Sweetings Pond—Seahorses and More

Hatchet Bay, Eleuthera, Bahamas N25° 21.917′ W76° 31.233′ Depth 0-7ft.

Some friends of ours told us to check this out if we ended up at Hatchet Bay, which we already had planned to do. It is about a 2 mile hike from the government dinghy dock. Head northwest along Queen’s Highway and the access road will be on the left across from an area where land has been cleared. Some maps may show a road from the cave further north but we found this road to be impassable by car or foot. At the end of the road is the ruins of a landing and easy access to the water.

The landing at Sweeting’s Pond has easy access to the water and the road is good enough for a car to drive all the way down.

We made this one of our last stops of an all day road trip. I was told that the best time to snorkel is near high tide. The water is shallow and the bottom silty so higher water will make it easier to keep your feet off the bottom. The pond is protected on all sides so it is doable in all kinds of wind conditions, however it is a large pond and winds from the north or northwest can stir up waves making visibility poor.

Sweetings Pond is one of many anchialine ponds, land-locked saltwater ponds that can be found all over the Bahamas and Caribbean (another anchialine pond is featured in the Flamingo Cay post. These ponds have a connection to the ocean and rise and fall with the tides. Hatchet Bay was one of these ponds until the entrance was cut to make it a harbor. Each pond is an isolated microcosm with its own ecology and set of organisms. Animals and plants here are selected for and evolve apart from the open ocean; and there is the potential for new species to develop.

It is believed that Sweetings Pond has been closed off from the ocean for about 6000 years.  Seawater is thought to come in through the porous limestone rock, not through a large opening therefore there is little or no recruitment of animals from the outside.  Dr. Heather Masonjones, a researcher who has studied this pond for several years, has found few large predatory fish.  The behavior of several of the pond’s inhabitants is very different than the same species that live in the ocean and may be due to the lack of predators.  High densities of some animals, an octopus, a brittlestar, and the seahorses, and lower diversity have been observed compared to the ocean outside the pond.

I have caught seahorses and kept them in an aquarium but had never been able to observe them in their own habitat.  When I was told that I could find seahorses here I had to try it.

So get in at the sea wall and take a look at this unique environment.  After you get in float on the surface and try not to stir up the sediment.

The rocks are covered with algae–good seahorse habitat!  A couple of needlefish are swimming near the surface.

The rock seawall is covered with brightly colored animals and algae.

Animals and algae on the rock wall.  #1-Bright red Rough Fileclams (Ctenoides scabra) are relatives of the scallop.  Long tentacles stream out the gape of their bivalve shells. The back end of the clam attaches to the rock with strong bissel threads. These clams filter the water for food with their gills, which can be seen on the animal by the #1.   Above and right of #2-Mangrove Tunicate (Ecteinascidia turbinata).  Green ball to the right of #3-A compound tunicate.  #2 and 3 will be explained in the following closeups.  In between are clumps of red fuzzy algae.
Close-up of the Mangrove Tunicate (Ecteinascidia turbinata) #2 above.  This is an aggregation of individual tunicate animals, each looks like an elongated sack with an incurrent and excurrent opening which you can see in the picture.  Water flows into the tunicate from one opening where food is filtered out with a sieve-like organ.  The filtered water exits the other opening along with waste.  Tunicates are considered more highly evolved (and closer to you) than a clam or worm, since the larvae have a notochord (a pre-backbone that all vertebrates have at some point during their development).  The larvae, called tadpole larvae, look like their namesake and swim around in the plankton until it is time for them to settle down.  At this time the tadpole attaches to the substrate it has chosen with suction cups on its head then proceeds to metamorphose, transforming its notochord, eyes, brain, and tail into parts more compatible with the sedentary life it will lead.
Close-up of the compound tunicate to the right of #3.  The tunicate body plan comes in three types. The type is specific to each species.  They can be solitary, aggregated, like the Mangrove Tunicate, or compound like this one.  All tunicate bodies are covered by a tunic, a rubbery or gelatinous matrix made of tunicin, a form of cellulose.   In compound tunicates, the tiny individuals are embedded in a common tunic and share a common excurrent opening.  In this one the incurrent openings are clustered around a central excurrent opening.  Compound tunicates can easily be mistaken for sponges and sometimes it takes me a long look to discern them.
Red fileclams, blue and brown sponges, and green tunicates splashed color in an otherwise muddy environment.
Up under the rock overhang we found several of these large crabs which are probably the Hairy Clinging Crab (Mithrax pilosus).  Several colonies of the compound tunicate are below the crab.
The Fringeback Dondice (Dondice occidentalis) was abundant.  This is a nudibranch, sometimes called a sea slug.  The nudibranchs are snails that have no shell.  The long finger-like projections on the back are the gills (therefore nudi=naked  branch=gill).  The head (left) has long tentacles and shorter rhynophores, both of which are sensory.  The ones I saw were 1-2″ long and do not get much bigger than that.
Every bit of hard substrate is prime real estate for sessile organisms (those that are attached and stay put).  This dead branch was home for oysters (large bivalves), mussels (small bluish ones), and barnacles (light colored clusters).

