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Friday, April 5, 2019

Sea Creatures Part 1 - Sharks

Sea Creatures Part 1 – Sharks

Introduction

Shark Station gets its name from the sharks that live near and around it. So, I figured they deserved their own special blog post as far as the sea creatures in Shark Station go. Each shark species featured in the novel and also those that were merely mentioned will be getting their own section in this blog post. There will be two more sea creature posts to feature the rest of the animals in the novel. Like for Allie’s Return, any animals that are repeat visitors from previous novels will not have their own feature in these posts (with the exception of the great white shark which wasn’t thoroughly highlighted before). With that said, let’s learn some more about the shark species in Shark Station starting with the most obscure to the most iconic.


Frilled Shark
saname777 from Tokyo, Japan [CC BY 2.0 (https://creativecommons.org/licenses/by/2.0)]

The frilled shark (Chlamydoselachus anguineus) is a deep-sea shark ranging from the pelagic to benthic zone in depths of 120-1570m (393.7-5,150.9ft)(1)(2). Rarely are they seen at the surface, but sometimes they can be found near the continental shelf(2). They are found worldwide from subpolar to tropical latitudes, with distributions in the Atlantic and Pacific Oceans(1)(3). In the Pacific, they range from Japan to New South Wales and Australia. The females are typically larger than the males growing to a length of 2.0m (6.6ft), while the males only grow to 1.7m (5.6ft) long. Both sexes exhibit gray or darkish brown coloration on the dorsal surface with a lighter shade on the ventral side. Six gill slits lie just before the pectoral fins with the first gill slit closest to the mouth being elongated and wrapping underneath the chin(3). They swim using an eel-like motion, undulating their body through the water(1). Interestingly, their anal fins are actually longer than their dorsal fin, which differs compared to most other sharks(3).

Little is known about the frilled shark in the wild due to its rare sightings. However, they are thought to give birth to live young, which use yolk sacs for nutrition while in the womb(1). The estimated gestation period is thought to be the longest of any chordate (animal with a backbone) at up to 3 ½ years. Each pregnancy they give birth to 2-15 pups and frilled sharks are thought to live up to 25 years old (2)(3). Some distinguishing characteristics that differ from other sharks are that they have fins placed farther back on their body, creating the eel-like appearance and a mouth more towards the front of their snout than underneath it like many other shark species(1)(3). Their lateral lines are exposed to the surrounding water so the hair cells are actually “open” to the water(3).

As for their feeding ecology, frilled sharks feed mainly on squid, other cephalopods, and bony fishes, but have been known to eat other sharks as well (1)(3). Some scientists have proposed that they may be able to close their gill slits to create suction to pull in squid. Their needle-like teeth are thought to make it difficult for their prey to escape. They are capable of ingesting prey almost half of their size(3).

An interesting fact about them is that they have an expandable fold of skin along their ventral side thought to help their stomach expand for digestion. Another interesting fact is that a new subspecies was discovered in 2009 near South Africa, it was named C. Africana(3).

According to the IUCN Red List, frilled sharks are listed as least concern as of February 20, 2015(4).



Goblin Shark (Mitsukurina owstoni)
Dianne Bray / Museum Victoria [CC BY 3.0 au (https://creativecommons.org/licenses/by/3.0/au/deed.en)]

The goblin shark (Mitsukurina owstoni) is known for its expandable jaws that can extend to catch prey in a blink of an eye. However, there is still a lot to learn about this species. Only one was ever maintained in captivity and it only survived for a week (click here to see a video of it swimming in an aquarium at Tokai University in Japan)(7). In the wild, goblin sharks are found in the Atlantic, Indian, and Pacific oceans. Ranging from depths of 95m (311.7ft) to 1300m (4,265.1ft), they inhabit waters off seamounts and continental shelves(6)(7). There are some records that indicate they may be a pelagic species(7).

As for their physical appearance, goblin sharks range from a pinkish color to a grayish purple. Lining the edges of their fins is a bright blue coloration(5). They average between 3-4m (10-13ft) long. The largest caught goblin shark weighed 210kg (463lbs) and over 5.5m (18 ft) long(7). The gestation period is unknown, but the shark is thought to be ovoviviparous like many other large sharks(6). Goblin sharks lack the nictitating membranes that other shark species like the great white have(8).

