Anchovies In The Net: Concealed Identities Revealed

For those who delight in eating Mediterranean anchovies, the taste of inshore varieties has long been preferred to that of the open-sea kind. An IRD researcher has shown that this organoleptic difference coincides with a real biological distinction. In the Mediterranean Sea there is not just one species of European anchovy but two, each occupying its own habitat.

Correspondence analysis was performed of all existing genetic data obtained between 1980 and 1996 concerning anchovies from the Mediterranean Basin and the eastern Atlantic, in order to establish the links between genetic variations and geographical distribution of different populations. There are two hypotheses that might explain the existence of two habitat-specific Mediterranean species, one coastal, the other pelagic.

The results highlight the value of using the tools of molecular biology to take a fresh look at anchovy classification, seeing that an ability to distinguish species can have a direct influence on the organization of fishing and trade in these fish, particularly in the Mediterranean.

The anchovy family embraces dozens of species, abundant in both tropical and temperate waters. The European anchovy, Engraulis encrasicolus, about 15 cm long, lives in schools in the waters of the eastern Atlantic and the Mediterranean in zones down to 150 m depth. Anchovy make up at least 15% of the 86 million tonnes of fish caught in the world each year (1). Economically important, they are also highly significant ecologically.
They constitute the staple diet of many predator species, such as tuna, some sharks, or certain birds. For these reasons, they have been the subject of several studies in molecular biology devoted to improving understanding of the genetic structure of stocks pursued for fishing.

An IRD researcher analysed all the results of work conducted in the eastern Atlantic and the Mediterranean Basin and succeeded in finding links between genetic variations and geographical distribution of different populations of European anchovy. He thus shed new light on their genetic composition, suggesting the existence of two species in the Mediterranean.

The division of fishing resources into stocks, useful for fisheries management, does not necessarily correspond to any true biological situation. Anchovy from the Adriatic Sea therefore, which until very recently were managed as a single stock, were newly recognized as originating from two distinct populations by their size and appearance. The ones which inhabit inshore reaches, estuaries or lagoons turn out to be paler and smaller than the oceanic (pelagic) ones. Nonetheless, these differences are not taken into account in the organization of the anchovy fishing and trade.

Morphological differences are accompanied by significant genetic differences, revealed by analysis of frequencies of genetic markers (2). In one and the same region, coastal anchovies are thus genetically different from the pelagic ones which live in the open sea. However, at the geographical scale of the whole Mediterranean Basin as a whole, inshore anchovy several thousands of kilometres apart prove to be genetically very close. This is the case for inshore anchovies inhabiting the northern reaches of the Adriatic, which are genetically indistinguishable from those in brackish estuary communities of the Golfe-de-Lion. This genetic proximity also concerns the pelagic varieties. Those inhabiting the Bay of Biscay show no significant genetic differences from those living in the western Mediterranean or the Ionian Sea.

These results indicate that genetic exchanges between the two anchovy varieties –coastal and pelagic- are restricted. The two forms therefore correspond to two distinct species, each tied to its habitat. Two hypotheses have been postulated to explain such speciation. Either there has been a divergence of the two forms following a geographical separation, an isolation by distance, then succeeded by an adaptation to different habitats, over a time-scale long enough for them no longer to be capable of interbreeding, once brought into secondary contact. Or, from generation to generation they adapted to their preferred habitat, while any hybrids between the two were eliminated by natural selection owing to lack of adaptation to either of these habitats. Only further more detailed investigations, calling on new genetic markers plus biological studies, on a large number of anchovy populations, could lead to any accurate identification of the evolutionary mechanisms brought into play in this speciation process.

Whatever the case may be, these analyses emphasize the contribution molecular biology is making to anchovy species identification. An ability to distinguish the anchovy species moreover has economic significance. In the Mediterranean, the inshore anchovies are sought by connoisseurs because of their gastronomic value, superior to that of the oceanic variety. The small inshore anchovy – no more than 12 cm long- are consequently sold at a much higher price to French, Spanish or Italian fish merchants. However, their export is still forbidden, as current European legislation does not allow the export of anchovy of less than 12 cm length. This clause, adopted in line with a concern for preserving the juveniles and guaranteeing stock durability, refers only to the mature, adult size of pelagic anchovies, which are larger. An ability to distinguish between the two anchovy species in the fisheries statistics and a review of the legislation should lead to improved management of these resources.