This month we’re taking a look at the Dusky Shark, a highly migratory species with a particularly slow growth rate and late age at maturity.  

Dusky sharks are one of the largest species within the Carcharhinus genus, generally measuring 3 metres total length but able to reach up to 4.2 metres. They are grey to grey-brown on their dorsal side and their fins usually have dusky margins, with the darkest tips on the caudal fin.

Dusky Sharks can often be confused with other species of the Carcharhinus genus, particularly the Galapagos Shark (Carcharhinus galapagensis). They have very similar external morphology, so it can be easier to ID to species level by taking location into account as the two species occupy very different ecological niches – Galapagos Sharks prefer offshore seamounts and islets, whilst duskies prefer continental margins.

Hybridisation:

A 2019 study found that Dusky Sharks are hybridising with Galapagos Sharks on the Eastern Tropical Pacific (Pazmiño et al., 2019). Hybridisation is when an animal breeds with an individual of another species to produce offspring (a hybrid). Hybrids are often infertile, but this study found that the hybrids were able to produce second generation hybrids!

Long distance swimmers:

Dusky sharks are highly mobile species, undertaking long migrations to stay in warm waters throughout the winter. In the Northern Hemisphere, they head towards the poles in the summer and return southwards towards the equator in winter. The longest distance recorded was 2000 nautical miles!

Very slow to mature and reproduce:

The Dusky Shark are both targeted and caught as bycatch globally. We already know that elasmobranchs are inherently slow reproducers which means that they are heavily impacted by overfishing; it takes them so long to recover that they cannot keep up with the rate at which they are being fished. Dusky Sharks are particularly slow to reproduce – females are only ready to start breeding at roughly 20 years old, their gestation periods can last up to 22 months, and they only give birth every two to three years. This makes duskies one of the most vulnerable of all shark species.

 

The Dusky Shark is now listed on Appendix II of the Convention on the Conservation of Migratory Species (CMS), but further action is required to protect this important species.

 

Scientific Name: Carcharhinus obscurus

Family: Carcharhinidae

Maximum Size: 420cm (Total Length)

Diet: Bony fishes, cephalopods, can also eat crustaceans, and small sharks, skates and rays

Distribution: Patchy distribution in tropical and warm temperate seas; Atlantic, Indo-Pacific and Mediterranean.

Habitat: Ranges from inshore waters out to the edge of the continental shelf.

Conservation status: Endangered.

 

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Images: Andy Murch

Diana A. Pazmiño, Lynne van Herderden, Colin A. Simpfendorfer, Claudia Junge, Stephen C. Donnellan, E. Mauricio Hoyos-Padilla, Clinton A.J. Duffy, Charlie Huveneers, Bronwyn M. Gillanders, Paul A. Butcher, Gregory E. Maes. (2019). Introgressive hybridisation between two widespread sharks in the east Pacific region, Molecular Phylogenetics and Evolution 136(119-127),

 https://doi.org/10.1016/j.ympev.2019.04.013.