Seahorses in the Bahamas
In the Bahamas, one mysterious, landlocked pool is home to a little-known population of seahorses taking its own evolutionary route.
We visit the top-secret, land-locked lagoon that’s home to a treasure trove of seahorses uniquely adapted to their environment
On one of the islands of the Bahamas, in the Caribbean Sea, there’s a large, landlocked pool that holds some remarkable secrets. For a long time, local islanders have told stories of monsters that live there, a giant octopus and turtle, and fish that sing. But within the pool’s shadowy waters lives a collection of real animals far more fantastical than any imagined beasts. Hiding in seaweed gardens are hordes of crabs, brittlestars and octopuses (normalsized ones). Most incredible of all, the pool is home to the world’s largest known population of seahorses.
Masters of camouflage
“At first you may not think there’s anything in there,” says Shane Gross, an underwater photographer who has visited the pool 40 or 50 times. With their heads hunched down and colours matching their surroundings, the seahorses look a lot like clams or mussels. When he brings friends to snorkel in the pool, they can spend half an hour searching in vain for seahorses. As soon as he points one out and they know what to look for, they realise there are seahorses everywhere. “They’ll spot one every minute or two after that,” he says. Nowhere else in the world will you have a better chance of spotting one of these curious little fish.
Seahorse biologist Heather Masonjones, of the University of Tampa in Florida, has been studying the pool for the last five years. Before that, she mainly focused on seahorses in the Gulf of Mexico and right on her doorstep in Tampa Bay. Then she got a tip-off from staff at the Bahamas National Trust about a little-surveyed pool on one of the archipelago’s islands that they thought might be home to seahorses. Heather had no idea that she would find so many there. “It’s just remarkable,” she says.
The pool is technically known as an ‘anchialine’ pool. The water is salty – connected to the open sea through porous limestone rock – and the tide gently rises and falls. It is just over 1.5km wide, about 2.5km from end to end and 13m at its deepest. Counting how many seahorses live there has not been an easy task.
‘Mark and recapture’ is a classic technique for estimating animal numbers and often works well for seahorses. The procedure involves carefully catching the creatures, injecting a dot of harmless coloured dye under their skin, then letting them go again. Repeat visits to the area will yield an estimate of how many animals come and go, and an idea of the total population size. But the pool’s seahorses seem to be
“The more we look at this system, the more it doesn’t fit with anything we understand.”
much more mobile and active than normal, and Heather has had some trouble finding marked seahorses again. “The more we look at this system,” she says, “the more it doesn’t fit with anything that we understand about these animals.”
While Heather has yet to come up with a definitive tally of the pool’s seahorses, the density is an order of magnitude greater than the global average for known seahorse habitats. The pool’s precise location is not widely known and Heather is keen to keep it that way, at least for now.
How seahorses originally arrived in the pool remains a mystery. Perhaps a few young seahorses wandered through the Swiss-cheese matrix of limestone rock. Or perhaps somebody put them there. There is a long history of people in the Bahamas using natural pools for informal aquaculture, stocking them with fish to grow bigger and multiply.
Another great puzzle surrounds the true identity of these seahorses. When Heather first visited the pool, she wasn’t sure which species they belonged to. There are two large Atlantic species in the Bahamas: slender seahorses, Hippocampus reidi, which have long, slim heads and bodies; and lined seahorses, H. erectus, which have shorter heads, deeper bodies and stragglier spines. Both can grow to at least 17cm from head to tail, and neither is a close match to the ones living in the pool.
“These animals look like the two species put together,” says Heather. She began to wonder if the species were interbreeding – lined and slender seahorses readily mate with each other in aquariums. To find out if the pool contains the world’s first known wild seahorse hybrids, her colleagues, Emily Rose of the University of Tampa and Adam Jones from the University of Idaho, sequenced two of the seahorses’ genes. The results showed that these are in fact lined seahorses, but clearly unlike any others.
Living in their isolated pool, the seahorses seem to be heading down their own evolutionary pathway, in a similar way to animals on remote islands. These secluded fish could be adapting to the unusual environment of the pool, where conditions differ from the surrounding seas. For one thing, their tails are shorter than normal. “The tip of the tail almost looks like a pinky finger,” says photographer Shane. In the still waters of the pool, where currents are weaker, perhaps the resident seahorses don’t have to hang on so tightly.
