Giant kelp Macrocystis pyrifera and scuba diver.

Deep in the forests celebrated for its alpine scenery, Tasmania offers equally wondrous sights beneath its seas. One of a handful of places in the world where giant kelp can flourish, a dive in these submarine forests opens a window on a hidden domain of astonishing diversity. Some 30 metres below, at the foot of the forest. The caves and drop-offs are decorated with a carpet of southern jewel anemones ‘Corynactis australis’ (left).

Giant kelp Macrocystis pyfifera holdfasts (left). Gas filled giant kelp Macrocystis pyfifera floats ( right).

FORMIDALBE GROWTH
On their way to the surface, 'Macrocystis pyrifera' kelp stems are supported by hundreds of gas-filled floats (top right), each forming a base from which the ribbon-like fronds sprout. A plant standing in 20 metres of water (left) may include a similar length resting on the surface. This allows it to maximise photosynthesis and generate a prodigious growth rate. In fact, it is the fastest growing plant in the world. Under optimum conditions of sunlight, nutrients and temperature, growth can reach half a metre a day, providing a renewable food source and home for up to half a million invertebrates per plant. Some, like the pink gastropod 'Cantharidus eximius' (below right) graze continually and though in some patches we found them in numbers, closer inspection revealed no visible harm to the plants.

Tassled anglerfish Rhycherus filamentosus showing "rod and lure"

HIDE AND SEEK
With a well developed “rod and lure” (top left) dangling in front of a cavernous mouth, the disruptive garb of the tasselled anglerfish 'Rhycherus filamentosus' (bottom left) seals the fate of many an unsuspecting fish. Indeed, had it not moved as we finned past it would have been dismissed as a clump of seaweed. Being one of the strongest swimmers in its family, luck was on our side when it quickly settled down for a photo session, and as long as we stayed outside its “critical distance zone”, it seemed quite convinced we couldn’t see it. Perhaps not so confident of its camouflage, Johnston’s weedfish ‘Heteroclinus johnstoni’ is another cryptic species. Giving birth to live young, this variety is one of the largest in the family.

LYING AT THE CONFLUENCE OF several biogeographical regions, Tasmania offers a diversity of marine life that is unsurpassed in the world's temperate seas. Off the island's southern shores, cold water drawn to the surface by passing currents supports species usually confined to greater depths.

IN REMOTE AREAS OF the south-east, an extraordinary collection of endemic species occur, while in the north, the more familiar species from New Zealand, New South Wales and the southern coast of the Australian continent are to be found.

BUT BY FAR THE most striking feature of Tasmania's waters are the luxuriant stands of giant kelp 'Macrocystis pyrifera' (above left) that fringe the coastal shallows, especially the Tasman Peninsula, like corridors of submarine forest. Held buoyant by gas-filled bladders, ranks of these outsized seaweeds sprout a dense carpet of fronds that rest on the surface, washing back and forth on the swell.

BELOW, BENEATH THE sun-drenched canopy, lies a world of shadows and dappled light, the stage for a million mini-dramas of life and death. Only on a passport supplied by bottled air can one appreciate the complexity of this hidden microcosm. All my years as an underwater photographer had not prepared me for the ghostly beauty and proliferation of life to be found in these submarine jungles. Sea urchins, anemones, gastropods, shrimp, fish and an "encyclopaedia" of smaller organisms on, and amongst, the kelp. Masters of disguise, weedy sea dragons and crested weedfish, now you see them, now you don't, float through the liquid tapestry.

Tasmania's colourful temperate reefs.

GROWTH INDUSTRY
Despite its formidable looking claws, this king crab ‘Pseudocarcinus gigas’ (left) showed little aggression when we approached it. At 15 kilograms it is one of the world’s largest crabs and has recently become the subject of a newly developing fishery. Generally a deepwater species, in the past most specimens were taken as by-catch in lobster pots or brought up in trawls, but now they are being targeted with robust, specifically designed traps. Even though knowledge of its biology is scant, management strategies, including restrictions on size and reproductive condition, have already been implemented to stop short-term exploitation.

Yellow sea spiders Pseudopallene ambigua.

ANOTHER CREEPY CRAWLY from Tasmanian waters, and one with significantly less commercial potential, is the yellow sea spider ‘Pseudopallene ambigua’. Often living on their food source, such as this bryozoan, sea spiders incubate their eggs attached to special carrying legs, a duty performed by the male. Sea spiders are no more closely related to terrestrial spiders than the swimming sea anemone 'Phlyctenactis tuberculosa’. Moving from place to place by releasing its adhesive pad and rolling with the current, it is commonly found attached to kelp and seagrasses, an unusual preference for an anemone.

Tasmania's temperate reefs - colourful whip and sponge gardens.

LIGHTS...AND ACTION
At 35 metres down absence of light releases sponges and other filter feeders from competition with algae. Off Bicheno, on Tasmania’s east coast, we were rewarded with some of Australia’s most spectacular and diverse invertebrate gardens. Our powerful underwater lights transformed a dull, grey bottom into a magical world where huge finger sponges thrust skyward like clusters of colourful smokestacks. The sea whips, zoanthids and sponges pictured here represent a small sample of the profusion of shapes, patterns and colours on display.

MINI KILLER
This flashing display put on by a southern blue-ringed octopus 'Hapalachlaena maculosa' is not for aesthetic purposes. A bite from this diminutive (12cm) creature can kill an adult in less than 15 minutes. This specimen was uncovered when we overturned a small rock to see what was sheltering beneath. Obligingly, it gave a display of its graceful swimming action, allowing us to snap several shots which clearly show its means of jet propulsion through the siphon nozzle. But when it attached itself to my regulator hose I began to wonder how badly I really needed a set of action shots!

Text: Gary Bell
Photography: Gary Bell