The Sargasso Sea was identified as the spawning area of the European eel (Anguilla anguilla) 100 years ago, and numerous subsequent surveys have verified that eel larvae just a week old are regularly recorded there. However, no adult eels or eel eggs have ever been found, leaving room for alternative hypotheses on the reproduction biology of this enigmatic species. Chang et al.1 theorize about an area along the Mid-Atlantic Ridge as a potential spawning ground. The main argument for this hypothesis was that the chemical signature found in eel otoliths would indicate that early stage larvae had been exposed to a volcanic environment, such as the one present along the Mid-Atlantic Ridge. Since this correlation was solely based on a mis-interpretation of cited literature data, no new, conclusive information to pinpoint the Mid-Atlantic Ridge as an additional or even alternative spawning area was presented by Chang et al.
For more than 100 years, the life history of Atlantic eels remains a matter of scientific debate. In a recent paper by Chang and colleagues, published in Scientific Reports (Sci Rep 10, 15981 (2020)), it is hypothesized that the spawning areas of the European eel (Anguilla anguilla) and the American eel (A. rostrata) are located along the Mid-Atlantic Ridge at longitudes between 50° W and 40° W1. This area lies outside the Sargasso Sea, which has so far been widely assumed to be the spawning region of both species since the beginning of the twentieth century2. The Danish researcher Johannes Schmidt collected eel leptocephali 30 mm long or less, some as short as 9 mm, all south of 30° N and west of 50° W3,4. Since then, Schmidt’s assumption was supported by a number of investigations that found recently hatched European eel larvae (< 12 mm) in a 2000 km wide region from 70° W eastward to 50° W (e.g.5,6,7,8).
Nonetheless, final evidence about the precise locations of specific spawning sites and the timing of spawning is still lacking and information on hydrographic conditions at the depth and place of spawning also remains limited9.
In their paper, Chang et al. raised four arguments to support a Mid-Atlantic Ridge hypothesis: (1) The importance of seamounts as orientation and navigation cues towards a spawning area, (2) The results of a drift simulation showing that the distribution of virtual eel larvae that were hatched at the Mid-Atlantic Ridge would result in the observed distribution of eel larvae in the Atlantic Ocean, and (3) elevated manganese (Mn) concentrations in the otolith cores of glass eels caught in western European estuaries compared to those of leptocephalus larvae caught in the Sargasso Sea, which the authors interpret as a hint for successful spawning only in areas with volcanic activity. In addition, the authors (4) mention insufficient sampling efforts and a limited area coverage of recent surveys as a possible reason for “false negative” observations along the Mid-Atlantic Ridge.
However, none of the arguments seem to withstand careful scrutiny.
Source: Ecology - nature.com
