20. Jun, 2021

The Coelacanths are members of the order of Coelacanthiformes that currently includes two species: the West Indian Ocean coelacanth (Latimeria chalumnae) primarily found near the Comoro island of  Africa and the Indonesian coelacanth (Latimeria menadoensis) in the waters of Sulawesi. Coelacanths are deep-sea creatures, living in depths up to 2,300 feet below the surface,  but may sometimes drift to more shallow layers of the  Ocean.  The coelacanths also called ‘’the living fossils’’,   were thought to have become extinct around 66 million years ago, but were rediscovered in 1938 off the coast of South Africa. Little was known of the coelacanth's normal habitat and behavior until observations in 1987 by German biologist Hans Fricke and his research team, using a submersible. Fricke discovered that adult coelacanths cluster in caves during the day and venture into open water at night. Coelacanths can become around 2 meters long and weigh 90 kilo’s. By examining imperceptible annual calcified structures (circuli) on the scales, maritime biologist have recently discovered  (see Current Biology)   that the fish has a slow growth,  can grow very old, around 85 years, and that the species only reach maturity at the age of 55 years, when it also becomes capable of producing offspring.  

Anatomy Instead of a spinal cord, Coelacanths have a stiff, hollow, fluid-filled tube known as the notochord,  functioning as the backbone, which runs from the skull to the tip of the tail and whose outline is clearly visible on the rear portion of the body.  Noticeable features are the two sets of paired lobed fins, pectorals and pelvics, lying to the side and beneath the belly. The muscles steering its pectorals fins consist of a pronator and supinator,  a muscle arrangement equivalent to two human antagonistic pairs of monoarticular muscles. Moreover, the two pairs of fins move in a synchronized pattern, characteristic of four-footed vertebrates. The right pectoral fin or forelimb is coordinated with and moves in the same direction as the left pelvic fin or rear limb.  Likewise, the left pectoral fin moves in the same direction as the right pelvic fin or rear limb (see picture above). This is similar to the gait of a trotting horse. The theory is that the fins behave as primitive ‘legs’, representing the bottom branch on the "family tree"  of evolution that led to legs in higher four-legged vertebrates.

6. Jun, 2021

In ancient times, the remora  was believed to stop a ship from sailing. In Latin, remora means "delay", while the genus name Echeneis comes from Greek ἔχειν, echein ("to hold") and ναῦς, naus ("a ship"). Its average size is around 40 cm but some species may even reach 80 cm. 

Species Although the taxonomic list of  Remora in Fishbase  mentions various species, it remains unclear if they reflect a genetical differentiation, synonyms, their local names, or the specific host they attach to. For example, next to the Common remora (Remora remora)  there are also Echeneis naucratesRemora albescens and Remora australis. The Common remora is often found on sharks, Echeneis naucrates on smaller fish such as tuna dolphins, and swordfish, R. Albescens that prefers mantas may even enter and perhaps reside in, a manta’s mouth or gill cavity. R. Australis is found almost exclusively on whales, particularly blue whales.

Anatomy of the disc In the common remora or ‘suckerfish' from the family of  Echeneidae, the frontal dorsal fins have evolved to enable them to adhere by suction to smooth surfaces. The suction cup (or disc) on top of the head is an amazingly effective adaptation,  allowing the remora to spend their lives clinging to a  host animal such as a whale, turtle, shark, or ray.  The oval-shaped disk is a modified dorsal fin that has split and flattened to form two symmetrical series of transverse, plate-like fin rays called disk lamellae. Suction under the disc is achieved by rotation of the lamellae when the disc is in contact with the host – this creates a relatively negative, sub-ambient pressure space under the disc. The disc also contains a fleshy-soft outer lip for suction, while the lamellae inside the disc carry tooth-like tissue projections (spinules), which the fish raises to generate friction against various host bodies to prevent slipping during attachment. The entire disc is operated by white muscle tissue ensuring that once a seal is made with the outer lip by creating a vacuum, it remains firmly attached.

From an evolution perspective, the adhesive disc evolved from dorsal fin elements, with an increase in lamellar number as a function of selection for enhanced shear adhesive power to the type of skin of the host. Although the oval disc has probably evolved from the dorsal fin spines typical of other fishes, the softer tissue, like the muscles controlling the disc suction, could be related to adaptations of the remora's cranial veins. These are highly modified and repositioned in comparison to those of other vertebrates,  lying more in front directly under the oval disc.  The suggestion is that these veins have functional importance associated with the adhesive mechanism (see Flammang  and Friedman for more detailed accounts).

Behavior When attached to their hosts, remoras appear to swim upside down. They feed on parasitic copepods, food scraps from meals, and sloughing epidermal tissue and feces of the host. In the Bahamas, they often choose the lemon shark as their favorite target. Sharks may not always appreciate their presence and have been seen acting irritated or even aggressively to remora when they become too obtrusive. A remora desperately seeking a host, may sometimes even cling to a naked spot on a  scuba diver passing by (that almost happened in the picture above, taken at Tiger Beach).  Some  African/Asian cultures use remoras to catch turtles. A cord or rope is fastened to the remora's tail, and when a turtle is sighted, the fish is released from the boat; it usually heads directly for the turtle and fastens itself to the turtle's shell, and then both remora and turtle are hauled in.  

 

10. Apr, 2021

The bull shark (Carcharhinus 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: Upper panel: schematical view of the organs of the shark involved in keeping a balance between osmolarity of body fluids and the environment.  Lower panel: bull shark in  saltwater Bahamas (picture taken with Olympus E-PL5 and 8mm lens, natural light)

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).

Buoyancy Sharks in deep saltwater use the caudal fin and pectoral fins to generate vertical forces that balance the negative buoyancy. This, in turn, results in drag due to lift by the body and pectoral fins. Negative buoyancy is favorable for marine sharks traveling fast whereas neutral buoyancy provided by large oily livers, such as in the Greenland shark, favors lower travel speeds, as a result of decreasing costs of lift production at higher speeds. Bull sharks swimming in freshwater may experience a two- to three-fold increase in negative buoyancy as a result of decreasing water density. Liver size or density offers only limited compensation for increased negative buoyancy. Suggesting that increased negative buoyancy in freshwater bull sharks might be less of a handicap when swimming in the shallow murky water of rivers and estuaries.

References

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