10. Apr, 2021

The bull shark (Carchrhinus leucas, also called Zambesi shark in Africa) is a large and stout shark, with females being larger  (around (2-3 meters) than males. Along with the tiger and great white shark, bull sharks are among the three shark species most likely to bite humans. Shark bites in shallow water, sometimes ascribed to great whites, later appeared to come from bull sharks. Remarkably, the bull shark is one of the few species of Carcharhinus that occasionally swims into fresh, brackish, and shallow water of river deltas, estuaries, and lakes that connect with the sea. 

Insert: schematical view of the organs of the shark involved in keeping a balance between osmolarity of body fluids and the environment.

Habitats The ability of bull sharks to tolerate freshwater could be rooted in competition for scarce saltwater food resources, where perhaps bull sharks suffered and needed to develop an edge. Giving them gradually the genetic advantage of access to a greater variety of fishes in freshwater regions, where other competitive predators sharks cannot enter. Females are thought to give birth to one to 13 pups in estuaries and river mouths, from where the young migrate and may remain far upstream for up to five years  In freshwater they are free from predators, similar to baby lemon sharks that often seek safety in the shallow mangroves as nurseries. Bull sharks hunt on bony fish, small sharks, and stingrays. Their diet may include turtles, crustaceans and enichoderms. They  also hunt in murky waters where it is harder for the prey to see the shark coming

Osmoregulatory mechanisms in the bull shark. The most abundant dissolved salts in seawater are sodium and chloride, magnesium, sulfate, and calcium: together around 36 gram per 1000 gram seawater. Seawater is thus denser than freshwater because the dissolved salts increase the mass by a larger proportion than the volume. The fluids inside and surrounding cells in the body of the shark are composed of water, electrolytes (mostly the salt particles in the body fluid or blood that produce ions, that is an electrical charge), and nonelectrolytes.  In addition to chemical compounds such as sodium and chloride, the blood plasma of sharks also contains high concentrations of organic compounds such as urea and trimethylamine oxide  (TMAO) to maintain the animal's isotonicity.

In marine sharks, the watery portion of blood, the plasma, has a concentration of salt and ions that is remarkably similar, and only slightly higher than that of seawater (see insert). In more technical terms, its osmolality (the concentration of dissolved particles of chemicals and minerals per liter)  is about 1070mOsm/l (=number of particles per liter of solution). Osmoregulation (or: osmosis) is the process of maintaining salt and water balance (osmotic balance) across a semi-permeable membrane (mainly. the gills) within the body fluids.  This allows molecules of a solvent to pass through the membrane from a more concentrated solution into a less concentrated one. This principle (called: diffusion)  is of vital importance in bull sharks resident in and migrating between fresh and saltwater. The challenge for these sharks is to maintain osmotic and ionic homeostasis. that is a constant hyperosmotic value (osmolality) of around >1000 mOsm/l relative to the external milieu, over a wide breadth of conditions.

Bull sharks possess several organs that are adapted to maintain appropriate salt and water balance; these are the rectal gland, kidneys, liver, and gills epithelium (see insert for a rough sketch). All elasmobranchs have a rectal gland that functions in the excretion of excess salts accumulated as a consequence of living in seawater. Marine and euryhaline elasmobranchs in saltwater reabsorb and retain urea and other body fluid solutes such that osmolarity remains hyper-osmotic to their surrounding seawater; consequently, they experience little or no osmotic loss of water. In contrast, euryhaline elasmobranchs in freshwater, balance osmotic water gain by increased urinary excretion. Overall, plasma osmolarity in freshwater-captured animals was significantly reduced compared to saltwater-captured animals, mostly caused by the decrease of sodium, chloride and urea, excreted by higher urine flow rates in freshwater sharks. They also synthesized less urea as well as retained less urea, Na, and Cl than marine individuals such that osmolarity remains relatively low but still greater than the surrounding freshwater. In sum, this implies that euryhaline bull sharks, acclimated to freshwater have urea and TMAO levels of about a half and one-third of their marine counterparts, respectively. In addition, they have sodium, chloride, and magnesium ion concentrations about 12, 13, and 15% of levels below marine species.  (Pillans & Franklin, 2004).


