Available at: https://digitalcommons.calpoly.edu/theses/2767
Date of Award
3-2024
Degree Name
MS in Biological Sciences
Department/Program
Biological Sciences
College
College of Science and Mathematics
Advisor
Sean Lema
Advisor Department
Biological Sciences
Advisor College
College of Science and Mathematics
Abstract
Fish experiencing stressful conditions increase production of the glucocorticoid hormone cortisol, which alters energy intake and allocation to help recover physiological homeostasis. Fish experiencing prolonged elevations of cortisol, however, often exhibit reduced somatic growth. While it has been shown previously in fish that cortisol inhibits anabolic growth processes in part via changes in endocrine growth hormone (Gh)/insulin- like growth factor-1 (Igf1) signaling, it is not fully understood which components of Gh/Igf1 pathways are modulated by cortisol. Here, we explored how cortisol influences the Gh/Igf1 system in blue rockfish (Sebastes mystinus) by administering exogenous cortisol and examining effects on Igf1 pathways. Cortisol-treated rockfish had higher plasma cortisol both 5 h and 24 h after injection compared to fish treated with vehicle solution only. Cortisol-treated rockfish also had lower plasma Igf1, even though relative expression of igf1 mRNAs in the liver was unaffected by cortisol. Rockfish treated with cortisol did, however, exhibit higher liver mRNA levels of Igf binding proteins (Igfbps) igfbp1a and igfbp1b, which encode Igfbps with proposed inhibitory influences on Igf1 stimulation of somatic growth. Cortisol-treated fish also expressed reduced liver mRNA levels of igfbp2a, an Igfbp thought to facilitate Igf1 transport in fish, as well as reduced mRNAs in skeletal muscle for myoblast determination protein 2 (myod2), a transcriptional activator for other genes promoting myocyte differentiation during myogenesis. Taken together, these findings point to cortisol-related changes in liver Igfbp expression as a mechanism for shifting Igf1 regulation of growth in fish experiencing elevated glucocorticoid secretion.
Temperature is a key regulator of metabolism and growth in teleost fishes. Fish experiencing atypically high temperatures undergo a variety of physiological changes to cope with the elevated temperature, which often includes a reduction in the rate of somatic growth. Studies examining the effects of elevated temperatures on fish growth have typically focused on longer duration increases in average temperature. However, with anomalous ‘heat wave’ events becoming more common in aquatic habitats with climate change, fishes may increasingly be exposed to shorter-term periods of extreme elevated temperature. In this study, black rockfish (Sebastes melanops) were exposed to a short-duration period of elevated temperatures resembling the effects of a heat wave event on a rocky intertidal tidepool, a habitat used by young-of-the-year juveniles of this rockfish species. Black rockfish maintained on two different food rations (1% or 9% g feed per g body mass per day) for 21 d were exposed to elevated temperatures simulating a ~6°C maximum temperature increase for a ~54 h period, after which temperatures were returned to ambient ocean conditions. Rockfish in both ration treatments showed reduced body condition and lower mass-specific growth rates (SGR) during the 3 week period following the heat event period, but then subsequently increased growth above pre- stressor growth rates during the 6 and 9 week period after the high temperature event. Plasma insulin-like growth factor 1 (Igf1) hormone concentrations were correlated with an individual rockfish’s mass-specific growth rate prior to the heating event, but then decreased in both feed treatments following the event and no longer showed that positive relationship to growth. Plasma glucose was also altered by the heating event and increased to remain higher in both feed treatments following the heat event. Plasma osmolality was initially lower in both food ration groups 20 d after the heating event, but then diverged to increase back toward pre-event osmolality values in 9% ration fish, but not 1% ration fish, 6 weeks following the heat event. These findings suggest that exposure of juvenile black rockfish to an acute temperature elevation resembling the heat wave conditions that occur in the species’ tidepool nursey habitats may have consequences for both somatic growth and osmotic balance for weeks following the heating event.