The sandtiger shark (Carcharias taurus), also known as Grey nurse shark or Spotted ragged-tooth shark, can be found in most moderate seas troughout the World. It is a typical social shark that congregates in smaller and larger groups above the sand, or prowls for bait in ship wrecks. Its ambiguous naming often leads to confusion. The sandtiger is not related to the tiger shark, nurse shark or the grey reef shark as the names above suggest.
Picture left: Sandtigers in North Carolina. Picture taken by Tanya Houppermans.
Phylogeny Sandtigers belong to the order of Lamniformes that are distinguished by eyes without nictitating membranes and a higher body temperature than the surrounding water. Many different families belong to the great order of Lamniformes. The tresher shark (Alopiidae), great white shark (Lamnidae) and mako shark (Isurus) are just some examples. The sandtiger belongs to the family of Odontaspididae, that splits up in two genera: Carcharias and Odontaspis. The sandtiger was originally named Odontaspis taurus, but later changed to its current official name Carcharias taurus. It is the only member of Carcharias, while Odontaspis contains two extant species: the smalltooth sand tiger (Odontaspis ferox) and the extremely rare big-eye sand tiger (Odontaspis noronhai). So it appears that there are three different species of sand tigers. Notice however that genetic markers suggest that the two Odontaspis species could be more closely related to sharks belonging to other families of Lamniformes, like the tresher shark (Alopius vulpinus) or even the crocodile shark (Pseudocarcharias kamoharai). Perhaps Odontaspis and Carcharias are the result of convergent evolution, meaning a process by which unrelated or distantly related organisms evolved similar body forms, coloration, organs, and adaptations.
Anatomy The sand tiger shark has a flattened, cone-shaped snout and a distinctive, oblong tail with a notched, upper lobe that is significantly longer than the lobe below. Individuals range in size from 6.5 to 10.5 feet in length. It usually swims with its mouth open displaying three rows of protruding, smooth-edged, sharp-pointed teeth. Adults tend to have reddish-brown spots scattered mostly on the hind part of the body. Its jagged protruding teeth certainly give the sand tiger a ferocious look: the sharp teeth seem to protrude in all directions, even when the mouth is shut. But looks can be deceiving, because sand tigers are a docile, non-aggressive species known to attack humans only when bothered first. They are also one of the few sharks that are tolerant of aquarium life.
Habitat and Behavior Sandtiger encounters are common occurrences in shallow offshore waters. Ship wrecks are one of their favorite habitats, probably because of the massive schools of small bait fish. Along the coast of North Carolina sandtiger sharks congregate in numbers of more than one hundred in the shipwrecks lining the coast, many dating from World War II battles (see insert above). At the wrecks they can be seen swimming through dense schools of small fish (‘bait balls). The abundance of food obviates baiting by humans and allows UW photographers to close in to the sharks when they glide their way through the bait balls.
Sandtigers are the only shark known to come to the surface and gulp air. They store the air in their stomachs, which allows them to float motionless in the water, seeking prey. When feeding at night they become voracious predators, that generally stay close to the bottom. Their prey is small fish, but they will eat crustaceans and squid as well. They occasionally hunt in groups, and have even been known to attack full fishing nets. During the day they are found near caves and ledges, hovering just above the surface either singly or in small groups. In these conditions the sand tiger is a relatively placid and slow-moving shark that will not cause any danger to humans. In fact the database of Shark Attack Survivors does not list any fatalities due to sandtiger sharks.
Survival. Two factors have made the survival status of the sandtiger critical. First, the slow-moving, approachable sandtiger with a tendency to aggregate in large groups has been an easy target for spearfishers for years. In waters of New South Wales and southern Queensland in Australia the numbers of sandtigers have declined dramatically throughout the 1960s and 1970s, due to the combined effects of targeted spearfishing, incidental capture by commercial and recreational fishing and beach protective shark meshing. Although the species have now become protected, accidental bycatch still occurs. A second factor is their low reproduction rate. Female sandtigers are ovoviparous and only develop two embryos, one in each uterus. The young embryos then feed on their mother eggs until only one pup is left in each uterus. Females don’t reach reproductive age until they’re nearly 10 years old, and then only produce one or two pups per litter every two years. Over nine months to a year, the pups grow within their mother feeding on hundreds to thousands of eggs and, by the time they are born, measure up to a meter long.
