Meeting Abstract
The Trinidadian guppy, a livebearing fish, has proved a fruitful model for studying rapid evolution in the field. Reduced predation in headwaters rapidly leads to stiff competition and the evolution of larger offspring size among low predation (LP) populations, relative to their high and high-high predation (HP, HHP) conspecifics. At birth, guppy neonates must locomote and forage independently, thus we ask: are guppies of varying size all born at the same level of morphological and functional maturity? We collected neonates (n=25 per pop), measured feeding and escape start performance and then stained specimens to quantify musculoskeletal maturation. While the external body proportions of offspring (ranging from 5.5-7.2mm standard length) scale with isometry (no change in relative shape), we find size and development of internal morphologies vary significantly among guppy offspring. Each neonatal size class has ossified approximately 30% more of their head skeleton than the next smallest class. In the tail region, the hypural plate is unossified in HHP neonates, where it is ossified and separated into two distinct arms in the LP neonates. Additionally, HHP neonates have more curved, U-shaped myomeres, where their larger conspecifics possess more adult-like, V-shaped myomeres. Head muscle area is relatively large in larger offspring, scaling with standard length as L2.5. We find that these differences improve the functional abilities of larger offspring, such that escape start performance and gape size at occlusion is relatively high (positive allometry) in larger neonates. The observed covariation of size and maturity appear to amplify the negative performance effects of being born small, which likely reduces survivorship and increases the severity of the critical period for the smallest guppies.
