The Seahorse That Isn't
AAlong the deep reefs of southern Australia lives a group of tiny fish that very few people have ever heard of. At first glance, they look exactly like seahorses. Their heads are shaped like a horse's and point downwards from the body, their tails curl round to grip objects, and the males carry their young in a closed pouch. Yet a new study has shown that these animals are not seahorses at all. Scientists have nicknamed them "sea muppets", and they belong to a genus called Australyichthys, a name built from the Latin word for "southern" and the Greek word for "fish". The discovery shows how much can still be learnt from specimens that have been sitting quietly in museum collections.
BThe most recently described member of the group is Australyichthys aegis, which has been given the common name "helmethead sea muppet". Like its relatives, it is no longer than a human thumb. It owes its name to a rounded crest on top of its head that resembles a helmet. The word aegis refers to a protective shield that appears in Greek mythology. The genus contains two other known species: the bullneck sea muppet and the paradoxical sea muppet.
CWhat makes these fish especially mysterious is that no one has ever seen one alive. Only seven specimens exist, and they were collected over a period of roughly 65 years along more than 2,500 kilometres of coastline. For an entire genus of animals, that is an extraordinarily small number of records. Every one of them was caught by accident, brought up in trawl nets or dredges that were dragged across the seabed at depths of about 50 to 125 metres. No new specimen has been found for almost twenty years.
DThe helmethead specimen has a particularly unusual history. It was collected in 2006 near the Gambier Islands, at the entrance to Spencer Gulf in South Australia, by a dredge that was actually searching for brachiopods, small animals with shells. After the expedition, the fish was mistakenly stored among the invertebrates at the South Australian Museum, where it sat unnoticed for years. The other specimens are kept in several collections, including those of the Australian Museum, Museums Victoria and the national science agency CSIRO.
EBecause the specimens were so rare, the researchers could not cut them open. Instead, they used micro-CT scanning to create three-dimensional models of the skeletons, which allowed them to compare each bone with those of true seahorses. This technique uses X-rays to produce detailed images of internal structures without harming the specimen. The differences were clear. A true seahorse has a stiff cage of joined bony plates that supports its belly, as well as a bony crown on its head known as a coronet. The sea muppets have neither of these features; instead, their bellies are held up by an arrangement of bones not found in any seahorse. DNA provided further evidence, although only the helmethead specimen could be tested. It was the only one that had been preserved in ethanol, while the others had been kept in formalin, which damages genetic material. The genetic results placed the fish outside the seahorse group, on a neighbouring branch of the family tree.
FSeahorses, sea muppets, pipefishes, pipehorses and seadragons all belong to the same family, called Syngnathidae. The new findings suggest that the seahorse body shape evolved at least twice within this family. This is an example of convergent evolution, in which animals that are not closely related develop similar features because they live in similar ways. The sea muppets come from the Great Southern Reef, a network of kelp forests and rocky reefs around the southern coast of Australia that is home to many species found nowhere else, and whose deeper parts remain largely unexplored. The scientists have labelled the fish a "lost species", but they stress that this does not mean the animals have died out. Rather, it marks them as a priority for surveys designed to look for them deliberately instead of relying on lucky catches, and the researchers believe the fish are probably still living in deep water that people seldom visit. Until such surveys take place, the sea muppets will remain known only from a handful of preserved bodies in museum jars.