Introduction
The blue crab Callinectes sapidus is currently one of the most problematic invasive species in the Mediterranean Sea [1-3], particularly along the Italian coasts. Since its first record in the northern Adriatic in 1949, the species has progressively expanded, colonising most Italian coastal and lagoon systems. Its invasion success is driven by a combination of biological and ecological factors, including high tolerance to variations in temperature and salinity, an extremely flexible and opportunistic diet, and a remarkable reproductive capacity. From an ecological perspective, the blue crab is an aggressive omnivorous predator capable of feeding on a wide range of organisms, including fish, crustaceans, and especially bivalve mollusks. This trait makes it particularly impactful in the lagoon systems of the northern Adriatic, where clam and mussel aquaculture are widespread [8]. Predation on these organisms represents one of the main economic threats to the sector. Alongside its trophic role, reproduction is another key aspect: females can produce millions of eggs per reproductive cycle and spawn multiple times, contributing to rapid population growth [7]. In addition, environmental factors such as increasing temperatures may further enhance the spread of the species [3-6]. In light of these issues, the present study aims to integrate the analysis of feeding behaviour and reproductive potential of the blue crab in order to better understand the mechanisms underlying its invasive success and to provide useful information for species management.
Materials and Methods
Blue crab specimens used in this study were collected in the Bevano River Delta (northern Adriatic Sea) and a fishing house in Cesenatico, during the summer of 2025. After capture, the animals were transported to the laboratory and acclimatised in a controlled recirculating aquaculture system (RAS) that ensured stable environmental conditions in terms of temperature, oxygen, salinity, and pH. Biometric measurements were performed to classify individuals according to sex and size, and the animals were then divided into groups and subjected to different experiments. Regarding feeding behaviour, three dietary treatments were tested: a monospecific diet of clams (Ruditapes philippinarum), a monospecific diet of mussels (Mytilus galloprovincialis), and a mixed diet. Food was provided daily, and both the number of consumed prey items and the ingested biomass were recorded. In parallel, prey handling and consumption times were analysed, recording the time required by crabs to open and consume each prey type. A further experiment evaluated the effect of temperature on feeding activity by progressively varying water temperature and recording food consumption across eight temperature levels (13–34 °C). Finally, reproductive performance was assessed using ovigerous females. Egg masses were weighed and subsampled to estimate total egg number, egg diameter, and developmental stage, as well as to evaluate the relationship between female body size and absolute fecundity.
Results
The results show that C. sapidus is able to consume both tested bivalve species without a clear numerical preference. Under mixed diet conditions, all size and sex groups showed very similar numbers of clams and mussels consumed. However, a clear difference emerged when edible biomass was considered. The mean edible tissue per prey item was 1.74 ± 0.52 g for clams and significantly higher at 4.50 ± 1.01 g for mussels. In the mixed diet trial, despite similar prey counts, mussels provided a significantly greater total edible biomass (47.75 ± 7.98 g vs 25.21 ± 2.98 g; p < 0.05). Regarding handling and consumption times, differences between prey types were limited. Mean valve-opening time ranged from 1 min 12 s to 1 min 16 s for clams and from 50 s to 1 min 17 s for mussels. Tissue consumption time was slightly longer for mussels than for clams . Total processing time ranged from approximately 2 min 50 s to 3 min 25 s across prey types, with no statistically significant differences detected (p > 0.05). Temperature had a clearly non-linear effect on food intake. Consumption increased progressively from 13 °C, reaching a peak at 25–28 °C, and then declined at 31–34 °C At 13 °C, consumption was reduced by approximately 84% compared to the optimal peak, indicating a strong thermal limitation of feeding activity. Reproductive analysis revealed high fecundity, with absolute egg number ranging from approximately 958,000 to 2,736,000 eggs per female and a mean of 1,632,558 ± 530,098 eggs per individual. Egg mass represented on average 20.45 ± 3.1% of total body weight. A strong positive correlation was detected between carapace width and absolute fecundity (p < 0.001), indicating that larger females produce significantly more eggs.
Discussion
The results confirm the role of C. sapidus as an extremely efficient and adaptable predator. Its ability to exploit different prey types and optimise the balance between prey number and ingested biomass represents a significant competitive advantage, particularly in resource-rich environments such as the northern Adriatic lagoons [4,5]. Although clams are easily accessible on the seabed, mussels provide higher energetic returns per prey item, which may influence prey selection under natural conditions [4]. However, the actual impact on aquaculture also depends on farming system structure, prey accessibility, and habitat characteristics, and the present results — obtained under controlled laboratory conditions without sediment — should be interpreted as estimates of potential consumption rather than direct predictors of field impact [5]. Temperature is confirmed as a key factor regulating feeding activity [6]. The unimodal thermal response, with peak consumption at 25–28 °C and reduced intake at both thermal extremes, is consistent with the physiology of ectothermic predators. The marked reduction at 13 °C is consistent with cold-induced inactivity, while the decline above 28 °C suggests entry into a supra-optimal thermal range. From an applied perspective, climate warming may extend the period of peak feeding activity, potentially increasing predation pressure on shellfish aquaculture [3,8]. Finally, high fecundity represents one of the most critical aspects for the management of this species [7]. The production of over 1.6 million eggs per female per brood, combined with multiple reproductive cycles, ensures rapid population expansion. The positive relationship between body size and fecundity underlines the key role of larger individuals in sustaining population growth. Taken together, the high trophic efficiency and exceptional reproductive capacity of C. sapidus, combined with its broad environmental tolerance, make it a particularly challenging invasive species and highlight the need for integrated and evidence-based management strategies.
References
Streftaris, N.; Zenetos, A. Alien Marine Species in the Mediterranean - the 100 'Worst Invasives' and Their Impact. Medit. Mar. Sci. 2006, 7, 87.
Mancinelli, G. et al. Distribution, Impact and Management of Callinectes sapidus in Europe. Mar. Pollut. Bull. 2017, 119, 5–11.
Shauer, M. et al. Invasion Patterns and Climate Change in the Mediterranean. Sci Rep 2025, 15, 34967.
Prado, P. et al. Prey Size and Species Preferences in Blue Crab. Estuar. Coast. Shelf Sci. 2020, 245, 106997.
Cabiddu, S. et al. Feeding Behaviour of Blue Crab on Mediterranean Bivalves. Hydrobiologia 2025, 852, 2907–2922.
Marchessaux, G. et al. Thermal Response of Callinectes sapidus. Front. Mar. Sci. 2022, 9, 1055404.
Prager, M.H. et al. Fecundity of Blue Crab. Bull. Mar. Sci. 1990, 46.
Chiesa, S. et al. Impacts on Clam Aquaculture. Estuar. Coast. Shelf Sci. 2025, 312, 109037.