A unique species of seahorse

A On the island of Eleuthera in The Bahamas, in the Caribbean Sea, there is a special pool of water. Its location is a bit of a hidden secret and local people are keen to keep it that way, at least for now. Known locally as Sweetings Pond, this stretch of water is 1.5km wide, 2.5km from end to end and a mere 13m in depth. Within its shadowy waters lives a collection of sea creatures including crabs and octopuses. But the most incredible of all, this little-surveyed ecosystem is home to the world’s largest known population of seahorses.

“At first you may not think there’s anything there,” says Shane Goss, an underwater photographer who knows the pool well. With their heads and trunks and colours matching their surroundings, seahorses are masters of disguise, looking a lot like clams or mussels. When Goss brings friends to snorkel in the pool, they can spend ages searching in vain for seahorses. But as soon as he points one out and they know what to look for, they realise the creatures are everywhere. ‘They’ll spot one every minute or two after that,’ he says. Nowhere in the world will you have a better chance of spotting these curious little fish.

B Counting how many seahorses live in the pool is not 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 provide an estimate of how many animals come and go, and a rough idea of the total population size.

Using this same method, seahorse biologist Dr Heather Masonjones estimated that there were many thousands of seahorses inhabiting the pool. Because of their particularly active nature, she experienced some trouble finding the marked seahorses again. While she has yet to come up with a definitive number of seahorses, she is confident that the pool is home to the most densely populated community of seahorses in the world. This is likely because the pool is in such a secluded setting. And this relative isolation has resulted in a lack of predators, meaning that young seahorses have a greater chance of reaching adulthood.

C How seahorses originally arrived in the pool remains a mystery. It seems incredible that these sea creatures could not only survive, but thrive, in a land-locked pool. Sweetings Pond is technically known as an ‘anchialine’ pool. The water is salty as the pool is connected to the open sea by porous rock and it may be that a few young seahorses managed to swim through these holes in the rock separating the two bodies of water. Or it could be the case that somebody put them there. There is a long history of people in the Bahamas using natural pools for informal aquaculture, perhaps stocking them with fish to grow bigger and multiply. One other theory is that they were left behind by falling sea-levels in the past. All that we know for sure is that they are there now, and in very large numbers.

Solving that great puzzle surrounds the true identity of the seahorses. When Dr Masonjones first visited the pond she was not sure what species lived there. There are two large Atlantic species in the Bahamas: Hippocampus reidi, which have long slim heads and bodies; and lined seahorses, H. erectus, which have shorter heads and straighter 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. Dr Masonjones first thought that was these seahorses looked like the two species put together. She began to wonder if the species were interbred, but genetic studies which involved sequencing two of the seahorses’ genes showed that these are in fact lined seahorses, but unlike others of that species.

D Living in their isolated pool, these seahorses seem to be heading down their own evolutionary path, in a similar way to animals on remote islands. These secluded creatures could be adjusting to the unusual environment of the pool, where conditions differ from the surrounding areas. For one thing, their tails are shorter and more stubbier than normal. This is likely because their self-contained environment lacks the currents of the open ocean and so it is much easier to swim in – this would also explain their active and highly mobile lifestyle. 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 type of food available in the pool. They are not equipped with teeth so use a process known as ‘pipette feeding’ to eat tiny shrimps and plankton. 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 prey before it has a chance to escape into the golden beds of algae.

E Being so cut off puts the pool and its seahorses at particular risk. In recent years, there have been incidences of people deliberately removing them from the pool and the risk of that continuing is high because these creatures are unique and therefore very valuable. Another threat is the possibility that the pool could be turned into a marina – the Bahamas lacks protected harbours and there are plenty of people willing to pay good money for somewhere to moor up their boats. Seahorses are extremely vulnerable to these kinds of physical disturbances as they rely on one mating partner over their lifetime and when they are separated from their life partner, it can seriously affect their ability to sustain the population. Agricultural practices in the area are also moving increasingly close to the pool’s borders, bringing real fears that chemical run-off from pesticides and fertilisers could contaminate the water quality.

F Raising local awareness of these threats is a vital part of conservation efforts to protect this unique ecosystem. And thankfully a sense of community pride is growing and there are people beginning to emerge who may play an important role in the pool’s future. There are plans to take government officials and decision makers to the pool to meet scientists so that they get first-hand experience of why effectively managing access to the pool and reducing the threats matters. If this proves successful, then these special creatures will stay safe and sound, even if they are receiving more attention than they would normally like.