The secretive seahorses hiding in a Bahamian lake

A land-locked lake in the Bahamas is home to one of the most unique and densely populated group of seahorses on the planet, but this marine marvel is also under threat from human interference.

Sweetings pond
Sweetings Pond from the air. The lake is located towards the northern end of the island of Eleuthera

Hidden away in the North of Eleuthera, one of the largest islands in The Bahamas, lies a special pool of water 1.5km wide and 2.5km long with a max depth of 13 metres, all of which is cut off from the sea by a stretch of land just a handful of metres across. It is known by locals as Sweetings Pond and in this isolated ecosystem lives a unique and very special population of seahorses, that until recently have been completely hidden from the rest of the world. I was lucky enough to visit this incredible site on a trip to Eleuthera in 2018 and have seen first-hand the wonders of this secluded setting. But this lagoon-like lake is now under threat from human activities and must be protected if its intriguing inhabitants are to survive.

Hidden in plain sight

If you go for a snorkel in search of seahorses in Sweetings Pond, at first you might think you’ve been misinformed about their existence. As you slowly survey the shallow edges of the lake staring hard at the beds of golden algae you start to think that you are completely alone, but then out of the corner of your eye you see one. A small seahorse with its short snout protruding inquisitively out of the seaweed. These tiny creatures around 10cm in length are masters of disguise, perfectly camouflaged with their surroundings, yet once you’ve found one you are suddenly able to find them all. As you take another look around you will realise that you have actually been surrounded by them the entire time. Before too long you will likely have seen more seahorses here than you have in your entire life.

Hidden
The seahorses in Sweetings Pond can be very hard to initially find for tourists and even researchers (photo by Dr Heather Masonjones)

How did they get there?

The most surprising thing about this group of seahorses is that they are even there at all. It seems almost impossible that a strictly marine species could survive, let alone thrive, in a land-locked lake. However despite being geographically isolated from the sea, Sweetings Pond is actually a lot more connected to the ocean than it first appears. It is what is known as an ‘anchialine pool’, meaning that it is actually made up of seawater that flows through the porous limestone rock in the ground separating the two bodies of water. As for how the seahorses were able to hop from one to the other, that is more of a mystery. It may be that some juveniles were one day able to swim through the limestone matrix as well, it could be that they were left behind by falling sea-levels in the past or it could even be that they were put there on purpose by an ancient generation of Bahamians. All we know now is that they are there now, and there is lots of them.

Baby boomers

Dr Heather Masonjones from the University of Tampa in Florida has been studying the seahorse population in Sweetings Pond for over 5 years. When she first arrived in Eleuthera to check out the site for herself she was blown away by just how many seahorses there were living there. Speaking to BBC wildlife magazine last year, she said “it was remarkable”. Using the classic ‘Mark and Recapture’ technique to assess numbers, by capturing a seahorse dying part of their skin and releasing them, revealed there were thousands upon thousands of them in the lake. Dr Masonjones still doesn’t know the exact number, because these seahorses are uncharacteristically mobile and move around a lot, but she is confident it is the most densely populated seahorse community anywhere on Earth. This is likely because their isolation has resulted in a lack of predators, apart from a handful of octopuses, which means there is a higher survival rate for juveniles.

Population density
To see so many seahorses in one place is a rare sight anywhere else in the world (photo by Dr Heather Masonjones)

New evolutionary path

As well as being unbelievably abundant in Sweetings Pond the seahorses also stumped Dr Masonjones for another reason, she couldn’t decide which species they belonged to. There are two seahorse species in the Atlantic that reach this comparatively large size, slender seahorses (Hippocampus reidi) and lined seahorses (Hippocampus erectus). She told BBC Wildlife magazine that “these animals look like the two species put together”. As a result her first theory was that the two species had merged by hybridization in the pool, but genetic analysis showed that this wasn’t the case. The seahorses were in fact lined seahorses, but not as we know them.

Golden
Not found anywhere else on Earth, these lined seahorses are one of a kind (photo by Dr Heather Masonjones)

The reason for their physical changes is because they have slowly adapted to their unique environment over time. Firstly their tails are much shorter and more stubby than other lined seahorses. This is likely because their self-contained environment lacks the currents of the open ocean and so it is much easier to swim in it, which also explains their active and highly mobile lifestyle. Secondly they also have elongated slender heads, although the reason for this is much less clear. “The more we look at these animals” says Dr Masonjones, “the more they don’t fit with anything we understand about them”. This all points to the idea that these seahorses are in fact on their own evolutionary path to becoming a brand new species.

Facing new threats

Like most seahorse species the lined seahorse is threatened with extinction. Every year approximately 15-20 million different seahorses are traded around the world, but it is hard to tell exactly because most are caught illegally. They are either killed and dried out to be used in traditional medicines or become part of the aquarium trade and are sold as pets. It is for this reason that this healthy self-contained population is so important for the species survival. Unfortunately researchers suspect that some seahorses have already been taken from the lake and the risk of that continuing is high because these individuals are unique and therefore very valuable.

DCIM118GOPROGOPR4978. Processed with Snapseed.
Researchers like this one know how to safely handle the seahorses, but tourists often don’t

As well as this seahorses are extremely vulnerable to physical disturbance as they rely on monogamous mating partners and when they are separated, either by clumsy tourists or poachers, it can seriously affect their ability to sustain the population. There is also a concern that agricultural practises in the area are moving increasingly close to the pool’s borders and there are fears chemical run-off may contaminate the water quality.

Protecting the pond

In years gone by the seahorses in Sweetings Pond have been protected by their anonymity. Nobody knew they were there and so they faced no immediate danger. For a long time researchers and locals managed to keep its location a secret and although an increasing number of people became aware of it, many didn’t even know which island it could be found on. However as the site has become increasingly popular with tourists and their inhabitants have grown famous in the marine biology world, everyone knows where they are now.

Therefore to protect the seahorses, the Bahamas National Trust is trying to do the complete opposite of keeping them a secret. They are in the process of designating Sweetings Pond the new ‘Seahorse National Park’, to help introduce new legislation to properly protect them. In fact they are quietly confident that they may even be able to turn it into a UNESCO World Heritage Site. If they are successful then these special seahorses will stay safe and sound, even if they are receiving more attention than they would normally like.

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