Hagfish feeding habits along a depth gradient inferred from stable isotopes

Feeding habits of 3 hagfish species were investigated along a depth gradient (similar to 50 to 900 m) in New Zealand using nitrogen (delta N-15) and carbon (delta C-13) stable isotopes. Neomyxine biniplicata had the lowest mean d15N value (14.2 parts per thousand), followed by Eptatretus cirrhatus (14.9 parts per thousand) and Eptatretus sp. 1 (15.8 parts per thousand). Neomyxine biniplicata (similar to 50 m depth) was characterized by (1) relative low lipid content in muscles and (2) consistent body condition index which together with its trophic position indicated that this species probably acquires its food by active predation, supplemented by opportunistic scavenging. Eptatretus cirrhatus (48 to 912 m) and Eptatretus sp. 1 (290 to 922 m) had similar morphology, but their delta N-15 signature indicated that they were feeding on slightly different trophic levels. For Eptatretus sp. 1, the combination of (1) variable lipid content, indicating phases of feeding and fasting, (2) decreasing body condition index with depth, indicating less regular feeding at depth, (3) increasing delta N-15 with depth and (4) decreasing delta C-13 signature with depth, pointed towards a feeding behaviour specialized in scavenging on large but rare falls of high-level predators such as whales, sharks or bony fishes. On the other hand, E. cirrhatus was characterized by (1) less variable lipid content, (2) a body condition index not influenced by depth, (3) delta N-15 values decreasing with depth and delta C-13 values constant across its depth range, which is likely to indicate a more opportunistic and mobile feeding behaviour on a range of prey.