The dusky dolphin (Lagenorhynchus obscurus) is a dolphin found in coastal waters in the Southern Hemisphere. Its specific epithet is Latin for "dark" or "dim". It is very closely genetically related to the Pacific white-sided dolphin, but current scientific consensus holds they are distinct species. The dolphin's range is patchy, with major populations around South America, southwestern Africa, New Zealand, and various oceanic islands, with some sightings around southern Australia and Tasmania. The dusky dolphin prefers cool currents and inshore waters, but can also be found offshore. It feeds on a variety of fish and squid species and has flexible hunting tactics. The dusky dolphin is known for its remarkable acrobatics, having a number of aerial behaviours. The status of the dolphin is unknown, but it has been commonly caught in gill nets.
Taxonomy
It is commonly thought that the dusky dolphin was first described by John Edward Gray in 1828 from stuffed skin and a single skull shipped from the Cape of Good Hope to the British Museum. Gray first described the species as Delphinus obscurus, with the subgenus Grampus in his 1828 Specilegia Zoologica. Gray reported that the animal was captured around the Cape of Good Hope by a Captain Haviside (often misspelt "Heaviside") and sent to the British Museum though the Royal College of Surgeons in 1827.
However, Gray later wrote that a similar dolphin was described as Delphinus supercilious by French surgeons and naturalists René Primevère Lesson and Prosper Garnot from a specimen collected off the coast of Tasmania two years before his own classification. Gray classified D. supercilious as a junior synonym of his D. obscurus and credited Lesson and Garnot (1826) for their original description. Meanwhile, Charles Darwin also described what turned out to be this species as Delphinus fitzroyi from a specimen harpooned off Argentina in 1838. The dusky dolphin was reclassified as Prodelphinus obscurus in 1885 by British naturalist William Henry Flower, before gaining another name, Lagenorhynchus obscurus, from American biologist Frederick W. True in 1889.
Genetics
The dusky dolphin and the Pacific white-sided dolphin are considered phylogenetically related species. Some researchers have suggested they are the same species, but morphological and life-history evidence shows otherwise. The two sister species diverged at around 1.9–3.0 million years ago. Recent analysis of the mitochondrial cytochrome b gene indicates that the genus Lagenorhynchus, as traditionally conceived, is not a natural (monophyletic) group. Another study finds that the dusky and the Pacific white-sided dolphin form the sister group to the (expanded) genus Cephalorhynchus. If this placement is accurate, a new genus name will need to be coined to accommodate these two species. It has therefore been proposed that the dusky dolphin, together with Pacific white-sided, hourglass, and Peale's dolphin are moved to the resurrected genus Sagmatias.
Dusky dolphins from Argentina and southwest Africa separated 2000 generations ago from an ancestral Atlantic population and subsequently diverged without much gene flow. Most populations have low genetic diversity, with the Peruvian population being an exception. Possible hybrids of dusky dolphins have been described with a long-beaked common dolphin and a southern right whale dolphin. There are three subspecies classified: the African dusky dolphin (Lagenorhynchus obscurus obscurus), Fitzroy's dolphin (L. o. fitroyi), and the Peruvian/Chilean dusky dolphin (L. o. posidonia). A fourth subspecies, L. o. superciliosus or the New Zealand dusky dolphin, has been proposed but is not currently accepted.
Description
The dusky dolphin is small to medium in length compared with other species in the family. There is significant variation in size among the different population areas. The largest dusky dolphins have been encountered off the coast of Peru, where they are up to 210 cm (6 feet) in length and 100 kg (210 pounds) in mass. The size for dusky dolphins in New Zealand have been recorded to be a length range of 167–178 cm and a weight range of 69–78 kg for females and a length range of 165–175 cm and a weight range of 70–85 kg for males.