Ballantyne, J.S.,  J. W. Robinson (2010). Freshwater elasmobranchs: a review of their physiologyand biochemistry. J Comp Physiol B, 180:475–493.

Ballantyne, J.S., D.I. Fraser (2012). Euryhaline Elasmobranchs. Editor(s): Stephen D. McCormick, Anthony P. Farrell, Colin J. Brauner, Fish Physiology. Academic Press, Volume 32, Pages 125-198,

Hammerschlag, N. (2006) Osmoregulation in elasmobranchs: a review for fish biologists, behaviourists and ecologists. Marine and Freshwater Behaviour and Physiology, 39:3,209-228

Heupel, Michelle R.; Colin A. Simpfendorfer (2008). "Movement and distribution of young bull sharks Carcharhinus leucas in a variable estuarine environment" (PDF). Aquatic Biology. 1: 277–289.

Ortega, Lori A.; Heupel, Michelle R.; van Beynen, Philip & Motta, Philip J. (2009). "Movement patterns and water quality preferences of juvenile bull sharks (Carcharhinus lecuas) in a Florida estuary". Environmental Biology of Fishes. 84 (4): 361–373. 

Pillans, R.D.; Franklin, C.E. (2004). Plasma osmolyte concentrations and rectal gland mass of bull sharks Carcharhinus leucas, captured along a salinity gradient. Comparative Biochemistry and Physiology A. 138 (3): 363–371

Reilly,B.D. et al. (2011). Branchial osmoregulation in the euryhaline bull shark, Carcharhinus leucas: a molecular and analysis of ion transporters The Journal of Experimental Biology 214, 2883-2895.





20. Jan, 2021

A shark pup's success in life is largely determined by its size at birth and whether the female shark has used a nursery area or a shallow part of the sea with fewer predators than the open sea.  Newborn carcharhinid sharks are equipped with fully functional jaws and teeth and have therefore been considered independent of maternal care or support at the point of birth.

Photograph of a Sand  Tiger shark  by Chris and Monique Fallows. Nature picture library 

Shark pups are also very independent, and those that are born live swim away from their mothers as soon as they're born, perhaps to avoid being eaten. Being larger would clearly also have a  survival value. One –admittedly hard-  way to achieve this is ‘embryonic cannibalism’. This is the name of an unusual mode of reproduction in sharks with many litters, whereby the first embryos in the uterus to reach a certain size consume all of their smaller siblings (called adelphophagy)   as well as the unfertilized eggs (called oophagy, oviphagy, or egg-eating) during gestation.

Approximately 14 species of sharks are thought to practice some form of intrauterine cannibalism. The best-known intrauterine cannibal is the sand tiger shark. Although the sand tiger shark has two uteri and produces many eggs, each litter yields just two pups -- one from each uterus. Because of their pre-birth diet, sand tiger pups enter the world bigger than other pups; they measure approximately one meter long. The cannibalistic battle for primacy in utero, with only one surviving represents an evolutionary strategy that allows the largest or strongest male sharks to father the successful baby and thereby outcompete sexual rivals. Paleontological  researchers examining incremental growth bands of the tooth of the extinct megatooth shark  (Otodus megalodon)   recently suggested that the large  (2 meters) of the megalodon babies and the rapid growth profiles of their tooth may have also reflected effects of intrauterine cannibalism to reach such enormous proportions,  to survive in an ocean infested with much greater predators than today.




