Stenella coeruleoalba

Stenella coeruleoalba

The striped dolphin (Stenella coeruleoalba, Meyen 1833) is a cetacean belonging to the Delphinidae family.

Systematics –
From a systematic point of view it belongs to:
Eukaryota domain,
Kingdom Animalia,
Phylum Chordata,
Subphylum Vertebrata,
Mammalia class,
Superorder Cetartiodactyla,
Order Cetacea,
Suborder Odontoceti,
family Delphinidae,
Genus Stenella,
S. coeruleoalba species.
The term is basionym:
– Delphinus coeruleoalbus Meyen, 1833.
The terms are synonyms:
– Clymene burmeisteri (Malm, 1871);
– Clymene dorides Gray, 1866;
– Clymene euphrosyne Gray, 1866;
– Clymenia aesthenops Dall, 1874;
– Clymenia burmeisteri Malm, 1871;
– Clymenia crotaphisca Dall, 1874;
– Clymenia crotaphiscus Gray, 1871;
– Clymenia dorides Gray, 1868;
– Clymenia esthenops Gray, 1871;
– Clymenia euphrosyne Gray, 1868;
– Clymenia euphrosynoides Gray, 1868;
– Clymenia novaezelandiae Hector, 1873;
– Clymenia styx Gray, 1868;
– Delphinus amphitriteus Philippi, 1893;
– Delphinus asthenops Cope, 1865;
– Delphinus crotaphiscus Cope, 1865;
– Delphinus delphis subsp. mediterranea Nobre, 1900;
– Delphinus euphrosyne Gray, 1846;
– Delphinus holbolli Eschricht, 1847;
– Delphinus holbollii Nilsson, 1847;
– Delphinus lateralis Peale, 1848;
– Delphinus marginatus Desmarest, 1856;
– Delphinus mediterraneus Loche, 1860;
– Delphinus styx Gray, 1846;
– Delphinus tethyos Gervais, 1853;
– Lagenorhynchus caeruleoalbus Gray, 1850;
– Lagenorhynchus coeruleoalbus Meyen, 1833;
– Lagenorhynchus lateralis Cassin, 1858;
– Lagenorrhynchus caeruleoalbus Burmeister, 1867;
– Orca tethyos Gerrard, 1865;
– Prodelphinus amphitriteus True, 1903;
– Prodelphinus burmeisteri Trouessart, 1904;
– Prodelphinus caeruleoalbus (Meyen, 1833);
– Prodelphinus coeruleoalbus True, 1889;
– Prodelphinus coeruleoalbus subsp. euphrosyne Bobrinski, 1944;
– Prodelphinus crotaphiscus Trouessart, 1898;
– Prodelphinus doreides Trouessart, 1898;
– Prodelphinus euphrosine Trouessart, 1898;
– Prodelphinus euphrosinoides Trouessart, 1898;
– Prodelphinus euphrosyne Flower, 1885;
– Prodelphinus lateralis True, 1889;
– Prodelphinus marginatus Desmarest, 1856;
– Stenella aesthenops (Cope, 1865);
– Stenella asthenops (Cope, 1865);
– Stenella caeruleoalba Scheffer & Rice, 1963;
– Stenella caeruleoalbus Tomilin, 1957;
– Stenella caeruleoalbus subsp. caeruleoalbus;
– Stenella caeruleoalbus subsp. euphrosyne Tomilin, 1957;
– Stenella crotaphiscus (Cope, 1865);
– Stenella euphrosyne (Gray, 1846);
– Stenella styx (Gray, 1846);
– Tursio dorcides Gray, 1866.

Geographic Distribution and Habitat –
The Stenella coeruleoalba is an odontocetic cetacean found in abundance in the north Atlantic, up to Greenland, Iceland, the Faroe Islands and Denmark, in the Mediterranean Sea, in the Gulf of Mexico, in the Indian Ocean, from Australia to southern Africa and in the Pacific from Japan to the coasts of the United States.
Their marine habitat is the pelagic one, in temperate and tropical waters of practically all oceans where they can live in waters whose temperature varies from 10 to 26 °C, although their optimum is around 18-22 degrees.
These cetaceans are quite common throughout their range, although there are some areas with a low population density. In the eastern Pacific their range overlaps with those of Stenella longirostris and Stenella attenuata, although they tend to be more numerous in areas where the two aforementioned species are less abundant. In the Mediterranean they live in waters whose depth exceeds 100 m.
Some populations perform migrations which, especially in the eastern Pacific, seem to follow the movements of the warm ocean currents. In the autumn months they move closer to the coast and in the winter they move further south. In summer, they do the reverse, but swim in the deeper, more open waters.
In the Mediterranean Sea, when the temperature of the southern basin increases, these cetaceans move towards the northern part. Nythemeral migrations have also been observed in this sea: in the evening the dolphins approach the coast to hunt, while in the morning they return to the open sea.

