Blennies belong to the suborder of Blennioids, or ‘slimy’ fishes according to the Greek τό βλέννος that stands for mucus or slime. Together the Blennoids form a incredibly large number of small fish species, that are often hard to tell apart at first sight. The Blennoids consist of six families. The largest and best known family are the Blenniidae also known as Combtooth or Scaleless blennies, that contains around 50 genera and 400 known species. They are followed by two other large families: the Clinidae or Scaled blennies found in all oceans, and the Chaenopsidae, a strictly tropical family, ranging from North to South America The blenny resembles the goby, with one difference; the goby has fused pelvic fins that form a disc-shaped sucker. This sucker is functionally analogous to the dorsal fin sucker possessed by the remoras. Another difference is that the goby has a habit of digging out his burrow and sifting sand, which is not a habit of the blenny'.
Left: Tompot Blenny. Picture taken by Alex Tattersall (2007). In the UK, swanage pier dorset is a favorite spot to meet the Tompot blennies.
Anatomy Combtooth blennies have blunt heads and large eyes, with large continuous dorsal fins, which may have three to 17 spines. The frontal part of the dorsal fin is often higher than the back part. Their name comes from the comb-like teeth lining of their jaws. Males and females are quite different, with the male being much more colourful particularly whilst breeding. The swim bladder is usually absent in adults which will make them sink to the bottom. The bodies are compressed, elongated and scaleless. Most species have two rays just anterior to their enlarged pectorals inserted near the throath, and a pair of branched tentacles above the eyes called cirri. The often branched tentacles can vary widely in form and size dependent on the species. Cirri could be are an additional sensing organ that helps the blenny to read the current and know which way the food will be coming from, as well as to help them to anticipate the approach of predators. They could also function as an attribute to impress its enemies or distract its prey. The blennies eyes can function independently, giving them that goggle-eyed look that enables them to look in two directions at the same time, keeping careful track of both prey and predators. Their big eyes, colourful faces and bizarre horn-like structures on their tiny heads make the blennies a favorite target for underwater macro photographers. If… they are lucky to get a good close shot of them, in particular of the head. Although I am not a blenny expert, I find the best blenny shots those where the photographer has succeeded to capture the blennies head with all its fine features and colours, in particular its eyes, crisp and sharp. Best opportunities are probably those when the blenny is lying still, for example on a rock or peeping out of a tube or bottle against a neutral background. Experts advise to bring the camera down to the level of the eyes of the blenny, and then patiently wait for the moment when the blennny will look at you*. Macro should be your first option, but if the blenny is not too small, a shot taken with a fish eye lense and mini dome at close distance will probably also look nice.
Behavior and habitat Combtooth blennies are found in tropical and subtropical waters in the Atlantic, Pacific and Indian Oceans; some species are also found in brackish environments and in tide pools. They often spend their entire life cycle on one general location. For some species small crstiaceans, mollusks, and other sessile invertebras are the primary food items but others may eatalgae or plankton. Combtooth blennies spend much of their time on or near the bottom. They may inhabit the rocky crevices of reefs, burrows in sandy or muddy substrates, or even empty shells. Generally found in shallow waters, some combtooth blennies are capable of leaving the water for short periods during low tide, aided by their large pectoral fins to crawl around. Blennies have an undulating swimming style and strong teeth. Females lay eggs in shells or under rock ledges; males guard the nest of eggs until hatching. Most blennies are voracious, mutually agonistical and curious. If you watch long enough, you will often see a blenny dart out of its hole and grab a meal out of the water column. Often the blenny wil lie on a rock or sandy bottom, but some prefer little crevices, and even empty cans, bottles or tubes to hide in.
Some species. Here follows just a grasp from that innumerable number of blennies. Some better known combtooth blennies (genus in Italics) are the Molly Miller (Scartella christata), Tompot Blenny (Parablennius gattorugine: see picture above), the Butterfly Blenny (Blennius) with a big blueish-black spot on the frontal part of the dorsal fin, the Adriatic Blenny (Microlippophrys adiaticus, no cirri) and the Blackhead Blenny (Microlippophrys nigriceps) from the Mediterranean. The Horned Blenny (Parablennius tentacularis) is a larger species often found in the Mediterranean (see frontpage for an example).