The Silvertip shark (Carcharhinus albimarginatus) is one of the rarer requiem sharks that can be encountered along the reefs walls of the Indo-Pacific. In the western Indian Ocean region, it can be found in the Red Sea and off East Africa including Madagascar, Seychelles, Aldabra Group, Mauritius, and the Chagos Archipelago. When a silvertip shows up, it means immediate supense. It is beautifully proportioned, and a swift and elegant predator. Like other requiem sharks, the silvertip is viviparous. It is recognized by the bright silvery tips of the dorsal and pectoral and the upper and lower lobes of is caudal fins. This makes it easy not to confuse with three other white tipped shark species; the white tip reef shark(Traenodon obesus), the grey reef shark (Carcharhinus amblyrhynchos) and Oceanic white tip (Carcharhinus longimanus). But its behavior does resemble that of the Oceanic, since the silvertip can also be inquisitive and bold, especially in the presence of food.
Unfortunately, in the northern Red sea the silvertip has become an infrequent visitor. In the southern Red Sea it may turn up above sandy plateaus of more remote islands, often at places that are frequented by a more common species of requiem shark, the grey reef shark. The favorite spots visited by the greys are steep promontories or ‘points’ where masses of schooling fish and larger predators patrol in the strong currents. During our visits of Saganeb reef in the Sudan the greys were regular customers along the famous South-West point of the atoll. Here we must have been the very first divers that started to bait the sharks with mall pieces of tuna, often hidden under a coral head or tied to a rock. Grey reef sharks are not considered as dangerous to divers and much less bold than the Silver tip or Oceanic sharks. Still, in a shark frenzy even the greys can become fierce, fast moving and darting around. Usually a sign for the diver to stay out of the turmoil. Occasionally scalloped hammerheads (Sphyrna lewini) may show up at the SW point of Sanganeb. But they prefer the deeper and darker layers of water where they congregate in schools, swimming against the current with their bodies occasionally reflecting the sunshine from above.
My first (and only) encounter with the silvertips were not at Sanganeb but at Shab Rumi, another atoll about 20 miles north of Sanganeb (see insert). Shab Rumi became famous because of Jacques-Yves Cousteau's experimental underwater village Precontinent II, which is still a major underwater hotspot of Sudan. Some remnants of this project, like two shark cages covered with soft corals and the mushroom shaped docking station of under water scooters, are still there. The beauty and richness of the Sudanese reefs makes a trip to these remote locations a must for Red sea adepts. A live aboard leaving from from Marsa Alam in south Egypt, would probably need another day of sailing from Rocky island to cross the Sudanese border and reach these reefs.
Jeremy Stafford Deitsch. Shark. Headline Book Publishing. London.1987
Blennies belong to the suborder of Blennioids, or ‘slimy’ fishes according to the Greek τό βλέννος that stands for mucus or slime. Together the Blennoids form a incredibly large number of small fish species, that are often hard to tell apart at first sight. No wonder one gets easily confused! One way for not getting lost in the labyrinth is to follow the taxonomy: Order--Family-- Genus--Species. The Blennoids consist of six families. The largest and best known family are the Blenniidae also known as Combtooth or Scaleless blennies, that contains around 50 genera and 400 known species. They are followed by two other large families: the Clinidae or Scaled blennies found in all oceans, and the Chaenopsidae, a strictly tropical family, ranging from North to South America. Then we have the Tripterygiidae or threefin blennies a family of blennies of which all members have a dorsal fin separated into three parts (hence the name); the first two are spinous.
The blenny resembles the goby, with one difference; the goby has fused pelvic fins that form a disc-shaped sucker. This sucker is functionally analogous to the dorsal fin sucker of the remoras. Another difference is that the goby has the habit of digging out his burrow and sifting sand, which the blenny' has not.
Left: Tompot Blenny. Picture taken by Alex Tattersall (2007). In the UK, swanage pier dorset is a favorite spot to meet the Tompot blennies. This blenny can get around 25 cm long.
