Steno bredanensis

Steno bredanensis

The Rough-toothed dolphin (Steno bredanensis G. Cuvier in Lesson, 1828) is a cetacean belonging to the Delphinidae family.

Systematics –
From a systematic point of view it belongs to:
Eukaryota domain,
Kingdom Animalia,
Phylum Chordata,
Mammalia class,
Subclass Eutheria,
Order Cetacea,
Suborder Odontoceti,
Delphinidae family,
Steno genre,
Species S. bredanensis.
The term is basionym:
– Delphinus bredanensis G.Cuvier, 1828.
The terms are synonymous:
– Delphinorhynchus bredanensis Lesson, 1828;
– Delphinus chamissonis Wiegmann, 1846;
– Delphinus compressus Gray, 1843;
– Delphinus oxyrhynchus Gray, 1850;
– Delphinus perspicillatus Peters, 1876;
– Delphinus planiceps Van Breda, 1829;
– Delphinus reinwardtii Schlegel, 1841;
– Delphinus rostratus Desmarest, 1817;
– Steno bradanensis (Lesson, 1828);
– Steno compressus (Gray, 1846);
– Steno perspicillatus (Peters, 1876);
– Steno rostratus Lutken, 1889.

Geographic Distribution and Habitat –
Steno bredanensis is a cosmopolitan cetacean, present in all oceans in the tropical and warm temperate belt of both hemispheres.
It is considered a widely distributed cetacean, but not very abundant everywhere, today it seems more frequent than in the past due to the greater competence of sailors in identifying the cetacean species encountered.
However, the distribution and population of this dolphin are poorly known. They inhabit the Pacific, Atlantic and Indian Oceans and the Mediterranean Sea, in warm temperate to tropical waters, with occasional reports from colder environments. These dolphins can also be seen regularly in locations stretching from the Windward Islands to Cape Verde, but only a small handful have been seen in the Azores and Madeira.
Most research activity involving these dolphins has been directed in the eastern Pacific, where researchers obtained a population estimate of 150,000 in the 1980s. Fossils belonging to the Steno genus are known from Europe and date back to the early and middle Pliocene.
In the Mediterranean Sea, the species was once thought to come from the North Atlantic, until recent discoveries revealed that there is a small but resident population in the eastern part of the sea.
Its marine habitat is decidedly pelagic: it can in fact be found more easily in tropical and subtropical waters (possibly where the surface temperature reaches and exceeds 25 °C), outside the continental slope.

Description –
The Steno bredanensis is a relatively large dolphin, with adults ranging from 2.09 to 2.83 meters in length and weighing between 90 and 155 kilograms; males are larger than females. Its most obvious feature is its conical head and slender nose; other dolphins have shorter snouts or a more visibly swollen melon on the forehead. The teeth are characteristic, having a rough surface formed by numerous narrow and irregular ridges. They have been reported to have between nineteen and twenty-eight teeth in each quarter of their jaw.
The jaw of this dolphin on the outside is very distinct. The color of the lower jaw is usually white, but may have a shade of pink mixed in.
The fins are further back along the body than in other similar dolphins, although in the sea this dolphin can be confused with spinner dolphin, spotted dolphin and bottlenose dolphin. The dorsal fin is pronounced, being 18 to 28 centimeters high. The animal’s flanks are light gray, while its back and dorsal fin are a much darker gray. Older individuals often have distinctive pink, yellow, or white markings around the mouth and along the underside.
The newborn measures 80 cm.

Biology –
Steno bredanensis gives birth to a single young, after an unknown gestation period; it is also unknown whether or not they have a distinct breeding season, as well as the duration of lactation. The young are about 100 centimeters long at birth and grow rapidly during the first five years of life. Females reach sexual maturity between six and ten years of age, while males reach sexual maturity between five and ten years.
Longevity would be around 30 years. In captivity, a female steno mated with a bottlenose dolphin, giving birth to a hybrid that survived for over five years.