From stomach contents, goblin sharks are known to eat small bony fish, cephalopods, and crab(8). Their diet consists of crustaceans, fish, and squid(5). To detect prey, they use their ampullae of Lorenzini, then capture the prey with their needle-like front-teeth and pharyngeal suction (7)(8). Their posterior teeth are shaped for crushing, which indicates that they may eat some hard-bodied organisms like crabs and shrimp(7).

As for conservation, the IUCN red list lists them as least concern since at least July 7th, 2017(9).  Threats to them include being commercially fished off of Japan, but usually only caught as by-catch elsewhere in the world(8).



Blue Shark


The blue shark (Prionace glauca) is one of the only truly pelagic shark species. They range from cold temperate waters to tropical waters and can dive as deep as 350m (1148.3ft) 15)(18). They are migratory and have been known to cross oceans. Their long pectoral fins are thought to be used to help them glide on currents as a part of energy conservation. They migrate in search of food and mates(15). The species exhibits sexual segregation, where the females tend to live at higher latitudes than the males and they only congregate to mate (16)(15). As a species, they range from 60° N to 50 ° S latitude(16). The gestation period lasts form 9-12 months and consists of a litter of pups ranging from 35-100 pups, although one litter of 135 was reported(15)(16). As adults, they can grow to 2.7-3m (9-10ft) long, although the largest reported one was 4m (13ft) long(16).

As for their diet, they are known to eat anchovies, hake, dogfish, squid, euphausiids, and mackerel, and seabirds(16).

The IUCN red list categorizes them as near threatened based on the last assessment on October 1, 2005. The major threats to their survival are longline fishing and sport fishing.



Shortfin Mako
Patrick Doll [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)] 

The shortfin mako (Isurus oxyrinchus) is a pelagic (open ocean) shark that can grow to 3.7m (12ft) long and weigh 544.3kg (1200 lbs). They can swim at 45 miles an hour, making it the fastest shark in the sea(10).  They have a countercurrent exchanger which keeps their blood warm enough to maintain a body temperature up to 0.6°C higher than the water around it, which helps them to move quickly through the water (11)(10).

Spread throughout the Pacific Ocean, the shortfin mako ranges from as far north as the Aleutian Islands to as far south as New Zealand(12). They are wide-traveling sharks. Voyages of 2,092km (1,300mi) in a month have been known to occur(11). Adult shortfin makos aren’t known to have any predators, putting them at the top of the food web in the pelagic zone but the juveniles may be eaten by other shark species and perhaps even adult makos(10). Mainly, their diet consists of tunas and billfish, but also squid, blue sharks, sea turtles, mackerels, porpoises and dolphins(11). They’ve even been known to eat swordfish, one of the other fastest fish in the sea(12).

Like the other sharks in this blog post so far, the shortfin mako is ovoviviparous. When they are captured, female makos are known to abort embryos which means little is known about their reproduction. What is known is that they have a gestation period of 15-18 months and litters consist of 8-10 pups which are born during winter(11). The coloration of the mako is an indigo to rich purple color on the dorsal side and white to silver ventral surface with an obvious line of coloration between the two(11)(12). They are thought to live somewhere between 29-32 years(12).

An interesting fact is that the scientific name for shortfin mako name means “equal tail, sharp nose”. Isurus is Greek for equal tail, oxy for sharp and rynchus for nose. There are two species of makos, the longfin and the shortfin. The way to tell the difference between a longfin mako and a shortfin are by the elongated pectoral fins and larger eyes on the longfin mako(12).

As of November 5th, 2018, they are listed as Endangered by the IUCN red list (13). Major threats to them include being fished for commercially, for recreation, and accidentally captured(10).

If you would like to see a video with blue sharks and mako sharks interacting together in the pelagic zone just like in the book, click here



Great White Shark

The great white shark (Carcharodon carcharias) is the most iconic of all shark species. They are known for their size as the largest predatory fish in the ocean(21). They can detect prey movement via the lateral line from 250m away. In quick bursts of speed, great whites can swim at 35mph, but only maintain 15mph(23)(20). Ranging from 3.6-7.62m (11-25ft) in length, they can weigh up to 2,268 kg (5,000lbs) with the females being larger than the males (22)(21). The dorsal side is a dark gray to almost black coloration with a white ventral side, they use this countershading adaptation to keep prey from detecting them.

