Green sea turtles (Chelonia mydas) migrate long distances between feeding and nesting sites; and some swim more than 2,600 kilometers to reach their spawning grounds. Female turtles can store sperm from a single mating for the entire nesting season and males visit the breeding areas every year, attempting to mate. The eggs are round and white, and about 4-5 cm in diameter and hatch in the sand. The hatchlings remain buried until they all emerge together at night after around two months.Then they rush instinctively over the beach to the waterline. Nests may contain up to 200 eggs. But only few turtles reach adulthood in the oceans where they are a favorite prey for various predators. The highest rate of predation occurs in the water within the first 30–60 min of swimming as they pass through the cordon of predators found in the shallow water surrounding natal beaches
Left: Top: two separate nesting sites of green sea turtles along the Great Barrier Reef. ( adapted from **).
Along the east coast of Australia female and male green turtles mate in the vicinity of their nesting beach. The Northern Great Barrier Reef (GBR) is home to one of the largest green turtle populations in the world, with an estimated female population size of around 200,000 nesting females. Furthermore, two genetically distinct breeding populations of green turtles are found at the opposite ends of the Great Barrier Reef: the southern Great Barrier Reef (sGBR) stock and the northern Great Barrier Reef (nGBR) stock, with virtually no nesting occurring along the middle part of the GBR (see picture left). On the average 80% of the turtles at the GBR are female and 20% male.
Sex determination in turtles Reptiles are related to birds and have roamed the earth for more than 300 million years. Most reptiles including sea turtles do not have sex chromosomes. Instead, sex of newborns depends on temperature-dependent sex determination (TSD). TSD relates to temperatures experienced during the middle third of embryonic development. It was already known to hold for the green as well as the painted turtle (Chrysemys pica) populations in areas where cooler summers always produced mostly male, and warmer summers mostly female turtle offspring (see Janzen *). Janzen also found that nests of the common snapping turtle (Chelydra serpentina) incubated at 26°C produced 100% males, at 30°C produced 100% females, and at 28°C produced equal numbers of females and males.
How does TSD work? Pivotal temperatures are species-specific temperature ranges in which males and females are produced in equal number. The pivotal temperature is heritable but only a few degrees Celsius are needed to drastically alter this ratio to produce 100% males or 100% females spans. It still remains unclear how the developing embryo detects a thermal stimulus that apparently directs its sexual fate. Microscopic studies of the embryo gonads have established that steroid hormones could start the process once the temperature change is detected. In some turtles the critical temperature-dependent component appears to be synthesis of the enzyme aromatase, which converts androgens, such as testosterone, into estrogens. At higher temperatures, increased aromatase activity produces more estrogens, which biases the sex ratio toward more females.
TSD and fitness A common pattern of TSD in turtles is that warmer temperatures favor development of female and colder temperatures development of male hatchlings. This pattern would make sense from the point of view of natural selection if females gain more in lifetime fitness by developing in higher water temperatures compared to male turtles. Which would create a stronger tendency of females than males to avoid cooler beaches. For example, warmer temperatures may lead to a stronger greater adult body size, which has a greater effect on female fecundity than on male fertility. In line with this theory Janzen reported that hatchlings from the all-male and all-female producing temperatures had significantly higher first-year survivorship than did consexuals from the incubation temperature that produced both sexes from the incubation temperature that produced both sexes.
Climate warming and GBR nesting Species with TSD have existed for millions of years and coped with the selective pressures of a changing environment through adaptive changes of heritable traits. However, it is unlikely that these traits would evolve rapidly enough to keep pace with the current rate of climatic warming. A recent Australian/American study carried out on 441 green turtles of the GBR (**) has shed more light on the possible impact of climate change on changing sex ratio’s of the green turtles colonies of the GBR. Blood and tissue samples were collected from the turtles to determine sex, steroid hormones and DNA. The study found a strong sex bias depending on the location of the GBR where eggs hatched. In the cooler southern GBR nesting beaches there was a moderate female sex bias (65%–69% female) in turtles, but turtles that originated from northern GBR nesting beaches were extremely female-biased (99.1% of juvenile, 99.8% of sub-adults, and 86.8% of adult-sized turtles; see also figure above, lower panel). The study further indicated that northern GBR green turtle locations have been producing primarily females for more than two decades and that the proportion of females has increased in recent decades.
