentiation among populations and morphs has been related to the evolution of new species. Various genetic, environmental and ecological factors have been shown to be important for segregation of morphs, including competition for food or other resources, phenotypic plasticity and sexual selection. Recently, it has been suggested that the importance of epigenetic and maternal effects for intra specific diversity have been underestimated. I studied the short- and long-term effects of egg size on development, behavior, body growth and physiology in Arctic charr Salvelinus alpinus and steelhead trout Oncorhynchus mykiss. I also examined how domestication can affect egg size.
Egg size was smaller in domesticated fish populations after accounting for female body size and age. Egg size was negatively correlated with embryonic development before hatching, i.e. embryos in small eggs developed more rapidly. At emergence, egg size was positively correlated with length and weight of first feeding progeny. Juveniles coming from larger eggs tended to feed more at the surface whereas juveniles coming from smaller eggs fed more on the bottom. These relationships and effects of egg size on embryos and first feeding fish were observed in both species in laboratory conditions. In Arctic charr there was a higher energy content per egg in larger eggs in both aquaculture and wild populations, and the total energy content per egg varied among populations. Behavior of Arctic charr at first feeding was affected by egg size, social environment and their interaction. At 300 days post fertilization, fish coming from different egg sizes differed in morphology and behavior: larger fish coming from larger eggs fed more at the surface than smaller fish coming from smaller eggs. Independently of their genetic origin large and small juveniles, coming respectively from large and small eggs, differed in body shape. This was most clearly seen in head and body morphology, e.g. larger fish were overall slimmer than smaller fish. The influence of egg size on behavior and morphology of Arctic charr varied with female parentage, indicating strong maternal x genetic interactions. In steelhead trout, both origin of fish and egg size were related with body growth of yearling fish reared under laboratory conditions: hatchery juveniles coming from small eggs were larger than wild juveniles coming from small eggs. Both were in turn larger than hatchery and wild juveniles coming from large eggs. Hatchery progeny showed lower osmo-regulatory status compared to wild progeny but nevertheless preferentially chose salt water.
This study presents novel findings that demonstrate that variability in egg size is an important source of phenotypic variation in fishes. My results support the hypothesis that females experiencing relatively high growth rate as juveniles produce a large number of small eggs as adults and that such a reduction in egg size happens rapidly, i.e. in only one generation in domestication. I discuss the implications of egg size for evolution of fishes and, especially how diversity created by egg size can influence diversification and
speciation of fishes.