On yet another very hot summer’s day I’m heading to the centre of the Sacca degli Scardovari, a lagoon area in the Po River delta in Italy, ready to witness the devastation that invasive species can leave behind first-hand. According to Daniel Sarto, a local fisherman of clams and mussels, the lagoon is completely overrun by invasive crabs, and they have begun to eat everything in their proximity.

Our small metal boat cuts through the green, silky water until we dock near some mussel farming lines. The water looks murky, and I prepare myself to hop in the water to dive. As I submerge my head below the surface, I realise the visibility is so  poor, I can’t even make out my manometer. However, I know the bottom is just a few metres below and so begin my descent. This is an area dedicated to mussel farming, and as the seabed slowly comes into view, I notice it’s strewn  with their empty shells. Through the viewfinder of my camera, I can see hundreds of shadows moving quickly around the seabed amongst them. Immediately I realise, I’m taking a first glance at the infamous Atlantic blue crab, a species responsible for far-spread devastation across marine habitats in the northern Adriatic Sea.

Considered by the International Union for Conservation of Nature (IUCN) as one of the 100 worst invasive species of the Mediterranean, this particular type of swimming crab originally hails from the western Atlantic Ocean. Researchers believe it’s been introduced to the Mediterranean Sea through ballast water from ships containing the blue crab’s larvae. The presence of the blue crab has been documented here for decades. The first sightings in Italy date back to the 1950s, followed by others in many of the countries bordering the Mediterranean Sea. In recent years, however, locals have been witnessing a population explosion, particularly in locations that offer the preferred conditions for this crustacean, such as the brackish lagoons and river mouths. Resilient and voracious, this crab adapts to varying salinity and can live in both salty and relatively fresh water.

The Atlantic blue crab has a very thick shell and females of this species store sperm after mating, which allows them to fertilise their eggs in multiple cycles without the need for further mating. During reproduction, the male embraces the female while waiting for her to molt. In this way, he protects her from predators and can fertilise her only at that moment. This usually occurs during the warmer months when water conditions are ideal for reproduction. The spawn can happen up to three times a year, and a single female crab can produce from two to eight million eggs a time. In winter, females will move toward the open seas, to areas with higher salinity, while males bury themselves in the sediment in the shallow. During these months they are almost inactive, and they will meet their female counterparts again during the next mating season.