Insulin-like growth factor-I gene expression as a growth indicator in Nile tilapia Oreochromis niloticus L.

Somatic growth in fishes is regulated by a variety of hormones. A central step in this hormonal network is the growth hormone-insulin-like growth factor-I (IGF-I) axis. Studies conducted evaluated the relationship of hepatic IGF-I (hIGF-1) mRNA with growth as affected by feeding regimes (satiation or restricted level; daily or alternate-day feeding), temperatures (high, ambient, low) and by social stress. To develop a cellular means for the quantification of hIGF-I mRNA levels in O. niloticus, hIGF-I cDNA was isolated and cloned. The partial sequence of IGF-I cDNA encodes for signal peptide, mature protein and a portion of the E-domain. A sensitive TaqMan quantitative reverse transcription-polymerase chain reaction (qRT-PCR) assay was developed based on the mature IGF-I. Using the developed qRT-PCR assay a significant positive correlation was observed between hIGF-I mRNA levels and growth rate of fish reared under different feeding regimes (r = 0.64) and temperature conditions (r = 0.64). On the dynamics of hIGF-I gene expression in response to elevated temperature, hIGF-I mRNA levels were significantly elevated after at least 2 days of exposure to warm temperature. This validates the concept that hIGF-I gene expressions are sufficiently sensitive to be used as a rapid growth rate indicator for O. niloticus. The hIGF-I levels have a significant positive correlation with specific growth rate (length; r = 0.92), and with condition factor (r = 0.55). On the effect of social stress, differential alterations in growth rates between the dominant and subordinates were observed which was attributed more to behavioral changes as transduced by physiological regulators. The fish's relative position in the social hierarchy was consistently reflected in the levels of hIGF-I mRNA and the eye color pattern. Subordination depressed hIGF-I levels while dominance stimulated it. These findings have shown that hGF-I level remained positively correlated to growth rate as affected by feeding regime, temperature and social stress. This suggests that hIGF-I plays a key role in controlling growth in O. niloticus and indicates that IGF-I mRNA quantification could prove useful for the rapid assessment of growth rate in this species of fish.

A Suite of Prey Traits Determine Predator and Nutrient Enrichment Effects in a Tri-Trophic Food Chain

Predation, predation risk, and resource quality affect suites of prey traits that collectively impact individual fitness, population dynamics, and community structure. However, studies of multi-trophic level effects generally focus on a single prey trait, failing to capture trade-offs among suites of covarying traits that govern population responses and emergent community patterns. We used structural equation models (SEM) to summarize the non-lethal and lethal effects of crayfish, Procambarus fallax, and phosphorus (P) addition, which affected prey food quality (periphyton), on the interactive effects of behavioral, morphological, developmental, and reproductive traits of snails, Planorbella duryi. Univariate and multivariate analyses suggested trade-offs between production (growth, reproduction) and defense (foraging behavior, shell shape) traits of snails in response to non-lethal crayfish and P addition, but few lethal effects. SEM revealed that non-lethal crayfish effects indirectly limited per capita offspring standing stock by increasing refuge use, slowing individual growth, and inducing snails to produce thicker, compressed shells. The negative effects of non-lethal crayfish on snails were strongest with P addition; snails increased allocation to shell defense rather than growth or reproduction. However, compared to ambient conditions, P addition with non-lethal crayfish still yielded greater per capita offspring standing stock by speeding individual snail growth enabling them to produce more offspring that also grew faster. Increased refuge use in response to non-lethal crayfish led to a non-lethal trophic cascade that altered the spatial distribution of periphyton. Independent of crayfish effects, snails stimulated periphyton growth through nutrient regeneration. These findings illustrate the importance of studying suites of traits that reveal costs associated with inducing different traits and how expressing those traits impacts population and community level processes.