Anatomy Combtooth blennies have blunt heads and large eyes, with large continuous dorsal fins, which may have three to 17 spines. The frontal part of the dorsal fin is often higher than the back part. Their name comes from the comb-like teeth lining of their jaws. Males and females are quite different, with the male being much more colourful particularly whilst breeding. The swim bladder is usually absent in adults which will make them sink to the bottom. The bodies are compressed, elongated and scaleless. Most species have two rays just anterior to their enlarged pectorals inserted near the throath, and a pair of branched tentacles above the eyes called cirri. The often branched tentacles can vary widely in form and size dependent on the species. Cirri could be are an additional sensing organ that helps the blenny to read the current and know which way the food will be coming from, as well as to help them to anticipate the approach of predators. They could also function to impress its enemies or distract its prey. The blennies eyes can function independently, giving them that goggle-eyed look that enables them to look in two directions at the same time, keeping careful track of both prey and predators.
Pictures Their big eyes, colourful faces and bizarre horn-like structures on their tiny heads make the blennies a favorite target for underwater macro photographers. If… they are lucky to get a good close shot of them, in particular of the head. Although I am not a blenny expert, I find the best blenny shots those where the photographer has succeeded to capture the blennies head with all its fine features and colours, in particular its eyes, crisp and sharp. Best opportunity is when the blenny is lying still, for example on a rock or peeping out of a tube or bottle against a neutral background. It is advised to bring the camera down to the level of the eyes of the blenny, and then patiently wait for the moment when the blennny will look at you*. Macro should be your first option, but if the blenny is not too small, a picture taken with a fish eye lense and mini dome at close distance will probably also look nice. Good contrast is essential. Using macro there are globally two ways to create a interesting contrast between your blenny and the background. One is using a narrow aperture and high shutter speed, which will create an almost black background. The other is a wide aperture (with high shutter speed), which will drastically reduce the depth of field and make the background blur (also called 'bokeh'). It will also make focusing a lot more difficult. To get at least one sharp shot of your blenny, 'focus bracketing' is a handy method. Once the focus is set on your camera, rock slightly a little back and forth to achieve the sharpest focus while you keep on shooting. A wide aperture goes with the lowest intensity of your strobes.
Behavior and habitat Combtooth blennies are found in tropical and subtropical waters in the Atlantic, Pacific and Indian Oceans; some species are also found in brackish environments and in tide pools. They often spend their entire life cycle on one general location. For some species small crstiaceans, mollusks, and other sessile invertebras are the primary food items but others may eatalgae or plankton. Combtooth blennies spend much of their time on or near the bottom. They may inhabit the rocky crevices of reefs, burrows in sandy or muddy substrates, or even empty shells. Generally found in shallow waters, some combtooth blennies are capable of leaving the water for short periods during low tide, aided by their large pectoral fins to crawl around. Blennies have an undulating swimming style and strong teeth. Females lay eggs in shells or under rock ledges; males guard the nest of eggs until hatching. Most blennies are voracious, mutually agonistical and curious. If you watch long enough, you will often see a blenny dart out of its hole and grab a meal out of the water column. Often the blenny wil lie on a rock or sandy bottom, but some prefer little crevices, and even empty cans, bottles or tubes to hide in.
Some species of combtooth blennies Some better known combtooth blennies are (genus in Italics): the Molly Miller (Scartella christata), Tompot Blenny (Parablennius gattorugine: see picture above), the Butterfly Blenny (Blennius) with a big blueish-black spot on the frontal part of the dorsal fin, the Adriatic Blenny (Microlippophrys adriaticus, no cirri) and the Blackhead Blenny (Microlippophrys nigriceps) from the Mediterranean. The Horned Blenny (Parablennius tentacularis) is a larger species often found in the Mediterranean (see frontpage for an example).
A particularly interesting group are the rockskippers (Salarius fasciatus). They are amphibious blennies, living in the surf and splash zone of rocky coasts. With twisting, jumping movements, they propel themselves over coastal rocky surfaces. These blenny species constitute a parallel evolutionary development to the mudskippers, amphibious gobies living in the mud out of the water. Similar ecological conditions have brought about similar adaptations. Then we have the beautifully coloured Peacock Blenny (Salaria pavo) and the False cleaner fish (Aspidontis taeniatus). These blennies have fang-like teeth with venom glands at their bases and are noted for their clever mimicry of cleaner wrasses Labroides dimidiatus.