Conclusion With average global temperature predicted to increase 2.6C by 2100, many sea turtle populations are in danger of high egg mortality and female-only off-springs. A high female bias as such might not create a problem, since only few males are needed to fertilize an entire colony of females. But an extreme female bias would create a problem in finding a male at all, and/or lead to genetic impoverishment due to lack of genetic variation in a relatively small group of males. Visits of breeding males from other populations might mitigate extreme feminization, but this would be less likely to occur at the northern GBR, where visits of male turtles from the south are less likely to occur.
Extreme incubation temperatures not only produce female-only hatchlings but also cause high mortality of developing clutches. A some beaches in Florida the strain of surviving at elevated temperatures and resulting heat stress seems to weaken the hatchlings and slow down their run to reach the shoreline in time. Researchers are now exploring plans to cool the nests, like using shade cloth or putting water on top of them.
*Janzen, F.J. (1994). Climate change and temperature-dependent sex determination in reptiles. Proc. Natl. Acad. Sci. USA 91, 7487–7490.
Janzen,F.J. (1995) Experimental evidence for the evolutionary significance of temperature-dependent sex determination, Evolution 49 864 – 873.
**Jensen, M.P. et al. (2018), Environmental Warming and Feminization of One of the Largest Sea Turtle Populations in the World. Current Biology 28, 154–159
***Spencer, R. J.; Janzen, F. J. (2014). "A novel hypothesis for the adaptive maintenance of environmental sex determination in a turtle". Proceedings of the Royal Society B. 281
Hake, L. & O'Connor, C. (2008) Genetic mechanisms of sex determination. Nature Education 1(1):25
Warner DA, Shine R (2008). "The adaptive significance of temperature-dependent sex determination in a reptile". Nature. 451(7178): 566–568. .
Pen, Ido, et al. (2010). "Climate-driven population divergence in sex-determining systems". Nature. 468: 436–439.
Valenzuela, Nicole; Fredric J. Janzen (2001). "Nest-site philopatry and the evolution of temperature-dependent sex determination" (PDF). Evolutionary Ecology Research. 3: 779–794. Retrieved 7 December 2011
Mrosovsky, N., and Yntema, C.L. (1980). Temperature dependence of sexual differentiation in sea turtles: implications for conservation practices. Biol
Luckily our world is still full of birds and insects that produce a variety of sounds. Even the sounds of some insects are pleasant to listen to. Like the songs of crickets and the Cicadas or Cigales of the Provence in France. The male cicadas can produce exceptionally loud sounds by using two small membranes under each wing that vibrate rapidly when pulled by tiny muscles. The male abdomen is largely hollow, and acts as a sound box, similar to a cello instrument. On hot summer days whole groups of males congregate in the plane- or pine trees and synchronize their sound to establish chorusing centers that fill entire streets or market squares. The reason why is to attract the females. And perhaps also to entertain the summerguests having lunch under the pine trees with a glas of cold vin rosee within reach.
..Oops, almost forgot that this is about fishes not cicadas! In contrast with birds and insects the reputation of fishes is not mainly based on the sounds that they make. Nevertheless, many fishes in the Atlantic or tropical oceans create different types of sounds using different mechanisms and for different reasons. Research of sound production in fish is still in its infancy. But scientists using recorded fish vocalizations with underwater microphones have identified many fish species that make amazing sounds, either as individuals or in groups. Based on this evidence, it now seems that hundreds of species of marine (and probably also freshwater) fishes are capable of generating acoustic signals
Left: Upper figure: the sound producing mechanism of the catfish in the base of its left pectoral fin (source **). Lower figure: swimbladder (SB), sonic muscle (SM) and sonic nerve (SN) in the northern searobin (adapted from ***)
Active sound production in fish often depends on time and space. Many fishes become ‘talkative’ in the breeding season, following a seasonal and/or diurnal cycle. In contrast with the sophisticated sonar sounds of marine mammals like whales and dolphins, the vocabulary of fish species is limited, and meant to communicate gross information. It could signal danger, distress, competition or attracting a female. Fish vocalizations can take a wide variety of forms, including clicks, purrs, grunts, groans, growls or barks. They may be intentionally produced as signals to predators or competitors, to attract mates or as territorial display.