They also have longer, more slender heads than their open-seas cousins. This may have something to do with the way seahorses feed and the types of food available in the pool. These fish eat tiny planktonic animals in a process known as pipette feeding. With their swivelling eyes, they spot a target, slowly line up their head within striking range, then rapidly flick their snout upwards, sucking in the unsuspecting prey before it escapes. A seahorse with a longer snout may reach further and target faster-moving prey. For now, that remains another mystery. Heather and colleagues are conducting more studies to determine whether this really is speciation in action.
Closely tied to the scientific research are major efforts to protect this unique ecosystem. For the last few years, staff at the Bahamas National Trust have been working towards adding a suite of new Marine Protected Areas to the 32 national parks they currently manage. When Heather uncovered the abundant seahorses, the pool became a top priority. The trust plans to call it the Seahorse National Park. “It’s of huge national and international ecological importance,” says the trust’s executive director, Eric Carey.
Being so isolated and cut off puts the pool and its seahorses at particular risk. Agriculture is encroaching around the pool’s edges, raising chances of pollution from fertilisers and pesticides. Without the frequent exchange of clean sea water from the ocean, pollutants could build up and impact the resident animals, which can’t easily get away from the pool.
Another threat is the possibility the pool will be turned into a marina. “A proposal like that would be a catastrophe,” says Eric. The Bahamas lacks protected harbours and there are plenty of people willing to pay good money for somewhere to moor up.
As well as plans for the Seahorse National Park, the trust is recommending that the Bahamian government add seahorses to the national list of protected species, making it illegal to catch and sell them.
There have been a few incidences of people taking seahorses from the pool.
Already, there have been a few incidences of people taking seahorses from the pool. The trust is also exploring the possibility of the pool becoming a UNESCO World Heritage Site. “We’ve looked at the requirements for designation,” says Eric, “and we’re confident the area would qualify.”
Raising local awareness is a vital part of the ongoing conservation efforts, with particular attention on the younger generation. Heather and staff from the Bahamas National Trust have been visiting most of the islands’ schools to tell children about the seahorses living in their own backyard. “They’re learning that animals they thought were a myth do exist, and in good numbers on their island,” she says. “It’s their own little secret.”
A sense of community pride is growing and there are people beginning to emerge who may play a role in the pool’s future. “We’re finding champions for conservation,” says Eric, “people who I think will become stewards of this area.”
Eric and his colleagues are also pushing for buy-in at a national level. There are plans to take senior government officials and decision-makers to the pool, to meet scientists and get first-hand experience of why it matters. The hope is that the park will be approved by the government as soon as possible, ideally by the end of 2019. “Having the area designated as a national park will mean we can begin to effectively manage access to the pool and reduce the threats,” says Eric.
Simple rules will help keep the ecosystem healthy. Swimmers should not use insect repellents or sunscreens that may harm the pool’s rich invertebrate life, including corals. Obviously, people shouldn’t touch or pester the seahorses. “We assume they are monogamous like other seahorse species,” says Heather. Disturbing them could split up couples and interrupt reproduction.
As word gets out, there’s no doubt more visitors will come to the pool to snorkel with the seahorses, hike through the lush surrounding vegetation and explore one of the largest mapped caves in the Bahamas. “Sunset and sunrise are hell because the mosquitos and sand flies will eat you alive,” says Shane. “Other than that, it’s a peaceful, calm, beautiful place.”
FIND OUT MORE
Read about marine and seahorse conservation: projectseahorse.org
A secluded lagoon shelters a large population of lined seahorses cut off from the sea. Their longer snouts and shorter tails may be adaptations to this particular environment.
Clockwise from left: tails act as anchors; lined seahorses are often found within coral; the landlocked pool; dye injections help with counts.
Clockwise from left: distinctive white barred lines on the neck are typical of lined seahorses; research lead Heather Masonjones urges divers to look but not touch; the seahorses blend in with the patterning of Atlantic pearl oysters; channel clinging crabs have been observed eating seahorses but are most likely to be scavenging.