Newly qualified Open Water student, Reynhardt Duvenage wrote and performed his own version of Sweet Home Alabama so show his thanks for a great week here!!
This Saturday we were fortunate enough to spot a Zambezi (also known as ‘Bull’ Shark) on Manta! Despite Manta being one of the top ten dive sites in the world, it was awesome to bump into the lone shark among the enormous schools of fish.
Our encounter did not, however, live up to the reputation of this shark. Their name (Bull) refers to the bullish, unpredictable and often aggressive behaviour recorded; but our experience was anything but. He was sitting at 18m until we swam closer, when he cruised away in the opposite direction – but fortunately no faster than our efforts, giving us a wonderful display.
Zambezi’s are unusual in that they are diadromous – able to swim between fresh and salt water. As a result, they are found in most corners of the world; from Massachusetts to Southern Brazil in the Atlantic, Kenya to South Africa in the Indian and Vietnam to Australia in the Pacific. The fish have a preference for shallow, warm waters, tending not to swim deeper than 30m.
It is not just their behaviour that accounts for their name – they are also bullish in appearance. These sharks are identifiable by a broad, flat snout and stocky shape lacking an interdorsal ridge. Like Great Whites, the Zambezi’s are grey on the top and white below. Females are larger than males, averaging 2.4m and 130kg, while the males 2.25m and 96kg. The largest recorded Zambezi was an astonishing 315kg.
The cartilaginous fish feed on a variety of species: turtles, dolphins, terrestrial mammals, birds, stingrays and fellow Zambezi’s are all recorded prey. They tend to hunt solitary, only occasionally teaming up with members of their school. A ‘bump and bite’ technique is deployed: the initial ‘bump’ sufficiently disorientating and/or injuring unsuspecting prey, before the ‘bite’ when prey is eaten. Due to this, murky waters provide the ideal conditions for feeding as the sharks cannot be seen.
In spite of numerous encounters with humans, specific reproductive behaviours such as courtship are only speculated upon. It is thought that males will bite a female on the tail, until the female turns upside down and the male can copulate. The gestation period is interestingly long, 12 months, but the pups are viviparous (born live and free swimming) and immediately independent. A single litter can have between 1-20 pups. They reach sexual maturity at 10 years, and average life expectancy is 16 years.
Fortunately Zambezi’s are not currently listed as threatened or endangered, but it is likely their numbers are shrinking due to demand for their meat, hides and oil.
On Sunday we took a group on clients to one of our local reefs, Devils Peak onto Hard Rock. Towards the end of the dive, the 5 of us left were intently looking at a guitar shark nestled in the sand when everything went dark… We were trying to make out the black and white shadows around us, when a HUMPBACK WHALE came past us within touching distance!!!! There are no real words to describe the experience. Humpbacks are one of the most extraordinary mammals in the world and to see them underwater, in the wild, in such proximity, was mesmerising. Unfortunately there is no footage as we were too captivated to even consider taking out a camera, but two swam past and came back again. Whales are undoubtedly the largest species you’ll see on scuba, a mammal so huge is overwhelming, a little terrifying, and so beautifully majestic all in one. It is the closest anyone at the dive centre has come to one on scuba, now we wait with eyes peeled for the next!
Although we are fortunate to see the whales daily from the surface, from the shore and within metres of the boat, underwater was an unparalleled and unforgettable experience.
Would you like the chance to see one? Come dive with us!!!
The only cooperative, interspecies hunting behaviour recorded between fish occurs in Guinjata Bay! It is a unique behaviour as yet largely unknown to the world of marine biology.
Moray eels, an abundant fish here on our reefs, are ‘recruited’ by Roving Coralgroupers to hunt together. The grouper will ‘invite’ a moray by shaking its head, forming a new partnership. It is unclear the advantage to the eel of doing such, but the eel aids the grouper by flushing prey out of narrow crevices that are otherwise inaccessible to the groupers.
Cooperative and symbiotic relationships are well known and researched, both in the ocean and on land, but this is the first hunting partnership (between fish) recorded.
Moray eel are snake-like fish, often mistake for serpentines, which almost exclusively inhabit marine waters. Although there are over 200 species with an innumerable number of colours and intricate skin patterns, when diving these eels are usually highly distinguishable by their heads just poking out of crevices and holes in an almost ominous way.
Moray eels have their mouths open most of the time in order to provide constant water circulation to their gills; not, as they are widely mistaken to be, because they’re in a continual search for prey. The eels are carnivorous though. They feed mostly on octopus, crabs, squid and cuttlefish while themselves predated by groupers, barracudas, sharks and sea snakes. In comparison to the wide array of prey available, morays are actually hunted by comparatively few species. This allows them to burrow in holes and crevices, a potentially risky residence where fast escape is not easily accessible.
