Wet Beast Wednesday: Port Jackson shark
It's shark time again, and today's Wet Beast Wednesday topic is kind of the pug of sharks. Not because it has chronic health problems due to irresponsible breeding, but because of how it looks. Like the pug, the Port Jackson shark is either utterly adorable or one of the ugliest things you've ever seen. I'm on the cute side of the fence and I hope some facts about its biology can lure over to appreciating these sharks too.
(Image: a Port Jackson shark swimming by some kelp. It is a small, light brown shark with a large head. There are prominent ridges over the eyes. The mouth is on the bottom of the head and has large nostrils, thick lips, and small teeth in the center. Along the sides of the body are darker brown lines forming a pattern similar to a harness. End ID)
The Port Jackson shark (Heterodontus portusjacksoni) is one of the bullhead sharks, a rather unusual looking family of small sharks. The Port Jackson shark is the largest of the bullheads, reaching up to 1.65 meters (5.5 ft), though most are smaller. A typical member of the bullheads, the PJ shark, as I will be abbreviating it to from now on (not to be confused with pajama sharks, which are their own thing), has a large head with ridges above the eyes, large nostrils, and a downward-facing mouth. The mouth is the weirdest part of these sharks, and a big clue as to why is in the family name Heterodontidae, which means "different teeth". The bullheads are unique among sharks for having multiple types of teeth. In other sharks, all their teeth are the same shape, differing only in size. The front teeth are only found in the center of the mouth, with fleshy lips on either side. They are small and pointy and occur in multiple rows back to back. The teeth in the back are broad and flat, similar to molars, and also come in rows to fill the mouth.
(Image: a Port Jackson shark resting on sand facing the camera with its mouth open. The shape of its jaw is visible, with a small hoop at the front with small teeth and the larger, molar-like teeth in the rear. End ID)
They look more messed-up on the inside. (Image: the skeletal jaws of a Port Jackson shark. Both jaws look like they have been pinched together, forming a narrow mouth. End ID)
PJ sharks have spines in front of their dorsal fins, which are used to protect them from larger predators, those mainly being seals, sea lions, and larger sharks. The front gill slit is muscular enough to draw water over the gills, letting the shark breathe while at rest. Many shark species have to constantly move forward to force water over their gills, but the PJ shark doesn't. This gill-induced respiration also give the PJ shark the near-unique ability to breathe and eat at the same time. Externally, the PJ shark can be distinguished from other bullhead sharks by the markings on its body. It's skin is sandy, but with dark markings that cross the eyes and then loop around the front of the body to form a pattern that has been compared to a harness. This has also force me to resist making bondage jokes in this post about sharks. See, this is why I shouldn't be allowed to name animals. I would have ended up calling this thing the funny-mouthed gimp shark or something and it just doesn't deserve that.
(Image: three Port Jackson sharks resting on rocks. The harness-like markings are on display and the spines are visible on the leading edge of the dorsal fins. End ID)
PJ sharks are bottom-dwelling predators who live in shallow coastal regions of central and southern Australia, with a few sightings being reported as far south as Aotearoa/New Zealand. They are migratory, moving south in the summer and north in the winter. PJ sharks prefer rocky-bottomed habitat, but will live among sandy or muddy bottoms and seagrass meadows. They are predators who preferentially hunt hard-shelled animals like urchins, snails, crustaceans, and bivalves. This is where their weird teeth come into play. The front teeth are used to pick up prey and transfer them into the mouth, where the flat back teeth crush the shells to get at the gooey insides. The shark will swallow the prey shells and all and later can invert their stomachs to vomit up the shell fragments once the meat has been digested. The sharks hunt using a combination of sight, smell, and electroreception to find prey buried in the sediment. Younger PJ sharks feed more on soft-bodied prey like worms, octopi, and fish and transition to harder prey as they age. A feeding behavior seen in juvenile PJ sharks but not the adults is to take in a mouthful of sand and then use their gills as filters, forcing the sand out while any animals within are trapped in the mouth. PJ sharks are nocturnal and prefer to spend their day in caves or under rocks to shelter from larger predators.
(Image: a Port Jackson shark with a spider crab in its mouth. More spider crabs are in the foreground. End ID)
PJ sharks have been noted to be intelligent and curious animals. Their small size, docile nature, and ease of care makes them a relatively common laboratory animal for shark science, where they are used as models organisms for bottom-dwelling sharks. Lab tests have shown that PJ sharks can learn to associate certain stimuli with food and can differentiate between different amounts of objects. In addition, they are capable of social learning, a phenomenon where one animals learns new behaviors by watching others. In lab tests (source), some sharks were trained to navigate a maze for a food reward. They later introduced untrained sharks to the maze and saw that the untrained sharks paired with trained sharks would mimic the trained ones to navigate the maze. Meanwhile, untrained sharks without trained partners took longer to learn to navigate the maze. Social learning is usually seen in social species, while PJ sharks are generally considered solitary due to hunting alone. However, that may not be the case, as PJ sharks have been seen resting in large same-sex congregations and many have been observed preferentially resting next to certain individuals, possibly indicating something akin to friendship. Females have been described as braver than males, while individuals have been described as having their own personalities by handlers. I will finish this paragraph by noting another study which found that PJ sharks could not be trained to distinguish jazz from classical music. Imagine being the person whose is to play music for sharks. I love science.
(GIF: a clip of a video from the above-mentioned maze study showing one juvenile Port Jackson shark following another through a gap in a wall. End ID)
PJ sharks mate in winter, usually around May, but the females do not lay eggs until around August to November. They have the ability to store the sperm in special glands called shell glands and preserve it to fertilize the eggs later. As with other sharks, fertilization is internal. The male will grab onto a female with his teeth to hold on and insert one of his paired phallus-like organs called claspers into her cloaca. The sperm then runs down a groove in the clasper and into the female. The female will lay a pair of eggs every 10-14 days and can lay up to 8 pairs per season. The egg cases have a strange appearance, with a ridge running along the outside in a spiral shape. The female will use her mouth to screw the egg case into a crevice to keep it from being swept away while helping keep it safe from predators. She provides no further care. Despite the protection, PJ shark eggs have an extremely high mortality rate of 89.1%, mostly due to predation. Fetal development takes 10 to 11 months, after which the juvenile shark will emerge and begin its life. PJ sharks develop slowly, with males becoming sexually mature at ages 8 to 10 and females at ages 11 to 14. They can live for around 30 years.
(Image: a Port Jackson shark egg case. It is a brow, rubber object with a curled flap running up the surface like the blade of an augur. End ID)
(Image: a Port Jackson shark with an egg case in its mouth, pobbibly being a mother moving one of her eggs or a shark eating another shark's egg. End ID)
PJ sharks are currently classified as least concern by the IUCN, meaning they are not in danger of extinction. PJ sharks are not fished commercially and are rarely targeted by sport fishing. Their largest threats seem to be habitat loss and bycatch. It is worth some concern though, as PJ sharks have the traits of an animal that is hard to conserve. Animals with long maturation periods and low recruitment rate are innately vulnerable to population loss, as it takes a long time for the population to recover. Port Jackson sharks are harmless to humans unless you manage to step on one and get stabbed by the spine. In that case it's your fault, not the shark's. I found all of one case of a PJ shark biting someone and that guy just ended up with a bruise. The shark couldn't even pierce his skin.
(Image: a large congregation of Port Jackson sharks all resting on sand and facing the same direction. End ID)
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