Diagnosis

An unmistakable,  gracefully-proportioned, slender-bodied shark with a long snout and no interdorsal ridge.  Eyes large; pectoral fins long and rather slim, with curved apexes; 1st dorsal fin rather small and with a curved apex, situated well behind the pectoral fins with its midpoint nearer to the pelvic fin origins. Low keels present on caudal peduncle; anal fin posterior margin is deeply notched. Papillose gill-rakers present and are unique amongst carcharhinids. Upper jaw teeth noticeably obliquely curved and triangular with serrated margins.   Colour dorsally vivid cobalt blue, shading to metallic mid-blue on the flanks and white ventrally.

Size

To about 380cm but commonly from 50 to 250cm in the region; size at birth 35 to 44 cm.

Status and Distribution

N.E. Atlantic: Common and cosmopolitan south of Lat. 50N (i.e, from S.W. England and S. Ireland southwards). Northerly limit to range is Icelandic and North Norwegian coasts, where these sharks may stray as sporadic outliers during particularly warm summers.
Mediterranean Sea: Common; entire Mediterranean, including Adriatic and Sea of Marmara but not Black Sea.

Biology

Typically an offshore, pelagic species from the surface down to at least 150m; highly nomadic, undertaking journeys between and across the great ocean basins, including inter-equatorial movements.  Occupies sea-temperatures of 7 to 16C, sometimes up to 25C and may swim in the deeper, isothermal Southern Mediterranean waters during the high summer. Blue sharks also make inshore incursions, particularly at night; juveniles may also inhabit waters less than 50m deep, such as off Portugal and within Northern Adriatic waters (both areas are nursery-zonse for this species) during the summer months. This species also occurs nearshore off Mediterranean islands adjacent to deep water (e.g., Isola Ponza and Aeolian Islands in the Tyrrhenian Sea).  Blue sharks are graceful, efficient slow cruisers, typically averaging 0.8km/h groundspeed over their long-distance meandering; often seen near the surface but capable of deep, gliding dives (using their long pectoral fins for minimum effort in controlling descent) in search of deep-ranging or vertically-migrating prey such as squid.  They will often congregate in great numbers at food-sources (for example, floating whale carcasses) but generally are rather solitary.  Typical prey includes scombroids such as mackerel, scad, bonito and bullet tuna; clupeids includings herring and sardines; jacks, garfish (especially in the Adriatic) and other pelagic schooling fish; occasionally demersal, benthic or littoral fish prey (e.g., thick-lipped mullet, conger eels and flatfish);  pelagic flying-fish eggs (apparently a very common food-item in Adriatic specimens); squid and cuttlefish (also important regional prey);   crustaceans; an occasional scavenger upon mammalian remains (dolphins, whales and porpoises) and even consumes seabirds (a gull was recently found inside an Adriatic specimen), although these may be attacked only post-mortem.  The writer and Dr Irene Bianchi have observed neonatal blue sharks to actively pursue similarly-sized specimens of garfish, Belone belone, at the surface but without achieving contact; possibly motivated through intraspecific competition for small fish baits. Commonly observed to follow ships in Mediterranean waters, quite possibly for great distances (the writer was told of two blue sharks that diligently followed a ship non-stop from Cyprus to Malta, feeding voraciously on discarded kitchen garbage, only then to be hooked and killed by the crew!).  Geographical sexual segregation is common in these sharks; the majority of specimens taken off Great Britain being females whilst off Long Island, New York, male sharks predominate.   Litter sizes are large, up to 135 pups but highly variable; gestation period 9-12 months; maturing 5-year old females can store sperm  in their shell glands and defer fertilization until in their 6th year; young are born to females of 7 years, with neonatal specimens common in Mediterranean waters from late May to July.  Limited tagging returns from an ongoing north Adriatic study co-ordinated by Dr. Irene Bianchi and Big Game Italia (pers. comm.) suggests movement of neonates from the Po Delta region to both the Eastern and Western-Central Mediterranean, but no recaptures are yet known from Atlantic waters.  It is possible that these sharks only exit the Mediterranean when nearing adulthood, but further tag-return data will confirm or nullify this thesis.  Females mature at about 221cm; males between 182 and 281cm TL.