Neil Shubin a paleontologist at the University of Chicago, and his colleagues recently described in the PNAS journal the anatomy of a fossil that may provide the ‘missing link’ between tetrapods (four-legged animals) and finned fishes.
Left: Reconstruction of the skeleton of Tiktaalik roseae, with a pelvic girdle at the back, suggesting early stage of hind-fin driven locomotion
The fossil, called Tiktaalik represents a fish species that must have lived around 375 million years ago. In more official terms their conclusion was that: ‘the mosaic of primitive and derived features in Tiktaalik reveals that the enhancement of the pelvic appendage of tetrapods and, indeed, a trend toward hind limb-based propulsion have antecedents in the fins of their closest relatives’.
Fossils of their close finned relatives of the tetrapods often have a large pectoral appendage but only tiny pelvic appendages. This gave rise to the hypothesis of ‘front-wheel-drive’ early locomotion. That is, that primitive fishes were probably able to move on land using their strong pectoral fins. The discovery of Shubin and his team suggested that in species like Tiktaalik the hip joint could have been the start to the development of ‘four-wheel drive’ locomotion, such as animals that walk on land using four limbs. Looking closely at Tiktaalik’s hip joint (figure above) you will notice it has a deep socket, similar to the corresponding human socket, which allows us to move our legs in many directions.
Indeed, the big surprise (discovered only recently in a more refined analysis of the back part of the fossil, described already in 2006 ) was the sheer size of Tiktaalik’s pelvic girdle and hind fin relative to its pectoral girdle. In that respect suggesting that hind-fin-driven locomotion probably began before the tetrapods. That notion is further supported by a 2011 PNAS report of an African lungfish, a living cousin of Tiktaalik, that also used its hind fin to “walk” underwater, very much like a tetrapod. This intermediate link between fish and amphibians probably represented features that foreboded a leap from water to land.
Hind limb walking gives an animal—especially a creature with heavy, air-filled lungs in the front of the body—incredible ability to maneuver in complex aquatic environments, such as swamps, streams, and estuaries. An unanswered question is still the timing of onset of the attachment of the pelvic girdle to the vertebral column: did that occur in finned or limbed creatures? Answers to these questions can only come from the fossils yet to be discovered.
Manta rays are large rays belonging to the genus Manta (sometimes called Mobula) comprising 11 different species. The two largest species are the giant oceanic manta (M. birostris reaching 7 m (23 ft) in width and the smaller reef Manta, (M. alfredi more than 5m (16 ft) in width. Both species have triangular pectoral fins and two symmetrical horn-shaped cephalic (head) fins flanking the flat forward-facing mouths. The cephalic fins at the front of the body are extremely malleable, and can even be rolled up and unrolled (see picture), depending on if the animal is traveling or feeding. While it is underway, it can roll up its fins to help them move quickly through the water. The fins then corkscrew into neat and thin forward-projecting appendages. In contrast with earlier views, the manta rays prefer to stay in patches of the ocean as small as 140 miles (220 kilometers) across and rarely if ever journey outside of them.
Left: the cephalic fins rolled up (upper picture) and unrolled (middle picture). Lower picture: the 'aileron' effect causing a sharp roll to the right: pectoral and cephalic fins bent upward at the right side, and downward at the left side of the body. Pictures were taken at Raja Ampat.
The mantas have horizontally flattened bodies with eyes on the sides of their heads behind the cephalic fins, and gill slits on their ventral (belly) surface. The belly contains distinctive markings, allowing individuals to be identified by the unique belly spot pattern, like a human fingerprint. Dorsally (backside), they are typically black or dark in color with pale markings on their shoulders. All-black color morphs are also known to exist. Mantas are sometimes observed to make spectacular breaches, leaping partially or entirely out of the water. The reason for breaching is not known; possible explanations are mating rituals, the removal of parasites and commensal remoras, or perhaps just ‘fun’.
The function of the Mantas horns is still a matter of speculation. When we observe the manta swimming it seems that its movements are driven primarily by the flapping of its massive pectoral fins. Mimicking the movements of a large bird's wings. The major function of the fleshy face fins is believed to funnel the plankton (its major food source) during filter feeding. Additional functions that may have developed during evolution are communication, steering, and sensations. A reef manta may sometimes flip open one cephalic fin while swimming past, potentially serving a sensory or communication function. Assisting movement could be a third function. Watching the swimming manta you get the impression that movements of the big pectoral fins and cephalic fins are nicely coordinated to guide locomotion. When making sharp turns, the horns seem to behave like ‘ailerons’ in a small aircraft (ailerons moving in different vertical directions produce the aircraft to roll: to move around the aircraft's longitudinal axis; see lower picture above).
