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.

BBC Wildlife Magazine - - Contents - By He­len Scales Pho­to­graphs Shane Gross

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.”

Ma­rine mar­vel

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.

Pool pro­tec­tion

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.”

Cham­pi­oning con­ser­va­tion

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.

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