Knysna’s endangered seahorses find a safe island stable
● It’s 7cm long, 40-million years old, and loves dancing in the morning.
Now scientists have discovered yet another curious characteristic of the Knysna seahorse — it prefers a man-made “mattress” to the seabed.
A new study of the endangered Hippocampus capensis, found only in three Garden Route estuaries, has found that the species is thriving inside the stony canal walls of a residential marina.
By spying on the seahorses with underwater cameras, a research team from Rhodes University showed that the creatures prefer the Reno Mattresses, used to construct marina canals at Thesen Islands, to their natural seagrass habitat.
“Reno Mattresses have been found to supplement and enhance the available habitat for this species, something that is important seeing that their natural habitat is being threatened,” said lead researcher Louw Claassens.
The Thesen Islands development was built about 20 years ago on marshy land once home to a timber mill. Seahorses are often seen swimming beneath jetties or clinging to the canal walls with their spiral tails.
While the success of the new habitat is beyond doubt, researchers are less clear on the reason why.
“I would suspect that a Reno Mattress provides stable and sturdy holdfasts for seahorses [whereas seagrass is quite variable],” said Claassens, a member of the International Union for the Conservation of Nature Seahorse, Pipefish & Seadragon Specialist Group.
“The Reno Mattress also provides nice crevices for them to hide in and protect themselves. Without a robust experimental approach the reason behind this choice is a bit tricky to figure out.”
The “mattresses” are steel-mesh cages filled with stones, commonly used to prevent coastal erosion or to reinforce canals. They were an essential building block in the development of 25ha of tidal waterways of the Thesen Islands development, which has 19 man-made islands linked by 21 bridges.
The seahorses’ unusual accommodation has implications for conservation and maintenance. Researchers believe Reno Mattresses could be used to substitute seagrass, particularly where the grass is under threat from coastal development.
“We essentially found that the Reno Mattresses provide a suitable, stable habitat for this endangered species, in addition to its natural vegetation habitat in the larger estuary,” said Claassens.
She said research into the link between the Reno Mattresses dated back several years and involved international co-operation. “This aspect has been growing as a conservation approach for seahorses, particularly in areas where habitat has been lost.
“One of the biggest threats to seahorses globally is habitat loss and alteration [in addition to harvesting and bycatch]. For the Knysna seahorse specifically, its natural seagrass habitat is being threatened by eutrophication of the Knysna estuary [an increase in nutrients]. This increase in nutrients causes nuisance algal blooms, which smothers and displaces the natural seagrass.”
Researchers have made several recent seahorse discoveries, largely thanks to underwater cameras and fluorescent tags.
“The cameras’ work has been quite cool. We found that these guys display morning greetings, similar to other seahorse species,” said Claassens.
“The morning greeting is when a pair of seahorses come together and intertwine their tails and do a little dance. They move up and down in the water column. This is to confirm the pair bond during the breeding season.”
Claassens was part of an international research team that recently discovered a new pygmy seahorse species, Hippocampus nalu, in Sodwana Bay. It is 2.7cm long.
The Knysna seahorse is one of the world’s most endangered of the 46 known seahorse species because of its restricted habitat — three southern Cape estuaries.
It has gills like other fish, a distinctive armoured body and horse-shaped head, and its two eyes move independently, like a chameleon. It is SA’s only endemic seahorse species.
Said Claassens: “Research on seahorses in SA is not that well developed and it is exciting to have this new focus on these amazing animals.”