There are globally two ways in which fishes make sounds. One is stridulation: striking or rubbing together skeletal components. Crickets use stridulation, as well as marine catfish and seahorses. For example seahorse species make clicking and/or snapping sounds by rubbing together bony edges of the skull and the coronet, a crown-shaped plate on the its head. These sounds are possibly amplified by the swim bladder. Some marine catfishes (Arius felis and Bagre marinus) have specialized pectoral fin spines that make a stridulatory squeaking sound. They do so by rubbing the base of the pectoral fin spine against the pectoral girdle (see picture above). The sound can even be heard at the surface by anglers. Trigger fishes can produce drumroll sounds that result from alternate sweeping movements of the right and left pectoral fins, which push a system of three scutes (bony scales) that are forced against the swimbladder wall. Other fishes use their swim bladder to produce sounds. A muscle attached to the swim bladder called the sonic muscle contracts and relaxes in a rapid sequence (see picture above). This action causes the swim bladder to vibrate and produce a low-pitched drumming sound. Examples are the goliath grouper, black drum, toad fish and silver peak. Not to forget the oyster toadfish, that is able to contract its muscle at a rate of 200 times a second. The swim bladder can either function as the actual sound generator itself, or as an amplifier for sounds generated by other body parts including e.g. the pectoral girdle, fin rays, various other bones or tooth in front of the mouth.
Along coral reefs of the Red Sea clownfish, triggerfish, damselfish and angelfish often make loud clicks, drum rolls, or ‘pangs’ during agonistic interactions, in distress and or when divers get too close to their territory (see audio gallery). Other male fishes, mostly found in the greater oceans or their coastlines are known to create very strange and often very loud sounds. The sounds mostly serve to attract females in the mating or spawning seasons. Here follow examples of some notorious sound producing species.
Catfish Catfish can become very large and are found in the sea, coastal water and rivers. The squeeker catfish (Synodontis eupterus) make a croaking sound by rubbing the spines located in their pectoral fins into grooves on their shoulders as shown in the picture above. Talking catfish (Platydoras armatulus) can produce sound in two ways—by vibrating their swim bladder or by vibrating their pectoral fin spines in their sockets.
Oyster toadfish (Opsanus tau) is a froglike fish from the family of Batrachoididae. The fish has a distinctive "foghorn" sound used by males to attract females in the mating season (audio gallery and toadfish song). Males make nests, and then attract females by "singing", that is, by releasing air by contracting muscles on their swim bladders. Attracted by the foghorn sound, the female comes into the nest, lays eggs, and then leaves.The sound can be loud enough to be clearly audible from the surface.
Plainfin midshipman (Porichthys notatus) is bioluminescent. The plainfin midshipman is another type of toadfish, found off the west coast of North America from Alaska to Baja California. During the mating season the male midshipman hums—sometimes for long periods—by hitting his swim bladder with his sonic muscle. His humming serves to attract a female. Once she deposits her eggs, the midshipman resumes humming to attract another female to his nest. The male guards the eggs until they hatch. Typically these fishes are nocturnal and bury themselves in sand or mud in the intertidal zone during the day.
The black drum (Pogonias cromis) is a black or greyish fish that lives in the brackish water found in areas such as estuaries. The young fish have black stripes that fade as the fish matures. Black drums are mainly bottom feeders. Adults can become very big and may weigh over a hundred pounds. Black drums become very noisy during the mating season (audio gallery). The low pitched sounds that they produce travel long distances. Again, males produce the sounds to attract females. The fish use their swim bladder and sonic muscle to create the vocalizations. Sometimes black drum mating calls are conducted by the ground and the seawalls and then enter nearby houses on the shore.
Herring communicate with each other by expelling gas from the anal area, producing bubbles and a high-pitched sound. Inventive researchers have called this sound production a FRT (Fast Repetitive Tick). Both the Atlantic herring (Clupea harengus) and the Pacific herring (Clupea pallasii) produce FRTs. The fish swallow air from the water surface and then store it in the swim bladder, so the air is not the result of digesting food. During the night, in darkness and when surrounded by other herring, air is released through the anal duct. The purpose of the collective FRT sounds may be to ensure that the fish stay close together.
