Some feature articles on our favorite underwater friends and sights:
Top of the food chain: predator or prey? - Written by Dr Alexander Mustard / Photo by Elly Wray
We have made it to the top of the East End food chain and it gives me the excuse to tell you about probably the most popular sight in East End diving – the sleek and streamlined Caribbean reef shark. These top predators are not just favourites with divers, but they play an important role in any ecosystem culling the injured or weak, improving the overall fitness of the community and balancing the food chain from the top down. [Cayman Reef Shark]
This classic looking shark is one of the top predators on the reef, and eats mainly fish. Cayman's East End has a healthy population with a group of around 6 large females residing along the top of the wall at sites such as Lighthouse Wall, The Maze, Jack McKenny's Canyon and Pat's Wall. From time to time a few male sharks take up residence in the area and as many as 15 sharks turned up for food during the Shark Awareness program (which is no longer running). Caribbean reef sharks are generally wary of divers, and are not considered dangerous to divers who are not spear fishing.
The Caribbean reef shark is a superbly adapted marine hunter. It is armed with sharp senses to detect prey, particularly a keen sense of smell, electro-reception and an acute ability to detect low frequency sounds, such as those produced by a struggling fish. Once in range the shark strikes with bite that lasts just 383 milliseconds!
Caribbean reef sharks are found from southern Florida down to Brazil, although they are absent in many areas as a result of fishing. In general sharks do not make good fisheries simply because they have evolved a reproductive strategy that produces just a few young, because without natural predators they do not need to continually replace their population numbers. Caribbean reef sharks take many years to reach reproductive age, the females then carry their young for about 1 year before giving birth to between 4 and 6 live young, which are about 75cm long. It is a slow process and one that gives their species little chance when they come under heavy fishing pressure. Luckily at the East End of Grand Cayman, right outside OF, scuba divers have the chance to see these sharks in their rightful place at the apex of the Caribbean food chain.
"Las Tortugas" - Written by Dr Alexander Mustard / Photo by Elly Wray
When Columbus discovered the Cayman Islands in 1503 he named them Las Tortugas because everywhere he looked the waters were filled with turtles. And since then the turtle has remained strongly linked with the islands; one of the island’s most famous exports is Tortuga Rum Cake, which doesn’t contain turtles but is made with delicious Cayman rum, and even the national flag features a Green Turtle. Turtle hunting was an important industry on the islands in the 1700s-1900s, and the legacy of this trade is that turtles are no longer as common as they once were. By far the most common turtle you will find in Cayman these days is the Hawksbill.
Hawksbills are one of the smaller marine turtles and grow typically to about 3ft and 180lbs, but are often smaller. They live for about 30-50 years and it probably takes them 20 years to reach maturity. It can be quite difficult to distinguish male and female hawksbills, but the males can have slightly brighter colours and longer claws and a thicker and longer tail. Their main predators are humans and large sharks, such as the Tiger Shark.
Very young Hawksbill turtles feed on plankton and algae, but as they grow up they move into reef habitats and in Cayman generally feed on the plentiful sponges growing on the reef. Some scientists speculate that Caribbean reefs must have been much drabber places before the turtle hunting industry, because with more hawksbills there would have certainly been less colourful sponges about! Hawksbills also adapt their diet to exploit other seasonally abundant food sources, for example at this time of year they will feed on jellies, particularly blooms of thimble jellyfish that tend to occur around Easter. Hawksbills are distributed right around the tropics and their diet does shift regionally, tending to favour more soft corals in the Indo-Pacific.
Female Hawsbill turtles do not mate annually, and probably breed every 2-3 years. The nesting season runs from July to October and during this season the females will lay on average 4-5 cluctches of eggs. Like other turtles the hawksbill must return to land to nest, and the females haul themselves out of the water up onto the beach at night. The whole process takes several hours while they lay and bury their eggs in the sand. Interestingly the sex of the hatchlings is determined by the incubation temperature of the eggs, while they develop underground!