Other families Scaled blennies belong to the large family of Clinidae that inhabit temperate oceans primarily south of the Equator. Dazzling and varied colors and markings differentiate the species. The largest clinid, one of the many pointy-headed blennies, is the 24-in (61 cm) giant kelpfish (Heterostichus rostratus), which inhabits the Pacific shoreline from British Columbia to Southern California. The 8-in (20-cm), blunt-headed, hairy blenny (Labrisomus nuchipinnis) belongs to the family of Labrisomidae and lives in the tropical waters off both Atlantic coastlines. Another family is that of the Tripterygiidae or threefin blennies. This family contains some of the smallest blennies —the female of the species Tripterygion nanus found in the Marshall Islands, is fully grown at less than 0.75 in (1.9 cm) in length. Tripterygion tripteronotus (Black faced blenny, steep forehead, breeding males with red body up to 8 cm) is found in the Mediterranean. The Secretary and Spinyhead Blennies are tube blennies from the family of Chaenopsidae. Tube blenny lives in the vacated tubes of Calcareous Tube Worms and seem to prefer locations in plenty of light at the top of coral heads. At no more than two inches in length they are very hard to see and even harder to tell apart.The pike blenny (Chaenopsis ocellata) is a tube-dwelling species found in Florida. Male pike blennies jealously defend their territories from other intruding males by aggressively displaying a stiffly raised dorsal fin and a widely gaping mouth. Two males may literally face off, gaping mouths touching, until one snaps its mouth shut on the other. These blennies will often create great photo opportunities.
Source and links
The Tresher shark is a rare mostly solitary shark that, like other marine apex predators, has a difficult time in surviving. It’s meat and fins make it a favorite target for offshore fisheries, which has put it on the IUCN Red List of vulnerable species. Many UW photographers that have succeeded to ‘shoot’ Tiger, Oceanic and Great Hammerhead sharks from a short distance, have not yet been able to come close to a tresher. My only encounter was many years ago in the southern Red Sea when I saw it passing below me in the hazy blue along a steep drop off. The tresher is simply a too rare, distant and shy animal to get lured into a baited shark session. It also prefers deeper water than the zone in which UW photographers normally feel comfortable. Probably it’s just that detachedness, combined with the magic of its enormous tail that keeps stimulating our interest and curiosity in the creature.
Left: From top to bottom: the Common, Pelagic and Big eye tresher. Picture adapted from Alessandro de Madalena.
Anatomy and general characteristics Although genetics has become a more accurate measure of relatedness than appearance, the conventional Linnaean taxonomy is still a helpful tool in distinguishing between the 400 different shark species of the World. Going from general to more specific categories this implies: order, family, genus and species. So, the tresher shark belongs to the order of Lamniformes also known as mackerel sharks. Which includes the great white, basking, megamouth and mako shark. Within this order the treshers form the family of Alopiidae or ‘foxlike’ sharks. Like other mackerel sharks thresher sharks are ovoviparous, meaning that the young develop inside a weakly formed shell within the female. Along each of their flanks runs a strip of red aerobic muscle, which can contract powerfully for long periods, enabling the thresher shark to swim without fatigue.* Treshers feed mainly on small fishes like herring, anchovies and sardines. They can be found in deep waters as well as along reef drop-offs and coastlines in many places of the world. Like other Lamnid sharks, the thresher shark (at least one of the species, the common tresher, see below) has a network of blood vessels called rete mirabile that allows it to maintain its body temperature slightly above that of the surrounding water. Its a system they share with the tuna fish. The network ensures that metabolic heath produced by its strong red muscles is not lost in the surrounding seawater, but retained inward towards the core of the body.* The treshers pectoral fins are long, a bit backwardly curved and spread out, like in the Oceanic shark. But it most conspicuous part is its enormous whiplike tail.
The tail The tresher’s elongated upper lobe of the tail fin almost equals the length of the sharks body. The only shark with a tail of almost similar length is the variegated shark (Stegostoma varium, or Stegostoma fasciatum). Because it is covered with dark spots or stripes (juveniles) it is often also called zebra shark or leopard shark (not to be confused with Triakis semifasciata). But the variegated shark belongs to the different order of Carpet sharks. Like the nurse shark it has nasal barbels and prefers shallow water, in particular sandy bottoms. Variegated shark is an oviparous (egg laying) shark that is also commonly seen in the Red Sea.
