User:Gillian Setiawan/Bluntnose sixgill shark

The bluntnose sixgill shark (Hexanchus griseus) is a specific species of sixgill sharks, of genus Hexanchus, that also consist of another species: the bigeye sixgill sharks (Hexanchus nakamurai). It should be noted that through their base pairs of mitochondrial genes COI and ND2, these 2 species of sixgills widely differ from one another.

Distribution (& Habitat)
As a deep sea shark, the bluntnose sixgill shark (Hexanchus griseus) is commonly found in deeper depths of the ocean, the deepest recorded depth being 2,500m. Being in such a deep area of the ocean, these sharks have developed the behavior of undergoing diel vertical migration in order to have more access to food. It was found that it takes more time for the sixgills to have to swim back down to their natural habitat of the bathypelagic rather than to swim up during the night to find food in the more densely populated zones. As such, it can be inferred that they have some sort of adaptation that aids buoyancy to ensure that these sharks are able to float more easily.

An example of this DVM occurrence was found off the coast of Oahu, Hawaii, whereby 4 sixgills' behaviors were studied. It was found that at around midnight to 3am, the 4 sharks swam up to a minimum depth of 300m whereas at about noon, they reached their maximum depth of between 600 to 700m. This shows a daily pattern whereby the sixgills are going up during the nights when it is darker and colder to forage for food up in the shallower depths but as morning comes and light and higher temperatures starts to come in more intently again, the sharks go back down to their original habitat to maintain a lower metabolic rate, ensuring that they will be able to use the nutrients from whatever they ate during the night slowly, reducing the need for them to search for more food throughout the day.

Another study found that the motivating factor for the bluntnose sixgill sharks' DVM behavior was foraging. Researchers were able to rule out predator and competitor avoidance as potential reasons for the vertical movement patterns because they found pairs of sharks with synchronized movements, indicating that the sharks were responding to the same stimuli. The sharks demonstrated distinct and consistent patterns of vertical migration despite size, sex, and spatial scales, showing that foraging behavior can most likely be seen as the reason for the diel vertical patterns of sixgill sharks.

The bluntnose sixgill shark has consistent seasonal movements. They move north during the winter and spring and south during the summer and fall. In this study as well, researchers were able to determine that these movement patterns can be attributed to the seasonal movements of prey over other reasons.

Growth and Development
In general, the side (in length and weight) of the sixgills increase with maturity. With the male sharks specifically, their sexual maturity is usually determined by the length of their claspers. While juveniles have short and flexible ones, mature male sixgills have rigid, calcified longer ones. On the other hand, the length-weight relationship of females tend to increase very rapidly as they get to the onset of their sexual maturity.

Feeding Behavior
Due to the food shortage where they live, the sixgills tend to eat anything in their vicinity. A study done in 1986, with 28 sixgills, discovered that the most abundant meal they were able to obtain include cartilaginous and bony fishes, followed by marine mammals and several invertebrates. As time passes, however, it seems that their stomach contents have changed. In 1994, it was found that of 137 samples, the major prey groups were cephalopods, teleost fishes, chndrichthyans and marine mammals.

This difference in results could be due to several reasons. Firstly, as noticed in the different sample sizes of the 2 studies, it could be accounted to the fact that the technology back then was just not advanced enough to be able to fully study the stomach contents and capture enough samples, leading to a skewed result for the 1986 study. Next, as human activities have increased over the years, it could affect the availability of food for the sixgills in the deep. Though we can't see how we are affecting marine life, our actions still affect them in some manner. There are other potential reasons for this change in diet, many factors may have affected the sixgills.

Sixgill sharks possess variability in their feeding mechanisms that could have contributed to their evolutionary success and global distribution. These sharks are able to protrude their jaws and vary their methods of feeding depending on the situation. They utilize sawing and lateral tearing techniques to manipulate food. Sixgill sharks also lower their pectoral fins right before they strike in order to stop forward progressions, making it easier for them to forage.

Biology and ecology
Bluntnose sixgill sharks are positively buoyant. During vertical movements, the sixgill sharks demonstrate more swimming efforts for the descent than the ascent. This is indicated by the greater number of tail beats and the sharks' ability to glide upwards for several minutes. The positive buoyancy can help the sharks to hunt stealthily by approaching prey from below undetected since the upward gliding permits minimal movement. It can also be advantageous for their diel vertical migrations. Since the sharks spend their days in colder water, their metabolic rates decline. Positive buoyancy can help them to glide upwards with minimal swimming involved during their evening migrations.

BBC Blue Planet
BBC's Blue Planet show, one that provides video footage of things you see in the oceans, had one episode where they portrayed what's seen in the deep. In this episode, there was a focus on the bluntnose sixgill shark whereby they were feeding on a whale fall. In this YouTube link, there was video footage of how the shark had attacked the crew members trying to collect the video footage. Thinking that they were competition, the sharks used their bodies to try to fend them off, only leaving them behind once they realized that the sub was not there to feed. Though it did seem dangerous for the team to do this, they were able to obtain useful video footage of the sixgills that they later featured on the episode. As a worldwide, well-known, scientific platform, it hence helped with the awareness of the existence of these species of sharks.

Joint Venture (Tagging Sixgills in its Natural Habitat)
Since 2005, scientists have successfully been able to tag sixgill sharks as a means of studying their behavior. With this being said, however, as of 2019, there has yet to be a sixgill tagged in its natural deep-sea habitat. Researchers from Florida State University, the Florida Museum of Natural History, the Cape Eleuthera Institute, and OceanX hence decided to join forces to tag a deep-sea shark through use of a submersible, and they succeeded in doing so. After 3 months of leaving the tag on the sixgill, it is determined that the tag would float up to the surface where scientists will be able to collect the data from that tag. Overall, this study showed how advancements in technology has helped scientists be better able to study marine life. Instead of having to go on an expedition for years at a time, the scientists here simply had to attach a tag onto the sixgill once and collect the data another time. The tag simply showed behavioral results of the sixgills.