Unfortunately there is no longer a nesting population of Hawksbill turtles on Cayman, but hopefully they will return to join the Greens and Leatherback turtles which, although much less commonly seen underwater, do nest here. Certainly young hawksbills seem to be abundant in Cayman waters and when these mature maybe they will once again nest here in Las Tortugas.
Where Eagles Soar - Written by Dr Alexander Mustard / Photo by Elly Wray
Seven graceful eagle rays fly in formation along Cayman's North Wall. A group of scuba divers freeze, breathing as lightly as possible, hoping not to spook the squadron. It is a classic Cayman wildlife encounter. The eagle ray is probably the most elegant of fish of all, and although they are a regular sight throughout the year, they always seem most common on the North Wall during summer.
[Eagle Ray Cayman Islands by Elly Wray] Spotted eagle rays are found in tropical waters around the world, and although they are usually given a single scientific name, many scientists now suspect that they are actually a complex of closely related species. Spotted eagle rays grow up to 10’ across and 8’ long (16’ if you include their long trailing tail). The beautiful patterns of spots and rings on their back are thought to act as camouflage for the rays, breaking up their shape like the stripes of a zebra, while they are feeding on the seabed. They use their shovel shaped snout to dig in the sand for food; their diet consisting mainly of molluscs, such as conch and clams, and crustaceans, such as shrimp. That said, they also feed on octopus, squid and even small fish given the chance. Their teeth are flat and platelike so that they can crush the hard shells of their prey.
Eagle rays are social creatures and you will often see them hanging out in groups of 2 to 5, and on very rare occasions in super squadrons of 30 or more. But while they enjoy the company of their own kind they are easily scared by divers, and rarely allow a close approach. Your best chance of getting a close encounter with an eagle ray is to spot one feeding on the sand, and to creep up slowly, hiding behind coral heads and not swimming directly at it.
Large sharks, such as the Tiger and the Great Hammerhead are the main predators of eagle rays. The rays have between 2 and 6 venomous stinging spines on their tails, which they use for their defence, although I have never been convinced that these would be very effective against a large shark!
Eagle rays eggs are fertilised internally, with the male gripping the female’s wing in his mouth and rolling underneath her to mate belly to belly. The eggs then hatch inside the female’s body and unusually the young continue to feed from their egg sacs without a placenta connecting them to their mother. Eagle rays give birth to between 1 and 4 live young, which are about 1 foot across at birth. You can often see young eagle rays swimming in the shallows at night. So if you take a walk along the beach or down the dock always remember to take a torch!
In praise of groupers - Written by Dr Alexander Mustard / Photo by Elly Wray
For many people the only thing that comes to mind when they hear the word grouper is high in protein, low in saturated fat, high in cholesterol and particularly tasty cooked on the barbeque! But as scuba divers we know that there is much more to these real characters of the reef than tartar sauce.
[Nassau Grouper Cayman Islands By Elly Wray] Groupers are part of the seabass family, there are about 150 species worldwide, and they form important commercial fisheries accounting, for example, for over 40% of the US reef fish catch. And since fishing targets groupers, they are well studied compared with other reef fish. We know that Graysby and Coney can live for nearly 10 years, Nassau Grouper for 28 years and Jewfish to the ripe old age of 38! Groupers are born as females and then later some change sex to become males. In some species such as the Red Hind and Coney there are far more females than males, while the Nassau grouper there is a 50:50 split between the sexes.
Probably the most spectacular aspect of grouper biology is their mass group spawning. Not all species spawn en masse, the smaller groupers tend to spawn in pairs of harems more regularly during the year. Larger species, such as Tiger, Nassau, Gag, and Jewfish spawn just once or twice a year, but gather together in massive numbers to do it. Groupers will migrate over huge distances to join these aggregations: one Nassau Grouper travelled 240 km (150 miles) to spawn. The Nassau Grouper spawning aggregation is well studied on Little Cayman, but the East End is also home to a traditional spawning site. At Ocean Frontiers we have yet to see the fish spawning (because poor weather in January stopped us getting out to this remote site) but we did see Nassau Groupers migrating to it, across the dives sites. Watch this space!