Why has nature equipped the ‘foxies’ with such an strange and enormous tail? Is it just a caprice of evolution, or an attribute with a certain advantage for its survival? A similar question is often asked with respect to the hammerheads bizarre shaped head, and the swordfish with its elongated snout. A generally accepted view is that the tresher uses its tail as a weapon or instrument for striking fish. In some amazing video captures it can indeed be seen slapping its whiplike tail and slashing its way through schools of sardines, before returning to devour stunned and wounded victims.**** Crocodiles are probably one of the few land species that are known to use their tails with a similar purpose. Some eye witnesses have even reported seeing the tresher slapping seabirds sitting on the surface. Alternatively, its long and strong tail might also enhance its propulsive power and ability to make swift turns.
Three species There exist three different species of the genus Alopius: the common tresher (Alopius vulpinus), the pelagic tresher (Alopius pelagius) and the bigeye tresher (Alopius superciliaris). The outside differences between these species may not be directly obvious. But despite the overlap in their anatomy, general appearance, behavior and habitats there still seem to be some important differences that justify their separation (see also the insert).
Common tresher The body is blue-grey to dark grey or blackish on top, with silvery or coppery sides and white undersides. It is the largest tresher that can reach a length of 6 meters. The small mouth is arched and, unlike in other thresher sharks, has furrows (labial folds) at the corners. The first dorsal fin is tall and positioned slightly closer to the pectoral fins than the pelvic fins. In addition, the shark has a stout cylindrical body with a short head, rounded between eyes. It is seasonally migratory and spends summers at lower latitudes. Common threshers tend to be epipelagic (oceanic near the surface), and are common in coastal waters over continental shelves. They are virtually circumglobal in warm seas. Common threshers are regarded by recreational anglers as one of the strongest fighting sharks, together with the short-fin mako, often leaping out of the water when caught on a fishing line. In addition, the common tresher has a vascular heat exchange system (rete mirabile) thet serves to generate and retain body heat, using the energy produced by its strong aerobic muscle.*
Pelagic tresher The Pelagic tresher was hardly mentioned some 20 year ago, probably because in many early publications it was mistaken for the common tresher or just called ''tresher'. The slender pelagic thresher is the smallest member of its family, rarily exceeding 3 meters. It has small teeth and its first dorsal fin is relatively short and is placed halfway between the pectoral and pelvic fins. It has a conical head. Around the mouth corner its has no labial furrows like the common tresher. It can also be distinguished by the dark, rather than white, color over the bases of its pectoral fins. Curiously, the pelagic tresher seems to lack the vascular heat exchange system found in the common tresher to generate and retain body heat (see also the big eye tresher below).* Although encounters between divers and the pelagic tresher are rare, it has been regularly seen in the Red Sea along offshore reefs such as Daedalus or the litte Brother islands. There have also been regular sightings at Layang Layang and Sipadan in Malaysia and Monad shoal near Malapascua in the Philippines. A Malapascua in the Philippines pelagic treshers can be seen when they visit more shallow cleaning stations for their early morning grooming*** Clearly the best opportunity for UW photographers to get closer to these shy sharks (see also the picture of Noam Kortler on this weeks frontpage).
But the pelagic tresher also shares many traits with the common tresher. It is an active, strong swimmer and has been known to leap clear of the water. It is a wide-ranging Indo-Pacific Ocean shark, apparently highly migratory, with low fecundity (two pups/litter) and a low (2-4%) annual rate of population increase. Like the common tresher it is epipelagic, although the species is reportedly relatively often in some coastal localities. It might also visit deeper mesopelagic zones when it hunts for schools of fish.
