A territorial dispute breaks out between two ferocious stallions. They approach one another with determined focus, their bodies quivering in anticipation of combat. Suddenly, the larger of the two makes the first move and a violent head-butting competition ensues, carried out until a winner is declared, his territory now secure for the upcoming breeding season.
These stallions are not the wild horses of the Mongolian steppe, but rather some of the smallest vertebrates to be found in the animal kingdom and they live, hidden within soft corals and seaweed, under the surface of the sea – the pygmy seahorses.
Pygmy seahorses are an extraordinary group of fishes that have captured the imagination of marine biologists and divers around the world. From the tip of their snouts to the tip of their curly tails, the seven species currently known to science occupy a size range between 14mm and 27mm, so finding these diminutive denizens is no mean feat.
As if their tiny size wasn’t enough to protect them from predatory attention, they’re also supremely camouflaged – so much so, that it wasn’t until 1969 that the first species was discovered, by George Bargibant, a New Caledonian scientist collecting sea fans for a museum in Nouméa.
While handling a sea fan specimen, Bargibant came upon a pair of these creatures hidden within its branches. Formally named Bargibant’s pygmy seahorse (Hippocampus bargibanti), it is now known to be a widely distributed species that inhabits gorgonian soft corals from the Philippines to Australia’s Great Barrier Reef.
It is within the stinging tentacles of their soft coral host that these seahorses spend their entire lives. They prey upon minute crustaceans carried by passing water currents, sucking them up using tubular, flute-like mouths.
Unlike their relatives in the seahorse and pipefish family, Syngnathidae, the true pygmy seahorses have only one gill opening on the back of the head rather than two, and the males, rather than brooding their eggs in a pouch on the tail, use a pouch that opens on the underside of their belly. The female passes her unfertilised eggs across to the male, who then broods them for up to a fortnight inside his protective, oxygen-rich trunk. Once hatched, the young pygmies are squirted into the water column, to be carried far from their natal reef on a quest for new territories.
The planktonic phase of the larval pygmy seahorse is a dangerous one, as many predators prowl this shelterless realm. Bargibant’s distribution maps clearly indicate that some of the clutches’ three dozen or so youngsters do, however, make it through this vulnerable stage to continue the circle of life on their own soft coral host.
Some species show a much more restricted range, such as the Walea soft coral pygmy seahorse (Hippocampus waleananus), found only within central Sulawesi’s Tomini Gulf, Indonesia. The tail of this species is much longer than that of its cousins, as it has to wrap itself around much thicker branches of soft coral.
It’s clear that these tiny creatures are nowhere near numerous and great care should be taken to ensure the protection of their habitats. They may be small, but they are outstanding, charismatic members of the marine community!