The Atlantic blue crab, granchio blu in Italian, is not a newcomer to Venice. It was first officially recorded in the Mediterranean in 1949, on the coasts of the northern Adriatic, which is this coast. What dates from spring and summer 2023 is the outbreak, the sudden abundance (Boschiero et al., 2026, p. 50).
The best evidence for how the animal lives here comes from a year-round study around Chioggia, at the southern end of the lagoon. Four stations, two inside the lagoon, one on the Pellestrina coast and one at the Brenta river mouth, all within 2 to 9 km of one another, were sampled 14 times each between June 2023 and June 2024, and 1,171 crabs were measured (Boschiero et al., 2026, pp. 52 to 53). The paper's argument is geographic, and it is the most useful idea on this page: the run from river water to sea water here is squeezed into roughly two orders of magnitude less space than in Chesapeake Bay, the crab's Atlantic stronghold (Boschiero et al., 2026, p. 52).
One thing to settle before the rest. The blue crab is a different animal from the moeche sold in Venetian bars, which are soft-shell Mediterranean green crabs, Carcinus aestuarii (Bordignon et al., 2026, p. 1006).
Is the blue crab new to the Venice lagoon?
No. The blue crab, Callinectes sapidus, is native to the western Atlantic from Nova Scotia to Argentina, and it was first recorded in Europe on the French coast in 1900. Its first official Mediterranean record, in 1949, was in Italy, on the coasts of the northern Adriatic (Boschiero et al., 2026, p. 50). The animal has therefore been part of the local fauna for three generations of fishermen. What changed is the number. The expansion along the Adriatic produced what the authors call an unprecedented outbreak in the northern Adriatic since spring and summer 2023, and that is the event visitors have read about (Boschiero et al., 2026, p. 50).
The species is listed among the 100 worst invasive species in the Mediterranean. The paper describes it as large and aggressive, with high fecundity, rapid mobility, the ability to osmoregulate across a wide salinity range and strong tolerance of temperature change. The likely route in was repeated introduction through ship ballast water (Boschiero et al., 2026, pp. 49 to 50). None of those traits is new either. What the research now points to is the combination of them with the particular shape of this coastline.
Why are there so many blue crabs in Venice?
The best answer in the literature is about distance. In the northwestern Adriatic the salinity gradient from river water to sea water occurs on an extremely small spatial scale, around two orders of magnitude smaller than in Atlantic environments such as Chesapeake Bay, where the blue crab is most prevalent and females undertake post-mating migrations of 150 to 200 km to reach the sea and brood (Boschiero et al., 2026, p. 52).
Venice compresses that journey into a few kilometres. The lagoon covers about 550 km2, the largest in Italy, and connects to the sea through three large navigable inlets at Lido, Malamocco and Chioggia. A few kilometres south of the Chioggia inlet the Brenta river discharges into the Adriatic at an annual average of 65 m3/s (Boschiero et al., 2026, pp. 51 to 52). Fresh, brackish and marine water therefore sit within a short swim of one another.
The consequence the authors draw is that most females here can probably brood their eggs in the same year they mate, with no need to overwinter part way through a long migration (Boschiero et al., 2026, pp. 63 to 64). Their conclusion is that this small spatial scale may be equivalent to or stronger than any other factor behind the invasion's success (Boschiero et al., 2026, p. 66).
How many eggs does a blue crab lay?
Between about half a million and four million at a time. The Chioggia study analysed 86 egg masses and estimated 518,717 to 4,106,743 eggs per mass, with an average of 2,072,450. One mass holding 5,982,200 eggs was recorded and excluded from the analysis as an outlier (Boschiero et al., 2026, p. 61). The masses themselves weighed 10.16 to 55.50 g, an average of about 15 per cent of the female's own wet weight, and individual eggs averaged 251 micrometres across.
The breeding window is long. Egg-bearing females were sampled from mid-April to mid-September, with none at all in winter (Boschiero et al., 2026, p. 59). Adding reports from local fishermen, the authors extend the start to late March and describe the species as reproductive for nearly six months (Boschiero et al., 2026, p. 64).
