Stable-isotope analyses reveal the importance of seagrass beds as feeding areas for juveniles of the speckled worm eel Myrophis punctatus (Teleostei: Ophichthidae) in Florida

Abstract

The feeding habits and habitats of the speckled worm eel Myrophis punctatus were studied on the mangrove edge of the Indian River Lagoon (IRL, Florida) using gut-content and stable-isotope analyses of carbon (d13C) and nitrogen (d15N). Four taxa were identified through analyses of gut contents, and the index of relative importance suggested that amphipods, microphytobenthos and annelids are the most important food sources in the fish's diet. To assess the feeding habits of the fish after their recruitment to the IRL, these food sources were collected from mangroves and nearby seagrass beds for isotope analyses. Stable isotopes constituted a powerful tool for discriminating fish prey items from mangroves (mean ±s.d.d13C = -20·5 ± 0·6 ) and those from seagrass beds (mean ±s.d.d13C = -16·9 ± 0·6 ), thus providing good evidence of food source origins. The 56 M. punctatus collected [10·0 < total length (LT) < 16·2 cm] had average isotopic signatures of d13C = -16·7 ± 0·2 and d15N = 8·2 ± 0·1 . A significant depletion in 13C was observed for larger juveniles (15·0 < LT < 16·2 cm), suggesting that they found a portion of their food in mangroves. Estimation of the trophic level from stable isotopes (TLiso) was similar among different size groups of juvenile fish (TLiso = 3·2 3·5); therefore, M. punctatus was considered a secondary consumer, which is consistent with its zoobenthic diet. The concentration-dependent mixing Stable Isotope Analysis in R (SIAR) model revealed the importance of food sources from seagrass beds as carbon sources for all the fish collected, with a significant increase in mangrove prey contributions, such as annelids, in the diet of larger juveniles. This study highlights the importance of seagrass beds as feeding habitats for juveniles of M. punctatus after their recruitment to coastal waters.