Birds have large wings, but a light body. Despite their walnut-sized brains, they do have the reputation of being pretty smart. This holds not only for singing birds that produce amazingly complex songs, but also corvids (members of the crow family) and parrots. Avian biologists have been puzzled for a long time how intelligent and complex behavior of birds can be produced by such tiny brains. Avian brains may not have large, but highly compact brains many of these neurons are located in the forebrain, the area that is connected with cognition in humans. They also found that parrots and corvids have forebrain neuron counts equal to or greater than primates with much larger brains.
The amazing quality of seabirds is their ability to navigate over the immense oceans using their smell as their major tool. This holds in particular for Procellariiformes, an order of tube-nosed seabirds that comprises four families: the albatrosses, shearwaters, fulmars, and 2 families of storm petrels (Hydrobates pelagicus). All species are accomplished long-distance foragers, and many undertake long trans-equatorial migrations. They have elongated tubes or nostrils on their thick upper beaks that probably contribute to their enhanced ability to smell prey in the open ocean (see insert). But more importantly perhaps is that these birds have olfactory tissues in the brain that take up about 37% of total brain volume (as compared with the 3 % of songbirds).
Until Gabrielle Nevitt from the University of California, Davis, came along, no one had fully explored how seabirds use their elaborate nose gear to track their prey over vast ocean expanses. How do they do it? One chemical in particular called dimethyl sulfic (DMS) seems to play a crucial role. DMS is generated when zooplankton (krill) devours phytoplankton. For us, DMS may smell like oysters on a half shell , or the seashore. DMS forms odor plumes, like puffs of cigarette smoke, that drift over the ocean surface and are picked up by the birds, zigzagging and continuously sniffing until zeroing in on their prey. Storm petrels are especially good at detecting the chemicals at a great distance. In addition, albatrosses may cover thousands of kilometers in a single foraging trip. Their survival thus depends on finding the proverbial needle in the haystack on a daily basis, often in dark and cloudy conditions within very limited visibility.
Nevitt, C.A. (2008). Sensory ecology on the high seas. The odor world of the procellariiform seabird. J. Exp Biology, 211, (2008).1706-1713.
Ackerman, J., The Bird way, Chapter four. The scent of sustenance. Penguin Books. 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.
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 naucrates, Remora 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.
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.
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.
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.