Females here are also larger than elsewhere in the Mediterranean, and they carry more. Egg-bearing females in this study ran from 12.51 cm to 17.80 cm across the carapace, against 10.10 cm to 15.60 cm reported from other Mediterranean areas, where the egg count is correspondingly lower at about 1.65 million (Boschiero et al., 2026, p. 65).
Does the lagoon breed them, or do they arrive from the sea?
Both, and the lagoon appears to do the raising. Wire pots, which have a large mesh, caught only adults, down to a minimum carapace width of 9.71 cm. Traditional fyke nets with a 0.6 cm mesh, set inside the lagoon between the two Chioggia stations, caught juveniles down to 2.47 cm, which the authors read as evidence that the area offers suitable nursery habitat (Boschiero et al., 2026, pp. 53, 59, 64). The spread of sizes in the catch points to several recruitment events through the year, probably from spring to autumn.
The population also grew through the study year. The mean carapace width of crabs taken in the pots rose from 14.22 cm in summer 2023 to 16.28 cm in spring 2024 (Boschiero et al., 2026, p. 59).
Condition was consistently better inside the lagoon than at the Brenta mouth, with the coast in between. The authors put the river result down to the energy cost of osmoregulation in oligohaline water, or to the absence there of the bivalve molluscs the crab prefers (Boschiero et al., 2026, p. 64). Activity was reduced in winter and never stopped, in contrast with the animal's dormancy through colder periods in its native range.
Is the blue crab the same thing as moeche?
No. Moeche are soft-shell Mediterranean green crabs, Carcinus aestuarii, taken in the hours after they moult, and moeca is the Venetian word for the crab at that stage (Bordignon et al., 2026, p. 1006). They are small animals: across the soft-shell crabs sampled at two lagoon sites the mean carapace width was 32.6 mm, with a range of 20.6 to 46.4 mm (Bordignon et al., 2026, p. 1010). An adult blue crab from the Chioggia pots starts at roughly three times that width.
The practice behind moeche is its own craft. Fixed nets called bertovelli are set in shallow lagoon water, fishermen known as molecanti pick out the individuals in the final pre-moult stage, called spiantani, and hold them in submerged containers called vieri, checking them twice a day to take the crabs during the brief soft-shell phase. Harvest runs in spring, from March to May, and in autumn, from October to December, when water temperatures near 15 degrees C trigger moulting. Annual production is between 20 and 60 tonnes and supports around 100 professional lagoon fishermen as a secondary occupation (Bordignon et al., 2026, p. 1007).
Those authors name the blue crab boom in northern Adriatic lagoons as a threat to the availability of green crabs, and so to the product and the practices around it. They cite earlier work from other Mediterranean areas for that, rather than a measurement of their own (Bordignon et al., 2026, p. 1007).
What the research does not show
Quite a lot, and it is worth being precise about it. The Chioggia study measured no impact on any other species in the Venice lagoon. Its statements about damage to clams and cockles, about losses to small-scale fisheries and about the constant maintenance that traditional fyke nets now need come from its own review of work in the Ebro delta in Spain, the Po delta in Italy, Greece and Croatia (Boschiero et al., 2026, p. 50). The same page carries the finding on the Mediterranean green crab, reported as threatened and probably already locally extirpated in some areas of Greece and Spain, and not in the Venice lagoon. The study gives no economic figure of any kind for this lagoon, in euros, tonnes or percentage loss.
Geography limits it too. None of the four stations lies north of Chioggia, so nothing in the work reaches the central lagoon, the northern marshes or the canals of the city. It covers one year, June 2023 to June 2024, with no comparison against the years before the outbreak.
On the question most readers want settled, the authors are blunt: complete eradication is unlikely, given the animal's adaptability and mobility, and what their results support is targeted management, meaning intensive and localised fishing (Boschiero et al., 2026, pp. 50, 66).