14. Jun, 2020

Several species of fish have the habit of making occasional leaps out of the water, which is also known as breaching. They may do so for different reasons. The leap could be part of their natural locomotion:  for example to save energy or just for fun like with the bottlenose dolphin. Spinner dolphins seem to enjoy their ability to spin multiple times in one jump. A jump could also reflect an attempt to catch a prey on the surface. A seal swimming on the surface may trigger a breach of the great white shark, and a fly above the surface of a river could tempt a trout to leap out of the water to snatch it. Other breaches reflect an attempt to escape a predator or noise of a boat propeller leading, for example,  to mass jumping of Asian carps in the  USA. These carps multiplied spectacularly after they escaped from a fish farm, outnumbering the local fish species. One fish jumping can set off a chain reaction and spook other fish — as seen in footage a river full jumping Asian carps in the Illinois River.  Spectacular are the jumps of salmons heading upstream to spawn that can leap up more than three meters to scale a waterfall. Some bony fishes such as mudskippers  (Periopthalmus) and amphibious blennies (Alticus) may spend more than 50% of their lives out of water. Anatomical (body) and behavioral adaptations let them move better on land and water. When threatened, these species typically produce prone jumps, using their fins to move around in skips. They may even flip their strong body to jump up to 2 feet (60 cm) into the air.

Sofar some examples of the leaping fish, but the flying fish are the ‘real’ flyers. They belong to the family Exocoetidae in the bony fish order of  Atheriniformes and closely related to the needlefish, halfbeaks, and sauries. Flying fish are limited to surface waters warmer than 20–23 °C, and contain about 64 species,  grouped in seven to nine genera. While they cannot fly in the same way a bird does, flying fish can make powerful, self-propelled leaps out of the water where their long wing-like fins enable gliding for considerable distances above the water's surface. Spotting a group of flying fish is a bit like seeing jumping dolphins. It is always fun to see a group turning up next to your boat and then performing its extended flight over the surface. In the open Oceans, flying fish are sometimes ending up on the decks of sailing boats or on larger cargo ships when the weather is rough. There are also stories of castaways surviving by eating flying fish that fell on their boat and swearing afterward never to eat sashimi for a long time.

Flying fish mainly fly to escape from predators, particularly dolphin-fishes (Coryphaena hippurus). Adult flying fish are of variable size (150–500 mm maximum length) and may be broadly divided into two categories: ‘two-wingers’ (e.g. Fodiator, Exocoetus, Parexocoetus) in which the enlarged pectoral fins make up most of the lifting surfaces, and ‘four-wingers’ (e.g.Cypsilurus, Hirundichthys) in which both pectoral and pelvic fins are hypertrophied. The pectoral fins are controlled by two groups of muscles, the lateral muscles that extend the wings, and the medial muscles that furl them. Both groups appear from external appearances to be red (aerobic) muscles (see the  Springer article for more details)

Beyond their useful pectoral fins, all have unevenly forked tails, with the lower lobe longer than the upper lobe. When the lower lobe touches a flat water surface it often draws a sinusoidal track. The process of taking flight, or gliding, begins by gaining great velocity underwater. After it has jumped out of the water produced by the rapid movement and vibration of the tail, they use their large pectoral fins almost as wings.  The pectoral fins then expand and stiffen like the wigs of a glider while in the air before the fish reenters the water. A flying fish can remain airborne for at least 40 seconds and can reach a top speed of at least 40 MPH (64 km/h). With a good wind however they might even fly as far as hundreds of meters.  When gliding, flying fish barely skim over the surface of the water. When the fish returns in the water it may become airborne again by violent flapping and extra thrust of its forked tail. As said, their flying action is meant to escape from predators (such as fish-eating bonitos, albacores, dorados, or the dolphin fish).  A Zodiac running across a group flying fish swimming close to the surface may also provoke a jump out of the water, as I witnessed several times on the Mediterranean.  In some Oceans airborne flying fish are confronted with another danger from the sky above: seagulls and frigatebirds looking for a tasty snack. Here they are literally caught between the ‘devil and  the deep blue sea


23. May, 2020

There are three species of bluefin tunas: the Atlantic (Thunnus Thynnus the largest and most endangered), Pacific (T. orientalis), and Southern (T. maccoyi).  More distant relatives in the genus Thunnus are the bigeye tuna (T. obesus) and the yellowfin tuna (T. albacares).

The Atlantic bluefin tuna Thunnus Thynnus is the largest tuna and the largest species in the mackerel family that can live for 40 years.  It is native to both the western and eastern Atlantic. One spawning ground exists in the western Mediterranean, particularly in the area of the Balearic islands. Their other important spawning ground is the Gulf of Mexico.