The big eye tresher’s This species can be easily distinguished by its large eyes and a pair of deep grooves on the top of its head, from which its scientific name is derived. Its enormous eyes are placed in keyhole-shaped sockets that allow them to be rotated upward. The first dorsal fin is placed more backwards than the common tresher. It is also know to visit very deep layers of the ocean in the mesopelagic zone below 200 meters. The large eyes of the bigeye thresher are probably adapted for hunting in the low light conditions of these greater depths. It is one of the few sharks that conduct a diurnal vertical migration, staying in deep and cold water during the day and moving into warmer surface waters at night to feed**. This migration likely relates to finding prey at night and avoiding predators during the day. The sharks' daytime swimming patterns are usually steady, while at night they have a pattern of slow ascents and rapid descents. Similar to the pelagic tresher it seems to lack the vascular heat exchange system found in the common tresher to generate and retain body heat. These differences could relate to the fact that the red aerobic muscle in this species as well as the pelagic tresher runs closer the skin than in the common tresher, allowing less effective functioning of the heat preservation blood vessel network and thus less metabolic heat conservation*.
Survival Overfishing in targeted shark fisheries, by-catch in fishing gear targeting other species, and high levels of illegal and unregulated fishing have caused drastic reductions in the tresher’s populations. The common thresher is widely caught by offshore longline and gilnet fisheries and is especially vulnerable to fisheries exploitation because its epipelagic habitat occurs within the range of many largely unregulated and under-reported gillnet and longline fisheries.
References and links
Sharks of the Open Ocean: Biology, Fisheries and Conservation. (2008) Blackwell Publishing Ltd Editor(s): Merry D. Camhi, Ellen K. Pikitch, Elizabeth A. Babcock Published Online: 28 JAN 2009 Print ISBN: 9780632059959
Susan E. Smith, Randall C. Rasmussen, Darlene A. Ramon and Gregor M. Cailliet. The Biology and Ecology of Thresher Sharks (Alopiidae) (pages 60–68) (Chapter 4 in the Camhi et al. book)
Compagno, L.J.V. (2002). Sharks of the World: An Annotated and Illustrated Catalogue of Shark Species Known to Date (Volume 2). Rome: Food and Agricultural Organization. pp. 8
*Sepulveda, C.A., Wegner, N.C., Bernal, D. and Graham, J.B. (2005). "The red muscle morphology of the thresher sharks (family Alopiidae)". Journal of Experimental Biology. 208 (Pt 22): 4255–4261. doi:10.1242/jeb.01898. PMID 16272248.
**Weng, K.C. & Block, B.A. (2004). "Diel vertical migration of the bigeye thresher shark (Alopias superciliosus), a species possessing orbital retia mirabilia". Fishery Bulletin – National Oceanic and Atmospheric Administration. 102 (1): 221–229.1–83. ISBN 92-5-104543-7.
The Amazone basin has an incredible diversity of animal species many of which are not yet catalogued by biologists. Glassfrogs, belonging to the amphibian family of Centrolenidae -which consists of about 12 genera- are one example. They are nocturnal animals that reside most oftheir time in the treetops of the forests. Glass frogs are small transparent creatures that will fit on a matchbox. A group of biologists from Ecuador and US discovered a new species of the glassfrog genus Hyalinobatrachium which consists of around 40 different species. The new species was found in three localities in the Amazonian lowlands of Ecuador and was baptised Hyalinba-trachium yaku (Yaku Glassfrog.) The specific epithet yaku is the Kichwa word for water. Water, in the form of streams, is fundamental for the reproductive biology of all glassfrogs. Water pollution through oil and mining activities represents one of the biggest threats for Amazonian amphibians, as well as for numerous other water-dependent species.
The Yaku glass frog. Top row: adult male in dorsal and ventral view. Bottom row: adult male seen from front and the side*
The glassfrog eggs are usually deposited on the leaves of trees or shrubs hanging over the running water of mountain streams, creeks, and small rivers. All species of the glassfrog genus have a completely transparent ventral peritoneum, which means that the belly organs are fully visible in ventral view. But Yaku differs from related glassfrogs by having small, middorsal, dark green spots on the head and dorsum (picture, upper row left), and a transparent pericardium which also exposes its beating heart (picture, upper row right). Males attend egg clutches located on the underside of leaves overhanging streams. Their reproductive behavior is also unusual, with males calling from the underside of leaves and providing parental care to egg clutches. Its transparent underside body probably gives it a clever evolutionary advantage. The silhoutte of a Yaku frog clutched upside down to the underside of a leave will be very difficult to spot by aerial predators that fly over the bushes.