Corvina The physical act of reproduction can be a noisy affair for many fishes, but beats all records for the Gulf Corvina (Cynoscion othonopterus). Every spring, millions of these large gray fish migrate to the Colorado River delta and sync their spawning to the tides and the phases of the moon. The magnitude of their sounds (produced by rapidly beating their swim bladders with the sonic muscles) can become even deafening during simultaneous chorusing of males within the larger spawning aggregation. These sounds can even extend up to 27 km distance along the main channel of the Delta and include 1.5 million individuals during a single spawning period. At the loudest the sound level of the chorusing fishes was more than 150 decibels (see corvina). The corvina is endemic to the Northern Gulf of California and faces imminent risk of species extinction due to overfishing of its spawning aggregation, and regulations that allow overfishing to persist*. In the spawning season local fisherman find it easy to locate the fishes by their sounds even from their small boats. With one net they often catch hundreds of corvina’s in a few minutes.
Sources and links
*Erisman BE, Rowell TJ. 2017 A sound worth saving: acoustic characteristics of a massive fish spawning aggregation. Biol. Lett. 13: 20170656
Popper AN, Fay RR, Platt C, Sand O. 2003 Sound detection mechanisms and capabilities of teleostfishes. In Sensory processing in aquatic environments(eds SP Collin, NJ Marshall), pp. 3–28. New York,NY: Berlin, Germany: Springer-Verlag.
Ben Wilson, Robert S. Batty and Lawrence M. Dill 2003. Pacific and Atlantic herring produce burst pulse sounds. Proc. R. Soc. Lond. B (Suppl.) 2003
Discovery of Sound in the Sea. University of Rhode Island and Inner Space Center. http://dosits.org/galleries/audio-gallery/fishes/
http://www.sfu.ca/biology/faculty/dill/publications/,FRTing_herring_Wilson_et_al.pdf (accessed August 26, 2017).
The whale shark (Rhincodon typus) is one of the many shark species I have never encountered. Perhaps one day when I am done with the Bahamian sharks (will I ever?), I shall cross the Atlantic to visit these gentle plankton gulping giants at Islas Mujeres. During the summer months hundreds of whale sharks gather just north of the island in a seven mile radius to take advantage of the plankton rich waters created by the joining of the Gulf of Mexico and the Caribbean Sea.
Left: The region behind the gills of whale sharks (above) exhibits suitable variation in spot pattern to enable individual recognition using image-matching software (below: see Arzoumanian et al. 2005).
Whale sharks have recently been upgraded from vulnerable to endangered under the IUCN Red List of Threatened Species. There are still numerous reports of individuals being killed for food, including having their fins removed for soup. Their characteristics – docility, being the largest of all extant sharks, and tendency to aggregate seasonally at several accessible locations worldwide – have encouraged ecotourism and citizen-science opportunities, but also mean they can easily be targeted by fisheries in jurisdictions where they are not afforded protection.
Adult’s whale sharks grow slowly and only reach sexual maturity when they are 30 years old, with their maximal age varying between 70 and 100 years. It seems that nobody has yet seen whale sharks mating or a female giving birth to her pups. A female species captured in 1996 appeared to have around 300 pups in her womb each about 40 cm long. There are several whale shark aggregation sites -or hotspots- spread over the world such as the Philippines, Islas Mujeres in Mexico, Belize, South Africa, Mozambique, Qatar and the Gulf States, Galapagos Islands and the Maldives.