But I have to say that the reason I like groupers so much has little to do with scientific fact. Groupers just seem more intelligent, more filled with personality than other fish. Have you ever watched a tang or a parrotfish swimming round and round the reef seemingly struggling to rub two brain cells together? In comparison to the dumb herbivores, groupers always seem to move with purpose. Groupers all around the world have been given pet names by scuba divers: Ben, VW, Frankie, Boris, Roi, Fred, Wanda, the list is long. They know where they want to go and if that is no-where then they will just stay still and get a good cleaning! Watching a tiger grouper at a cleaning station must be one of the best ways to while-a-way a dive on the East End.
Small is beautiful - Written by Dr Alexander Mustard / Photo by Elly Wray
There are ladies all around the world subtly reminding the men in their lives that the best presents usually come in small packages! And as we get more experienced as scuba divers we often find the same is true of marine life.
A long-time favourite find for divers with a keen interest in the reef’s small but perfectly formed critters is the Flamingo Tongue, which is an inch long marine snail, from the family Ovulidae or False Cowries. Flamingo Tongues are common and you should be able to find them on almost every dive or snorkel on Grand Cayman's East End, living amongst the branches of their favourite food of gorgonians and seafans. [Cayman Flamingo Tongue by Elly Wray]
Flamingo Tongues are one of the major predators on gorgonians, and you can often see trails behind them showing where they have been feeding. Being snail-paced they might seem easy pickings for predators, and are a favoured food item of the Giant Hogfish. But the Flamingo Tongue is well defended. As it feeds on the gorgonian it is able to concentrate the toxic chemicals from the coral into its own tissues, which is then wraps around the outside of its shell.
But there is no point being poisonous, unless you advertise the fact, and the Flamingo Tongue displays bright warning colours of its toxicity. It is often a surprise to divers to learn that the shells of Flamingo tongues are actually white, and it is the spotty mantle, the layer of living tissue that the snails wraps over its shell, that gives it its colour. The mantle also allows the snail to breath, by absorbing oxygen from the water.
You will often spot Flamingo Tongues in male/female pairs, and occasionally you can find gorgonians packed full of much smaller juveniles, which have similar colours but are much thinner than the adults. Once you have got your eye in for Flamingo Tongues you’ll find them on every dive and at this point you are ready for the next challenge: trying to spot the closely related Fingerprint Cyphoma. Fingerprints are pretty rare on Grand Cayman and they take sharp eyes and real dedication to find. Happy hunting. We also also think the Flamingo Tongue should be renamed as the Giraffe Snail.
A giant stride back in time - Written by Dr Alexander Mustard / Photo by Elly Wray
For about the last two million years, the branching Elkhorn coral (Acropora palmata) has been the dominant shallow reef building species across the Caribbean. Then about 25 years ago things changed. This once abundant species died off to such an extent that it is now rare or even extinct over much of its former range. US waters were the best studied, and there the coverage of this species declined by 98% during the 1980s.
The decline was caused by white band disease combined hurricane and storm damage. Many scientists believe that it is no coincidence that it is now, when mankind’s impact on the marine environment is greatest, that this species has suddenly become susceptible to a naturally occurring coral disease. [Elkhorn Coral Cayman by Elly Wray]
In most parts of the Caribbean this coral is very rare, but jump in the water at a shallow site at the East End of Grand Cayman and we have the chance to step back in time and see the way reefs used to be. Elkhorn coral gardens are not only important habitats for reef species, but their thick branches also break up waves and protect the coast.
Acroporids, like most reef building corals, have single celled algae living symbiotically within their tissues that help them grow at rates of up to a record 10 inches per year. Asexual reproduction occurs throughout the year when branches are naturally broken off colonies and re-attach elsewhere on the reef. Sexual reproduction occurs just once in 12 months, which if predictions are correct should be 2-2.5 hours after sunset sometime between the 13th and 15th September for all the population in Cayman. If you have never dived in an underwater snowstorm, I suggest that this might be a good time for a night dive!