A particularly interesting group are the rockskippers (Salarius fasciatus). They are amphibious blennies, living in the surf and splash zone of rocky coasts. With twisting, jumping movements, they propel themselves over coastal rocky surfaces. These blenny species constitute a parallel evolutionary development to the mudskippers, amphibious gobies living in the mud out of the water. Similar ecological conditions have brought about similar adaptations. Then we have the beautifully coloured Peacock Blenny (Salaria pavo) and the False cleaner fish (Aspidontis taeniatus). These blennies have fang-like teeth with venom glands at their bases and are noted for their clever mimicry of cleaner wrasses Labroides dimidiatus.
Other families Scaled blennies belong to the large family of Clinidae that inhabit temperate oceans primarily south of the Equator. Dazzling and varied colors and markings differentiate the species. The largest clinid, one of the many pointy-headed blennies, is the 24-in (61 cm) giant kelpfish (Heterostichus rostratus), which inhabits the Pacific shoreline from British Columbia to Southern California, while the 8-in (20-cm), blunt-headed, hairy blenny (Labrisomus nuchipinnis) lives in the tropical waters off both Atlantic coastlines. But the Clinidae family also contains some of the smallest blennies —the female of the species Tripterygion nanus found in the Marshall Islands, is fully grown at less than 0.75 in (1.9 cm) in length. The Secretary and Spinyhead Blennies are specific tube blennies in the genus Acanthemblemaria and are closely related. These blennies belong to another family of blennies called Chaenopsidae which include pikeblennies, tubeblennies and flagblennies. As the name suggests, the tube blenny lives in the vacated tubes of Calcareous Tube Worms and seem to prefer locations in plenty of light at the top of coral heads. At no more than two inches in length they are very hard to see and even harder to tell apart.The pike blenny (Chaenopsis ocellata) is a tube-dwelling species found in Florida. Male pike blennies jealously defend their territories from other intruding males by aggressively displaying a stiffly raised dorsal fin and a widely gaping mouth. Two males may literally face off, gaping mouths touching, until one snaps its mouth shut on the other. These blennies will often create great photo opportunities.
Source and links
The Tresher shark is a rare mostly solitary shark that, like other marine apex predators, has a difficult time in surviving. It’s meat and fins make it a favorite target for offshore fisheries, which has put it on the IUCN Red List of vulnerable species. Many UW photographers that have succeeded to ‘shoot’ Tiger, Oceanic and Great Hammerhead sharks from a short distance, have not yet been able to come close to a tresher. My only encounter was many years ago in the southern Red Sea when I saw it passing below me in the hazy blue along a steep drop off. The tresher is simply a too rare, distant and shy animal to get lured into a baited shark session. It also prefers deeper water than the zone in which UW photographers normally feel comfortable. Probably it’s just that detachedness, combined with the magic of its enormous tail that keeps stimulating our interest and curiosity in the creature.
Left: From top to bottom: the Common, Pelagic and Big eye tresher. Picture adapted from Alessandro de Madalena.
Anatomy and general characteristics Although genetics has become a more accurate measure of relatedness than appearance, the conventional Linnaean taxonomy is still a helpful tool in distinguishing between the 400 different shark species of the World. Going from general to more specific categories this implies: order, family, genus and species. So, the tresher shark belongs to the order of Lamniformes also known as mackerel sharks. Which includes the great white, basking, megamouth and mako shark. Within this order the treshers form the family of Alopiidae or ‘foxlike’ sharks. Like other mackerel sharks thresher sharks are ovoviparous, meaning that the young develop inside a weakly formed shell within the female. Along each of their flanks runs a strip of red aerobic muscle, which can contract powerfully for long periods, enabling the thresher shark to swim without fatigue.* Treshers feed mainly on small fishes like herring, anchovies and sardines. They can be found in deep waters as well as along reef drop-offs and coastlines in many places of the world. Like other Lamnid sharks, the thresher shark (at least one of the species, the common tresher, see below) has a network of blood vessels called rete mirabile that allows it to maintain its body temperature slightly above that of the surrounding water. Its a system they share with the tuna fish. The network ensures that metabolic heath produced by its strong red muscles is not lost in the surrounding seawater, but retained inward towards the core of the body.* The treshers pectoral fins are long, a bit backwardly curved and spread out, like in the Oceanic shark. But it most conspicuous part is its enormous whiplike tail.
