Interactions of long-term food ration variation and short-term fasting on insulin-like growth factor-1 (IGF-1) pathways in copper rockfish (Sebastes caurinus).

Variation in food intake affects somatic growth by altering the expression of hormones in the somatotropic endocrine axis including insulin-like growth factor-1 (IGF-1). Here, we examined IGF-1 pathway responses to long- and short-term variation in food availability in copper rockfish (Sebastes caurinus), a nearshore Pacific rockfish important for commercial and recreational fisheries. Juvenile copper rockfish were raised under differing ration amounts (3% or 9% mass feed·g-1 fish wet mass·day-1) for 140 d to simulate 'long-term' feeding variation, after which some fish from both rations were fasted for 12 d to generate 'short-term' conditions of food deprivation. Rockfish on the 9% ration treatment grew more quickly than those on the 3% ration and were larger in mass, length, and body condition (k) after 152 d. Fish on the 9% ration had higher blood glucose than those on the 3% ration, with fasting decreasing blood glucose in both ration treatments, indicating that both long-term and short-term feed treatments altered energy status. Plasma IGF-1 was higher in rockfish from the 9% ration than those in the 3% ration and was also higher in fed fish than fasted fish. Additionally, plasma IGF-1 related positively to individual variation in specific growth rate (SGR). The positive association between IGF-1 and SGR showed discordance in fish that had experienced different levels of food and growth over the long-term but not short-term, suggesting that long-term nutritional experience can influence the relationship between IGF-1 and growth in this species. Rockfish on the 3% ration showed a lower relative abundance of gene transcripts encoding igf1 in the liver, but higher hepatic mRNAs for IGF binding proteins igfbp1a and igfbp1b. Fasting similarly decreased the abundance of igf1 mRNAs in the liver of fish reared under both the 9% and 3% rations, while concurrently increasing mRNAs encoding the IGF binding proteins igfbp1a, -1b, and -3a. Hepatic mRNAs for igfbp2b, -5a, and -5b were lower with long-term ration variation (3% ration) and fasting. Fish that experienced long-term reduced rations also had higher mRNA levels for igfbp3a, -3b, and IGF receptors isoforms A (igf1rA) and B (igf1rB) in skeletal muscle, but lower mRNA levels for igf1. Fasting increased muscle mRNA abundance for igfbp3a, igf1rA, and igf1rB, and decreased levels for igfbp2a and igf1. These data show that a positive relationship between circulating IGF-1 and individual growth rate is maintained in copper rockfish even when that growth variation relates to differences in food consumption across varying time scales, but that long- and short-term variation in food quantity can shift basal concentrations of circulating IGF-1 in this species.

Endocrine and metabolic impacts of warming aquatic habitats: differential responses between recently isolated populations of a eurythermal desert pupfish.

Temperatures of inland aquatic habitats are increasing with climate change, and understanding how fishes respond physiologically to thermal stress will be crucial for identifying species most susceptible to these changes. Desert fishes may be particularly vulnerable to rising temperatures because many species occupy only a fraction of their historical range and occur in habitats with already high temperatures. Here, we examined endocrine and metabolic responses to elevated temperature in Amargosa pupfish, Cyprinodon nevadensis amargosae. We studied C. n. amargosae from two habitats with distinct thermal conditions: the Amargosa River, which experiences diurnally and seasonally variable temperatures (0.2-40°C); and Tecopa Bore, a spring and marsh fed by hot groundwater (47.5°C) from an artesian borehole. These allopatric populations differ in morphology, and prior evidence suggests that temperature might contribute to these differences via altered thyroid hormone (TH) regulation of morphological development. Here, we document variation in hepatic iodothyronine deiodinase type 2 (dio2) and type 3 (dio3) and TH receptor ß (trß) gene transcript abundance between the Amargosa River and Tecopa Bore wild populations. Fish from these populations acclimated to 24 or 34°C retained differences in hepatic dio2, dio3 and trß mRNAs and also varied in transcripts encoding the TH membrane transporters monocarboxylate transporter 8 (mct8) and organic anion-transporting protein 1c1 (oatp1c1). Tecopa Bore pupfish also exhibited higher dio2 and trß mRNA levels in skeletal muscle relative to Amargosa River fish. Muscle citrate synthase activity was lower at 34°C for both populations, whereas lactate dehydrogenase activity and lactate dehydrogenase A-chain (ldhA) transcripts were both higher and 3,5,3'-triiodothryonine responsive in Tecopa Bore pupfish only. These findings reveal that local population variation and thermal experience interact to shape how pupfish respond to elevated temperatures, and point to the need to consider such interactions in management actions for desert fishes under a changing climate.