Reproduction
Thresher sharks are thought to follow a biennial reproductive cycle meaning they give birth once every two years. Typically litters consist of 2-4 pups but the largest litter size recorded was made up of seven pups. Mating seasons typically happen in the summer whereas birthing happens the spring after mating. Their gestation period typically lasts nine months (Teo et al. 2018).
Male thresher sharks tend to reach maturity around age five whereas female thresher sharks typically reach maturity at around age twelve or thirteen (Natanson et al. 2015). Studies have questioned however whether this is the same throughout the Atlantic and Pacific populations and if reproductive maturity age may differ between the two populations. A study was conducted that determined that female Pacific thresher sharks may reach reproductive maturity at ages as young as five. A large issue with this study however was the fact that it was based largely on fishery dependent data which could not verify multiple things including thresher species, actual age, as well as if the individual was actually reproductively mature. Because of this the two stocks are looked at as though they mature at the same age until further or more thorough research can be done (Teo et al. 2018).
Thresher sharks practice ovoviviparity meaning the pups get their nutrients from an egg sac while in the uterus but are born live. Thresher sharks also have been found to eat other egg sacs in the womb in order to continue to gain nutrients after their own yolk-sac has been depleted (Teo et al. 2018). It is not thought however that thresher sharks practice adelphophagy, or the eating of their siblings in partially formed/fertilized embryos. Studies have been done examining the gut contents of unborn thresher sharks and there was no evidence of smaller sharks in their stomach or of eaten smaller sharks in the womb of the mother. The teeth of thresher sharks also do not become shown, or unhidden, until very soon before birth which also indicates that they do not take part in adelphophagy (Bruno et al. 2018).
The most well documented nursery for this species is the Southern California Blight in the Pacific and Northern Carolina to Massachusetts in the Atlantic. Typically these nursery areas are along the continental shelf due to the fact that juveniles tend to spend their time in waters no deeper than 50m (Kneebone et al. 2020, Cartamil et al. 2010).
Age and Growth
Typically adult threshers have a slow growth rate but may reach lengths around sixteen feet long, including the length of their large tail. Alopias vulpinus grows the largest in comparison to the other species in the Alopias genus (Kneebone et al. 2020).
Juvenile threshers have a much quicker growth rate which is thought to be a form of protection from predators. Pups are typically born with a 65cm fork length (Cartamil et al. 2020).
(9. Image from website Research Gate)
Females are found to be mature at 12 to 13 years old while males around found to reach maturity at 8 years old. When threshers reach maturity they typically slow their growth rate by about 50% so females grow larger than males do. It is hard to determine the exact age that threshers live to as band pair counting is only accurate up until the thresher is five years old. Carbon dating band pair countings have found age estimates to be underaging individuals by up to 18 years. A 2015 study found a new maximum age estimate of 38 years old which was much older than previous estimates. Maximum age may also differ depending on sex of the individual but not enough research has gone into this area to determine if there is a sex difference or not (Natanson et al. 2015).
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