The tail The tresher’s elongated upper lobe of the tail fin almost equals the length of the sharks body. The only shark with a tail of almost similar length is the variegated shark (Stegostoma varium, or Stegostoma fasciatum). Because it is covered with dark spots or stripes (juveniles) it is often also called zebra shark or leopard shark (not to be confused with Triakis semifasciata). But the variegated shark belongs to the different order of Carpet sharks. Like the nurse shark it has nasal barbels and prefers shallow water, in particular sandy bottoms. Variegated shark is an oviparous (egg laying) shark that is also commonly seen in the Red Sea.
Why has nature equipped the ‘foxies’ with such an strange and enormous tail? Is it just a caprice of evolution, or an attribute with a certain advantage for its survival? A similar question is often asked with respect to the hammerheads bizarre shaped head, and the swordfish with its elongated snout. A generally accepted view is that the tresher uses its tail as a weapon or instrument for striking fish. In some amazing video captures it can indeed be seen slapping its whiplike tail and slashing its way through schools of sardines, before returning to devour stunned and wounded victims.**** Crocodiles are probably one of the few land species that are known to use their tails with a similar purpose. Some eye witnesses have even reported seeing the tresher slapping seabirds sitting on the surface. Alternatively, its long and strong tail might also enhance its propulsive power and ability to make swift turns.
Three species There exist three different species of the genus Alopius: the common tresher (Alopius vulpinus), the pelagic tresher (Alopius pelagius) and the bigeye tresher (Alopius superciliaris). The outside differences between these species may not be directly obvious. But despite the overlap in their anatomy, general appearance, behavior and habitats there still seem to be some important differences that justify their separation (see also the insert).
Common tresher The body is blue-grey to dark grey or blackish on top, with silvery or coppery sides and white undersides. It is the largest tresher that can reach a length of 6 meters. The small mouth is arched and, unlike in other thresher sharks, has furrows (labial folds) at the corners. The first dorsal fin is tall and positioned slightly closer to the pectoral fins than the pelvic fins. In addition, the shark has a stout cylindrical body with a short head, rounded between eyes. It is seasonally migratory and spends summers at lower latitudes. Common threshers tend to be epipelagic (oceanic near the surface), and are common in coastal waters over continental shelves. They are virtually circumglobal in warm seas. Common threshers are regarded by recreational anglers as one of the strongest fighting sharks, together with the short-fin mako, often leaping out of the water when caught on a fishing line. In addition, the common tresher has a vascular heat exchange system (rete mirabile) thet serves to generate and retain body heat, using the energy produced by its strong aerobic muscle.*
Pelagic tresher The Pelagic tresher was hardly mentioned some 20 year ago, probably because in many early publications it was mistaken for the common tresher or just called ''tresher'. The slender pelagic thresher is the smallest member of its family, rarily exceeding 3 meters. It has small teeth and its first dorsal fin is relatively short and is placed halfway between the pectoral and pelvic fins. It has a conical head. Around the mouth corner its has no labial furrows like the common tresher. It can also be distinguished by the dark, rather than white, color over the bases of its pectoral fins. Curiously, the pelagic tresher seems to lack the vascular heat exchange system found in the common tresher to generate and retain body heat (see also the big eye tresher below).* Although encounters between divers and the pelagic tresher are rare, it has been regularly seen in the Red Sea along offshore reefs such as Daedalus or the litte Brother islands. There have also been regular sightings at Layang Layang and Sipadan in Malaysia and Monad shoal near Malapascua in the Philippines. A Malapascua in the Philippines pelagic treshers can be seen when they visit more shallow cleaning stations for their early morning grooming*** Clearly the best opportunity for UW photographers to get closer to these shy sharks (see also the picture of Noam Kortler on this weeks frontpage).