The branching arms of Elkhorn coral are one of the best wide-angle opportunities in the Caribbean either lit with flash or as a silhouette. I like to try an incorporate a sunburst in the shot, or possibly a diver to liven up the background. There are also good macro shots to be had. The polyps are fun for abstract shots and there is often a range of critters on this coral, such secretary blennies.
A blast from the past – Written by Dr Alexander Mustard / Photo by Elly Wray
I reckon tarpon are Cayman’s most underrated fish! They are big, they have massive mouths and gigantic silvery scales and we often see them hunting, but we never seem to get excited about seeing tarpon. If we had to travel across the globe to see tarpon I am sure we talk for hours about these majestic fish. But as its name suggests Megalops atlanticus is a resident of the Tropical Atlantic and is rarely given exotic status!
Its possible to see tarpon at dive sites right round Grand Cayman, but the most fitting backdrop are the shallow caverns of the East End, where tarpon hang out and can be seen on just about every dive. Tarpon come from an ancient lineage of fish, a much older design than other fish we see on the reef. Curiously, they are distant cousins of the eels, and if you are lucky on a night dive you may see the evidence for yourself. The larvae of tarpon look just like eel larvae: a flat ribbon, a couple of inches long, with a small head at one end. Amazing to think that it may grow into an 8 ft monster! [Cayman Tarpon by Elly Wray]
Tarpon are predatory fish and specialise in hunting small schooling fish in shallow water. At low tide in the shallows, rotting vegetation can use up most of the oxygen dissolved in the water, making all the fish sleepy. Tarpon have a cunning adaptation to this environment - they can breathe air! Tarpon can swallow air at the surface and absorb the oxygen through their swimbladder, giving them the turbo boost they need to catch their sluggish prey.
For the underwater photographer tarpon are quite a challenge because their shiny scales are quite a tricky to expose correctly. TTL flash only tends to do a good job when the tarpon fills the whole frame. I have found that they look best with just a little bit of flash to pick out their silvery sides. Alternatively try shooting them in black and white film without flash, which works very nicely too. Digital photographers aren’t excluded either, just shoot without flash and greyscale your images in Photoshop (if you don’t have a black and white mode on your camera) that’s what I did to get the shot here. A good tip for black and whites is to fit an orange or red filter to your lens which can really improve the contrast in your shots.
The Real Sponge Bob - Written by Dr Alexander Mustard / Photo by Elly Wray
Even the least observant scuba divers will be familiar with the subject of this article: sponges. Indeed, colourful sponge growth is so luxuriant on Cayman’s reefs that you would have to swim round with your eyes closed not to notice them on every dive. The most eye catching species are the extravagantly coloured and shaped tube, vase, barrel and rope sponges. You might be surprised to learn that sponges are animals, although primitive ones, and feed by filtering out particles from the water column. Sponges are specialists at straining the smallest particles from the water, removing as much as 99% of all bacteria from the water that they pump through their bodies. [Cayman Barrel Sponge by Elly Wray]
Sponges grow into their elaborate shapes to help them feed. Their tubes act like chimneys; as currents flow past their siphon holes they suck more water through the sponge. Sponges can filter a volume of water equal to their own body every five to twenty seconds. If you look carefully at their siphons you can see how fast the water flowing out.
Despite their juicy-looking appearance sponges make an unappetising meal because their bodies are filled with hard glass spicules and chemical toxins. On land the bright colours of flowers and fruit are used for attraction, underwater they are often warnings. Hawksbill turtles are one of their few predators and eat so many sponges that more than half the weight in their digestive system may be glass fragments. How they deal with the highly toxic chemicals is unknown. Sponges are a treasure trove of chemicals, and many compounds are extracted from common reef sponges for pharmaceutical uses, for example in antibiotics and cancer drugs.
Quite a few sponges in Cayman are covered in zoanthids, a symbiotic relationship that is thought to keep both parties safe. The zoanthids benefit because few species can stomach the sponges poisonous flesh and the sponge is protected from some of its predators by the stinging zoanthids. The sponge also creates feeding currents than help provide food to the regularly space zoanthids.
