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.
Like other top ocean predators, the sea otters presence helps to maintain a diverse community of animals and plants in the kelp forests. Otters eat sea urchins and other grazing animals such as crabs and abalones keeping them from devouring the kelp. Once hunted to extinction for the fur trade, the southern thick-haired sea otter (Enhydra litris nereis) has had a slow road to recovery under more than a century of legal protections. Last year it even reached a number of almost 3200 species. The sea otter profited from the sea urchin boom along the Californian coast, but now shows a jump in mortality that curtails their expansion. The reason is bites from great whites, who are also growing in number due to their pinniped food source expanding. Instead of preying on the otters (they don’t have the blubber of other sea mammals) sharks are taking investigative bites, some of which cause severe injuries causing death
A sea otter research program is carried out in Monterey Bay Sea Aquarium led by Michelle Staedler using data loggers to track their goings, and rescuing abandoned otters. Because of the increased risk of shark bites, researchers as Kerstin Wasson and Ron Eby are now exploring new shark-free habitats for the otters. One such area is Elkhorn Slough, a major estuary system in Moss Landing that feeds into Monterey Bay. Although the swampy slough is not really a coastal area, the otters so far seem to enjoy their new habitat where they also have more opportunities to stay on land. Captured abandoned sea otters from the Monterey coast are also released in Elkhorn Slough by Michelle Staedler and her team.
Eelgrass beds in the Slough were being smothered by algae that grew unchecked on the leaves, absorbing the sunlight eelgrass needed for photosynthesis. Snails, slugs and other invertebrates would eat the film of algae, cleaning the grass and allowing it to get the sunlight it needs. The snails and slugs, however, tend to be devoured by the increasing number of crabs in the estuary, which have few natural predators. Since the sea otter turned up (now counting around 150 individuals) and began feeding on the crabs (see insert) the balance is now improving. The bay of the San Francisco is another area that Wasson and Eby are exploring as a future shark free habitat of the otters, a place where many sea otters lived in the past until they were killed by fur hunters.
The cuttlefish (Sepia) belongs to the family of Sepiida, that in turn are part of the Cephalopoda which also include squids octopuses and nautiluses. Cuttlefish (see also my earlier Blog) form an enormous variety of species of which some, like Sepia officinalis and Sepia elegans, have -sadly enough- gained great popularity as appetizers and tasty entries on dinner tables. Sepia has a cuttlebone used for buoyancy control. They feed themselves by extending two hidden feeding tentacles, which snag prey and pull it back to its strong beak. When Sepia mates the male grabs the female by the face and inserts a specialized tentacle into an opening near the females’ mouth and then inserts sperm sacks. The male then guards the female until the eggs are laid a few hours later.
Giant Cuttlefish (picture taken by Dave Abbott)
The reputation of Sepia as ‘ chameleon of the sea’ is based on their unique capacity to rapidly change color by controlling pigment cells in their skin. Their skin functions like an extended brain, a bit like the eight arms of the Octopus, with two-thirds of its nerve cells located in the nerve cords of its arms (see my earlier Blog on Octopus). But in Sepia, the brain is reflected in its pigment carrying cells in the skin. These chromatophores produce colors varying between orange, red, yellow, brown or black. They are like color pixels on a computer screen, but instead of a computer program their skin cells are controlled by motor neurons in the brain, that in turn activate muscles around the chromatophores. The muscles constantly expand and contract in a complex choreography in direct response to the activity of motor neurons, which is displayed in rapid changes in the color of the chromatophores.
The color changes reflect the inner state of the cuttlefish. They may mirror certain ‘emotions’ like fear (for predators), sexual attraction (spotting a potential partner), anger (spotting a rival) or when using camouflage by mimicking the color seagrass of coral. The mating ritual of the Australian giant cuttlefish is particularly impressive with respect to color changes.
Groups of cuttlefish use complex codes, changing rapidly in often synchronous patterns of pigment changes reflecting processes of social communication, of which the meaning still remains a mystery for biological science.
Sam Reiter and his team from The Max-Planck Institute for Brain research, recently published an article in the journal Nature, describing in more detail the properties of cells in the cuttlefish skin, and neurons extending from the brain that control color changes. The team monitored changes in the state of the color-changing cells in living cuttlefish with high-resolution cameras, which allowed them to track the activity of tens of thousands of neurons simultaneously, for the first time. They also used electrical stimulation of muscles of the mantle of a small number of cuttlefish to produce artificial color changes and track motor units (nerve cells that make a muscle contract). Analysis of the pattern dynamics and rules that govern the development of skin pattern, showed that new pixels of the chromatophores (that changed recently) were mostly yellow and more mature pixels black.