But the pelagic tresher also shares many traits with the common tresher. It is an active, strong swimmer and has been known to leap clear of the water. It is a wide-ranging Indo-Pacific Ocean shark, apparently highly migratory, with low fecundity (two pups/litter) and a low (2-4%) annual rate of population increase. Like the common tresher it is epipelagic, although the species is reportedly relatively often in some coastal localities. It might also visit deeper mesopelagic zones when it hunts for schools of fish.
The big eye tresher’s This species can be easily distinguished by its large eyes and a pair of deep grooves on the top of its head, from which its scientific name is derived. Its enormous eyes are placed in keyhole-shaped sockets that allow them to be rotated upward. The first dorsal fin is placed more backwards than the common tresher. It is also know to visit very deep layers of the ocean in the mesopelagic zone below 200 meters. The large eyes of the bigeye thresher are probably adapted for hunting in the low light conditions of these greater depths. It is one of the few sharks that conduct a diurnal vertical migration, staying in deep and cold water during the day and moving into warmer surface waters at night to feed**. This migration likely relates to finding prey at night and avoiding predators during the day. The sharks' daytime swimming patterns are usually steady, while at night they have a pattern of slow ascents and rapid descents. Similar to the pelagic tresher it seems to lack the vascular heat exchange system found in the common tresher to generate and retain body heat. These differences could relate to the fact that the red aerobic muscle in this species as well as the pelagic tresher runs closer the skin than in the common tresher, allowing less effective functioning of the heat preservation blood vessel network and thus less metabolic heat conservation*.
Survival Overfishing in targeted shark fisheries, by-catch in fishing gear targeting other species, and high levels of illegal and unregulated fishing have caused drastic reductions in the tresher’s populations. The common thresher is widely caught by offshore longline and gilnet fisheries and is especially vulnerable to fisheries exploitation because its epipelagic habitat occurs within the range of many largely unregulated and under-reported gillnet and longline fisheries.
References and links
Sharks of the Open Ocean: Biology, Fisheries and Conservation. (2008) Blackwell Publishing Ltd Editor(s): Merry D. Camhi, Ellen K. Pikitch, Elizabeth A. Babcock Published Online: 28 JAN 2009 Print ISBN: 9780632059959
Susan E. Smith, Randall C. Rasmussen, Darlene A. Ramon and Gregor M. Cailliet. The Biology and Ecology of Thresher Sharks (Alopiidae) (pages 60–68) (Chapter 4 in the Camhi et al. book)
Compagno, L.J.V. (2002). Sharks of the World: An Annotated and Illustrated Catalogue of Shark Species Known to Date (Volume 2). Rome: Food and Agricultural Organization. pp. 8
*Sepulveda, C.A., Wegner, N.C., Bernal, D. and Graham, J.B. (2005). "The red muscle morphology of the thresher sharks (family Alopiidae)". Journal of Experimental Biology. 208 (Pt 22): 4255–4261. doi:10.1242/jeb.01898. PMID 16272248.
**Weng, K.C. & Block, B.A. (2004). "Diel vertical migration of the bigeye thresher shark (Alopias superciliosus), a species possessing orbital retia mirabilia". Fishery Bulletin – National Oceanic and Atmospheric Administration. 102 (1): 221–229.1–83. ISBN 92-5-104543-7.
The Amazone basin has an incredible diversity of animal species many of which are not yet catalogued by biologists. Glassfrogs, belonging to the amphibian family of Centrolenidae -which consists of about 12 genera- are one example. They are nocturnal animals that reside most oftheir time in the treetops of the forests. Glass frogs are small transparent creatures that will fit on a matchbox. A group of biologists from Ecuador and US discovered a new species of the glassfrog genus Hyalinobatrachium which consists of around 40 different species. The new species was found in three localities in the Amazonian lowlands of Ecuador and was baptised Hyalinba-trachium yaku (Yaku Glassfrog.) The specific epithet yaku is the Kichwa word for water. Water, in the form of streams, is fundamental for the reproductive biology of all glassfrogs. Water pollution through oil and mining activities represents one of the biggest threats for Amazonian amphibians, as well as for numerous other water-dependent species.
