28. Jun, 2017

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 pikeblenniestubeblennies 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

https://en.wikipedia.org/wiki/Blennioidei

https://en.wikipedia.org/wiki/Combtooth_blenny

http://www.fishbase.org/Summary/FamilySummary.php?Family=Blenniidae

Blennies - Species, Found, Fish, and Teeth - JRank Articles http://science.jrank.org/pages/960/Blennies.html#ixzz4lE5YqF4Y

http://www.uwphotographyguide.com/secretary-spinyhead-blenny

*http://www.divernet.com/photography/p315910-be-the-champ!-the-tompot-blenny.html

 

 

13. Jun, 2017

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.

 ***Oliver SPHussey NETurner JRBeckett AJ. Oceanic sharks clean at coastal seamount. PLoS One. 2011 Mar 14;6(3):e14755. doi: 10.1371/journal.pone.0014755.

**** Oliver SPTurner JRGann KSilvosa MD'Urban Jackson T Thresher sharks use tail-slaps as a hunting strategy. PLoS One. 2013 Jul 10;8(7):e67380. doi: 10.1371/journal.pone.0067380. Print 2013.

https://en.wikipedia.org/wiki/Common_thresher

https://en.wikipedia.org/wiki/Bigeye_thresher

http://www.elasmo-research.org/education/shark_profiles/a_vulpinus.htm

http://wetpixel.com/articles/thresher-shark-images-on-wetpixel-facebook-group

 

31. May, 2017

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 speciesThe 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. 

Sources:

*http://zookeys.pensoft.net/article/12108/element/8/32387/

 https://doi.org/10.2994/057.005.0101

21. May, 2017

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 B277: 679–688

https://en.wikipedia.org/wiki/White_Shark_Caf%C3%A9

http://www.whitesharktrust.org/migration.html

https://www.niwa.co.nz/coasts-and-oceans/research-projects/white-sharks

 

6. May, 2017

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

Smith-Vaniz, W. F. (2015). "Regalecus glesne". The IUCN Red List of Threatened Species 2015: e.T190378A21911480doi:10.2305/IUCN.UK.2015-4.RLTS.T190378A21911480.en

http://britishseafishing.co.uk/king-of-herrings/

https://en.wikipedia.org/wiki/Giant_oarfish

http://news.nationalgeographic.com/2017/02/pictures-oarfish-philippines/ poorly understood