Description –
The Stenella coeruleoalba has a tapered and slender body, to ensure greater hydrodynamics.
The adults are 180 to 256 cm long, the females reach smaller dimensions. At birth they measure from 80 cm to 1 m.
The specimens living in the Mediterranean are smaller. The weight varies between 100 and 165 kg. Birth weight is not known.
Furthermore, small differences in the size of the individuals of the populations living in the north-eastern Atlantic and in the north-western and south-western Mediterranean have been observed. Generally the animals living in the southern Mediterranean are larger than those living in the northern basin. This suggests the existence of a certain degree of genetic variability among various populations.
It is white on the belly, gray on the sides and blue on the back. The flanks are crossed by longitudinal streaks which start from the ear and reach the anus. Other streaks start from the eye and reach the pectoral fins. A further stripe, variable in shape and size, similar to a flame and white in color, extends from the flanks to the base of the dorsal fin.
It has a rostrum, formed by the elongation of the maxilla and mandible, long and thin, well evident.
The dorsal fin is arched and small and located about halfway down the body.
The pectoral fins are tapered. The caudal fin is thin and divided into two lobes by a very evident septum.
As in all Cetaceans, the caudal fin and the dorsal fin have no bones and are made up of connective tissue, while the pectoral fins are made up of bones homologous to those of other tetrapods.
The forehead is clearly separated from the rostrum with a rather pronounced part.
The teeth, present in number of 50 both on the maxilla and on the mandible, are short and conical, with a diameter of about 3 mm and slightly curved.
On the top of the head there is a blowhole, through which this dolphin expels the breathed air and whose opening and closing is due to voluntary muscles. When the blowhole is open, the nasal septum can be observed.
Like other cetaceans, they are hairless.
In general, it is an easily recognizable dolphin even at great distances due to the splashes caused by its numerous and spectacular jumps. The coloring of the sides makes it distinguishable from the other species of dolphins, even if it is often confused with the Delphinus delphis. It often swims in the bow wave of ships and boats and is the most frequently sighted dolphin in the Cetacean Sanctuary, a stretch of sea between Liguria, northern Sardinia and southern France.

Biology –
The reproduction of the Stenella coeruleoalba has been studied in the waters of the western Mediterranean; it has been seen that the births take place in September-October, to take advantage of the period of the year in which the waters are not yet cold and the availability of food is high. This synchronicity helps females overcome the high energy expenditure of lactation.
Mating also takes place between the end of summer and the beginning of autumn, and the gestation lasts about a year, at the end of which a newborn is born, measuring about 95 cm in build.
Like the rest of the cetaceans, they have only one offspring by birth.
Males reach sexual maturity between 7 and 15 years, females between 5 and 13.
Mating occurs in winter and early summer in Pacific populations, while it occurs in autumn in Mediterranean ones.
Gestation lasts 12 months and the interval between one gestation and another is about 4 years. Weaning takes about 16 months.
Birth and the first months of suckling are apparently critical moments in the life of these dolphins, with a relatively high mortality rate during this period. This explains the relative abundance of larvae and young pups that are found dead on beaches during the fall and early winter.
Furthermore, the presence index between the sexes is slightly favorable for males during the first years of life (1.11 males / 1 female), but since mortality in them is higher than in females, the proportion of individuals of each sex levels off in the adult stage. In general, adult males and females, together with the young that depend on them, form reproductive groups which usually bring together many individuals, while the young specimens, which no longer need maternal attention, separate from the rest, forming small groups.