I was so distracted by other forms of life that I was not sure I would find a seahorse but finally I spotted one.  The large male had spotted me first and flattened himself against the algae trying to look inconspicuous.

Lined Seahorse (Hippocampus erectus).  This one was about 5-6″ from head to tail and he had a very swollen brood pouch.  He was obviously pregnant.  Yes, the female lays her eggs into the male’s pouch where they are fertilized and brooded until developed.  They are born as perfect tiny seahorses about 1/4″ long.  It is not unusual, in the world of fish, for the male to be the caretaker but usually not to this extreme.
Seahorses spend much of their time hanging on to the algae with their prehensile tails but they can swim slowly and gracefully by sculling with the dorsal (back) fin.  The arrow points to a small shrimp, likely the seahorse’s food.  A seahorse carefully observes its food then places the end of its snout very near.  It then quickly expands its gill plates (right behind the eyes) creating a large suction through the tubular snout.  With a popping sound, the shrimp disappears into the mouth.  This one is also a male who’s brood pouch is not as full as the previous one.
I found this one hanging upside down, probably trying to decide if something in that hole is good to eat.  Seahorses have a very short digestive tract and have to eat a lot to keep healthy.
This is the only female I saw.  The female’s belly turns abruptly into the tail.  The brood pouch of the male makes his belly slope gently toward the tail.

Until genetic work was done, there was some confusion about what species the Sweetings Pond seahorses belonged to.  There are two species of large seahorse found in the Bahamas and some individuals in Sweetings pond had longer snouts and tails than normal for the species.  Genetic analysis found them all to be H. erectus and not a hybrid or new species.  Isolated populations will often develop unique traits over time.

At Sweetings Pond conditions are perfect for seahorses, for which the pond is quickly becoming famous.  Unfortunately, with fame, comes the potential for harm to this delicate and unique system that supports seahorses and other forms of life.  Seahorses are fished for decorations and are believed to have medicinal value in some cultures.  Like many other ocean organisms, habitat degradation and coastal development leads to decreased numbers.  There has been talk of opening Sweetings Pond and making a harbor and marina like was done to Hatchet Bay.

I considered not posting about this site.  Part of me wishes it could remain hidden.  However, it is known and the conversation about progress vs protection is already happening.  My hope is that I might foster some appreciation and awareness of this special place.  If you visit it, please be respectful and careful.

Some information for this post was obtained from a lecture given by Dr. Heather Masonjones entitled “One Species, Two Species, Hybrid Species, New Species: Secrets of the Sweetings Pond Seahorse.”   The lecture was part of The Evenings at Whitney Lecture Series hosted by the University of Florida Whitney Laboratory on April 20, 2017, at 7 p.m.  You can listen to the lecture at http://www.whitney.ufl.edu/next-evenings-whitney-april-20-one-species-two-species-hybrid-species-new-species-secrets-sweetings-pond-seahorse/

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