Circumstantial evidence indicates a couple of species may be poisonous, but they generally are no threat to divers. Similarly, in some species the mucus which is secreted over the skin is toxic in a minority of species. All morays (toxic and not) have the ability to produce a considerable amount of mucus in goblet cells in the skin though, which provides protection from abrasion when swimming fast and in crevices (also aids streamlining and lubrication). The mucus also adheres well to sand granules, making the walls of sand burrow residences more permanent.
Interestingly, the eels have two sets of teeth: one set directly in the most and visible to divers, the second inside their throat (‘pharyngeal jaws’). Both sets of jaws are launched open to actively capture prey and transport it; Morays are unique as the only animals known to use the pharyngeal to assist with the capture and restraint (most animals will just use for digestion). The teeth in both set are pointed backwards to stop prey getting away – the rare occasion where eels have struggled to differentiate fingers and food, fingers must be manually prised off the teeth. One interesting theory for having two sets of jaws is that their head is too narrow to create adequate low pressure which most fish use to swallow.
Although most abundant in shallow, tropical reefs, moray eels are found in tropical and temperate climates (even if only extend marginally beyond subtropics). Temperature also dictates mating season; when the temp and food availability are both optimum then the females release up to 10,000 eggs, while wrapping themselves around the male. Simultaneously the males release sperm and externally fertilize the eggs to form leaf-shaped larvae, which swim independently in the water with plankton. After 8 months, the larvae drop to the ocean floor and begin life as fully formed adult moray eels. Their life expectancy is 6-36 years. Although this is the basic life and reproductive cycle of the eels, research has shown morays to be hermaphrodites too. Some are synchronous, with both functional testes and ovaries, whereas others are sequential (changing from male to female). It is then possible for these to mate with either sex, increasing the chances of successful reproduction.
Moray eels are commercially fished, but not extensively. However, eating the fish can cause ciguatera fish poisoning, a food borne illness from reef fish where toxics in plankton accumulate up the food chain to dangerous concentrations. There are also often parasites on the skin – making the eels popular with cleaner shrimps and wrasses but also a greater threat for consumption.
Nudibranches are minute, slug-like molluscs that are dotted over the floors of all the oceans, from Antarctica to the Florida Keys, and Mozambique to the Arctic. Officially, they are shell-less molluscs, part of the sea slug family and in the Gastropod class. There are over 3000 species and more being discovered daily – many as yet unidentified on our reefs! It is virtually impossible to find the same one twice. So, if you like small colourful things, that potentially no-one else in the world has ever seen, our Nudi dive sites are the place for you.
Nudibranch (or Nudibranchia) literally means “naked gills’, a fitting name as they breathe through feathery gills standing out on the back, a little like external lungs. There are two known types: the ‘Dorid’ and the ‘Aeolid’. The Aeolid (also spelt Eolid) are covered in Cerata (finger-like attachments), which are used for breathing, digestion and defence. In contrast, the Dorid only have gills (no Cerata) on their back and breathe through these.
Nudi’s are Bethnic animals, spending virtually all their time on the sea floor or reefs. Only a minority can swim freely. They have one ‘foot’, which is used to hold on to the bottom. Much like a snail, this foot leaves a slimy trail along the floor. Occasionally, they will release the foothold on the reef and suck air into the stomach (to float), allowing them to tumble in the current before resettling in a new location.
Fascinatingly, Nudi’s have few natural defences, but use poison from their prey for protection. They will identify any irritating or toxic compounds in their prey, and store it in their Cerata before it can be digested. The poison remains in the Cerata until required for hunting or defence, when it is secreted through skin cells or glands.
The prey Nudis’s consume is also responsible for their wide-array of colours; for example the Spanish Shawl (type of Nudi) eats a species of hydroid which possesses a pigment that gives this kind of Nudi its distinctive purple, orange and red colouration. Nudi’s are also carnivorous. They largely feed on algae/anemone/barnacles/corals/sponges and fish eggs, but have been known to be cannibalistic too. They are a particular predator of the Portuguese Man of War. Despite this wide array of prey, some Nudi’s are very fussy, and entire species or families will consume just one kind of prey! Like the Frogfish, Nudi’s use a Radula to eat.
Given the relative immobility of Nudi’s, they are not ideal animals for mating. As a result, they have evolved to be Hermaphrodites (so do not have to wait for a Nudi of the opposite sex to pass for mating), but cannot self-fertilize. They will deposit eggs within a gelatinous spiral. Eventually the eggs hatch into free-swimming larvae, which settle on the floor as adults (comparative to many underwater species, a simple reproductive cycle).
