Octopus, an unexpected ally in the struggle against blue crabs

BarcelonaAdding the blue crab to rice, fish broths and other culinary recipes has helped combat the plague that threatens the ecosystems in the Ebro Delta. The years-long war against this crustacean fishermen have been waging may have found a new ally. Tests carried out by researchers at the Institut de Recerca i Tecnologia Agroaliamentàries (IRTA) have shown that octopuses are natural predators of the blue crab, which they are capable of eating in large quantities if necessary: "A single octopus can eat up to 33% of its weight in crab meat in a single day", explains project coordinator Patrícia Prado.

That octopuses like to include blue crab s(Callinectes sapidus) in their diet is something that had already been observed on the American Atlantic coast, the crustacean's native area. Videos recorded by IRTA, for the moment in a nursery, show that the octopus is quite skilled at stopping the crab's claws, which proves that it is "one of its favourite prey," according to Prado, who adds that an octopus weighing at least one kilo can eat crabs of all sizes (large and small) found in the ecosystem.

In view of their potential to help combat the spread of blue crabs, IRTA researchers are considering other experiments and analyses to determine the degree of voracity that octopuses may have in the natural environment in an area highly populated by blue crabs. "Observation is difficult in the wild, but we have ways of analysing whether or not an octopus has been feeding on crab," the researcher explains to ARA.

Since the appearance of the blue crab in the Delta de l'Ebre in 2017, the species has been a real nightmare for the biodiversity of the area. Colonisation has been very rapid, since the reproduction rate of this species is very high: "In each brood it manages to lay between 700,000 and 2 million eggs," stresses Prado. The discovery of its culinary value was an unexpected (but welcome) brake that has meant that in certain areas the proliferation of crabs seems to be clearly on the decline: in May of this year, the 2,650 kilos of crab caught are much lower than the 13,800 kilos of only one year ago.

However, the researcher asks for caution to wait to see the data from the end of this year (the crab's growth season is about to start) to be able to speak with more certainty of a decline. "What is clear to us is that in some areas of bays and upriver the crab has consumed many resources and now it is not feeding so well, but this does not happen in the open sea, where it finds a lot of food," Prado points out. Some studies have already shown that the massive presence of blue crabs takes its toll on ecosystems, since they feed on molluscs, crustaceans, fish and native algae. Species such as the big-scale sand smelt, the Iberian killifish or the Mediterranean green crab have suffered significant declines, according to several studies

In addition to the IRTA tests, the Universitat Politècnica de València and the Oceanogràfic de València are also carrying out their own experiments to find other natural predators to join the blue crab hunt, such as the eel and the loggerhead turtle, among others.