Under the Jetties in the Southern Australia Seas

It is early June, the onset of winter in the Southern Hemisphere, and an army has just reached its destination. It has marched from the ocean’s depths into the shallows, amassing among the pilings at Blairgowrie Pier in Port Phillip Bay, south of Melbourne, Australia.

Above the waterline, people relax on the 925-foot-long (282-meter-long) concrete pier structure, picnicking, fishing, and pointing at sailboats. Most of them are seemingly unaware of the crustacean horde encamped 10 to 20 feet (3 to 6 meters) below the jetty. Two divers, however, excitedly chatter about spider crabs — and big-belly seahorses, giant cuttlefish, and more — while preparing their gear and donning thick wetsuits for the 54°F (12°C) water.

Miles de cangrejos araña con patas que se extienden 38 centímetros (15 pulgadas) se trepan unos sobre otros y forman montículos de hasta 10 cangrejos. Cada año se reúnen en masa de esta manera para mudar la piel. El proceso es un ejemplo extrañamente fascinante de la biología de los crustáceos donde una versión notablemente más grande del mismo cangrejo se arrastra lenta y torpemente hacia atrás para abandonar su viejo caparazón y surgir lleno de esperanza pero sumamente vulnerable. Su nuevo caparazón es peligrosamente blando hasta que se endurece y se convierte en la formidable protección acorazada que eventualmente le queda chica y abandona.

Los depredadores los están esperando. Tiburones de Port Jackson hambrientos y rayas negras de cola corta de 1,8 metros (6 pies) patrullan el muelle y programan su visita para alimentarse de tiernos cangrejos recién mudados.

Para los cangrejos araña, la supervivencia es una cuestión de números. El riesgo de ser escogido y comido es estadísticamente más bajo en una multitud de lo que sería estando por su cuenta, por lo que mudar la piel simultáneamente tiene sentido desde una perspectiva evolutiva. Los cangrejos astutos aparentemente saben que para tener la mejor oportunidad de sobrevivir, prosperar y regresar para volver a mudar la piel el año siguiente deben escapar del campo de matanza.

As soon as their wobbly legs can support their weight, the crabs crawl along the sandy bottom and climb up pilings into the shadowy safety of the pier’s underside. Here they hide and wait until their exoskeleton hardens so it once again deters predation by rays and sharks. Only then is it safe to descend to the sand, leave the jetty, and return to the deep.

Esta dramática prueba de la vida que se presenta en Planeta Azul II is one of many wonders to witness in the seas of southern Australia. Exploring the shallow, temperate coastal waters on scuba and snorkel is easier and more productive with the presence of piers such as Blairgowrie, nearby Rye and Portsea jetties, and dozens more extending from the shoreline in Victoria and neighboring South Australia. Though built to support and facilitate industrial and recreational interests such as shipping, boating, and fishing, the artificial structures also provide divers with an ideal means to enter the water by descending stairways — no need to slog through the surf and risk a tumble.

Los espigones son imanes de vida marina y se han convertido en vibrantes arrecifes artificiales. Una asombrosa variedad de invertebrados y peces, desde esponjas y estrellas de mar hasta caballitos de mar y peces vaca, han colonizado el metal, el hormigón y la madera con el paso del tiempo. En la actualidad, comunidades eclécticas de criaturas marinas llaman hogar a estos paisajes marinos únicos e híbridos.

Some species living under jetties are endemic to Australian waters. One such icon — and the official marine emblem of the state of South Australia — is the leafy seadragon, a masterfully camouflaged fish with frilly skin appendages. They headline the star attractions at the Rapid Bay and Second Valley jetties on the Fleurieu Peninsula southwest of Adelaide. Keen-eyed divers who diligently search may spot the 10- to 15-inch-long (25- to 38-centimeter-long) seahorse cousins hiding in plain sight among kelps and seagrasses growing under and alongside these and other piers.

Old wives (Enoplosus armatus) and rough bullseye sweepers (Pempheris klunzingeri) school among the pier pilings beneath Rapid Bay Jetty near Adelaide. © BRANDON COLE

Giant spider crabs (Leptomithrax gaimardii) congregate into large groups to breed. They also form huge groups of hundreds to even thousands of individuals when they aggregate in shallow water during winter to molt. © BRANDON COLE

Leafy seadragons feed mainly on small crustaceans called mysids, sucking them into their tube-snouted mouths. Parenting is a team effort: A female transfers 200 to 300 eggs to her partner, placing them under the male’s tail. After fertilization he carries the developing brood until they hatch, and then the miniature seadragons must set off bravely into the big blue.

Divers can participate in citizen science by sharing their underwater seadragon photos with organizations that catalog these stunning fish. Distinctive body markings and branching patterns of the skin appendages help biologists identify individual seadragons and track them over time. Long-term monitoring projects at Rapid Bay have revealed that some seadragons are residents, including an individual nicknamed Wishbone they have seen here repeatedly over seven years. A similar identification program exists for weedy seadragons, which are close relatives of leafies. Divers can regularly find weedies at Flinders Pier on the Mornington Peninsula’s southern coast.

The construction of a jetty forms a new underwater habitat that attracts sea life. Rapid Bay’s original pier was constructed in 1940 to facilitate loading ships that were exporting limestone mined from a nearby quarry. It shut down in 1998, but a different legacy lives on today beneath the waves.

More than 50 fish species have been recorded at this jetty, showcasing greater biodiversity and abundance than the mostly barren sand flats adjacent to the pier. Shoals of old wives and sweepers weave among the forest of pier pilings. Globefish, Shaw’s cowfish, and mosaic leatherjackets stay close to the vertical supports. Fiddler rays glide over the sand, hunting for worms, mollusks, and crustaceans, while blue weed whiting favor the seagrass beds, which are important habitats for many smaller fish and invertebrates.