Sponges are a ancient group and were the first multi-cellular animals originating more than 650 million years ago. They were also the earliest reef builders. Long extinct vase-shaped Archaeocyathids sponges constructed reefs in the shallow seas more than 500 million years ago, creating a habitat for a community of species.
Sponges remain important members of the reef community, and they are second only to corals in their dominance of sessile reef life. One of the key roles they play on modern reefs is not building but boring into them. Many sponges actually dissolve into the rocks to create a space in which to live and can be a dominant source of reef erosion.
Schools in for summer – Written by Dr Alexander Mustard / Photo by Elly Wray
Summer is a special time on Grand Cayman; the seas are calm and the water is so warm you can dive all day without a wetsuit. It’s a great season for the marine life too. It’s mating time for the reef sharks and stingrays, and it’s coral spawning season too, but perhaps the most distinctive sight of the summer is the swelling of silverside schools in the caves and caverns of the East End.
Silverside schools are actually made up of several species of two-inch herring-like fish that mix together in large shoals for their protection. Schools protect their residents by providing more eyes on the lookout for danger, a dilution effect for individuals of safety in numbers and a confusion effect where it is difficult for a predator to pick out a target. This doesn’t stop the predators trying; silverside schools always attract a host of predators - tarpon, groupers, snappers and jacks are all keen to gorge themselves on this summer bounty. [Cayman Silversides with Diver by Elly Wray]
A common question I am asked is what is the difference between a school and a shoal? Well simply, all groups of fish that remain together are a shoal, and a shoal becomes a polarised school when all the fish are synchronised in a common direction. Shoals are very typical part of fish life – about 50% of all fish shoal at some point. The exact spacing of individuals and the precise and unified movement are controlled mainly by eye contact, although the lateral line which can sense of movement is also thought to be important. Interestingly, many protective shoals, like those of silversides and grunts, disperse at night to allow the fish to feed.
Schools of silversides are enchanting to watch, dancing in the rays of light, with the fish moving in unison as if they are a single creature. The timing and magnitude of silverside schools varies from year to year, but on occasion they can be monstrous. At times the schools can completely fill the caverns and gullies of divesites like Grouper Grotto and Snapper Hole, even overflowing across the top of the reef. On these rare days these dives are probably the best in the world, as you not only descend beneath the waves, but then also on into another living liquid of millions of fish. As you swim the school parts in front of you, then engulfs you in a bubble of clear water, entirely and symmetrically encircled by fish. Suddenly a predator bolts through the mass, hoping to catch a straggler, momentarily scattering the silversides before they reform into their sinuous mass. Summers don’t get any better than this.
Live Fast, Die Young - Written by Dr Alexander Mustard / Photo by Elly Wray
Reef squid are definitely one of the most interesting creatures you will meet diving here in the Cayman Islands. With intelligence, complex communication signals and an ever-changing colour pattern, their fascinating behaviour can keep you transfixed for a whole dive.
You can see Caribbean Reef Squid on the shallow dive sites and on the wall, and particularly in the summer, if you grab your snorkel - you have a good chance of encountering large schools of youngsters very close to shore. Reef squid can sometimes be very curious, and if you are patient you can actually attract one in to investigate your hand, by mimicking its displays with your fingers. Night dives are another great time to look for them, and you can usually draw them in close with your dive light. [Caribbean Reef Squid by Elly Wray]
Squid are Molluscs, the same group as snails and clams, and are close relatives of octopus and cuttlefish. In fact the reef squid looks rather like a cuttlefish, being less streamlined and heavier built that other squid. Actually there aren’t any cuttlefish in the Caribbean, and for your peace of mind, I should mention that the squid used to feed the rays at Stingray City aren’t reef squid either!
Perhaps the most remarkable feature of squid is their ability to change colour. Squid have chromatophores, sacs of coloured pigment, in their skin that they can expand and contract at will, allowing them to change colour instantly. Colour changes are used for camouflage, both from prey and predators, and to communicate emotions or intentions. Squid certainly seem to live by the mantra of live fast and die young. They have a voracious appetite, devouring about half their body weight a day in small fish, shrimps and crabs. They are also an important prey item for many fish, such as jacks, groupers and reef sharks.