Still little is known about the whale sharks migratory habits and habitats. A recent paper in Biomagazine* however described a large scale project of Australian researchers called EcoOcean. Central in the project is the Wildbook for whale sharks: a visual database of whale shark encounters of individually catalogued whale sharks. Here people can assist with whale shark research by submitting photos and sighting data. Since 1992 the library has collected around 30.000 pictures of whale sharks over 57 countries, many taken by snorkelers and scuba divers visiting one of their hotspots. This included in particular Ningaloo Reef and other sites along the along the extended Western Australian coastline. The ever growing data-base caused the number of 14 known hotspots to increase to 20. Top four sightings of whale sharks were from the locations Mexico-Atlantic, Western Australia (Ningaloo Marine Park), Mozambique and the Philippines (e.g. Cebu)
The project has contributed to a better understanding of the wale sharks mobility as well as their regular habitats. An unique aspect of the project is that creation of the picture library was realized with the help of Citizen Science: pictures taken by snorkelers or scuba divers while visiting popular sites of whale sharks. The technique used to identify the sharks in the EcoOcean project is straightforward, and based on pattern recognition software using the white dot patterns behind the gills of the whale shark as a standard (see picture above). This particular area on the shark’s skin needs to be photographed in the correct orientation. Participants also upload, when possible, other relevant sighting information for storage and future analysis, including sighting location, sex, and estimated total length
As noticed in my earlier Blog, using natural body markings is an alternative and shark-friendly way to identify individual sharks, although it does not allow real time tracking of the shark. But it is much cheaper than tagging of sharks with telemetric devices, a more invasive method that can only be applied on a relatively small number of species. An it also allows to search for individual sharks by comparing its unique pattern with a larger data base (somewhat like a fingerprint of a foreign visitor collected by the US customs at the airport). Another interesting side of this method is that UW photographers can contribute to collect such a data base.
The first whale shark that entered the library was a wale shark called Stumpy, because of the unusual shape of the upper lobe of its caudal fin. Stumpy’s unique patterning was later identified in 69 photographs submitted to the library between 1995 and 2016. During these two decades, Stumpy’s total length remained relatively unchanged at 7.40 m which closely matches the 7.44 m estimates calculated from lengths reported by citizen scientists between 2008 and 2015. Photographic evidence also showed that Stumpy’s claspers had become elongated for the first time in 1998 and 2001, respectively – an indication that they had attained maturity*. Other whale sharks from the data base are A-103, a faithful customer of Ningaloo reef in Australia estimated to be about 21 years, and BZ-011 that has been spotted over 5 years along the coast of Belize, but nowhere else. A surprising exception was A-424 who showed far more mobility. After been identified first in Australian waters, she was spotted later in Indonesia about 2.700 km further to the north-west. Smallest species were found in Indonesia (4 meter) largest in Galapagos (11 meter). In general, long distance migratory routes were extremely rare. However this did not rule out the possibility that the date base might cover only a small part of the world population, or that the identified whales sharks had visited other more remote and unknown locations.
Taking these cinsiderations in account, Citizen Science seems to have demonstrated that:
-Whale sharks are far less migratory than other shark species (like for example tiger sharks ak)
-Exhibit a high degree of site fidelity with longevity conservatively estimated to be at least 80 years
-Males and females are strongly segregated with a preference for their own distinct habitats (in accordance with other predator sharks ak). For example at the Galapagos, 99% of sexed individuals were female while in Maldives and South Africa, only 9.43% and 9.60%, respectively, of the sexed whale sharks that were submitted were females
Sources and links
*Norman B. et al. (2017) undersea constellations: the global biology of an endangered marine megavertebrate further informed through citizen science Bioscience xx: 1–15. Published by Oxford University Press on behalf of the American institute of Biological Sciences
Hsu HH, Joung SJ, Hueter RE, and Liu KM. 2014. Age and growth of the whale shark (Rhincodon typus) in the north-western Pacific. Mar Freshwater Res 65: 1145–54.
Norman B and Stevens J. 2007. Size and maturity status of the whale shark (Rhincodon typus) at Ningaloo Reef in Western Australia. Fish Res 84: 81–86
Arzoumanian Z, Holmberg J, and Norman B. 2005. An astronomical pattern-matching algorithm for computer-aided identification of whale sharks Rhincodon typus. J Appl Ecol 42: 999–1011.
Davies TK, Stevens G, Meekan MG, et al. 2013. Can citizen science monitor whale-shark aggregations? Investigating bias in mark–recapture modeling using identification photographs sourced from the public. Wildlife Res 39: 696–704
The Sperm whale (Physeter macrocephalus) is a highly social and gentle mammal. Its behavior certainly not justifies the reputation of ferocious monster created by Melville when the describes the deadly struggle between the white whale Moby Dick and the obsessed captain Ahab.