The cephalic fins seem to have developed analogous to forelimbs of other vertebrates. Recent research suggests that the genes that guide the development of the rays’ cephalic lobes play the same role in the fins of a closely related ray species, the little skate, which doesn’t have cephalic lobes. The results suggested that the ray’s horns aren’t a third set of appendages at all – they’re simply the foremost bit of fin, modified for a new purpose. Suggesting that cephalic lobes are not independent appendages but rather modified pectoral fins.
John D. Swenson et al. How the Devil Ray Got Its Horns: The Evolution and Development of Cephalic Lobes in Myliobatid Stingrays (Batoidea: Myliobatidae). Front. Ecol. Evol, published online November 13, 2018
Dawn and dusk are incredibly active times on coral reefs. Together, these twilight, or crepuscular, periods take up only about an hour of each day, but they are extremely important to all reef life. It is a wonderful moment for a diver to hang around along a reef wall to observe these transformations taking place. During daily twilight periods, fish and invertebrates emerge or retreat to their refuges. Diurnal fish leave their overnight resting places and swarm out onto the reef at dawn, returning to these shelters at dusk, while nocturnal fish follow the opposite pattern.
Picture left: Silky shark (Carcharhinus falciformis) at Jardines de la Reina, Cuba. Wikipedia.
Between one set of fishes going into shelter and their counterparts emerging to feed, there is a period of about 20 minutes when the reef seems absolutely devoid of all life. This slightly eerie period is sometimes referred to as the “quiet time.” One of the reasons for this dead zone in the reef's daily transitional period is the emergence of predators like groupers, tuna, trevallies, barracuda, moray eels and snappers. Probably these hunters are at their most effective during the twilight period when their eyes are attuned to the half-light and this gives them the edge.
Similar to the smaller reef predators, elasmobranchs are assumed to be also more active during low-light twilight periods. A reason why swimmers, snorkelers or scuba divers are often warned to better stay out of the water. Sharks could be more nervous or aggressive at these moments of the day, because they consider humans as intruders, especially when they start feeding on fishes along the reefs, and they might mistake intruders for their prey or other predators.
According to the research of the shark researcher Neil Hammerslag, generalizations about increased elasmobranch activity during dark periods are currently not supported. Implying that the dusk and dawn theory might just be a fallacy as far as sharks are concerned. One problem with the theory is that it can only be tested in specific areas, that is along with the shallow areas of the coral reefs, where sharks can be observed when they congregate to hunt on smaller species. In the open oceans, however, the habitat of most sharks, little is known about their feeding behavior.
Another complicating factor is that feeding patterns may vary considerably depending on the species of sharks. Notice that there are more than 1000 species of elasmobranchs and it remains unknown how widespread possible increases in nocturnal and/or crepuscular activity might actually be in this group of fishes. For example, the Oceanic shark (Carcharhinus longimanus) has the reputation of being a scavenger, constantly cruising the ocean in search of food, irrespective of transitions between day and night. In contrast, the white tip reef shark (Triaenodon obesus) spends much of its time during the day resting on the floor of sandy caves. At night, however, the white tip transforms in a fierce nocturnal hunter on the reef, that emerges to hunt bony fishes, crustaceans and octopus in groups, its elongate body allowing it to force their way into crevices and holes to extract hidden prey. The Great hammerhead is also believed to hunt primarily at dawn or dusk. Moving over the sandy seabed they swing their heads in broad angles over the sand, so as to pick the electrical signals from their favorite prey, the stingray, with their numerous ampullae of Lorenzini located on the underside of the cephalofoil. The great hammerhead is also seen hunting blacktip sharks during daytime, when these congregate in large numbers in shallow waters. It seems however that they lack the speed necessary to successfully catch these swiftly moving species.
Investigations have also reported that the lemon shark (Negaprion brevirostris) often uses waters and sandy bottoms less than 5 m depth, for example, to patrol shorelines at low light levels in order to intercept fishes moving between shallow waters and adjacent areas.
What about other shark species? In this respect its worth to mention here an encounter with the silky shark (Carcharhinus falciformis) described by Jeremy Stafford Deitsch *. The silky is an open water shark that occasionally moves into the coral reef area. The encounter took place along the point of Shab Rumi, an atol in the Sudan where silkies are known to turn up in the late afternoon. When Jeremy entered the water with his snorkel and camera, two silkies aggressively confronted him. This was shortly after some bottlenose dolphins had also passed the point. Despite frantic ‘kicks of the silkies with my fins, a clunking them with my camera’ the sharks kept pursuit, while making attacks with swift movements. This kept on even after he sought refuge over the shallow roof of the reef. Luckily, he managed to attract the attention of the Zodiac operator at the edge of the reef to pick him up. This event probably reflected a coincidence of two factors: the late afternoon dive when silkies were actively feeding, and the earlier presence of dolphins triggering an aggressive response to a human entering their territory.