The Yaku glass frog. Top row: adult male in dorsal and ventral view. Bottom row: adult male seen from front and the side*
The glassfrog eggs are usually deposited on the leaves of trees or shrubs hanging over the running water of mountain streams, creeks, and small rivers. All species of the glassfrog genus have a completely transparent ventral peritoneum, which means that the belly organs are fully visible in ventral view. But Yaku differs from related glassfrogs by having small, middorsal, dark green spots on the head and dorsum (picture, upper row left), and a transparent pericardium which also exposes its beating heart (picture, upper row right). Males attend egg clutches located on the underside of leaves overhanging streams. Their reproductive behavior is also unusual, with males calling from the underside of leaves and providing parental care to egg clutches. Its transparent underside body probably gives it a clever evolutionary advantage. The silhoutte of a Yaku frog clutched upside down to the underside of a leave will be very difficult to spot by aerial predators that fly over the bushes.
In the world of sharks the great white has always been an outsider. It has a high media profile attention, and is the only shark that still carries that aura of brutal force, a ‘raw predator’ with its powerful body often covered with scars and its fearless approach of human visitors. It prefers open and cold waters, with Australia/New Zealand, South Africa and the North Eastern Pacific as the mostly frequented locations. UW photographers that visit Guadalupe island in Mexico in autumn or early winter have a good opportunity to get some nice pictures of this legendary shark. The island is the home base of the elephant seal which makes it a favorite place for foraging in that season. Unfortunately, being locked up in a cage is still the only safe option for UW photographers to get close to the great white. In South Africa at Seal island, another hotspot for meeting great whites visitors can even join great white shark breaching trips combined with cage diving. Great whites are attracted by a seal decoy pulled behind the vessel and occasionally jump with lightning speed with unbelievable precision and accuracy to take the decoy.
Upper picture: Location data for four satellite-linked radio-telemetry-tagged female white sharks leaving from Guadalupe island during their offshore migration. Adapted from **
Lower picture: Site fidelity of all satellite tagged white sharks to three core areas including the North American continental shelf waters and the waters surrounding the Hawaiian Island Archipelago and the ‘White shark Café’.Yellow circles represent position estimates from light- and SST-based geolocations, red circles satellite tag endpoint positions respectively. Adapted from ****.
Apart from the white sharks disputed reputation as a man eater and spectacular sightings we know very little of its behavior: its migratory routes, breeding grounds, socialising behavior, how it attracts a mate and what drives its search for a prey. Getting to know how and were they travel is one the challenges for marine biologists as well shark conservationists. The great white is on the red list of theathened animals of the World Conservation Union, and research that clarifies their immigration patterns, mating and pupping grounds could indeed have important implications for their conservation. Are great whites like other apex predators, that often alternate long trips in the wide ocean with visits to specific aggregation areas?
