The green sea turtle (Chelonia mydas) is listed as endangered by the IUCN and CITES and is protected from exploitation in most countries where the species nests. Factors threatening its existence are direct harvesting for the consumption of its flesh, boat strikes, egg poaching, and habitat destruction, such as the sandy beaches that form their egg nesting sites. Worldwide, hundreds of thousands of sea turtles a year are accidentally caught in shrimp trawl nets, on longline hooks, and in fishing gillnets. The dramatic increase of tourism on the Carribean island also had a negative impact on the turtle populations.
Turtles may swim more than 2,600 kilometers to reach their spawning grounds, and then return to the beaches on which they were born to lay their own eggs. The green turtle is found on both the Caribbean and Pacific coastlines and can reach a weight of up to a hundred and eighty kilograms. The extensive, shallow continental shelf of eastern Nicaragua is home to hundreds of thousands, possibly millions, of green turtles that forage on the abundant seagrass that grows there. The turtles help to improve the health and growth of the seagrass beds. Although trade in turtles is prohibited, Nicaraguan law still allows the subsistence use of green turtles, and local demands from coastal Indian inhabitants have supplanted the historical export demand. Here the demand for its meat is estimated to involve approximately eleven thousand killings each year. Hunting these turtles is affecting the turtle populations, as the green turtle takes between twenty to fifty years to reach sexual maturity (they may live on to 80 years) and without sexually mature adults, the numbers of these turtles could decline rapidly.
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. 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