To close, the best advice so far is to be on the alert when you enter areas where sharks are active, especially in low vision/ and low light conditions. Especially when you dive alone, like UW photographers often do. An aggressive reaction of the shark can be triggered by any event that it associates to interfere with its feeding behavior, either in baited or non-baited dive conditions.
*Jeremy Stafford Deitsch. Red Sea Sharks. Trident Press Ltd. 1990.
'Electric eels have a strategy for inactivating the muscles of difficult, struggling prey that have been grasped but not subdued. In these cases, the eels concentrate the electric field by sandwiching the prey between the two poles of their long electric organ'.*
The remarkable physiology of the electric eel (Electrophorus electricus) made it one of the first model species in science. It was pivotal for understanding animal electricity in the 1700s, was investigated by Humboldt and Faraday in the 1800s, was leveraged to isolate the acetylcholine receptor in the 20th century, and has inspired the design of new power sources and provided insights to electric organ evolution in the 21stcentury. And yet few studies have investigated the electric eel’s behavior.
This review focuses on a series of recently discovered behaviors that evolved alongside the eel’s extreme physiology. Eels use their high-voltage electric discharge to remotely control prey by transcutaneously activating motor neurons. Hunting eels use this behavior in two different ways. When prey have been detected, eels use high-voltage to cause immobility by inducing sustained, involuntary muscle contractions. On the other hand, when prey are hidden, eels often use brief pulses to induce prey twitch, which causes a water movement detected by the eel’s mechanoreceptors. Once grasped in the eel's jaws, difficult prey are often subdued by sandwiching them between the two poles (head and tail) of the eel’s powerful electric organ. The resulting concentration of the high-voltage discharge, delivered at high-rates, causes involuntary fatigue in prey muscles. This novel strategy for inactivating muscles is functionally analogous to poisoning the neuromuscular junction with venom. For self-defense, electric eels leap from the water to directly electrify threats, efficiently activating nociceptors to deter their target. The latter behavior supports a legendary account by Alexander von Humboldt who described a battle between electric eels and horses in 1800. Finally, electric eels use high-voltage not only as a weapon, but also to efficiently track fast-moving prey with active electroreception
*Abstract from Catania K.C. (2019) The Astonishing Behavior of Electric Eels. (2019). Front. Integr. Neurosci. 3:23.
In his book "The voyage of the Beagle", Charles Darwin set out the framework of a theory for the formation of lagoon-islands, atolls, and reefs. It started with a coral reef formation around an extinct volcanic island, growing from a fringing reef to a barrier reef to an atoll, as the island and ocean floor subsided. Since Darwin's discovery, many have wondered how rich and diverse ecosystems teeming with fishes like coral reefs could grow in the marine equivalent of a desert. Some even called it the ‘Darwins paradox’. But is it really a paradox? Evolution basically describes how complex and rich forms of life developed from very elementary organisms and mechanisms. And even an empty ocean may not be the barren desert some take it to be.
The basic mechanism of coral growth is the symbiosis between the coral polyps and symbiotic algae called zooxanthella stocked in the tentacle of the coral polyps. The algae provide the carbon necessary to build up the skeleton of the coral. The coral polyps also extract nutrients like nitrogen and plankton from the seawater to provide the algae carbon dioxide and phosphates. Some marine biologists believe that the phytoplankton may be the primary source of the development of coral ecosystems. The enhancement in phytoplankton near an island-reef ecosystem is also called the Island Mass effect (IME, see insert).
This paradoxical enhancement in phytoplankton near an island-reef ecosystem was discovered about 60 years ago by Doti and Ogury, and recently confirmed by Jamison Gove. He and his team showed that IME is an important feature among a majority of coral reef ecosystems surveyed, creating near-island ‘hotspots’ of phytoplankton biomass throughout the upper water column. The team supervised by Jamison found phytoplankton coral island hotspots surrounded by barren oceans landscapes were nearly everywhere the team looked. Apparently, corals have an astonishing capacity for storing scarce nutrients for their survival. They pick up phytoplankton from the surrounding currents or rising from deeper colder layers around the island to feed the algae that that provide nutrients for the growth of corals using photosynthesis.
A recent study by Simon Brandl published in Science suggests that the ocean’s smallest vertebrates, called cryptobenthic reef fishes, promote internal reef-fish biomass production through exceptional larval supply from the pelagic environment. In addition to the phytoplankton, these tiny fishes and larvae form an important nutrient for larger fishes such as smaller coral fishes and even pelagic predators of the coral reef. So they could also provide (indirectly) the energy and fuel for species on the reef higher in the food chain. Finally, sponges and their excrements also seem essential for the energy cycle, providing ‘manure’ for the coral reefs as well as food for smaller species like crabs, worms, and snails.