Bio-telemetry is the tool that has shed more light on these questions in the last decade. It implies detection of a shark from a certain distance with an electronic receiver that picks up a signal from a shark implanted ‘tag’. Often used are acoustic listening receivers placed at at the ocean floor at hot spots near the coastline. Whenever a tagged shark comes within a distance of say 800 feet of a receiver, a code or ‘ping'' for that particular shark will be received and transmitted. Its a useful tool to detect the presence of a shark in specific areas. More advanced sattelite based tools are PAT (Pop-up Archival Transmitting) and SPOT (Shark Position and Temperature) tags. PAT or Popup archival tags are normally active for maximal one year and provide approximate location data, making it most useful for tracking long-distance migrations. These tags remain inactive as long as the shark is submersed, until the tag detaches itself from the shark and floats to the surface and starts transmitting its collected data. They do not have to be physically recovered for the data to be obtained. Popup tags are relatively non-invasive small transmitters. They are placed with a tag pole near the base of the dorsal fin, mostly when the sharks come close to a diver or takes a bait from a small boat drifting with the shark on the surface. In contrast, the more expensive battery operated SPOT tags will transmit information for several consecutive years, making it the first choice for studying long term migration periods. These tags do not work underwater but transmit a signal to a satellite receiver whenever the dorsal fin breaks the surface of the water. They are more difficult to deploy since the shark has to be caught and restrained while the tag is attached to the dorsal fin. Which requires hooking the shark with a line and bait supended from a vessel, hauling it on board of the vessel, where the transmitter is attached by drilling holes in the apex of the dorsal fin and secured with metal or plastic bolts. It certainly involves some form of mutilation of the shark, justified by the argument that it provides a more complete picture of the sharks migration routes than PAT tags.
GWs are pelagic as well as philopatric Some useful information has been collected with sattelite tagging in the past years on its migration and breeding sites. An important finding was that great whites are pelagic (go out in the open sea) as well as ‘philopatric’ (returning to the same sites). In the open ocean they often display long migration paths with strange and irregular patterns. Males and females also seem to differ with respect to the duration of their migration phases. Some more details are presented below.
South Africa In South Africa, great white sharks in False Bay do not stay at Seal Island year-round, although it provides their favorite food, cape fur seals*. Most of the PAT tagged white sharks revealed at least three different movement patterns, including wide-ranging coastal migrations up and down the eastern side of South Africa. Often they leave the island for long trips in the Ocean, even extending over thousands of miles and crossing oceans separating continents. Most notorious case is female shark Nicole (named after Nicole Kidman) that was tagged in Gansbaai South Africa and migrated 11,000 km to Western Australia. Nicole was first tagged in November 2003, near the Western Cape of South Africa where researchers affixed a pop-up archival satellite transmitting tag to her dorsal fin. This is not a unique event; there have also been a few other recorded incidents of great whites from South Africa migrating to Australia and back within a year. This also included return of females mating in Australia to give birth in South Africa.
Pacific white sharks: Guadalupe island migrations Most detailed information on great whites migratory patterns has been collected from northeastern Pacific white sharks, revealing a seasonal migration between a vast offshore region and coastal aggregation sites. The sharks spend roughly half of their time in the deep-ocean environment, sometimes traveling as far as the Hawaiian Islands before returning to the continent. Hawaii is likely to be an important foraging area for great whites. A study by investigators of the Marine Conservation Science Institute used satellite-linked radio-telemetry (SLRT) tags with a multi-year battery capacity on a limited group of sharks.** (see insert: upper figure for aggregated data of 4 females). It confirmed a strong seasonal philopatry of GWs to one of two aggregation sites in the northeastern area: one off central California, USA, and the other at Guadalupe Island. Mexico. There were also interesting sex differences. Females showed a 2-year migration period that rougly consisted of a offshore gestation period phase which began when the females departed Guadalupe island. They spent around 18 months patrolling in the great ocean between Baja California, Mexico and Hawaii before migrating back to the coastal regions of Mexico during the pupping season. During this phase, females remained in the coastal waters of Baja California, after which they returned to Guadalupe again for the autumn and winter months, probably with a double purpose: feeding on the seals and mating.
