Use of black soldier fly (Hermetia illucens) larvae meal in diets for gilthead seabream juveniles: Effects on growth-related gene expression, intermediary metabolism, digestive enzymes, and gut microbiota modulation

Abstract

A study was conducted to evaluate the effects of dietary inclusion of defatted Hermetia illucens larvae meal (HM) on the expression of growth-related genes, plasma metabolites, activity of intermediary metabolism enzymes, and gut microbiota modulation in gilthead seabream (Sparus aurata) juveniles. Four experimental diets were formulated (43% crude protein, 18% crude lipids) with increasing levels of defatted HM: 0 (HM0), 15 (HM15), 30 (HM30), and 45% (HM45) replacing fishmeal (included at 34% in the control diet) at 0, 22, 60, and 100%, respectively. Fish were fed these diets for 67 days. At the end of the trial, the expression of muscle growth hormone receptors (ghr1 and ghr2), insulin-like growth factors (igf1 and igf2), cathepsins (cts-a and cts-b), and myogenic determination factor 1 (myod1) were not affected, while a downregulation of myogenic determination factor 2 (myod2) and upregulation of myostatin (mstn) was observed with HM inclusion. Plasma metabolites profile was unaffected by diet composition, except for a linear decrease trend in total lipids. A linear increase of liver alanine aminotransferase (ALT) and glutamate dehydrogenase (GDH) activity and a linear decrease of glucose 6-phosphate dehydrogenase (G6PDH), malic enzyme (ME), and (FAS) activity were observed with HM inclusion. The activity of β-hydroxy acyl-CoA dehydrogenase (HOAD) followed a quadratic trend, being higher in fish fed with diet HM15 than in the control. Likewise, amylase, trypsin, and chymotrypsin activity were higher in the posterior intestine of fish fed with diet HM15. Digesta of fish fed the HM-containing diets showed an increase in the number of operational taxonomic units, microbial richness, and diversity. Conversely, a reduction in these indexes was observed in the mucosa. Among the identified bacteria, Firmicutes represented the predominant phyla. The inclusion of HM increased the abundance of Oceanobacillus, Lederbergia, Bacillus, and Corynebacterium in the digesta while Lactobacillus and Caldalkalibacillus abundance was reduced. Overall, total FM replacement with defatted HM had no major effects on plasma metabolites and muscle growth-related genes but induced a few differences in intermediary metabolism and digestive enzymes activity. HM inclusion improved gut digesta microbiota richness and diversity, promoting the growth of potential beneficial bacteria.