Date of Award

6-2026

Degree Name

MS in Biological Sciences

Department/Program

Biological Sciences

College

College of Science and Mathematics

Advisor

Sean C. Lema

Advisor Department

Biological Sciences

Advisor College

College of Science and Mathematics

Abstract

Marine heatwaves associated with climate change are increasing in frequency and severity, posing significant threats to fish populations in shallow habitats such as estuaries and salt marshes. Because temperature is a primary regulator of fish reproduction, understanding how heatwaves affect reproductive physiology is critical for anticipating the biological consequences of ongoing climate change for coastal fishes. Here we examined how simulated heatwave conditions affect the reproductive hypothalamic-pituitary-gonadal (HPG) endocrine axis in the estuarine sheepshead minnow (Cyprinodon variegatus), a eurythermal fish well suited to studying thermal stress responses in estuarine environments. Adult sheepshead minnows acclimated to a daily oscillating thermal regime (25–30°C) were exposed to a simulated heatwave (+5°C) for 8 days, followed by a 7 day recovery period. Heatwave-exposed females had reduced ovarian gonadosomatic index (GSI) and lower circulating 17b-estradiol (E2), whereas males showed no change in testicular GSI and or 11-ketotestosterone (11-KT). Gene transcripts encoding the b-subunits of follicle stimulating hormone (fshb) and luteinizing hormone (lhb), and a-subunit glycoprotein hormone (gpha) in the pituitary gland were each reduced in abundance in females exposed to the heatwave condition. Those heatwave-exposed female minnows also had diminished mRNA abundances in the ovary for the steroidogenic enzymes cyp11a1a, 3bhsd, and cyp19a1a. Hepatic vitellogenin (vtgab) expression was also reduced in exposed females. Together, these findings demonstrate that short-term marine heatwave events can disrupt the HPG axis to impair steroidogenesis and oogenesis in eurythermal estuarine fishes, with females being considerably more sensitive to thermal stress than males.

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