White shark ‘café’ Tracking data of this study also indicated that during the offshore phase mature males and females were spatially strictly segregated, but joined again during their concurrent seasonal presence at Guadalupe Island. For males the offshore migration phase lasted only one year. Curiously, males in particular had a strong preference to hang around at a location approximately halfway between the coast of Baja California, Mexico and the Hawaiian Islands, also called shared offshore foraging area (SOFA) or White shark Café. This area lies within the eastern boundary of the North Pacific Gyre. Their preference for this destination remains a mystery because it is presumed to have very little food for the animals; researchers describe it as the shark equivalent of a desert (see also ***)
North Pacific Californian coastline Another study was carried out on a much larger group of North Pacific sharks by biologists from the Department of Biology, Stanford University. **** They mainly focused on the migratory routes of great whites of the California coast, using satellite PAT tags in combinaton with passive acoustic monitoring via listening stations near the coastline. The Californian sharks also alternated site fidelity at hot spots along the Californian coastline with long-distance migrations to and from defined oceanic core areas (see insert: lower figure for aggregated data). This included the same foraging area (White shark café) visited by the Guadalupe island sharks. “What we know," said Salvador Jorgensen, who co-authored the study, "is that all of them leave the coast in the winter and all of them end up either in the cafe or offshore in Hawaii." By comparing overlap between male an female sharks the study suggests that mating might also occur at the Cafe. Observed overlap was minimal near Hawaii and ‘’no direct or indirect evidence of copulation at North American coastal sites has ever been reported, despite decades of observation’. So the issue of mating sites still remains a bit of an open question in both studies of north Pacific whites.
Sources and links
* Bonfil R, Meyer M, Scholl MC, Johnson R, O’Brien S, Oosthuizen H, Swanson S, Kotze D, Paterson M: Transoceanic migration, spatial dynamics, and population linkages of white sharks. Science 2005, 310:100–103
**Domeier ML, Nasby-Lucas N: Migration patterns of white sharksCarcharodon carcharias tagged at Guadalupe Island, Mexico, and identification of an eastern Pacific shared offshore foraging area. MarEcol Prog Ser 2008, 370:221–237.
***Domeier ML, Nasby-Lucas N, Palacios DM: The Northeastern Pacific white shark Shared Offshore Foraging Area (SOFA): A first examination and description from ship observations and remote sensing. In Global Perspectives on the Biology and Life History of the White Shark. Edited by Domeier ML. Boca Raton: CRC Press; 2012:147–158.
****Jorgensen, SJ; Reeb, CA; Chapple, TK; Anderson, S; Perle, C (2010), "Philopatry and Migration of Pacific White Sharks", Proceedings of the Royal Society B, 277: 679–688
The giant oarfish (Regalecus glesne) is a rare and strange customer indeed. The oarfish is assumed to live in deepwater in the mesopelagic zone between 200 and 1000 meters. Its resemblance to the giant moray eel is only superficial, because it has no teeth but gill rakers meant to catch tiny organisms and filter out plankton. It also has wide round eyes, a reflective silver body covered with black dots and a long pinkish dorsal fin starting between its eyes to the tip of its tail. The fin rays are soft and may number up to 400 or more. At the head of the fish, the rays are lengthened forming a distinctive red crest. The oarfish derives its name from the long paddle-like appendages sprouting downwards from the head. The stories of sea serpents may in fact be accounted for by sightings by fishermen of the giant oarfish with its undulating way of swimming when it came near the near the surface.
Despite its large size the oarfish is not a very strong swimmer. It moves through the water undulating its large body with the dorsal fin as a means of propulsion while the body axis is held straight and stable, called amiiform locomotion. The long antenna shaped tendrils and ’oars’ that protrude from the gills may be used to sense for orientation and keeping balance.
Its flesh seems to be rather untasty, the reason why they are unwanted bycatch of fishermen that occasionally find a specimen in their nets. Most sightings of the oarfish are on land, when they wash up dead or dying on a beach and become a popular target for visitors and photographers (picture above). The reason why the oarfish strands so often remains a mystery. One possibility is that it is essentially a deep see dweller and a poor swimmer that lacks a swim bladder. It becomes helpless when it is caught by strong currents that push it to the continental shelf where it strands on a beach, or becomes battered to death on the rocks by larger swells. It may also become confused by the sudden changes in pressure when it enters more shallow waters. A curious finding is that the oarfish often seem to be missing a part of its tail. Since there are no bite marks, it could be a sign of autotomy, the ability to willingly cast off a body part like lizards do. With its vital organs concentrated towards the head end of the body, a loss of the tail would therefore not necessarily do the fish much harm.
Source and links
http://news.nationalgeographic.com/2017/02/pictures-oarfish-philippines/ poorly understood