Nudibranches can survive at almost any depth, as well as any temperature. Most commonly though, they are found in shallow, tropical waters. Lifespan varies too, from one month to one year.
Marbled electric rays, scientifically known as Torpedo Marmorata, accompany us on many of our local dives, but this week we have come across a considerable number of them! They are thought to be the most electro-sensitive animals, even more so than sharks, and an unforgettable species to dive with.
Although commonly perceived as a tropical species, the marbled rays are found as far North as the North Sea, the Eastern Atlantic, throughout the Mediterranean and down to Southern Africa.
Each fish has two electric organs; one on either side of the head, stacked with columns made up of jelly filled electroplates that act like a battery. This facilitates a current up to 30amps to travel from the lower to upper surface of the ray, producing shocks of 70-80volts (a similar effect to submerging a mainspowered hairdryer into a bathtub).
When a ray comes in contact with prey or predators its initial shock will be the strongest, with the intensity declining as it becomes fatigued. Interestingly, below temperatures of 15degrees Celcius, the nerves innervating the electric organs stop functioning and therefore rays may go entire winters (particularly in the North) without the ability to produce a current.
Typically, like many stingray species, the marbled electric rays are sedentary during the day, concealing themselves in the sand and most active at night. They are well-adapted to waters with minimal oxygen, such as deoxygenated bottom water and small pools during falling tide. They have a very low oxygen carrying capacity and heart rate, and stop breathing all together when the partial pressure of oxygen drops to 10-15 torr. Amazingly, though, are able to comfortably survive for five hours in this state without breathing.
In addition to their primary defence mechanism, electric rays use their shocks for hunting. Two capture behaviours have been specifically identified: “jumping” and “creeping”. With prey swimming close to the rays head, it will “jump” and produce an initial shock causing tectonic contraction of the prey, breaking the vertebral column. Whereas, with stationary or slow-moving prey, the marbled electric rays will use their disc and tail movements to draw prey closer in increments. When close enough, it will suck the prey in, at times producing electric shocks throughout ingestion. Bethnic bony fishes, such as hake, sea bass, jack mackerel and goatfish compose 90% of electric rays diet (by weight) and are consumed whole.
Interestingly, the gestation period for Marbled Electric Rays is longer than humans, between 9-12 months and producing a litter range of 3-32. There are striking differences between males and females: males sexually maturing when 5, while females maturing only at 12-13 years. Life expectancy also varies between sexes; males typically 12 years while females up to 20. As a result of the lengthy gestation period, females are only able to mate biennially while the males can every year.
For all intents and purposes the flesh of the Marbled Electric Rays is inedible, with only a gelatinous texture to offer. Consequently, they are redundant to fishing vessels and often thrown overboard dead. Historically however, their economic value has been significantly greater. The Greeks and Romans used the electric shocks from the rays for chronic headaches and gout, while eating the flesh was recommended for epileptics. In the 1800s the oil was used as fish oil and a source of fuel for lamps and lights. The oil was also believed to have medicinal properties – claims that have since been entirely disregarded by the scientific community.
Nowadays the Electric Rays are most valuable in medical research, as model organisms, because the electric organs are rich in a particular acetylcholine receptor which plays an important role in the human nervous system.
Cuttlefish are from the same family as Octopodes and Squid, belonging to the class Cephalopoda. They are cephalods (meaning “head-footed”), as have only two distinct body parts: the soft body and then 5 pairs of tentacles/arms. Despite the name, Cuttlefish are in fact molluscs rather than fish.
Globally, the Molluscs are found in both tropical and temperate climates, almost everywhere in the world except for the coasts of the Americas (it is thought they were perhaps unable to cross the cold Atlantic to colonize there). They are able to tolerate depths up to 600m, but generally prefer to reside close to the seabed in lagoons and warm shallow waters. We find Cuttlefish on all of our reefs,
Cuttlefish are thought to be highly intelligent, with one of the largest brain to body ratios of all invertebrates. Their internal shell (referred to as the cuttlebone) is comprised of aragonite and has a number of chambers which are used to control buoyancy. They regulate the ratio of liquid and gas within the chambers in order to maintain a density close to that of the sea water, a method not dissimilar to that submarines use. The Cuttlebone is commonly used as parakeet food (due to its high calcium content) and by jewelers to cast small objects.
Cuttlefish can also change their colour and texture, using tiny elastic sacs on the skin. Each sac contains pigments that can expand or retract in response to the environment. Adults tend to use this to creep up on prey, whereas juveniles can hide from predators.
