Trophic ecology and bioaccumulation of mercury in the three hagfish (Myxinidae) species from the Gulf of Mexico

Hagfishes (Myxinidae) are common in deep waters of the Gulf of Mexico (GOM). Two of the species found in the GOM are endemic to the area and potentially provide key ecosystem services (e.g. generate substrate turnover and recycling of organic matter by consuming carrion falls) to the deep environments of the Gulf. Yet, very little is known about hagfish life histories and ecology. I investigated inter and intraspecific variations in trophic structure of Eptatretus springeri, Eptatretus minor, Myxine mcmillanae, including variations along relevant environmental gradients. At the time of this study, the Gulf of Mexico had recently experienced an environmental disaster with the Deep Water Horizon (DWH) oil spill. As a result, I also investigated the potential effects of the DWH oil spill on hagfish trophic structure, and the potential for higher bioaccumulation of mercury (Hg) in their muscle tissue. Feeding ecology and trophic structure were investigated using nitrogen (delta 15N), carbon (delta13C) and sulfur (delta 34S) stable isotopes and variability in the lipid content (Deltadelta 13C)) of each species was used as an indicator of periods of feeding and fasting. I was able to differentiate the isotopic niche of each of the species and determine the different feeding ecologies of E. springeri and M. mcmillanae. The isotopic niche of E. minor overlaps with that of E. springeri and M. mcmillanae. The most depleted mean delta15N values were observed in E. springeri (12.8‰), followed by E. minor (13.9 ‰) and M. mcmillanae (14.9‰). Eptatretus springeri had the highest (Deltadelta 13C)), lowest trophic level (delta15N) and exhibited the most isotopic variation with depth. This could suggest this species is an active predator that feeds regularly, and potentially relies on scavenging only to supplement its diet. Eptatretus minor had slightly higher Deltadelta13C), intermediate but depleted delta 15N, and higher delta15N isotopic diversity. These characteristics suggest a more opportunistic foraging behavior for this species, potentially feeding on prey items from several trophic levels. Myxine mcmillanae had the highest delta15N but lowest delta 13C), and no variation in delta15N with depth. These trends indicate this species could be experiencing irregular feeding with periods of fasting, potentially due to a mostly scavenging behavior and occasionally feeding on larger but infrequently available carrion falls of large animals. Spatial analysis showed all hagfish species displayed enriched delta 15N and delta13C), and depleted delta34 S values on the continental slope off Louisiana than along the North Florida Slope and in DeSoto Canyon. These shifts in isotopic signatures could be attributed to influx from riverine sources from the Mississippi River. Temporal variations were also correlated to the seasonality of nutrient discharge from the river. Isotopic shifts seemed to be a consequence of riverine inputs rather than as a result of changing environmental conditions from the DWH oil spill. This study also determined Total Hg (THg) and Hg species (Methyl-Mercury-MeHg, inorganic Hg-IHg) concentrations in hafish muscle tissue. There was significant intra and interspecific variation in THg concentrations of all three hagfish species. Enriched delta15N and much higher THg concentrations were observed in M. mcmillanae (11.9 ppm) when compared to E. minor (2.1 ppm) and E. springeri (1.1 ppm). Mean MeHg concentrations were highest in M. mcmillanae (10.9 ppm), followed by E. minor (1.4 ppm) and E. springeri (0.8 ppm), while mean IHg concentrations were 1.3 ppm, 0.7 ppm, and 0.4 ppm , respectively. However, Hg species displayed a wide range of variation, with IHg concentrations in muscle tissue accounting for up to 75% to 95% of the THg content for some individuals. THg concentrations were studied in relation to several biological and environmental factors (stable isotopes, depth, total, length, body weight and lipid content, spatial and temporal variations) but most of these relationships were not significant for one or more species, which suggests that more than one uptake and depuration pathway or factors could contribute to the bioaccumulation of Hg in hagfishes.