In small clusters around the site, several Giant Sea Hares at a time join together in a curved chain, draped over bushes of red algae. A big slimy orgy, all of them gorging on algae and all copulating at the same time.

Sea hares are herbivores, grazing in shallow water, eating almost their own bodyweight per day when they are small and recently settled out of the plankton. Some species prefer red algae, some like green sea lettuce, a few even eat sea grasses. Their choice of food reflects in the colour they become - the same species from different places can take on distinctly different hues.

The smaller and younger hares are rather vulnerable to predators - after all they are basically chunks of unprotected protein living in shallow waters. So they tend to stay hidden during the day, coming out at night to graze the alge. As they grow, they concentrate defensive chemicals from the algae they eat.

Some of these chemicals are used to make a purple ink which they can squirt at potential predators in a similar way to their distant cousins the octopodes and squids. Other chemicals in a different gland produce a white slime that is distasteful to predators. Larger adult sea hares, as they accumulate defensive chemicals in their skin and tissues, effectively become poisonous enough to become free from predators, so they can forage in daylight.

Sea hares' lives are short and simple, grazing, gorging and growing fast for the first handful of months and reaching their maximum size rather quickly. Resources are switched from growth to egg and sperm production and the hares spend the last two-thirds of their lives mating as much as possible, then wrapping egg strings around raised algae clumps during high tidal periods, to better disperse their millions of plankton-drifting larvae. They live a year at the most.

Orgies are a pretty normal aspect of the life of an adult sea hare. They are hermaphrodites, able to act as male and female at the same time. The penis sits along the side of the head facing forward, the vagina further back facing backwards, so a sea hare can't reciprocally mate with another single animal. A hare usually initiates mating 'as a male' - approaching a potential mate from behind having picked up her/his scent down-current.

Once the scent has been detected the hare follows the odour trail, waving 'his' head from side to side to home in on his mate. Once mating starts the mated animal then becomes aroused to seek its own mate, and the two crawl off joined together, attracting more slugs as they go.

Large chains of mating hares will sometimes close the loop, the first in the chain mating with the last to form a ring, often draped over a clump of algae to they can all eat at the same time. Once they become mature sea hares might spend a quarter to two thirds of their time copulating.

It's a rather simple life. Graze, grow, mate and die.

Malcolm Nobbs is a recreational diver and photographer from Nelson Bay. Jamie Watts is a marine ecologist from the United Kingdom. The pair regularly collaborate on marine life articles.

To showcase the Port's incredible underwater world the Examiner is collaborating with divers, marine scientists and photographers on a new series that explores life Beneath the Surface.