Digestive Tract and the Muscular Pharynx/Esophagus in Wild Leptocephalus Larvae of European Eel (Anguilla anguilla)

Several aspects of the biology of European eel (Anguilla anguilla) larvae are still unknown; particularly, information about their functional development and feeding is sparse. In the present study, we histologically characterize the digestive system of wild caught specimens of European eel leptocephalus larvae. The aim was to provide more understanding about how food may be ingested and mechanically processed in the leptocephalus larvae, and to discuss this in the context of its hypothesized feeding strategy. Larvae were caught in the Sargasso Sea during the “Danish Eel Expedition 2014” with the Danish research vessel Dana. The larval sizes ranged from 7.0 to 23.3 mm standard length (SL) at catch. We found that the mouth/pharynx, especially the anterior esophagus, was surrounded by a multi-layered striated muscle tissue and that the epithelium in the mouth/pharynx had a rough filamentous surface, followed by epithelial columnar cells with multiple cilia in the anterior esophagus. This suggests an expandable pharynx/esophagus, well-suited for the transportation of ingested food and likely with a food-crushing or grinding function. The digestive tract of the larvae consisted of a straight esophagus and intestine ventrally aligned within the larval body, and its length was linearly correlated to the larval length (SL). The length of the intestinal part constituted up to 63% of the total length of the digestive tract. The intestinal epithelium had a typical absorptive epithelium structure, with a brush border and a well-developed villi structure. Some cilia were observed in the intestine, but any surrounding muscularis was not observed. The liver was observed along the posterior part of the esophagus, and pancreatic tissue was located anterior to the intestine. Our findings support the hypothesis that the eel leptocephalus may ingest easily digestible gelatinous plankton and/or marine snow aggregates. The muscular esophagus and the ciliated epithelium appear sufficient to ensure nutrient transport and absorption of the ingested food through the digestive tract.

Modeling the drift of European (Anguilla anguilla) and American (Anguilla rostrata) eel larvae during the year of spawning

The distribution of the leptocephalus larvae of European (Anguilla anguilla) and American (Anguilla rostrata) eels collected during recent Sargasso Sea surveys was used to model larval drift. The drift trajectories of individual larva were back-calculated to the estimated time of spawning, using current data from two global oceanographic assimilation models. The results of both models give the same overall result; widespread spawning extended in time from December to March. The drift was also calculated forwards for approximately 1 year. The forward drift modelling showed that most leptocephali remained in the area south of the Subtropical Frontal Zone. One conclusion is that the majority of leptocephali remain trapped and possibly die in the retention area. A small proportion of leptocephali are entrained into the Gulf Stream system. An implication is that the spawning success may be highly sensitive to oceanographic and climatic factors that alter the dispersion of leptocephali out from the retention area. An alternative interpretation is that the surveys were made too late after the peak spawning period and that the core spawning area was missed.

Low occurrence rates of ubiquitously present leptocephalus larvae in the stomach contents of predatory fish

Leptocephali, the larvae of eels, grow to large sizes and are widely distributed in tropical and subtropical oceans. Their role in oceanic food webs is poorly known because they are rarely reported as food items in fish stomach content studies. Data from 13 years of research on the trophic dynamics of Pacific Ocean predatory fish indicate that among 8746 fish of 76 species/taxa (33 families) that had been feeding, only 16 fish of 6 species had remains of 34 leptocephali in their stomachs. Only 0.013% of the 256 308 total prey items were leptocephalus larvae, and 0.03% of the total prey items were juvenile or adult eels (mostly snipe eels: Nemichthyidae). There were 10 fish of 2 species of lancetfish (Alepisaurus spp., n = 152), 2 rainbow runners (Elagatis bipinnulata, n = 222), and 2 yellowfin tuna (Thunnus albacares, n = 3103) that had leptocephali in their stomach contents, but all except one T. albacares (contained 15 leptocephali) had each eaten ≤3 leptocephali. A swallower, Pseudoscopelus sp., and a frigate tuna, Auxis thazard, had eaten single leptocephali. Twenty-eight bigeye tuna, Thunnus obesus, had eaten 76 juvenile/adult nemichthyid or serrivomerid eels. A literature survey found that only 15 out of 75 examined publications listed leptocephali in the stomach contents of a total of 6 species out of ∼ 42 300 predatory fish of 40 species. The transparency of leptocephali and their apparent mimicry of gelatinous zooplankton could contribute to lower rates of predation. Their soft bodies likely digest rapidly, so although this study and existing literature indicate that leptocephali sometimes contribute to predatory fish diets, particularly for fish that do not exclude gelatinous prey types, and fish with low digestion rates in their stomachs such as lancetfish, their levels of contribution to fish diets and the impacts of predators on eel recruitment remain uncertain.

ATLAS DE IDENTIFICACIÓN DE LARVAS DE PECES DE LA SUBDIVISIÓN ELOPOMORPHA DEL PACÍFICO MEXICANO

El objetivo principal de este trabajo es presentar la composición de especies de la Subdivisión Elopomorpha, contenida en la colección científica de las larvas de peces del Pacífico Mexicano, que pertenece al Centro Interdisciplinario de Ciencias Marinas del Instituto Politécnico Nacional. Presentamos fichas descriptivas de 23 especies recolectadas con redes de arrastre de zooplancton en el Golfo de California, la Bahía de La Paz, la costa occidental de Baja California, el Pacífico Central Mexicano y el Golfo de Tehuantepec, incluyendo larvas pertenecientes a las familias Elopidae, Albulidae, Muraenidae, Ophichthyidae, Congridae y Nettastomatidae. Las descripciones proporcionan patrones morfológicos, merísticos y la pigmentación que permiten identificar las larvas leptocéfalas en nuestra colección. Identification Atlas of larval fishes of the Subdivision Elopomorpha from the Mexican Pacific The main objective of this work is to present the species composition of the Subdivision Elopomorpha, housed in the Scientific Collection of Fish larvae from the Mexican Pacific, at the Centro Interdisciplinario de Ciencias Marinas of the Instituto Politécnico Nacional. We provide descriptive cards for 23 species collected with zooplankton trawls throughout the Gulf of California, La Paz bay, the west coast of Baja California, the Mexican Central Pacific and the Gulf of Tehuantepec. Larvae belonging to the families Elopidae, Albulidae, Muraenidae, Ophichthyidae, Congridae and Nettastomatidae are included. The descriptions provide morphological, meristic and pigmentation patterns that allow to identify the leptocephalus larvae in our collection.