Left: skin pattern of a sperm male after exfoliation of the skin (author unkown)
Sperm whales are the largest predators on earth. They also have the largest brain in the animal kingdom, and can dive over 7000 feet deep to find their favorite food, the giant squids. It thanks its name to its enormous head, almost one third of the size of its body (15-18 meters). The large head of the sperm whale functions as one big sonar system. The sperm whale produces ‘clicks’ (sound burst of short duration) with a pair of phonic lips (also known as "monkey lips" or "museau de singe") at the front end of the nose, just below the blowhole. Sperm whales often swim in small social units or pods, although several pods may form larger groups or clans distributed over a much larger area. Recent studies have shown that families and clans have their own 'dialect': typical signatures of sound bursts called codas. Using these codas sperm whales recognize vocalizing individuals of other social units that share a similar dialect.
Although sperm whales usually swim in small groups there are periods when they gather in much greater numbers at certain locations. These periodical meetings have the character of ritual meetings or ‘get togethers’ were several clans meet members of other clans. Resembling the pow wows, the social gatherings held in the past by different Native American communities.
An area where these gatherings can be observed by snorkelers is Dominica in the Western Carribean. The ocean floor along Dominica's west coast drops steeply to several thousand feet very close to shore, providing a calm and sheltered area for a large group of resident sperm whales to feed, mate, and socialize. Year round, they can be spotted very close to shore, cruising up and down the island's coast. On occasions as many as 70 animals come together for hours or days at Dominica. Here some remarkable forms of behavior have been observed and photographed by UW photographers Keri Wilk from Canada and Tony Wu from the USA, which also has shed more light on the function of these gatherings. It appears that the major incentive for these cetaceans to gather periodically is highly practical, namely to groom each other by rubbing their massive bodies and itching skins together. The result of skin rubbing shows up in large chunks of skin floating on the surface giving the impression of large plastic bags. As a result the whales start to show white and black camouflage-like skin patterns, where patches of new skin show up against the old scraped of skin (see picture above).
According to Luke Rendell, a marine biologist at the University of St Andrews, UK. this shedding of skin is part of a natural antifouling mechanism to stop them being encrusted with other marine animals and parasites. “They love touching against each other and one of the rewards may be exfoliation,” says Rendell. Even more remarkable than the skin rubbing rituals (called ‘scratchatons’ by Tony Wu) are the concurrent defecations when the whales disperse clouds of liquid poop looking like chocolate milk in the water. The whales often show prolonged bowl movements, unlike the normal defecations when shark return to deeper water. One can only guess about the meaning of these Poonados (a term used by Keri Wilk). Pooping can occur as a a defensive reaction or a sign of anxiety, as sometimes observed in smaller whale species when feeling insecure. The clouds produced during defecation could also be a form of camouflage, like the ink-clouds emitted by the octopus. Another possibility is that adult bulls in the pod use these clouds to impress rivals or females from other clans. Finally, whales could simply enjoy the experience, with the reaction to empty their bowels triggered by with the pleasant sensation of skin rubbing. Whatever the reason, group defecation seems to be an integral part of large social gatherings for these animals...a group poop, so to speak. Another question that remains to be answered is if these mass gatherings perhaps also serve to swap between members of the pods, for example when females or young bulls hop over to other pods where their presence is more urgently needed to guarantee new offspring.
Source and links:
In Greek antiquity the Chimaera was a mythical monster, depicted as an incongruent animal with the head of a roaring lion spitting fire, the body of goat sticking out in the middle and a vicious snake-like tail. The word also stands for a delusion; like in the french chimère. In our oceans the chimaera fish (alias: ghost shark, rabbit fish, spook fish) earned its name because of its strange body shape: a composition of a birds beak, a fishlike body and long slender tail (see picture at left). The rat-like tail is the reason why certain species are called ratfishes. Chimaera seems to have diverged from its shark relatives around 400 million years ago. The ways of evolution are often mysterious and hard to unravel. Which holds also for its products, the creatures that have lived in the seas for millions of years, and probably even more so for the chimaera.
