HIDDEN REALMS
A stunning close-up look at some of our coral reefs’ more diminutive denizens.
CRUISING IN SCUBA GEAR 20m beneath the ocean’s surface, examining clump after clump of fist-sized ‘reef bursts’, as I call them, is a meditative pursuit. These tiny patches of habitat on coral reef walls, on the outermost seaward slopes of reefs, include sea squirts, hydroids and miniature pink tubular sponges. After half an hour of searching in these waters off the Indonesian island of Seram, I spot a pair of comic googly eyes peering up at me from a kaleidoscopic melange of reef organisms. This is Lynne’s pipefish – cocktail-stick sized and associated only with clumps of particular sponges, which it mimics in colour and texture. It’s just one of many remarkable specialist coral reef species discovered recently.
MANY THOUSANDS OF species are supported by coral reefs – Earth’s most biodiverse marine ecosystems. Some are conspicuous and easily recognisable, including well-known turtles, lobsters, corals, sharks, sponges and stingrays. But look more closely, scrutinise every detail and a new layer of life appears. Tiny eyes peer from within sea squirts and unexpected flashes move deep inside giant clams. Many of these creatures are very precise about where they live, and rather small. Most would comfortably perch on a 5c coin. A huge number of these habitat specialists have been discovered in recent years and little is known about many of them. But, as a group, they contribute hugely to the mindboggling biodiversity of reefs.
The calcium carbonate skeletal structures that form the basis of coral reefs are built by many millions of tiny individual coral animals called polyps, via a close relationship they have with algae living within them (see AG 142). This symbiosis has built colossal reefs in nutrient-poor, clear tropical waters where you would hardly expect such a profusion of life. It isn’t the only intimate connection between different species on coral reefs. Other kinds of symbiosis are also widespread. Many species survive with another in an association biologists call commensalism, during which neither is harmed nor helped.
Then there are organisms that coexist in a long-lasting, close relationship known as mutualism, from which each gains benefit. Clownfish are perhaps the best known of these particular specialists. In the Disney animation Finding Nemo, several artistic liberties were taken, notably ignoring the transition Nemo’s
father would have undergone from male to female following the death of Nemo’s mother. And Nemo would never have strayed more than a metre or so from his anemone home for fear of predators and losing his place in the social hierarchy.
Clownfish live exclusively in close proximity to just one anemone or a cluster of these large, disc-shaped jellyfish relatives that have stinging tentacles for protection from predators. The fish living within them are covered in a mucus that prevents the anemone’s stinging harpoons from firing. It’s a mutualistic relationship from which both fish and anemone benefit. The clownfish have a safe haven from predators. The anemone benefits from the waste products of the fish, and also profits from their pugnacious nature: clownfish will fend off any of the reef ’s hardier fishes that might try to eat the anemone and go to great lengths to fulfil their commitment against even larger animals.
There are many other examples of symbiosis on coral reefs, including Coleman’s shrimp living on fire urchins and the stunningly camouflaged leopard anemone shrimp. My research focuses on tiny, well-camouflaged species of pygmy seahorse. This group was only discovered relatively recently. In 1969 a researcher in New Caledonia was collecting a sea fan for a museum in Nouméa. Upon close inspection, he noticed two tiny fish clinging to the sea fan’s surface. The species was later named in his honour as Bargibant’s pygmy seahorse. No bigger than a $1 coin, it’s not surprising it eluded discovery for so long.
It’s easy to forget we’re among the f irst generations to have had the freedom to explore the world’s shallow seas using modern scuba.
Since the turn of the millennium, a further seven species of pygmy seahorse have been discovered. In 2018, along with a couple of colleagues from IUCN Seahorse, Pipefish and Seadragon Specialist Group, I named the recently discovered pygmy seahorse species Hippocampus japapigu, which survives in waters off Japan. In May 2020 I published the description of the first pygmy seahorse from the Indian Ocean, H. nalu.
My research on the population biology of South-East Asia’s pygmy seahorses was the first of its kind. I studied Bargibant’s and another species that lives on the surface of sea fans and found them to be much rarer than their larger relatives, which are more relaxed regarding habitat choice. This is a typical trend and, in fact, highlights the precarious nature of habitat specialists on reefs. Bargibant’s is found only on the surface of sea fans in the genus Muricella. So far, I have recorded the other species I studied, Denise’s pygmy seahorse, living on 10 different kinds of sea fan. In terms of conservation, the risk of extinction is greater for the extreme habitat specialist Bargibant’s, compared with the more cosmopolitan Denise’s. If the Muricella sea fan homes of Bargibant’s were to die out from a disease, for example, the seahorse would likely follow suit. Denise’s pygmies, on the other hand, could seek refuge on one of their other potential hosts. Worryingly, certain sea fans in the Mediterranean have suffered mass die-offs due to warming waters resulting in increased susceptibility to disease.
IT’S EASY TO FORGET we’re among the first generations to have had the freedom to explore the world’s shallow seas using modern scuba. Due to the sad decline already in larger reef organisms, particularly in Asia, divers have begun focusing at the smaller scale. One positive result is that recreational diving enthusiasts and citizen scientists have been responsible for the discovery of many new habitat specialists during recent decades.
Among the most amazing examples I’ve seen is a tiny species of porcelain crab. This yet-to-be-named pea-sized crustacean lives only among the tentacles of just one species of anemone that sticks only to the dead shell of a snail inhabited by a nocturnal hermit crab! It wasn’t until friends of mine, naturalists and conservationists Ned and Anna DeLoach, discovered this gem that it was realised it actually isn’t uncommon – often hiding in plain sight. Looking back over old photos I realised I’d photographed the crab years earlier but hadn’t noticed it in my pictures.
Coral reefs face an increasing threat from climatic warming, which causes bleaching. As well as corals and other conspicuous reef species that may disappear after a mass bleaching event, there are also many organisms whose disappearance goes almost unnoticed. Consider, for example, diminutive coral hermit crabs and Christmas tree worms sheltering in holes on coral heads, filtering passing water for food with specially adapted feather-like appendages. Inch-long pink-eyed gobies, which need healthy coral colonies to lay their eggs, also vanish. There are, of course, many specialised species that eat coral polyps, such as butterflyfish and the harlequin filefish, which also disappear quickly without their food source when corals die en masse.
Much like hard corals, anemones suffer bleaching, because they too have symbiotic algae in their tissues. When anemones bleach, the fish inhabiting them also suffer. Many die and what survivors there are produce considerably fewer offspring. While some clownfish can inhabit a variety of anemones, others are restricted to just a single species, which can put them in a precarious position.
McCulloch’s anemonefish, an Australian endemic, is an example. Found only off Lord Howe Island and a couple of remote Tasman Sea reefs, it lives with one species of anemone and has a very small geographic range, which means it also has a naturally small population size. Together, these factors put the species at an even higher extinction risk compared with less picky and more widespread species.