Their reproductive strategy is also unusual as squid usually die soon after mating. One of the most interesting times to encounter squid is when they are gathered together to mate, which usually occurs in small groups at dusk or dawn. At this time colour displays are most intense and a male squid can even show different colour patterns on each side of his body - a courting pattern on one side to woo his mate and an aggressive pattern on the other to ward off rivals. If the male is accepted, he fertilizes the eggs by passing a sticky sac of sperm to the female, who then lays her eggs in a safe crevice in the reef to start the next generation of these fabulous creatures.
What's for lunch? - Written by Dr Alexander Mustard / Photo by Elly Wray
Coral reefs are invertebrate heaven: from sponges and snails to crabs and corals, a staggering number of invertebrate species find that the reef offers plenty of things to eat and plenty of places to live. To give us a flavour of just how diverse reefs are we can think about how many kinds of animal live there. Biologists divide all the animals in the world into 34 phyla – the 34 groups of the very different kinds of animals. The number of phyla present in an ecosystem can help us to judge its biodiversity. In all the rainforests in the world you will animals from 9 of these phyla, while on coral reefs there are species from 32! [Cayman Cowrie with Mantle exposed]
With such a variety of invertebrates living on the reef, it is no surprise that more fish species feed on them than on any other food source, and that fish have developed a great variety of ways to feed on inverts. I can’t begin to go through all the adaptations that reef fish have evolved to feed on invertebrates here. The best way to appreciate the variety of specialisations is to dedicate part of a dive to fish watching and look at the ways in which species feed. Even when different species feed on the same thing they often go about it in different ways. Think about the species that dig in the sand – goatfish, hogfish, grunts and rays all look very different.
Four-eye butterfly fish are great for fish watching and can be seen on every dive at the East End. Notice how their tall, thin bodies give them great manoeuvrability for flitting between the coral. See how they use their long snouts to pick at just about any potential food source on the reef from coral polyps to the feeding fans of Christmas tree worms. So my advice this month is next time you are exploring an East End reef take a few moments to watch how the fish feed and appreciate the great diversity of adaptations and behaviours they have.
Creature of the night - Written by Dr Alexander Mustard / Photo by Stephen Broadbelt
Every year around the month of September we enjoy the coral spawning spectacular, the week always reminds me how great the night dives are here. It is a real privilege to spend some no-limits bottom time with all the nocturnal critters of the East End's reefs. The reef literally crawls with invertebrates: slipper lobsters, spiny lobsters, hingeback shrimps, clinging crabs and definitely the most entertaining to watch, the Caribbean Reef Octopus.
Octopuses are intelligent animals and as a result their behaviour is varied and fascinating. In captivity they can even learn by watching other octopus in other aquariums! In their natural habitat their wits are continuously tested as they are non-stop carnivores seeking out crustaceans, fish and molluscs, and constantly changing colour and texture as they prowl the reef. Your best chance of seeing a Caribbean Reef Octopus out and about is at night, when they are a common sight on most dives. During the day you can consider yourself very lucky if you see more than an eye and a few suckers peering out from the reef. [Cayman Octopus Mating]
Unlike many other animals that use hormones to slowly change colour over time, the octopus has nervous control over its appearance and can alter its body size, colour and texture in an instant. Caribbean Reef Octopus often take on a beautiful cyan blue as they patrol their territories at night. Other colours indicate their emotions: red colouration usually means anger and white means fear. If they are threatened octopus will often go very dark, then release a cloud of purpley-black ink, before immediately bleaching white and jetting away to safety. Defence is very important for octopus as their shell-less body makes them a favoured food for sharks, rays and large groupers.
Most octopus species lay many thousands of small eggs to ensure that a few reach adulthood. Like most reef species, these eggs hatch into larvae that join the plankton, feeding and growing there until they are ready to settle back to the reef. It is a wasteful strategy and one that the Caribbean Reef Octopus avoids with an unusual approach. It produces just a few hundred much larger eggs. These allow the young to grow safely until they are ready to hatch as miniature versions of the adults already set up for life on the reef.