Ecological Role –
Steno bredanensis is the only species of the genus Steno Gray, 1846.
The systematic history of this species is rather confusing. Cuvier was already aware of the existence of the species in 1812, when he reported in one of his publications the presence of some unnamed skulls in the collections of the Paris Museum. Shortly afterwards, in 1817, Desmarest mentioned these skulls in his Nouveau Dictionnaire d’Histoire Naturelle, with the name Delphinus rostratus Cuvier, even though the actual scientific description of this species had not been made. Then, in 1823, in an unsuccessful attempt to bring order, Cuvier resumed the list of species reported by Desmarest, renaming Delphinus geoffrensis Blainville, that is, the Amazonian dolphin today called inia, with the name of Delphinus frontatus, unfortunately including in the material described also those mysterious skulls he had mentioned eleven years earlier. It was the merit of his Dutch colleague van Breda, passing through Paris, to point out to him that the famous skulls, “fronted”, had nothing, and that they belonged to dolphins very different from the hynia. Thus Cuvier decided to create a new specific species, which he called Delphinus bredanensis in honor of his colleague. This clarification was later immortalized by Lesson in 1828, where the species is mentioned D. bredanensis Cuvier. The Steno genus was then created by Gray in 1846, to accommodate this species which was all in all so different from the dolphin. Since then, S. bredanensis has remained the only recognized species belonging to the Steno genus.
This dolphin is capable of developing high speeds and has good acrobatics. Its underwater performance is unknown: it is certainly known that it can go beyond 70 m. Some specimens have been observed swimming rapidly below the surface, with only their dorsal fin protruding from the water. There is no information on the movements of this species, nor on their periodicity. Although it is sometimes encountered in large groups (50-500 specimens), which however give the impression of being aggregations of smaller units, the usual size of the groups is 10-20 specimens.
S. bredanensis is a cetacean that has demonstrated, both in nature and in captivity, particular qualities of sociality. It is also one of the species that has shown the highest learning abilities among all farmed delphinids. Unfortunately, however, there are no details on the social organization of this intriguing dolphin.
Furthermore, it has been observed several times in association with other cetacean species and with schools of Thunnus albacares.
It does not hesitate to approach the bow of the boats, attracted by the wave on which it willingly starts playing with an agility that has nothing to envy that of other more common Delphinids. The whistles produced by this species are generally short (less than a second), with a frequency between 3 and 12 kHz.
This dolphin, which is most likely equipped with biosonar, also emits clicks with a broad frequency spectrum, in repeated series lasting a total of 0.1-0.2 seconds. Collective strandings of this species are very rare: two cases are known, one in Florida and one in Indonesia. Although there is no data on predation against it, due to its pelagic and tropical habits, it can potentially be threatened by large pelagic sharks, as well as killer whales.
This species feeds mainly on fish and pelagic cephalopods. Its ability to manipulate and split fish, such as dolphinfish (Coryphaena sp.), too large to be ingested whole, has been observed, as has its propensity to share its prey with its companions. Although the details of their diet are vague, the stomach contents of stranded dolphins include fish such as silverside silverside, costargia, houndfish, smelt, cutlass, and various squid and octopus.
Regarding its conservation status, the population is not believed to be threatened by human activities. A small number of individuals have been harpooned by Japanese whalers, and hatchlings are also massacred during the Taiji hunt. Others were caught in the trawl nets of fishing boats fishing for tuna.
S. bredanensis is covered by the Agreement on the Conservation of Small Cetaceans of the Baltic, North-East Atlantic, Ireland and the North Seas (ASCOBANS) and the Agreement on the Conservation of Cetaceans in the Black Sea, the Mediterranean Sea and the contiguous Atlantic area (ACCOBAMS). The species is also included in the Memorandum of Understanding on the Conservation of Manatees and Small Cetaceans of West Africa and Macaronesia (MoU on Aquatic Mammals of West Africa) and the Memorandum of Understanding on the Conservation of Cetaceans and Their habitat in the Pacific Islands region (MoU on Pacific Cetaceans).
In the IUCN Red List the steno is listed in the category of least concern species.

Guido Bissanti

Sources
– Wikipedia, the free encyclopedia.
– GBIF, the Global Information Facility on Biodiversity.
– Gordon Corbet, Denys Ovenden, 2012. Guide to the mammals of Europe. Franco Muzzio Editore.
– John Woodward, Kim Dennis-Bryan, 2018. The great encyclopedia of animals. Gribaudo Publisher.

Photo source:
– https://inaturalist-open-data.s3.amazonaws.com/photos/110871591/original.jpeg
– https://ru.m.wikipedia.org/wiki/%D0%A4%D0%B0%D0%B9%D0%BB:Steno_bredanensis_1889.jpg