Ecological role –
The Stenella coeruleoalba lives in offshore temperate or tropical waters. It is found in abundance in the North and South Atlantic Oceans, including the Mediterranean (sightings and strandings have been reported quite recently in the Sea of Marmara) and in the Gulf of Mexico, Indian Ocean and Pacific Ocean. It roughly occupies a range from 40°N to 30°S.
This cetacean has been found in water temperatures between 10 and 26°C, although the standard range is 18-22°C.
In the western Pacific, where the species has been extensively studied, a distinctive migration pattern has been identified. In other sectors this has not happened.
The dolphin appears to be common in all areas of its range, although it may not be continuous; there are areas with low population density. The total population exceeds two million. The more southerly record of individuals concerns an individual stranded near Dunedin in southern New Zealand in 2017.
This cetacean feeds on squid and small fish and can go as deep as 200 m to hunt. It is capable of making spectacular leaps out of the water and, like the bottlenose dolphin (Tursiops truncatus), is one of the most studied and best known dolphins.
The populations living in the Mediterranean feed mainly on Cephalopods, especially Albraliopsis pfefferi, Onychoteuthis banksii, squid (Todarodes sagittatus) and Brachioteuthis riisei, while the oceanic ones feed on fish belonging to the Myctophidae family, most of which have bioluminescent organs and they have a length between 60 and 300 mm.
In South Africa and Japan, dolphins hunt to depths of up to 700 m.
They often swim with tuna to cooperate in fishing in eastern Pacific waters.
This species generally lives in herds, called “schools”, which can vary in the number of specimens and in the composition. Schools of just under 500 specimens have been identified in the waters off the Japanese coast, although schools of thousands may exist. Generally, however, the average number of specimens in a school does not exceed 100 individuals.
The schools of individuals present in the northern Atlantic are composed of 10-30 specimens and only rarely reach 100 units.
In the Western Pacific, schools have been divided into three different types according to reproductive behaviors:
juvenile schools: composed of young individuals;
adult schools: composed of adult individuals;
mixed schools: composed of adults with their puppies.
Adult and coeducational schools are divided into reproductive and non-reproductive; pups stay with adults for about 2-3 years after weaning and then join juvenile schools. Shortly before reaching sexual maturity, females join adult non-reproductive schools, while sexually mature males join reproductive schools, in which mature females also converge. Males, after mating, may leave these schools and join the mixed non-reproductive schools.
This cetacean also presents a series of surface behaviors whose meaning is not yet clear, such as:
– swim by being carried away by the bow wave of the boats or by the waves created by the whales;
– jump completely out of the water;
– jump out of the water during fast swimming;
– “walk” backwards on the surface of the water pivoting on the tail fin;
– slamming the tail fin against the surface of the water;
– beat pectoral fins on the surface of the water.
These dolphins are also capable of reaching speeds of around 37 km/h.
As for their acoustic signals, little is known about the behavior of these dolphins.
They produce sounds with a frequency between 50 and 150 kHz, called clicks, which are used for echolocation and therefore for hunting. They are able to emit prolonged series of clicks, called bursts, which appear similar to meows to our ears.
Alongside these sounds, they are also capable of producing whistles, of a lower frequency, about 20 kHz, used for the intraspecific communication and which can be heard even at very high distances. The acoustic activity of the individuals seems to be greater at night, in agreement with the feeding behavior of these cetaceans.
With reference to the state of conservation, to date, there are no certain data that certify that the populations are in decline.
However the main dangers that threaten these dolphins are:
Hunting: in Japan these cetaceans, like others, are hunted for food purposes, in what are considered real slaughterhouses, often deprecated by Western public opinion. In the years 1942 to 1960, between 13,000 and 22,000 dolphins were hunted, most of which belonged to the Stenella coeruleoalba. In the following years, the number of specimens hunted decreased considerably, reaching less than 1000 specimens hunted in the eighties.
In the Mediterranean, hunting is prohibited, although in some areas it is practiced illegally for food purposes. In Spain they were hunted for use as bait in shrimp traps.
Pollution: High concentrations of heavy metals, DDT and PCBs, can affect the reproductive capabilities of these mammals. PBCs lead to the formation of cysts within the ovaries of females, preventing them from ovulating.
Fishing: Dolphins are often accidentally entangled in fishing nets, drowning as they are unable to surface for air.
Uncontrolled fishing of squid and fish that these dolphins also eat can lead to a decline in populations of this species due to the reduction in the availability of prey.
Morbillivirus: In the 1990s, the Mediterranean dolphin populations were decimated by an epizootic Morbillivirus infection. It is thought that this epidemic was favored by the weakening of the cetaceans’ immune system due to the high concentration of PCBs found in their tissues.
The striped dolphin populations of the eastern tropical Pacific and Mediterranean are listed in Appendix II of the Convention on the Conservation of Migratory Species of Wild Animals (CMS), as they have an unfavorable conservation status or would benefit significantly from international cooperation.
This dolphin is listed as Vulnerable on the IUCN Red List due to a 30% reduction in its subpopulation over the past three generations. These dolphins could also be an indicator species for long-term monitoring of heavy metal accumulation in the marine environment due to their importance in Japan’s pelagic food web and their ability to live for many years.
In addition, S. coeruleoalba falls within the scope of the Agreement on the Conservation of Small Cetaceans of the Baltic, North-East Atlantic, Irish and North Seas (ASCOBANS), the Agreement on the Conservation of Cetaceans in the Black Sea, the Mediterranean Sea and the contiguous Atlantic area (ACCOBAMS), the Memorandum of Understanding for the Conservation of Cetaceans and their Habitats in the Pacific Islands Region (MOU on Cetaceans of the Pacific) and the Memorandum of Understanding on the Conservation of Manatees and Small Cetaceans of West Africa and Macaronesia (MoU on Aquatic Mammals of West Africa)
Conservation efforts also target ship lines taking a new route to their destination, such as cruise lines, as well as reduced human interaction up close. Feeding the dolphins has also become a problem and has led to behavioral changes. This has also been suggested as another reason for the fatality events.

Guido Bissanti

Sources
– Wikipedia, the free encyclopedia.
– GBIF, the Facilidad Global de Información sobre Biodiversidad.
– Gordon Corbet, Denys Ovenden, 2012. Guide to mammals of Europe. Franco Muzzio Publisher.
– John Woodward, Kim Dennis-Bryan, 2018. The Great Encyclopedia of Animals. Gribaudo Publisher.

Photo source:
– https://inaturalist-open-data.s3.amazonaws.com/photos/255464615/original.jpg
– https://www.raalg.pt/en/especie/golfinho-riscado