Considering the diversity of 'shark like species' it is good to start with a taxonomic classification. Chimaera, skates, rays and sharks are all cartilaginous fish, belonging to the class of Chondrichthyes with a cartilaginous skeleton, and claspers in the males. There are two subclasses of cartilaginous fishes: the Elasmobranchii (with sharks, rays and skates and the sawfish) and the Holocephali (chimaera), indicating that members of Elasmobranchii are more closely related to another than to the chimaera. Further down the taxonomy we have families and species. Chimaera, or rather the order of Chimaeriformes consists of many different species with different outlooks and habitats. Some species have several synonyms or aliases which make the naming process a bit messy. But taken together they end up in 50 accepted species that are assembled in three families:
Families and species
-Callorhinchidae (Plownose chimaera: alias elephant fish and ghostshark, only one accepted species) are the oldest clade in the evolution thee. They have an elongate and flexible snout bearing a hooklike structure and are mostly found on the Southern hemisphere.
-Chimaeridae (Shortnose chimaera or ratfishes; 40 species) have a short and rounded snout. They are found in Atlantic, Pacific, and Indian oceans in temperate to tropical waters, mostly below 200 m. Some popular species are Chimaera monstrosa alias rabbit fish (the only species in the Mediterranean), the spotted ratfish (Hydrolagus colliei) in the north-eastern Pacific Ocean, and the small-eyed rabbitfish (Hydrolagus affinis).
-Rhinochimaeridae (Longnosed chimaera; 8 species) have a long and pointed snout, lacking a hooklike process and living worldwide in temperate and tropical seas. Some weird looking species are: Narrownose chimaera, alias Harriotta raleighana alias spookfish, the long nosed Rhinochimaera pacifica and the paddlenose chimaera (Rhinochimaera africana) with its flattened paddle-like nose.
Anatomy and habitat Chimaeriformes have large rabbit-like eyes, and a large head along with a tapered body. The large translucent-green eyes are adaptions of the darkness of water at greater depths. Although chimaera has some characteristics in common with sharks and rays, there also big differences. One example is the head with the small mouth and lips, and upper jaws that are fused with their skulls. Chimaera also miss the row of sharp teeth of sharks, but posses three bony tooth plates with fused teeth, forming a ideal beak to break hard shells. Their diet consists mainly of bottom-dwelling invertebrates like sea urchins, crabs, shell fish, crustaceans and starfish.
Chimaera has two large dorsal fins, the first erectile high. with a short base and preceded by an erectile poisonous spine, the second nonerectile low, and with a long base. Their bird-like style of swimming with the spread out big pectoral fins resembles the propulsion of rays. Like sharks they are equipped with electroreceptor cells on their snout for orientation. But unlike sharks and rays, chimaera has a single external gill opening, covered by a flaps or opercula as in the bony fishes, on each side of the body. Breathing water chiefly occurs through the nostrils.
Chimaera species vary in size between 60 cm and 1.5 m and live in temperate oceans. They tend to dwell on muddy or sandy seafloors, often down to 2,600 m deep, with few occurring at depths shallower than 200 m. This is also the reason why pictures of these species are rare. Exceptions include the members of Chimaeridae, like the rabbit fish and the spotted ratfish, which locally or periodically can be found at relatively shallow depths. Many species that live on or just above seafloors at greater depth have become the victim of deep sea trawlers.
Reproduction Chimaera are oviparous. Male chimaeras have claspers formed from the posterior portion of their pelvic, one of which is used to inseminate the femal. In addition, they possess a supplemental clasping organ, the tenaculum on the forehead, which is thought to aid in holding the female during mating. It is only visible during copulation and then used to clamp onto the female’s pectoral fin. This remarkable stalked club structure with little hooks must have been a clever adaptation of their evolutionary ancestors that lacked the rows of sharp teeth that male sharks use to hold their mating partner in a steady position. The females lay eggs in spindle-shaped, leathery egg case. Finally, there is a second grasping structure, the pre-pelvic tenaculum, just before the pelvic fins that also allow the male to anchor into position.
Sources and links
Nelson, J.S., 1994. Fishes of the world. Third edition. John Wiley & Sons, Inc., New York. 600 p.
Didier, D. A., Kemper, J. M., & Ebert, D. A. (2012). Phylogeny, biology, and classification of extant holocephalans. Biology of sharks and their relatives, 2nd edn. CRC Press, New York, 97-124.