Bluefins are tremendous predators, already seeking out schools of fish like herring, mackerel, and even eels just after birth. With their superb vision they hunt by sight, while their torpedo-shaped body and retractable fins made for speed allow them to reach almost  80 km per hour. Their specialized blood vessel system (called a countercurrent exchanger) serves to maintain a body temperature that is higher than the surrounding. Which facilitates movements such as rapid and powerful turnings of the body. To keep the blood oxygenated they swim constantly with their mouth open.

Atlantic bluefin tuna has been hunted down massively since it became the most highly prized fish used in Japanese raw fish dishes like sushi and sashimi.  In the restaurants in southern Spain (e.g. in the tuna paradise Barbate) the Atún Rojo is also a local delicacy. A small tin of tuna costs around 20 Euro. Driven by such high prices, fishermen have developed more refined techniques like anchored nets to catch tuna, with the fish disappearing as a result. Although tuna do provide food and livelihoods for people, they are more than just seafood. Tuna is a top predator in the marine food chain, maintaining a balance in the ocean environment

About 80% of the caught Atlantic and Pacific bluefin tunas are consumed in Japan, where it is considered as an ultimate delicacy. Most catches of the Atlantic bluefin tuna are taken from the Mediterranean Sea which is the most important bluefin tuna fishery in the world. 15 years ago the future for tunas looked really bad, reaching a low point in 2012.   A report of the International Commission for the Conservation of Atlantic Tunas (ICCAT) concluded that only 10% remained of the population that existed 50 years ago. The catch quota was then brought back in 2008 from 26.750 tons to 12.900 tons worldwide for a  4 year period, restricting the catch to species larger than 30 kilos. 

However illegal catch continued in the waters of Southern Spain. And when the tuna populations started to grow again, the ICCAT decided, under pressure from member countries such as Japan, France, and Italy,  to increase the catch quota of bluefin tuna from 24,000 tons annually to 36,000 tons annually. Scientists and conservationists fear that increasing the catch limit will set back the bluefin recovery effort enacted in 2008 after stocks of the prized sushi fish have badly overexploited in the Mediterranean and Atlantic.








22. Mar, 2020

The narwhals (Monodon Monoceros) live year-round in the Arctic waters around Greenland, Canada, and Russia.  They have been harvested for hundreds of years by Inuit people in northern Canada and Greenland for meat and ivory, and a regulated hunt continues. It is one of two living species of whale in the family  Monodontidae, along with the beluga whale. Narwhals can live up to 50 years and grow to 6 m long. Their pigmentation is a mottled pattern, with blackish-brown markings over a white background. They are darkest when born and become whiter with age. During the winter, narwhals make some of the deepest dives recorded for a marine mammal, diving to at least 800 meters, several times per day, with some dives reaching 1,500 meters.

Normally, the canine tooth only on the left side of the upper jaw becomes a tusk. The tusk is actually an enlarged spiraled ivory tooth with sensory capability (somewhat like a feeler) and up to 10 million nerve endings inside, that can grow as long at 10 feet. Rarely, males develop two tusks. Only about 15 percent of females grow a smaller tusk.

What is the function of the strange rapier-like tusk of the male narwhal: is it a weapon or rather a signal?  The general scientific consensus is that the narwhal tusk is not directly necessary for survival but serves as a sexual trait, much like the manes of a lion, or the feathers of a peacock  Perhaps the sensors in the tusk have also a communicative function, the reason why some males have been seen ‘tusking’:  that is crossing the tusks like rapiers in a simulated fight. Based on the disproportional growth and large variation in male tusk length biologist Zackary Graham from Arizona and colleagues found morphological evidence that narwhal tusks are indeed sexually selected during male-male contests. Parts of the body that are sexually selected are often disproportionally larger. The variation is tusk size among male narwhals is indeed much larger (between 45-250 cm) than the tail (or fluke, between 45-90 cm). Large tusks thus benefit male narwhals in sexual acts, probably signaling  ’I am bigger than you'', and avoid potentially dangerous fights by impressing rivals (see insert). Source:  Royal Society Biology Letters,  March 2020