Author: rgardner2 (Page 1 of 7)

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).

Thresher sharks have an inferior mouth type and their typical prey are small to medium sized schooling fish, cephalopods and sometimes seabirds. Their main hunting strategy is to use their whip-like tail in order to stun fish that they can then go retrieve (Knaub et al. 2024). 

Threshers can use their tail to whip over their heads as well as to the sides of their bodies. Their typical hunting form is broken into four phases: preparation, strike, wind down recovery, and prey collection. Essentially threshers speed up towards their prey (preparation) before lowering their snout and abducting their pectoral fins to slow themselves down as they raise their caudal fin (strike). In this motion they act akin to a trebuchet throwing an object from a sling. The apex of the strike happens as the caudal fin aligns over the dorsal fin and is so powerful it can diffuse gas out of the water (Knaub et al. 2024).

A sideways strike is only used after a successful overhead strike. The average tail speed of one of these whips is 14.03m/s or 31.38 miles per hour. The fastest ever recorded tail whip clocked in at 21.8m/s or 48.77 miles per hour (Knaub et al 2024).

I was unable to find information on sensory biology in Alopias vulpinus although it is thought that their eyesight is poor like many other shark species. Due to this it is likely that they rely on their Ampullae of Lorenzini and lateral line in order to determine what their surroundings are like. It is thought that part of the daily migration of threshers is in order for them to be below their prey which may suggest that they use shadows to determine light intensity and rough shapes for hunting (Cartamil et al. 2010).

There are no large scale fisheries of thresher sharks in the Atlantic ocean however there is a very large recreational fishery industry surrounding the species from Virginia to Maine. While the fishery industry may not be targeting threshers they are often caught as bycatch in nets. From 1980s to the 2000s it was thought that thresher populations decreased by 63-80% in the Northern Atlantic. Since then the population has thought to have leveled off with periods of annual variability as seen through fishery dependent data. In spite of this the population is listed as vulnerable by the International Union for Conservation of Nature as it is thought there is an overall worldwide population decrease (Kneebone et al. 2020).

Many different gear types such as: commercial bottom long haul-line, pelagic longline, gill net (sink and floating), haul seine, purse seine, otter trawl, mid-water trawl, and recreational rod and reel are detrimental to thresher populations. The most detrimental to the species has been gill nets, pelagic longlines, and rod and reel which tend to catch threshers the most. There are no size restrictions in these waters which can be an issue due to the fact that juveniles are caught at a higher rate than adults. Due to the low fecundity of thresher sharks this is a detriment to the population (Kneebone et al. 2020).

They have been petitioned to be protected under the US endangered species act in order to protect their populations though they are still only listed as vulnerable. I’d say a large issue with this is that just not a lot of things are known about the thresher shark. It’s hard to protect their habitats or nurseries when their main habitats and nurseries may not be known. Another issue is that a place has to be proven to be a nursery according to policy definition of a nursery, which may be a hard and lengthy process to prove.

Something that has seemed to work in the past however is public knowledge. The more people know about thresher sharks and want to protect them the more likely they are to be protected. More recent research has started coming out to map their migration patterns in order to protect these areas.

More recent media coverage has also been given to thresher sharks, not for their declining populations, but for their loveable faces. If this keeps up they may be on their way to becoming a protected species.

^{_{(10. Image from Gina Zwicky on twitter)}}

Habitat

Juvenile threshers are typically found along the continental shelf of warmer temperate waters as they prefer a lower temperature differential. Typically juveniles inhabit waters with depths no more than 20m (Cartamil et al. 2010).

^{_{(6. Figure 5 from Kneebone et al. 2020)}}

Acoustic telemetry research suggests that juveniles, specifically in the California Bight area, go through daily vertical migrations. They tend to stay in deeper water during the day and rise closer to the surface at night. This is thought to be a predation strategy (Cartamil et al. 2010). 

Adult threshers typically inhabit waters around 200m of depth, though they have been found in a variety of depths from 1-5427m, and inhabit temperate waters worldwide. During adulthood threshers go through a northward and southward migration as the seasons progress (Kneebone et al. 2020).

^{_{(7. Figure 4 from Kneebone et al. 2020)}}

In the Atlantic Ocean this migration is between Cuba to Newfoundland while in the Pacific Ocean this migration is between Mexico and British Columbia Canada. In each ocean the threshers spend the summer months in the north and the winter months in the south (Kneebone et al. 2020).

In the Pacific it is theorized that multiple different environmental factors go into this migration each year. While in the Atlantic it is thought that temperature is the main driving factor causing threshers to migrate each year as threshers prefer water with a surface temperature of 10-22°C. Despite this, adults have been found in waters with surface temperature anywhere from 4-31°C. Migratory routes and spatial distribution is unclear in the Atlantic as most of the data about threshers in the Atlantic comes from fishery dependent sources (Kneebone et al. 2020).

Behavior

No sex related habitat aggregation behavior has been recorded though that could be from lack of information as it is likely females stay in warmer waters longer in order to help with gestation. Threshers are not often seen aggregating together as they are typically a solitary species (Kneebone et al. 2020).

The main segregation behavior seen in threshers is the fact that juvenile threshers inhabit shallower, warmer waters compared to adult threshers which may migrate further northward and into deeper waters. This is thought to be so that juveniles may avoid predators but, it may also be due to the fact that the main prey juveniles go after also aggregate closer to shore (Cartamil et al. 2010). Unfortunately this means that juvenile sharks tend to be caught more frequently than adults so many individuals are not living to reach maturity (Kneebone et al. 2020).

Taxonomy

Kingdom: Animalia
Phylum: Chordata
Class: Chondrichthyes
Order: Lamniformes
Family: Alopiidae
Genus: Alopias
Species: Alopias vulpinus
_{(Ichsan et al. 2021)}

^{_{(1. Image from shutterstock)}}

Morphology & Locomotion

One of the most notable things about the Alopias genus is the long whip-like upper caudal fin lobe that extends from their body. This tail can sometimes be as long as their body. With relatively large pectoral and anal fins, and a smaller singular dorsal fin, the fin types in the Alopias genus are truly unique (Knaub et al. 2024).

^{_{(2. Image from Atlantis Dive Resorts)}}

Compared to the common thresher, Alopias vulpinus; big eye threshers or, Alopias superciliosus, are distinguished by their large eyes while pelagic threshers, Alopias pelagicus, are harder to tell the difference with. Pelagic threshers have dark patches of skin above their pectoral fins, lateral cupselts on their teeth, as well as defined labial furrows around their mouth while common threshers do not have any of these features (Ichsan et al. 2021).

A common threshers caudal fin can be used to whip prey over their head or to their side and it is a very different motion from the oscillatory movement used for swimming. Due to the increased stress on the back vertebra when this is happening thresher sharks have different cartilaginous morphology in their tail in comparison to the rest of their body. Posterior vertebrae are smaller, shorter and have more lamellae than anterior vertebrae. This increased lamellae is thought to increase the amount of minerals in the centra of the vertebrae to support the oscillatory motion of swimming, as well as provide support for the body during tail whipping events. Thresher sharks are also seen to have less stiffness in their vertebral centra compared to other lamniformes or carcharhiniformes (Knaub et al. 2024).

Although they do have a thunniform swimming type the caudal fin of thresher sharks means their body is not as streamlined. Which is unlike their lamniformes shark counterparts (Donley et al. 2012).