Found in all oceans except the Arctic and Southern Oceans, they range from cold to tropical latitudes from the coast to the pelagic zone and are known to dive deep in search of food. In the eastern Pacific, they are known to migrate from Mexico to Hawaii(21). They are found from 60°N to 60°S latitude. Some sharks have been known to travel across an entire ocean.  They have been reported as far north as the Aleutian Islands to as far south as Southern Mexico(22).

It is still unknown as to where great white sharks give birth or exactly what size they are at birth. The smallest recorded great white was 1.1m (3.6ft) long and only 16kg (35lbs). An ovoviviparous species, females give birth to live young. The pups hatch within the mother in egg capsules and then the female gives birth not long afterward. The gestation period is thought to be more than a year. Litters can be anywhere from 2-10 and even as large as 17 pups. At birth, they are expected to live up to 30 years(22). As the young grow into adults, they begin seeking larger prey than the small fishes they ate as juveniles, progressing up to large marine mammals(21). Juveniles don’t start eating marine mammals until there are at least 450kg(22).

The great white diet consists of a variety of animals including fish, seals, sea lions, dolphins, sea turtles, toothed whales, and even carrion (dead meat) (19)(20). They have 300 teeth used to grab and tear meat into bite-sized chunks(19). Group hunting has been seen in great whites where they work together to kill prey and then share the kill(23). To maintain their body heat, sharks have a countercurrent exchanger to help maintain a body temperature slightly higher than that of the surrounding water. It helps them to keep their brain and muscles warmer so they can think and move effectively even in colder waters(21).

Some interesting facts about great white sharks are that their only natural predator (outside of humans) is the orca (Orcinus orca). They have been known to kill and even consume sharks including the great white. Great whites also have an “ear stone” that allows it to orient itself in the water, to basically know if they are upside down or not. Specialized eyes with retinas divided into two sections allow them to see in the daytime and low-light. Although behavioral research is ongoing, they are thought to be intelligent and scientists have seen a variety of behaviors of great whites amongst each other from aggressive to non-aggressive(23).

Great white conservation is tricky but extremely important for many reasons. They are an apex predator and they influence the health of the ocean ecosystems in which they play a part. However, no reliable data exists on their populations, but scientists do know their numbers are decreasing(20).  According to the IUCN Red List, they have been listed as vulnerable based on the last assessment on October 1, 2005. Some of the threats against them are sport fishing for their fins and teeth and also as bycatch(19).



Megalodon
Mary Parrish, Smithsonian, National Museum of Natural History [Public domain]

The Megalodon (Carcharocles megalodon), whose scientific name means big tooth, was a shark that lived millions of years ago. They are thought to have lived during the Miocene and Pliocene epoch (15.9-2.6 million years ago). Megalodon teeth have been found on every single continent instead of Antarctica(25). It lived during the Silurian period, 200 million years before the dinosaurs. Much of what we know about megalodons come from inferences based on their teeth, dermal scales, and calcified vertebrae(26). Due to the cartilaginous nature of shark skeletons, the only way to tell the size of the megalodon is by the size of its teeth. Based solely on that, it is estimated that they may have been as long as 18.3m (60 ft), while some think that they could have been as large as 24.4m (80ft) long. The largest tooth ever found was 17.8cm (7in) long, almost 3 times the length of great white teeth(25). Estimates of size put it at 45MT (50 tons)(26).  

Megalodon had 276 serrated teeth and was thought to have fed on whales and perhaps other sharks in the warm waters of the Pliocene epoch(25).

I shouldn’t have to state this, but megalodons are extinct. In fact, the most recent fossil was from 2.6 million years ago(25). The major thought behind their extinction was the warmer waters which would have made it difficult for them to survive, or as their prey adapted to colder waters, they starved because they couldn’t follow them into the colder waters(26).

This concludes the end of the sharks sea creatures blog post. In two weeks, they’ll be another sea creatures blog post, this time focusing on the animals of the pelagic zone. I hope you enjoyed reading this blog post and if you have any comments, you can leave them here on my blog or email me directly at elizataye@gmail.com.

Reference

1 comment:

  1. Very interesting facts about sharks. No need to be afraid of them, especially if we stay out of the water! I like nature, so these facts are great, and I enjoyed reading the book.

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