Seagrass beds are critical nurseries for many species. The plants improve water quality in the area and benefit the ecosystem by functioning as a nutrient sink and the base of a detrital food web for bacteria and animals. Seagrasses are marine flowering plants capable of photosynthesis that normally grow in shallow, sheltered bays. The green plants fare well alongside the jetty because the pier’s structure reduces erosion of the soft substrate from wave action.

Nutrient-rich upwelling of waters borne on the Leeuwin and Flinders currents meet and mix at Rapid Bay Jetty, nourishing the sea life. It’s no surprise that this is a favorite site of divers along Australia’s southern coastline or that it draws people from across the globe. Talking to locals at nearby dive shops is an excellent way for visitors to gather intel about entry points, water conditions, critter locations, no-dive zones, and hazards for this jetty and others.

Las estructuras de los muelles transforman sitios de fondos blandos planos o ligeramente inclinados en hábitats tridimensionales más complejos y ecológicamente dinámicos. La estructura es el catalizador, los cimientos sobre los que se construye la abundancia de vida. Un sustrato rígido como roca, metal y hormigón es limitado bajo el agua y representa un espacio valioso. Los organismos sésiles, lo que incluye esponjas, tunicados y briozoos, necesitan un sustrato firme para que las larvas plantónicas errantes puedan establecerse fuera del agua, adherirse y crecer hasta convertirse en adultos. El ecosistema combinado de espigón y arrecife ofrece este beneficio, lo que permite que estos organismos prosperen.

Edithburgh Jetty lies across Gulf St. Vincent, opposite Rapid Bay. Because of its position on the southeastern tip of the Yorke Peninsula and shallow seafloor depths of 10 to 30 feet (3 to 9 meters), Edithburgh is exposed to strong winds and swell from the south and east. When conditions are calm, however, this is a rewarding, easy dive site for critter-watching naturalists and underwater photographers. The pier’s pilings and crossbeams are resplendent with the rich growth of encrusting invertebrates in bright orange, pink, yellow, and blue. These polychromatic assemblages comprise various filter feeders, with sponges, tunicates, and bryozoans as the most conspicuous examples.

A Tasmanian blenny (Parablennius tasmanianus) is nestled among tunicates on a pier piling. © BRANDON COLE

The ornate cowfish (Aracana ornata) is endemic to southern Australia. © BRANDON COLE

Snails, crabs, shrimps, polychaetes, sea stars, and fishes are among the clumps of filter feeders. The presence of sponges, tunicates, and bryozoans creates additional habitats. Nooks and crannies in the conjoined, living tapestry attached to the jetty’s framework offer abundant living space for other organisms. This microecosystem within the larger jetty and reef hosts its own community of animals going about their daily activities such as sheltering, feeding, reproducing, and competing.

Los buzos pueden descubrir hermosos nudibranquios, elegantes gusanos plumeros con sus tentáculos para alimentarse extendidos y una variedad de cangrejos, incluso cangrejos esponja ocultándose de los depredadores debajo de sombreros de gran tamaño hechos de tunicados. Cómicos y carismáticos blenios cornudos de Tasmania observan desde sus madrigueras, rodeados por colonias de tunicados azules (Clavelina moluccensis). Peces sapo de aspecto severo, como peces sapo de manchas blancas y enigmáticos peces sapo teselados, esperan pacientemente preparados para acechar. Vale la pena buscar con esmero en los lechos de algas justo al sur de Edithburgh Jetty con la esperanza de ver caballitos de mar cortos y peces pipa del golfo del sur colgados y deslizándose por el sargazo y las algas.

To the north, mixed seagrass and shellfish beds are excellent places for encountering a southern blue-ringed octopus. This dangerous beauty has a venomous neurotoxin to immobilize its prey, primarily crabs. Its paralytic venom is extremely potent and is strong enough to harm humans. Indeed, people have died after being bitten by this small octopus, whose tentacles usually span 8 inches (20 centimeters) or less. Fortunately, they are not aggressive and pose no risk to people admiring them from a safe distance. These octopuses often take refuge inside dead razor clams, tucking their impressively pliable bodies into the narrow opening between the clam’s shells.

No treasure-hunting expedition under Australian jetties, especially Edithburgh, is complete without poking around after the sun goes down. Although many diurnal creatures observed moving about during daytime hours will now be sleeping, the neighborhood’s nocturnal denizens are wide awake and at their most active. They leave their hiding places in the encrusting invertebrates on the pilings and under the uprooted, dead razor clam shells scattered on the bottom to take center stage beneath the pier. Others emerge from the sand where they buried themselves for the day.

La noche es el momento adecuado para encontrar cefalópodos. Una vez conté siete especies durante un buceo nocturno de dos horas en Edithburgh, incluso un pulpo de anillos azules, pulpos de arena del sur, sepias "bobtail” apareándose y un calamar de pijama rayado.

Todos son animales maravillosos, pero el calamar de pijama rayado merece especial atención. Se parece a un malvavisco con ojos y un uniforme de prisión. Las glándulas en la parte inferior de su cuerpo regordete de 7,6 centímetros (3 pulgadas) producen una mucosidad venenosa. El calamar de pijama normalmente se arrastra por la arena, pero puede nadar torpemente por la columna de agua por propulsión a chorro, para lo que succiona agua hacia la cavidad paleal y luego la expulsa con fuerza.

En las tierras australianas hay muchas cosas extraordinarias que esperan ser descubiertas debajo de los espigones.