Their eyesight is also highly developed and used in both hunting and protection from predation. The eyes will be easily noticeable to a diver – large with a strangely shaped pupil providing 360degree vision. Interestingly though, although colour plays an important role in both camouflage and courtship, they are unable to detect colour.
One of the most fascinating anatomical features of the Cuttlefish is their triple circulatory system. Unlike humans, which use the iron-based molecule haemoglobin to transport oxygen within the blood, Cuttlefish use hemocyanin, a copper-based carrier. Hemocyanin is less efficient than haemoglobin and causes the fish to have blue blood. In order to compensate for this, they have three hearts, one for each gill and another to pump blood to round the rest of the body.
Like Octopi, Cuttlefish have ink, which is used as their primary defence. mechanism. Ink is squirted if the fish is approached by a predator, creating a noxious black cloud. While this distracts and disorientates a predator, the Cuttlefish uses a jet propulsion to flee the area, by ejecting water through a siphon within the body cavity. Historically the ink has been used as dye called sepia, but this practice widely has been replaced by the use of artificial dyes.
Cuttlefish reach sexual maturity at 13-18 months, signalling the end of the solitary first phase of their lives. They begin to collect in groups where the males use rhythmic displays of courtship behaviour to impress females. Once the female is satisfied, the males use a specialised tentacle for mating, which deposits sperm sac in an opening close to the mouth. Even at this point however, intrasexual competition prevails and the fertilising male must guard the female, as competing males attempt to use jet propulsion to flush out the sperm sac. It is possible for a female to mate with twice and therefore have two sacs; when this occurs she is able to choose which to use to fertilise her eggs.
Each female will spawn, on average, between 150-4000 eggs, each in its own 1-2cm egg case. The eggs are covered in ink for camouflage and take residence on grass/weeds/shells/rocks. One week before hatching, the egg sac will clear and thin, providing a window to the external environment. The babies will begin learning at this stage, for example by observing small shrimps that become prey immediately after they hatch.
Cuttlefish hunt in a number of stages. Firstly the camouflage is used to sneak up on prey within suitable proximity to strike. Then they shoot their tentacles to grab the prey, using the suckers for extra grip. In spite of a lack of hard tissue, Cuttlefish have a surprisingly sharp beak which will bite the prey. Finally, whatever is caught is torn up using a Radula (a rasp-like structure) and swallowed.
Anemonefish, otherwise recognised as ‘nemo’ fish, are abundant on our local and deep dive sites. They are feisty fish and not afraid of divers, often straying unusually far (up to 5ft from their anemone) to check us out.Anemonefish live symbiotically with anemones and are very rarely seen without one another. While anemone will sting potential prey or predators that come in contact with it, the Anemonefish acclimatize over time by acquiring a layer of mucus that prevents stinging and establishes them as residents in the anenome.
The juveniles are small and black, with white spots, while the adults are the familiar orange with black and white stripes.
They are protandrous hermaphrodites (able to change their gender). When the fish hatch from the eggs, during the first stage of life, all are male. As they develop into juveniles, the fish are neither male nor female but instead have elements of both sexual organs. Once adult, they mature into fully functional males. They remain male until the female of the breeding pair dies. At this point, the adult male will transform into a female and the most dominant, non-reproductive juvenile will then mature to be the male in the new breeding pair. We usually see one family of fish on each anemone, with two larger fish which are the breeding pair and any number of juveniles. Of the pair, the largest will be female and slightly smaller the male. The juveniles do not have a typical growth cycle; they instead will remain small due a combination of bullying and hormones.
The Anemonefish also have an interesting spawning process. They spawn close to full moon, all year round. In Southern African waters however, it is particularly between November – April. The pair will “nip” on the side of the anemone, so it withdraws its tentacles and creates an opening 10cm in diameter where the female lays her eggs to be fertilized by the male. The eggs are pinhead in size and initially a dark, rusty colour. However, by the time they hatch are golden with the eye (a black spot) visible. The majority of the time the male will guard the eggs, keeping them oxygenated and removing the dead ones to remove contamination. Once hatched, the babies will eat minute plankton and after one week leave to search for their own anemone.
Last Friday we went with a group of guests staying with us at Casa Do Mar, and spent the afternoon by the Estuary close to Inhambane, near Pansy Island. We loaded the bukkies with essentials (dive gear, Castle Lites) and headed north. An hour of scenic sand roads and few thousand coconut palms later we reached the estuary, with just enough time for a dive before the high tide.
The shallow turquoise water of the lagoon proved an idyllic location to try something new. The sandy 3m bottom was dotted with giant urchins, sea horses, clams and crabs – an ecosystem very different to our deeper ocean sites. The depth allowed a longer bottom time than usual, while non-divers enjoyed collected pansy shells and snorkeled above.