Piles and clusters of the odd-looking sharks strewn around, most apparently asleep, some lying across each other, some still half-heartedly twisting around each other. The main event happened last night and into this morning, and actually over many nights in recent weeks.

But now it's morning, and in the winter we find these bizarre beasties basically dozing, hanging out on rocky reefs, caves or overhangs. Like many sharks they are more active - moving around to feed and breed - at night.

These are Port Jackson sharks. They're obviously sharks, but they're 'kinda funny-looking'. Yes the tail, fins and basic body plan all look shark-ish, albeit not particularly sleek. But the blocky head and the face is very odd, with crestlike ridges above the eyes, and a downturned, piglike snout, with the mouth at the end of the snout and odd, curled nostrils.

Like dogfishes, they have a spine at the leading edge of each dorsal fin, the mucus glands around these spines probably producing toxic proteins.

What look like ears are spiracles, modified pumping gill slits that allow PJs to spend about three quarters of their time resting - they can pump water over their gills without having to move, which also means they can breathe while eating and while not moving.

Or, as they seem to for much of the day, having a post-coital or post-prandial snooze. Being out and about and active at night, when their crunchy-shelled sea urchin, mollusc, crustacean and worm food is more active, makes sense.

The PJ, growing to almost the length of an adult human and about half the weight, is one of the largest of the nine species of bullhead or horn sharks (although maybe 'pighead sharks' would be more appropriate), a tiny order of sharks with a single family, the last remnants of by far the most ancient surviving group of sharks. They date from the early Jurassic, probably even further - almost certainly more than 100 million years before T-Rex. All other modern sharks and rays are far more recent offshoots.

The bulk of the PJ population drifts south in the summer as cool, productive waters as far south as Tasmania warm up and provide their summer food bounty, and back north to breed in the winter. They arrive in breeding bays, including areas around and to the north, from about late June, with males arriving first.

For NSW scuba divers, a great place to see them is on the Oakland wreck which lies just outside of the Port Stephens Heads.

Biologist Dr Dave Harasti says his best Port Jackson experiences have been at Governor Wharf at Fingal Island near Port Stephens.

At times during the winter breeding season he saw 150-200 PJs crammed in on the reef, competing with one another for mating "like an enormous rowdy night club, where all the males are fighting among each other to gain access to the females. I've seen a male biting the tail of another male as he's mating, trying to stop 'the process', and I've seen two males try to insert their claspers into the same female at the same time! It really is wild stuff."

He's also seen the mating activities during the winter at all times of day.

It's a grown-ups only orgy. The youngsters seem to be more solitary, and probably steer well clear of the rowdy adults. After mating, the females lay pairs of eggs every week and a half or so, depositing and hiding them in crevices or amongst rocky areas over the next couple of months. Most will not make it - they will be found and eaten by a variety of predators and scavengers over the ten months or so it takes for them to develop.

Those that survive and hatch come out as miniature versions of their parents, but with one important difference. For their first years they have pointed teeth, like most sharks, very much suited to grabbing soft-bodied prey. As they mature their teeth differentiate, until as adults they keep the regular 'grabbing' teeth in the front, and in the back of their jaws grow nutcracker-like crunching, crushing teeth, very much suited to breaking into clams, snails and urchins - rocky reef grazers that mostly come out at night.

To showcase the Hunter's incredible underwater world, the Newcastle Herald's sister paper the Port Stephens Examiner is collaborating with divers and photographers on a new series that explores life Beneath the Surface. This contribution was first published as part of that series.

Malcolm Nobbs is a Nelson Bay diver and photographer. Jamie Watts is a UK marine ecologist.