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A New species of Sleeper Shark

It is very possible that you never heard the term sleeper shark. This refers to a family of sharks called "Somniosidae", so called because of their slow lifestyle. These sharks live in especially deep and cold environments, so are very slow-growing and not very active. You may have heard of one of these family members; the Greenland shark (Somniosus microcephalus), as they recently made headline news regarding their indelible centuries-long lifespans. However, you probably have not heard about a new species of sleeper shark which was recently discovered off the coast of Taiwan...



A research team found the new species of shark in a fish market in Cheng-Kung, in the east of Taiwan! The shark had been caught by long-line from the deep sea. In a stroke of remarkable good fortune, not only were they able to find just one specimen of this animal at the market, but she was pregnant female. This meant they were also able to study the morphology of the developing embryos and describe the new species' reproductive strategy.


The shark was 134 cm total length and as she was carrying pups, she was clearly sexually mature. This suggests this species would be unlikely to grow significantly larger than around 1.5 m in length. They described the shark as uniformly brown, with a slender, almost cylindrical body shape and moderately long head with rounded snout. The teeth were quite different in the upper and lower jaws; the upper teeth being "lanceolate" (meaning they were a narrow oval shape tapering to a point) and the lower were "semi-oblique" (meaning they are not parallel and point outwards from the front of the jaw (* you can see this if you look closely at the image above)) (Hsu et al, 2020).

Somniosus (Rhinoscymnus) pregnant female (Hsu et al, 2020)
Fins of the new sleep shark (Hsu et al, 2020)

They also described the different fins of their specimen. The shark had two spineless dorsal fins (see image A and B) the back and small pelvic fins underneath (images C and D), with an asymmetrical, paddle-shaped "caudal" (aka tail) fin (image E) (Hsu et al, 2020).


The particular size and shape of these fins allowed the researchers to be certain that their shark was indeed a new species because several features were different to other species which had been documented previously (Hsu et al, 2020).


The researchers also noted that the shape of the "dermal denticles" were very distinctive and contrasted to any other known species, as they were rhomboid-shaped. Dermal denticles are hardened scale-like structures covering the shark's body, which give the skin its strength and hydrodynamic properties. They noted that the skin felt very smooth and sleek to the touch (Hsu et al, 2020).

Dermal denticles (Hsu et al, 2020)

They discovered a total of 33 embryos (8 males and 25 females) in the specimen. Ranging between 12.8 and 15 cm TL the embryos were "mid-term" (roughly half-way through their gestation period). They were light brown or somewhat yellowy grey in colour. The embryos all contained a large yolk sac, which suggests that this shark reproduces by "ovovivipary", meaning that an egg develops into an embryo within the mother, to be born alive (Hsu et al, 2020).

Embryos with yolk-sac attached (Hsu et al, 2020)

The researchers named the new species Somniosus (Rhinoscymnus) cheni, in honour of a special team-member (Wen-Jong Chen), who helped them to complete their work. They included two "Genus" names, as there is some dispute amongst taxonomists about whether this group of sharks are actually Somniosus or Rhinoscymnus (Hsu et al, 2020).




References

Hsu HH, Lin CY & Joung SJ (2020). Somniosus (Rhinoscymnus) cheni sp. nov., A New Species of Sleeper Shark (Squaliformes: Somniosidae) from Eastern Taiwan, with Aspects of Embryo Biology. Zoological Studies 59:0gg.


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