The name porpoise is a strange assembly of fish and swine. It’s stocky body and bulbous head with a small blunt snout was probably why the species was called porcopiscus in Latin which is a compound of porcus (pig) and piscis (fish). The species' taxonomic name, Phocoena, is derived from the Greek phōkaina, which in turn comes from φώκη (phōkē) or seal. Suggesting that the ancients might have mistaken the porpoise for a seal.
Left: Harbor porpoise. Picture by F.Graner
Porpoises are small toothed whales (Odontoceti) that are closely related to oceanic dolphins. The porpoise, however, has always remained more mysterious than its relative the dolphin. Perhaps because they are less wide-spread over the world than the dolphins, and are also not easily spotted in the open sea. Porpoises rarely jump out of the water like dolphins and even then you are not likely to see more than the top part of its back with its small triangular dorsal fin when surfacing for a breath of air. Porpoises also have a compact body shape with a stiff neck while the dolphins have a long beak and flexible head.
Seven extant species of porpoises The porpoises belong to the order of the toothed whales, consisting of around 70 species. Odontocetes feed largely on fish and squid, not rely on their sense of sight, but rather on their sonar to hunt prey. They echolocate by creating a series of clicks emitted at various frequencies. There are now only seven extant species of porpoises that fall in the family of Phocoenidae. They are in respective order:
- Genus Phocoena (four species): the harbor porpoise, vaquita, spectacled porpoise, and Burmeister’s porpoise
- Genus Neophocaena (two species): the finless porpoise and narrow-ridged finless porpoise
- Genus Phocoenoides (one species), the Dall’s porpoise
As said, porpoises are not widespread, with some species specializing near the polar regions, usually near the coast. The most frequent species, the harbor porpoise (Phocoena phocoena) lives in the shallow, relatively cold northern coastal seas. One believes that there were once huge populations living in these waters, when their favorite food, the anchovies, was still abundant. The reason why the species was called harbor porpoises was that they probably often followed the fishing boats into the harbors. Because it is most commonly found in bays, estuaries, harbors, and fjords the species is particularly vulnerable to gillnets and fishing traps, pollution, and other types of human disturbance, such as underwater noise.
In past decades, pollution, particularly PCBs, caused a sharp decline in the population of harbor porpoises along all of the coastal areas of the southern North Sea. According to a recent study an increasing number of stranded harbor porpoises have been found in the last three decades on the beaches of Belgium and Holland: an amount of 16.000, mostly young male species*.
Porpoises were also highly affected by bycatch. Many porpoises, mainly the vaquita, are subject to great mortality due to gillnetting. The vaquita is a species of porpoise endemic to the northern part of the Gulf of California that is on the brink of extinction. The Dall's porpoise from the northern Pacific is still extensively harvested for meat in Japan. Stranded porpoises often got killed by drowning after they became entangled in fishing nets. The ‘pinger'', a loudspeaker tied to a floating buoy, is used by fishermen to keep the harbor porpoises away from their boats. But these devices could also serve to tell the porpoise that ’dinner is ready’, so rather attracting the animals than keeping them at a distance. Along the northern Dutch coast porpoises have been found with bites resembling the tooth of the grey seals, which often show up in the North Sea. Suggesting that adult grey seals are true predators, even preying on porpoises. The good news is that in the open North Sea the porpoises now seem to be back again, feeding mostly on herring, sprat and mackerel, and even smaller species like gobies. The estimate is that there are presently living around 250.000 species living in the North Sea, making them the most common cetacean in these waters.
Hibernation is a state of constant hypothermia (low body temperature) and low metabolism. This can often take a long period. The evolutionary advantage of hibernation is that a non-migratory mammal can survive during the winter without having to spend energy searching for food, which is then difficult to find. There are facultative hibernators entering hibernation only when either cold-stressed, food-deprived, or both, and obligate hibernators, who enter hibernation regardless of ambient temperature and access to food.
Fishes and reptiles Body temperature in fresh- and saltwater fishes including larger predators like sharks reflects the temperature of their watery environment: they are ectothermic. They do not hibernate in the strict sense because they cannot actively down-regulate their body temperature or their metabolic rate. However, some species experience decreased metabolic rates called dormancy, associated with colder environments and/or low oxygen availability (hypoxia). Water also makes a good shelter for freshwater fishes as well as reptiles such as frogs and turtles. When the weather gets cold, they move to the bottom of lakes and ponds. There, they hide under rocks, logs or fallen leaves or may even bury themselves in the mud where they become dormant. Cold water holds more oxygen than warm water, and the frogs and turtles can breathe by absorbing it through their skin. The same holds for the common goldfish (Carassius auratus) in domestic ponds that are able to survive in temperatures below 10 degrees Celsius, even when the pond is covered with ice, as long there is some oxygen available. With European winters becoming increasingly mild, goldfish may delay or even skip their annual period of a dormant state as long a there remains some food available from the surface.
Sea mammals A different situation holds for sea mammals. Whales are warm-blooded (endothermic) and will keep a high body temperature that does not change in the colder water. In order for whales to keep warm in cold/polar climates they have developed a thick layer of insulating blubber, which protects against freezing winds and icy water. Whales also profit from migration to colder oceans where food like krill is more abundant. Manatees, however, that lack the protective blubber do migrate from the sea to warmer water in winter, often found in inshore freshwater springs, or even power plants along the shore.
Climate change and hibernation To what extent do rising winter temperatures affect hibernation and animal's chances of survival? If the bees, hedgehogs or bats get out of their winter rest too early due to high winter temperatures, then there is far too little food (e.g. insects) available. When two periods of frost are separated by one warm week, the hedgehog is in trouble. When it awakes, it is hard for the animal to get back to sleep. And a hedgehog that is awake but unable to find food will not survive in the cold. Even the state of dormancy, accompanied by minimal use of the body resources and slowing down of physiological functions (think of our goldfish) could be essential, not so much to overcome a temporary shortage of food or lower body temperature, but as a rest period allowing recuperation and recovery of metabolic functions.
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.